According to Apps chief Michael Pettersen, one recurring question is: “Will my microphone (wired or wireless) model ‘X’ work with my whatchamacallit?” Or “Why doesn’t my Shure mic work as expected with my new i-Thingamabob?”  While we’d love to have the microphone input specifications of every device in the world that needs a microphone, it’s just an impossible task.    Instead, we’ve provided a few signposts that will help your navigate this sometimes-frustrating journey.

To select the proper microphone, it is essential to have specifications for the microphone input of your device.  Typically, these specifications will be provided in the Owner’s Manual, most of which (for current or recent products) are online and free.

Vital Microphone Input Specification #1
Called ‘Input Sensitivity’ or ‘Nominal Input Level’, this specification indicates how strong of a signal the microphone must supply to satisfy the microphone input of the device. This specification might be given in millivolts (mV), or volts (V), or in a minus dB form (-dBV, -dBm, -dBu, -dBs).

In the Shure product line, there is a wide variation of microphone signal levels available depending on the model. If a microphone is selected whose signal level is too low for the device, the audio will be noisy and low in level.  If a microphone is selected whose signal level is too great for the device, the audio will be distorted and unintelligible.  Proper matching of the microphone’s signal level to the device’s required input level is imperative.

Vital Microphone Input Specification #2
 Called ‘input impedance’ or ‘actual input impedance’, this specification is important as it determines the proper impedance range of the chosen microphone. This specification will be given in ohms.  Contrary to popular audio mythology, the impedance of a microphone does not need to exactly match the input impedance of the device.

In the Shure product line, there are different impedances available depending on the microphone model. If a microphone is selected whose impedance is lower than, or equal to, the device’s input impedance, the microphone will work as long as it provides the proper signal level – see #1 above.  If a microphone is selected whose impedance is greater than the device’s input impedance, the microphone will not deliver its full signal level to the device and the audio will be noisy and low in level.

Vital Microphone Input Specification #3a, #3b, And #3c
The final requirements are:

a) The type of microphone input connector on the device
b) How many connection points are inside the connector
c) What is the function of each connection point

This specification will be the name of the connector, such as: XLR female, 3.5 mm mini-phone jack, TRS 1/4 inch female phone jack, screw terminals, TINI QG connector.

Each of these connectors has at least two connection points and most have three (or more) connection points.  Common circuit functions include: ground, audio send, audio return, and DC bias. It is imperative that the circuit function of each connection point be known so that the proper microphone wiring can be determined.

In the Shure product line, there are different wiring schemes available depending on the microphone model. If the microphone connections are not properly matched to the device’s input connector, there may be no audio, or funny sounding audio, or the microphone might be damaged if there is an unexpected voltage appearing on the device’s connection points.

And finally, just because two connectors mate does not mean that each is wired the same.

Lab Test
Now that we knew what we were looking for, we set out to determine how easy or difficult it might be to find this information. We put it to the test by thinking about which kinds of devices you might want to connect to your microphone and settled on BeachTek’s DXA-SLR, a camera adapter that’s tripod bushing-mounted and used to power two condenser mics.

The challenge: Answer these questions with available resources online or contact the manufacturer.  And if we contacted the manufacturer, how long would we wait to get a response?   Here’s what we discovered:

Input sensitivity
In this case, it relates to the camera device (rather than the adapter)

LO Gain:             -51dBu
HI Gain:              -36dBu

Input Impedance
According to our BeachTek expert: “If you look at the specs of most mics, they usually indicate a minimum input impedance of 1k ohm. Virtually all of today’s audio equipment has an input impedance much higher than that.”

Type of connector

• Balanced XLR (2)
• Unbalanced mini-jack (1)
• RCA inputs (2)  – for playback monitoring

 Number of connection points

• 3 pin for XLR

Function of each connection point
On XLR connectors, these are standard:

• Pin 1 – Ground pin
• Pins 2 & 3 – Signal carrying

BeachTek on condenser microphones: “These mics have much higher sensitivity than dynamic mics but require power to operate. They will either have a built-in battery or be powered by an external power source called phantom power – normally 48 volts. Phantom power can come from a mixer, camera or our DXA-SLR adapter – it sends 48 volts down the XLR cable on pins 2 and 3 to power the mic.”

We were able to find most of the information we were looking for by checking out the operating instructions for the DXA-SLR we found online. Remaining answers were provided by BeachTek’s CEO, Harry Kaufmann, who picked up the phone and called us back within an hour or two of our email message.  According to Harry, “In this case, we’re manufacturing the adapter, so customers have to be especially careful to match the mic’s characteristics to the device’s, which in this case is a camera.”  And just like Shure, he said, “We get calls, too. There’s almost nothing more important than educating our customers.”

CONCLUSION
So there you have it. There are many variables that affect whether a particular microphone (wired or wireless) will work properly with a particular device.  Still stumped?  If online resources or the printed materials that came with your device don’t include the specifications indicated here, get in touch with the manufacturer.  Not all will have the real time ‘talk to a real human being’ technical support that Shure offers, but most will offer some level of customer service.

 Still, it doesn’t have to be guessing game.  Understanding the basics of how wireless systems and radio waves function will help you consistently triumph over dropouts, interference and distortion.  You can start right now by avoiding these common errors.

 1. Signal blockage

Maintain line-of-sight between the transmitter and receiver antennas as much as possible. Avoid metal objects, walls, and large numbers of people between the receiving antenna and its associated transmitter.  Ideally, this means that receiving antennas should be in the same room as the transmitters and elevated above the audience or other obstructions.

The human body can also interfere with wireless signals. Largely composed of water, our bodies absorb RF energy. In addition, if a user cups his or her hands around the external antenna on a handheld transmitter, its effective output can be reduced by 50 percent or more. Similarly, if the flexible antenna on a bodypack transmitter is coiled or folded, the signal suffers.

 2. Incorrect antenna type or placement

Receiver antennas are one of the most misunderstood areas of wireless microphone operation. Mistakes in antenna selection, placement, or cabling can cause short range, dead spots in the performance area or low signal strength at the receiver that leads to frequent dropouts. Modern diversity receivers offer much better performance than single-antenna types, but the right antennas must still be put in the right place to maximize the performance and reliability of the system.

To ensure good diversity performance, space antennas apart by at least one-quarter of a wavelength (about 5 inches at 600 MHz). The receiver antennas should be angled apart in a wide “V” configuration, which provides better pickup when the transmitter is moving around and being held at different angles.
Try to keep antennas as close to transmitters with line of sight as is possible. Antennas can also be frequency band-specific. Don’t try to use an antenna from another system without double-checking the frequencies first.

If the receiver will be located away from the performance area (in an equipment closet or a closed rack, for example), ½-wave antennas or directional antennas should be remotely mounted (ideally above the audience) in order to have a clear line of sight to the transmitters. (Short ¼-wave antennas should never be remotely mounted, however, because they use the receiver chassis as a ground plane.) Increasing the separation between diversity antennas up to one wavelength (about 20 inches at 600 MHz) will improve diversity performance.  Beyond one wavelength, extra distance between the antennas will not significantly improve diversity performance, but may allow better coverage of a large stage, church, or meeting room.

If the antennas will be far from the stage, use directional antennas to improve reception by picking up more signal from that direction and less from other angles.

If the antennas will be connected to the receiver with a length of coaxial cable, in-line antenna amplifiers may be required to overcome the inherent signal loss in the cable. The amount of loss depends on the exact length and type of cable used, so follow the manufacturer’s recommendations.  Total net loss should not exceed 5 dB.

3. Poorly coordinated frequency set

A properly coordinated set of wireless frequencies must satisfy two criteria:

• Frequencies must avoid local active TV channels
• Frequencies must be mutually compatible

 Television transmitters may operate at power levels up to one million watts while wireless microphone systems typically have only 50 mW (fifty one thousandths of one watt!) or less  output power.  To combat broadcast television interference, avoid using frequencies of local active TV channels.

How local is local? “Local” is generally considered to be up to 50 or 60 miles, depending on the coverage area of the particular TV transmitter and on the location of the wireless microphone system. The good news is that indoor setups are at less risk than outdoor setups because building structures will usually strongly attenuate TV signals. Inside buildings of substantial construction, it may be possible to ignore TV stations as close as 30-40 miles. Still, since the locations and assignments of television stations are well known, it’s pretty easy to choose relatively safe wireless microphone system frequencies in a particular area.

To insure a mutually compatible set of frequencies once the local TV channels have been taken into account, it is necessary to use one of two methods.  The simpler method is to use the “Group” and “Channel” frequencies that are already programmed into the wireless systems.  By using Channels that are all in the same Group, compatibility is guaranteed for small setups of like equipment.  The appropriate Group and Channels can be determined from a link to the manufacturer’s website or often by using the built-in “Scan” function on the receiver itself.

If the wireless setup is more complex, for example using wireless microphones and wireless in-ear monitors together, it may be necessary to use a frequency coordination computer program to insure compatibility.  Wireless manufacturers can assist in these situations.

 One frequency does not fit all. If you are touring, one consequence of the newly dense TV channel distribution in the US is that it is not generally possible to use a given set of wireless microphone frequencies everywhere in the country.

There is no such thing as “set and forget”.

Even if your audio system doesn’t move from place to place, the radio environment can change unexpectedly. It’s largely true that television stations remain constant, but if there are other wireless systems in the frequency band – whether it’s multiple systems in your own location or interference from the coffeehouse down the street – your wireless frequencies may need to be adjusted. What worked at sound check may not be failsafe when the show begins. And that’s why frequency coordination is so important.

 4. Poor battery management

Despite the fact that transmitter battery life is a top concern with wireless mics, users continue to try and cut operating costs by using inexpensive batteries. Most wireless manufacturers specify alkaline or lithium single-use batteries because their output voltage is very stable over the life of the battery. This is important because most transmitters will exhibit audible distortion or signal dropouts when supplied with low voltage. Rechargeable batteries often seem like the ideal solution, but many rechargeables provide about 20 percent less voltage than a single-use battery — even when they are fully charged.

 To combat battery problems, carefully compare the transmitter’s voltage requirements with the battery’s output voltage over time to make sure that the battery will last through a full performance. For 9-volt applications, there are lithium-ion types that work well, while Ni-Mh and Ni-Cad batteries may last only a couple of hours. For AA applications, Ni-Mh rechargeables offer similar performance to single-use alkaline batteries.

Using rechargeable batteries is a great way to save money and landfills as long as you or someone on your staff is able to effectively manage them. Remove batteries from transmitters after each performance. This will keep you from using half-dead batteries the next time you need them and will also prevent an accidental leak from damaging your transmitter if stored for an extended period of time.

 5. Improper gain set-up

Setting the proper input gain is one of the most important adjustments on a wireless microphone system. Distortion may occur if the gain is set too high, while poor signal-to-noise may result if the gain is set too low.

Most wireless systems have a gain control on the transmitter itself in the form of a switch, a pot, or a programmable adjustment.  It may help to think of this gain control as serving the same function as the “trim” or “gain” adjustment on a mixer.  Its purpose is to set the input sensitivity low enough to prevent input overload or “clipping” but high enough so that the signal level is well above the system noise floor.

Adjustment of the wireless transmitter gain is done in the same way as mixer input gain:  set the gain control so that the loudest input signal just barely lights the overload or peak indicator.  For a wireless system this indicator is usually on the receiver, so it is necessary to observe the receiver front panel while the performer is singing or playing.  If the peak indicator is flashing constantly, reduce the transmitter gain until it flashes only occasionally.  If the indicator never flashes, increase the gain until it flashes just on the loudest signals.

Many wireless microphone systems have an output level control on the receiver.  Since this control only affects the receiver output, it has no effect on improper gain adjustment in the transmitter.  That is, if distortion or poor signal-to-noise is occurring in the transmitter, it cannot be “fixed” by changing the receiver output level.  Most professionals recommend leaving this control at maximum.  As long as the mixer input can accommodate this level, the overall system will exhibit the best possible dynamic range.

Bill Gibson has spent the last 30 years writing, recording, producing and teaching music. He is well known for his production, performance and teaching.

Bill is Developmental Editor for Hal Leonard Performing Arts Publishing Group, President of Northwest Recording, serves on the National Advisory Board for the P&E Wing of the Recording Academy, is an instructor for Berklee College of Music and the Art Institute of Seattle. He has authored over 30 books, including his most recent book with Quincy Jones (Q on Producing) and his upcoming work with the legendary Bruce Swedien (The Bruce Swedien Recording Method).

1. Does the directional characteristic make a difference in the sound of the mic?

Absolutely!

Omnidirectional Mics hear equally from all directions, not rejecting sound from anywhere in the 360-degree sphere around the capsule. They have an open and natural sound and they’re used frequently in the studio when the engineer wants to include the sound of the room in the recording.

The danger in using an omnidirectional mic is that any room sound (ambience) that is recorded is there to stay. For most people it’s safer to record with a more directional microphone (cardioid or hypercardioid) and add any ambient sound artificially during mixdown. However, in a controlled and well-tuned acoustical environment, an omnidirectional mic is frequently the way to go because the close-miked vocal sound it captures is more open and less cluttered in the low and low-mid bands than a cardioid mic used in the same way. A singer who moves in very close to the omnidirectional mic sounds very intimate while retaining a more natural and clear tone.

Cardioid Mics directional characteristic prefer the front of the microphone (on-axis) and they reject sounds coming from behind the mic (off-axis). Microphones with cardioid polar patterns, such as the Beta 58 or SM58®, are typically better suited to close- than distant-miking applications. These mics exhibit frequency response characteristics that roll off in the low band to compensate for their susceptibility to the proximity effect—the boomy, bass-heavy sound we hear when a voice or instrument is extremely close to the mic (within less than a couple inches or so). Because they prefer the on-axis sounds, they help reduce the relative levels of room ambience and other sounds that are off-axis.

Bidirectional Mics are most sensitive to sounds in the front and back of the mic but they exhibit almost complete reject of sounds that come from the sides. Some large-diaphragm condenser mics, such as the KSM44, can be set to bidirectional configuration and ribbon mics, such as the KSM353 and KSM313, are naturally bidirectional. These mics are well suited to miking solo instruments or voice, but they also provide an efficient and convenient way to close-mike two vocalists—or other instruments—at the same time. Bidirectional mics exhibit the most extreme proximity effect, in comparison with cardioid and omnidirectional mics. Therefore, the close-miked sound they provide is sometimes too boomy and full to be useful. From a more distant perspective, however, bidirectional mics provide a very nice, full tone, which includes a little more acoustical ambience than a similar cardioid mic.

In a live sound reinforcement application, omnidirectional mics are the most prone to feedback. They don’t reject sound from any directional and are inappropriate for most applications. Also, keep in mind that floor and stand monitor positions are usually different depending on the mic choice. When using a cardioid pattern, there is usually less feedback with the monitor directly in front of the vocalist. When using a hypercardioid pattern, the monitor should be placed slightly to one side or the other in front of the vocalist for minimal feedback. If you look at the polar response graph for the specific mic, you’ll notice exactly where the mic is least sensitive—that’s the right spot for the monitor.

A mic like the KSM9 is a great choice for vocals in a live setting. It sounds like a studio condenser mic and it offers pattern selection between cardioid and hypercardioid.  The flexibility provided by selectable patterns makes a mic that would already be exceptional, even better.

Keep in mind that every singer is different. If you have a choice of mics and directional characteristics, simply select the pattern that sounds best for the vocalist or choose the pattern that provides the best feedback rejection. In the studio, a mic like the KSM44 is an excellent choice because the selectable pickup patterns let the engineer choose the texture, tone, and feel of the vocal track by simply changing between any of these directional characteristics.

2. Is handling noise really an issue? Aren’t all mics about the same?

Yes. Handling noise is an important issue, especially in a live setting.

Many studio mics aren’t designed to be handheld. They’re placed in specially designed shock mounts that protect them from vibrations, bumps, and thumps. However, mics that are used in a live handheld environment must contain ample internal shock mounts and vibration control.

If you line up ten different mics on stands, you’re likely to notice dramatic differences in the sound caused by simply removing each mic from its clip. Some mics even rumble in normal handheld use. They don’t sound good although they don’t sound terrible, but the amount of handling noise they produce makes them completely unusable. Just shifting the mic in your hand causes a dramatic rumble—the sound of putting them in and out of the clip is unacceptable.

Mics that exhibit excessive handling noise also pick up excess amounts of noise from anything that moves on, or vibrates, the stage, such as footsteps, the kick drum, the bass cabinet, dancing, and so on. One of the reasons for the popularity of the SM and Beta series mics from Shure is excellent design of their internal shock mounting systems and their minimal handling noise.

3. How close should the lead vocalist be to the mic?

This question is usually borne out of the frustration that the sound operator feels when working with a singer who has bad mic technique.

It’s common for an inexperienced sound operator to tell the singers to just stay close to the mic (within an inch or less). That’s definitely not the best approach, but it puts control in the hands of the sound operator.

Vocalists must learn to move closer to the mic when they are quiet and farther away when they’re loud—the actual distances depend on exactly how quiet and how loud. In addition, speaking too close to the mic can decrease intelligibility and clarity. The overall volume of the house mix, the size of the audience, and the acoustics in the room are also considerations in mic technique.

Work with each singer to determine the mic technique that works the best for him or her. Determine three ranges of mic distances for three separate purposes:

The “I’m singing background” distance—usually 1.5 to 3 inches (2 fingers to 4 fingers).

The “I’m singing a quiet, intimate lyric” distance—usually 1 inch or less (1 finger or less).

The “I’m really belting it out and I don’t want to hurt someone’s hearing” distance—usually 6 inches to arm’s length, depending on the singer, the song, the instrumentation in the band, and the size of the room.

4. Our lead singer gets lost in the mix and yet there are times when she’s way too loud. How can I get a smooth and even vocal sound, like the sound I hear on professionally produced recordings?

Considering that your singer has good mic technique and you’re riding the vocal levels to help with global differences between levels for speaking and belting, the sound you’re looking for is probably a result of compression. A compressor is an automatic volume control that responds to the strength of the incoming signal. The sound operator sets a threshold level. When the signal strength exceeds that threshold, a built-in amplifying circuit—typically a VCA (voltage controlled amplifier)—turns the signal down.

A compressor is essentially an automatic sound operator. Like you, it turns the signal down when it’s too loud and then back up to where it started when it’s not too loud. The attack time, release time, and ratio controls let you determine whether the compressor acts like a Masserati or the giant in Jack and the Beanstalk.

Compressors only turn the signal down—they don’t boost levels. However, the effect of compression is to enable the nuance in the vocals to be heard better. Because the loud parts are turned down, the entire channel can be turned up. The gain reduction meter indicates the amount of gain reduction. If it shows that the channel is being turned down by 6 dB at the loudest parts of the performance, then the entire channel can be boosted at the channel fader or at the “Makeup Gain/Output” control on the compressor. This results in the loud passages being the same volume as they would have been but the softer passages, vocal nuance, and emotional inflections have been turned up by 6 dB—they are, therefore, more audible to the audience.

Setting up a compressor is really pretty simple:

1. Set the ratio control to determine how extreme the action is—typically between 4:1 and 7:1 for vocals. If the ratio is x:1, for every x dB that exceeds the threshold, the VCA will only let 1 dB through.
2. Set the attack time—typically between 5 and 10 milliseconds.
3. Set the release time—typically about 1/2 second.
4. Adjust the threshold so that there are times when there is no gain reduction and times when there are about 6 dB of gain reduction.
5. Boost the Makeup Gain/Output control to makeup for the gain reduction.

Often, recordings are extremely compressed. In a live setting, be aware that if the compressor is reducing the gain substantially during a performance, once the performance is over, the VCA will let the signal return to its original level—this can easily cause massive feedback. The amount of compression you use in a live performance is dependent of the amount of gain before feedback in your system. In a live application, it’s usually best to compress by 6 dB or less.

5. Should I always buy a mic with a flat frequency response curve?

No. Part of the reason for differences in response curves is the intended application. If you use a mic with a flat frequency response on a live, close-miked vocalist, the sound will be thick and muddy because of the proximity effect. If you use a mic that’s designed for close-miking, for instance, a distant mic on an acoustic ensemble, the sound will be far too thin and weak.

The SM58® or Beta 58 have frequency response curves that roll off in the low end with a presence peak in the high end. This fact doesn’t make it a lower quality mic than a mic like the KSM32 or KSM141 that exhibits a flat frequency response—it just makes them better suited to close miking than distant miking.

When a handheld vocal mic is close to the singer’s lips—within a few inches—the proximity effect rounds out the lows so they are essentially flat. Low frequency response is determined by mic distance. The built-in presence peak helps provide a clear and understandable vocal range. Notice that these presence peaks are typically between 4 and 7 kHz—strategically positioned in the range of vocal sibilance and intelligibility.

Mics with a flat frequency response curve are best suited to distant-miking applications in which the mic is a foot or more from the source, and yet a full sound is desired. Many condenser mics exhibit a very flat frequency response; however, they often provide a low frequency roll-off switch to compensate for the proximity effect when used in a close-miking application.

6. What’s the difference between miking a vocalist in a live performance and miking a vocalist in the studio?

The difference is really much less than it used to be before the KSM9.

In a live setting, we use vocal microphones designed for close-miking. They have historically been moving-coil mics because of their dependability, ruggedness, and simplicity; however, moving-coil mics don’t capture the fine transient detail as accurately as condenser mics.

In the studio we have historically used large–diaphragm condenser mics for vocals. Since the acoustics are controlled in a studio and leakage isn’t a consideration, most studio vocals are recorded from a distance of 6 to 12 inches. Sometimes, the singer moves closer, but the mic might be set to an omnidirectional configuration so the sound isn’t too thick and muddy or the low-frequency roll-off might be applied to compensate.

The vocal sound is adjusted by moving the mic across a much greater distance range than in a live setting. In addition, many professional studios have excellent acoustics—the sound of the room blends very well with the vocal to provide a desirable character and personality.

The KSM9 utilizes a studio-quality condenser capsule that provides the type detail that’s expected in a studio sound. It is housed in a body that feels good in the hand and the capsule sits in a well-designed shock mount system—it sounds great and rejects handling noises and vibrations very efficiently.

New for 2012: Bill’s “The Ultimate Live Sound Operator’s Handbook: 2nd Edition,” a 428-page book, including a DVD full of excellent audio and video examples.

In addition, Bill recently released “Q on Producing,” the first of three books he’s writing with the legendary Quincy Jones.

1. Gain set-up is crucial for the proper operation of the wireless transmitter.Handheld transmitters can be overdriven by a vocal presenter if the gain is set too high and under-driven if the gain is set too low. Both situations can lead to poor results.

   Always try to adjust for the talent of the day – not all talent are the same; some sing or speak softly requiring more initial gain and others are very loud and require minimal gain. Body-pack transmitters are frequently used with guitars that have a higher output and therefore require less gain (or more pad).

2. Use fresh batteries with a full charge for each event.Using rechargeable batteries is a great way to save money and landfills as long as you or someone on your staff is able to effectively manage them. Remove batteries from transmitters after each performance. This will keep you from using half-dead batteries the next time you need them and will also prevent an accidental leak from damaging your transmitter if stored for an extended period of time.
3. Make sure that you have chosen a clean frequency in which to operate your wireless system.You can use the frequency calculator on Shure’s website to help determine which TV channels to avoid. If you still have questions, Shure’s Applications Engineering Group is glad to help.
4. Antenna placement and set-up is important.Try to keep antennas as close to transmitters with line of sight as is possible. Antennas can also be frequency band specific – don’t try to use an antenna from another system without double-checking the frequencies first.

   Never let antennas touch one another. If you are using more than one wireless system, make sure you leave at least a foot between antennas from different receivers.

   Make sure all connections are solid. For antennas, double-check the center pin on the BNC connectors to make sure it isn’t bent or broken. If you need to remotely locate an antenna, be sure to use the correct cable – not all coax cable is the same. Cable used for television will not perform the same as cable designed for use with antennas. Consult the Shure website or tech support group for assistance in picking the right product for your application should you have any questions.

5. It is important to dry and clean off the body pack before storing.Many times a body pack transmitter can get wet from a performer (sweat). Using a hairdryer (set to low or no heat) is a great way to speed up the process. Storing bodypacks with silica gel desiccant packets also works well.

Why Sound Quality is Important
Popular media such as compact discs, movies, and music videos have helped to raise our standards for what qualifies as “good sound.” Unfortunately, the sound quality in distance learning classrooms often leaves much to be desired. Poor sound quality will make it more difficult for students to understand the material being presented. It may also reduce their level of interest and participation in the class. Classroom audio systems that are poorly designed or implemented will inhibit interaction between students at different class sites, because they will not always be willing to make the effort required to overcome technology barriers which prevent them from being heard.

Complaints from instructors and students about classroom sound typically run along these lines:

• “It sounds like the person speaking is at the bottom of a barrel.”
• “When someone talks, the first couple of words get cut off.”
• “We can’t turn it up loud enough to hear without getting feedback or howling.”
• “When we talk, we hear an echo of our own words.”

The technical and operational causes of these problems are rarely apparent to someone who does not have specific audio training. Fortunately, an understanding of just a few basic audio concepts can help the designer, installer, or user of a distance learning audio system to achieve very good sound quality in the classroom environment.

What it Takes to Make a Distance Learning Classroom Work
There are four main parts which make up a distance learning classroom:

• the classroom itself,
• an audio system,
• a video system, and
• a link to a transmission network.

This paper will focus on those variables in the classroom and the audio system which determine the quality and intelligibility (the ability to understand what is being said) of the sound that is transmitted to listeners at other classroom sites, and what must be done to control those variables.

While the choice of transmission medium – fiber optics, satellite, microwave, etc. – can have a significant impact on sound quality, a comprehensive discussion of the many different transmission schemes available and their strengths and weaknesses is beyond the scope of this paper. We will discuss, however, particular audio equipment which may be required to compensate for the effects on sound quality of certain transmission media.

PART 2 – THE CLASSROOM

Room Acoustics vs. the Audio System
The sound that is sent out of the classroom to other sites begins with the classroom itself. The acoustics of the room – that is, the way in which the room affects sound waves – are determined by physical characteristics, such as the size of the room and what materials are used to construct and cover surfaces such as walls and floors. If the acoustics of the room are poor, the sound picked up by the audio system and transmitted to other sites will be unclear and fatiguing to listen to. In extreme cases, voices may be nearly unintelligible, or interaction may be so difficult that teaching cannot take place.

The classroom and the audio system both have a major impact on sound quality. For sound that is crystal-clear, and for the audio system to be as cosmetically appealing and “user-friendly” as possible, both the acoustics of the classroom and the design of the audio system must be optimized. In many instances, however, certain restrictions may force compromises in one area, which must be offset by optimization in the other area. For instance, if circumstances dictate the use of a classroom with poor acoustics, then there will be little room for compromise in the design of the audio system. Similarly, if budgetary constraints or user preferences force compromises in the design of the audio system, it becomes critical that the acoustics of the classroom be very good. When a poor audio system is combined with poor room acoustics, results are usually so unsatisfactory that instructors and students prefer not to use the system at all.

Room acoustics vs. audio system: if one must be compromised, the other must be optimized in order to maintain acceptable sound quality.

When you are present in the same room as a person talking, your brain makes use of both the aural information supplied by your ears and the visual information supplied by your eyes. This combination of sight, sound, and brain power allows you to “ignore” or “filter out” some of the noise and undesired sound, and to concentrate on the desired sound (the talker.) A microphone does not have this ability, so it must be able to “hear better” than a human listener would in order to pick up clear and intelligible speech. But what makes one room sound good, and another sound poor? And is it possible to know in advance whether a particular room is a good choice – from an acoustic standpoint – for distance learning?

How to Tell if a Classroom is Acoustically Suitable for Distance Learning
Room acoustics is a broad term comprised of many components. For our purposes, there are two acoustic variables which have a major impact on the sound of a classroom: decay time and background noise level.

Decay Time — Sound waves emanate from a talker in all directions. Some travel directly from the talker to the microphone, while others take a roundabout route, bouncing off of the walls, ceiling, or floor. The reflected sound reaches the microphone later than the direct sound, and blends with it to become audible as a continuation or “smearing” of the original sound. Decay time is how long it takes for this reflected sound to weaken or “decay” to the point that it can no longer be heard. The decay time of a room is determined by its size, shape, and construction. The decay time in a large marble cathedral might be as much as 5 seconds; in a classroom with concrete block walls and tile floor it might be 1 or 2 seconds; in a conference room with thick carpeting and heavy drapes it might be 1/2 second.

If a room is too reflective, speech that is picked up by a microphone in that room will usually sound as if the talker is “at the bottom of a barrel” or “at the end of a long hallway.” This is in spite of the fact that the sound may be perfectly acceptable to a live listener in the room. In general, the longer the decay time is, the worse the sound will be.

Guidelines for acceptable decay time in a distance learning classroom

Decay time is measured with special test equipment which generates a burst of “white” or “pink” noise, and then measures how long sound persists after the noise has ceased. The measurement is typically made by a professional sound system installer or acoustic consultant in the process of evaluating a classroom for distance learning use.

Background Noise Level
Simply put, “noise” is any sound that the listener does not want or need to hear. Low levels of background noise can sometimes be suppressed or “tuned out” by the brain for a few minutes, but this quickly causes listening fatigue. As the level of noise increases compared to the level of speech, intelligibility suffers and listeners begin to miss words. Typical sources of background noise that make listening more difficult in a classroom include:

• cooling fans in computers or overhead projectors
• air ducts that vibrate or rumble
• air vents that produce an audible “rush” as air moves through them
• people walking or talking in hallways outside the classroom
• equipment rooms located next to, above, or below the classroom
• fluorescent light fixtures that hum or buzz

A precise measurement of background noise in a room can be made using a device called an audio spectrum analyzer. The noise level is measured in units called decibels, abbreviated dB. Because sounds with different tonal characteristics affect the human ear differently, measurements are made at many points across the audible spectrum. The resulting data are then compared to standardized Noise Criteria or “NC” curves to determine the acceptability of the room for a given purpose. Rooms used for distance learning, teleconferencing, and similar activities should have an NC rating of 35 or less (which is very quiet.)

Another device, called a sound level meter or SPL meter, provides an average reading of the noise level in a room. Be aware, however, that the sound level meters sold at neighborhood electronics stores are usually not capable of detecting low frequency noise (such as the rumbling of an air duct) and therefore may not provide an accurate and useful measurement of the noise level in a classroom.

How Room Acoustics Can Be Improved
In most classrooms, some improvement to the acoustics of the room will need to be made. The most common need is for a reduction in the decay time of the room, which is usually accomplished by covering some of the surfaces (walls, floor, ceiling, and windows) with commercially-available materials or panels designed to absorb, rather than reflect, sound waves.

A reduction in background noise is typically addressed by repair or replacement of light fixtures, and adjustment of or modification to air conditioning vents and ducts. Noise coming from outside the classroom is usually more difficult to control, however. Footsteps in a hallway might be silenced simply through the addition of carpeting, while reduction of noise from highway traffic or nearby equipment rooms might require major reconstruction of walls, ceilings, or floors. Reducing the transmission of noise into the room from outside sources can be very expensive, and should not be attempted without the help of an experienced acoustic consultant. In some cases, it may be wise to consider an alternate location for the classroom.

PART 3 – THE AUDIO SYSTEM

Components of the Audio System
The particular equipment that makes up a distance learning audio system varies depending on class size, seating arrangement, the subject being taught, the available budget, and other factors. Certain fundamental pieces of audio equipment are common to nearly all systems, however:

• a wireless microphone for the instructor
• desk- or table-mounted microphones for the students
• a microphone mixer to control, balance, and combine the signals from the microphones
• an amplifier and one or more loudspeakers to distribute the audio from other classroom sites throughout the room

Flow diagram of a basic distance learning audio system.

Specific information about different types of microphones and wireless microphone systems, how they work, and how to choose the best type for a particular application is included in Guide to Better Audio, a complimentary booklet available from Shure Brothers Incorporated. Specific information about amplifiers and loudspeakers is available from manufacturers and vendors of those products.

Thanks to popular spy movies such as the James Bond series and “Sneakers,” many people are under the impression that commonly available audio equipment can perform miracles, such as picking up intelligible speech from a mile away, or with a microphone hidden in an air duct. While it is probably true that a team of acoustic experts working for months with powerful computers could extract intelligible dialogue from a recording made that way, such feats are not possible during a live program or within the typical school system’s budget. Modern audio systems can perform well under a wide range of conditions, but they cannot compensate for having poor sound fed into them. Noise and reflected sound are impossible to remove once they are picked up and combined with speech.

Fortunately, there are time-tested “rules” for designing audio systems that can minimize the effects of less-than-ideal acoustic conditions. Three of these are so important that, if they are ignored, there is almost no hope of achieving satisfactory sound quality no matter how much time, money, or effort is spent.

Three Things About the Audio System that “Make or Break” Sound Quality

1. Talker-to-microphone distance

One of the most significant factors in determining the performance of an audio system is the distance from the talker to the nearest microphone. As the microphone is positioned farther away from the talker, the loudness of the speech reaching the microphone decreases, so the microphone’s sensitivity to sound – its ability to “hear” – must be increased to compensate. This causes the microphone to pick up more of the desired sound (speech), but also to pick up more of everything else – background noise, reflections, sound from the loudspeakers, etc. The farther away the microphone is from the talker, the more hollow and noisy the sound will be.

So, when it comes to microphone placement, how far is “too far”? It so happens that there is a certain distance from the microphone beyond which a talker will sound hollow and difficult to understand, regardless of what type of microphone is used or which way it is pointed. This is called the Critical Distance, abbreviated Dc. The Critical Distance is different for every room, and is determined primarily by the volume of the room (in cubic feet) and the decay time. Those talkers who are at or beyond the Critical Distance from the nearest microphone will be difficult to understand, no matter what type of equipment is added to the audio system or how it is adjusted. Given that most classrooms have a Critical Distance of from two to five feet, the following guidelines should be used for determining acceptable distance from the talker to the nearest microphone:

• less than 2 feet is ideal
• between 2 and 3 feet is good
• between 3 and 5 feet is marginal
• more than 5 feet is unacceptable

In a typical classroom, microphones should be located less than two feet from talkers.

When in doubt, always place the microphones nearer to rather than farther from talkers. Moving microphones closer to talkers is the single most significant improvement that can be made to most classroom audio systems.

The Best Places To Locate Microphones In a Classroom
We’ve discussed just how important it is to place microphones close to talkers, but where exactly should microphones be located to pick up students’ questions? The answers depend on how the classroom is used and on the seating arrangement. What subjects are taught in the classroom? If the schedule includes foreign language classes, or advanced courses that include lots of technical terms, intelligibility is critical. In this case, microphones should probably be placed on the student desks so that they are less than two feet from the talkers. Lecture-type classes that do not rely on heavy student interaction may permit slightly longer talker-to-microphone distances.

If it is likely that students will be spreading out books and papers on the desks, flat surface microphones may be inadvertently covered up, resulting in muffled sound or pickup of rustling noises. Slim, flexible “gooseneck” microphones solve this problem by elevating the sensitive part of the microphone above the surface of the desk – away from the noise and closer to the talker.

The toughest situation is when the room layout must be changed regularly. If microphones cannot be mounted on student desks or tables, the only remaining option is usually to suspend microphones from the ceiling. The longer talker-to-mic distances required for adequate head clearance when students are entering and leaving the classroom represent a significant compromise in the audio system design – one that usually cannot be completely compensated for. Excellent room acoustics are absolutely essential in classrooms which utilize suspended microphones. If room acoustics are poor or merely average, then suspended microphones should never be considered.

The Case Against Ceiling Microphones
For appearance and security reasons, it is always tempting to place microphones directly on the ceiling of the classroom, where they are out of sight and out of reach. This is absolutely the worst location for microphones, however, and virtually guarantees that sound quality will be terrible in all but the quietest and most acoustically-perfect rooms.

First of all, ceiling microphones are far beyond the Critical Distance for most rooms, making voices sound hollow and distant. Second, ceilings almost always contain air vents which produce noise, and air ducts which cause the ceiling to rumble and vibrate. Ceiling microphones are closer to these undesired sounds than they are to the students – exactly the opposite of the way things should be for intelligible voice pickup. Finally, students do not talk up toward the ceiling; if anything, they talk down at the desk or the floor!

2. Number of open microphones

An often-overlooked factor in audio system performance is the number of open microphones, abbreviated NOM. This is the number of microphones which are “live” or “open” at any moment, meaning that the sound that they pick up is being recorded on tape or heard by other sites on the network. Only those microphones which are “open” affect sound quality; mics that are turned off by a switch or turned down all the way by a volume control do not.

While you might expect that using several microphones to pick up the sound of a talker would sound better or louder than a single microphone, in reality it sounds much worse. This is because each additional microphone picks up some of the background noise and reflected sound in the room, even though it is not picking up the words of the talker.

Having four microphones turned up when only one person is speaking results in audio that is nearly 90% noise and reflected sound being transmitted to other sites.

So, if only one person is speaking and four microphones are open, the audio system is fed four times as much background noise and reflected sound as with just one open microphone, but no additional speech (because the additional three microphones are probably too far away to clearly pick up the talker.) What this means is that only the microphone nearest to the talker should be turned up. If you want to hear what a difference this can make, make a tape recording of one person talking in a room, first with several open microphones and then with only one open microphone. The improvement in sound quality that results from turning off the unneeded microphones is dramatic.

What can you do to keep unneeded microphones turned off in the classroom? There are really only three options: have a designated operator turn the microphones on and off as needed; have the students operate their own microphones; or have an automatic microphone mixer do it.

If a trained person is available to operate the sound system during every class, that person could be responsible for turning the appropriate microphones on and off when students speak. This is usually a poor choice, however, because even the best operator cannot react to a talker until a few words have already been missed – and a student’s question may only consist of a few words. Another option is to require the students to control their own microphones. This is done using Push-to-Talk or “PTT” microphones, which require the student to push and hold a button to be heard. This can be effective if the students remember to push the button when they want to talk. Depending on the age and interest level of the students, this may or may not be a reasonable expectation. Some instructors feel that forcing students to make such an effort before they can speak creates a barrier to interactivity, while others (especially those who teach several remote sites at once) appreciate the “shield” from unwanted distractions and interruptions which manually-operated microphones can provide for the instructor. An additional concern is that the students who are in the same room as the instructor usually do not feel compelled to activate their microphones when they talk, with the result that students at other sites cannot hear them.

A third solution to the problem of keeping unneeded microphones turned off is to use an automatic or “voice-activated” microphone mixer. This is a microphone mixer which automatically turns individual microphones on and off in response to the presence of sound at the microphone. Basic automatic mixers use a simple “fixed threshold” method to decide when to activate microphones. With this method, the sound level at any microphone must exceed a preset minimum (called the “threshold”) before the microphone will be turned on and the talker will be heard.

Proper adjustment of fixed threshold models is critical, because background noise levels within the classroom usually change from day to evening or even from hour to hour, and speaking levels vary significantly between individuals. For instance, if the threshold is set low enough so that a quiet talker can activate the microphone, then the system may be overly sensitive to background noise, and microphones may activate every time the air conditioning turns on, or when the hallway fills with students during a passing period. Conversely, if the threshold is set high enough to prevent such incidences of false activation, then quiet talkers may not be heard.

More sophisticated automatic mixers are available which constantly adapt to changes in background noise level. These units feature a “floating threshold” which is referenced to the level of noise in the room. Talkers need only speak slightly louder than the background noise level to activate a microphone. These systems are generally easy to set up and require almost no fine-tuning, since they adjust themselves to changing conditions in the room.

3. Microphone pickup pattern

It is beyond the scope of this document to review all of the different types of microphones available. In general, however, directional microphones should always be used in distance learning classrooms. Directional microphones (also called unidirectional) favor sounds from the direction in which the microphone is aimed, and reject sounds coming from behind the microphone. This trait offers two very important benefits which make directional microphones especially suitable for use in distance learning classrooms. First, they can be aimed toward a desired sound source (the students), and away from an undesired sound source (the loudspeakers.) This helps to prevent the sound from other sites (coming out of the loudspeakers) from being picked up by the microphones. This loudspeaker audio would otherwise be retransmitted, which could cause annoying echoes or howling. Second, most directional microphones pick up only one-third as much background sound as non-directional types, making them far less sensitive to the ambient noise and reflected sound present in the room. With less noise and reflected sound mixed in with the audio, speech is clearer and more intelligible.

Note that a directional microphone begins to lose its effectiveness as the distance from the talker increases. The further it is from the talker, the less improvement in sound quality it can offer over a non-directional model. In fact, when placed near (or beyond) the room’s Critical Distance, a directional microphone will sound just as poor as a non-directional type.

Common Problems with Distance Learning Audio Systems and What You Can Do About Them
A live, two-way interactive communications link is more complicated than a regular audio system used for public address or recording. The three major audio problems which plague distance learning classrooms are feedback, transmission echo, and reflected or “hollow” sound. While feedback and reflected sound are commonly encountered in many sound reinforcement systems, echo is unique to two-way communications.

Feedback
Feedback is the howling or squealing that is heard when sound from a loudspeaker is picked up by a microphone and reamplified. Feedback can easily occur between audio systems located in different rooms, if the microphones in each room pick up too much of the sound from the nearby loudspeakers and retransmit it to other sites, where it can be picked up by microphones and transmitted back again.

Leakage from loudspeakers into microphones, causing feedback or echo.

There are many factors which can contribute to microphone-to-loudspeaker leakage, so there are many possible solutions which may work in a given classroom. Here are the corrective measures which are most successful at eliminating feedback in a distance learning network:

Change the positioning and/or proximity of the microphones and loudspeakers relative to each other. For best results, loudspeakers should be positioned behind (not above) typical directional microphones, which are less sensitive to sounds arriving from the rear than to those arriving from the front.

Turn the loudspeaker volume down in the room. Lower volume levels, while making it more difficult for people to hear, reduce the tendency of the audio system to howl.

Reduce the number of open microphones (NOM), through the action of a live operator, or the use of Push-to-Talk microphones, or an automatic mixer. Reducing the number of open microphones has the same effect as turning down the overall volume of the audio system (but without the penalty of lower listening levels), thereby reducing the incidence of feedback.

Make the surfaces in the room less reflective by adding sound absorbent panels or coverings. In rooms with less-than-optimal acoustics, microphone-to-loudspeaker coupling may occur even though there is no direct path between the two. In this case, sound leaves the loudspeaker and is reflected off of the walls, ceiling, or floor, reaching the microphones indirectly.

Transmission Echo

Some types of transmission networks slightly delay the audio and video signals before or during transmission. This may be due to the time required for satellite transmission or the action of a device called a codec, which compresses the signal so that it can fit into a smaller (and less expensive) amount of space on the transmission line. When distance learning sites are connected over a network which induces some signal delay, the sound from the loudspeakers at the remote site can leak into microphones there, which causes an echo to be returned to the originating site. In other words, a talker in Classroom A speaks, his or her voice comes out of a loudspeaker in Classroom B and leaks into a microphone there, and that signal is transmitted back to Classroom A, where the talker hears an echo of his or her own words 1/4 to 1 second after having said them. If the leakage problem in Classroom B is not severe, the returned echo may be low enough in volume to be tolerable, but in almost all cases it is so annoying that conversation is impossible.

Various methods of dealing with echo have been devised; most of these were intended to minimize echo in long distance telephone lines. One type of device, called an echo canceller, monitors the incoming (or `Receive’) audio from other sites, and compares it to the signal that is about to be transmitted (the `Send’ signal). If the echo canceller detects the presence of the incoming audio in the outgoing signal, it creates a replica of the incoming audio and electronically subtracts it from the outgoing signal. This reduces the amount of echo, but does not completely “cancel” it. Notice that the echo canceller attempts to prevent the incoming audio from other sites from being sent back to them, but it does not do anything about the echoes that other sites may be sending to your site. For this reason, if one site on a network requires echo cancellation equipment, all of the sites on that network will almost certainly need it.

There are two general types of echo cancellers. The first, called a line echo canceller, is designed to remove electronic echoes from telephone lines; most long distance telephone circuits employ these devices. Line echo cancellers cannot significantly reduce the complex echoes which result from loudspeaker-to-microphone leakage in a classroom. The second type, called an acoustic echo canceller, is designed to reduce the chance of an echo being produced due to this leakage. Acoustic echo cancellers are commonly mistaken to be capable of removing the hollow sound associated with a room that is too reflective; no electronic device can do that. In fact, excess reflected sound makes it more difficult for the echo canceller to work properly, and reduces the degree to which it can reduce transmission echo.

Acoustic echo cancellers take time (1/10 of a second or more) to “learn” how to reduce echo in a particular room, and they have to go through this learning process whenever the path from loudspeaker to microphone changes. This might be caused by a wireless microphone user moving around the room, or microphones being turned on and off by an automatic mixer. During these “learning” periods, echoes will not be reduced.

Echo cancellers are expensive, and are not an alternative to good room acoustics and proper audio system design. At best, they can improve the sound quality of a distance learning network that suffers from echo problems, but they cannot make a classroom that has poor acoustics sound good.

Reflected or “Hollow” Sound
One of the most persistent and annoying problems with distance learning audio systems is the hollow sound – as if the talker is “at the bottom of a barrel” – caused by a room that is too reflective. Unfortunately, reflected sound cannot be removed by any type of electronic device; it must be kept out of the microphones in the first place.

The obvious solution, of course, is to make the room less reflective by covering surfaces with specially designed sound-absorptive materials. If changes to the room acoustics do not provide a sufficient improvement, the number of simultaneous open microphones (each of which adds a measure of reflected sound and background noise to the audio signal) must be reduced. Directional microphones can also reduce pickup of reflected sound, but only if they are positioned at less than the Critical Distance from the talker. Beyond the Critical Distance for the room, directional microphones offer minimal benefit. Finally, microphones can simply be moved closer to the talkers. Because the speech reaching the microphones will then be louder, the sensitivity of the microphone can be turned down at the microphone mixer, thereby decreasing its sensitivity to reflected sound and background noise as well.

The Broadcaster’s Secrets
Television news shows routinely feature live interviews by satellite with leaders around the world. Somehow, they never seem to experience any of the audio problems that plague many distance learning networks. What do broadcast engineers do that allows them to dodge the effects of feedback, echo, and reverberation? Here are three “audio commandments” that broadcasters live by:

Place the microphones as close to the talker as practical. In the studio, news anchors always wear a very small microphone (called a “lavalier” type) clipped to the necktie or jacket. In the field, reporters may wear a lavalier or use a standard handheld microphone. Because the camera shot is usually a tight close-up the microphone may not be visible, but rest assured that it’s no more than a foot or so away from the talker’s mouth. In situations where the talker cannot wear or hold a microphone, a long “shotgun” type microphone is mounted on a pole and held by a technician above and in front of the talker, just outside of the camera’s field of view. While effective for recording, shotgun microphones offer little advantage when used in a classroom. They, too, sound poor when used indoors at or beyond the Critical Distance of the room.

Use an ear piece. Broadcasters hear each other, the guests, the director, the commercials, and everything else through a small flesh-colored earphone worn in one ear. In high-noise environments such as sports events, the commentator wears a tight-fitting headset (to block out noise) with a boom microphone positioned just an inch or so from the mouth (for clear speech with minimal noise pickup.) Loudspeakers are rarely used, so loudspeaker-to-microphone leakage is eliminated. No leakage means no problems with feedback or echo. This is especially critical, given that remote interviews by satellite – which are subject to significant signal delays – often must be set up on a moment’s notice.

Use as few open microphones as possible. Fewer microphones pick up less noise, and require less time to set up and plug in. To optimize sound quality, an engineer constantly adjusts or “rides” the settings at the microphone mixer. You will rarely see more than four microphones being used at one time on television, but when more microphones are required than the engineer can keep up with, automatic mixers are frequently used.

Who Can Install the Audio System in Your Classroom

Hopefully, you are beginning to have a better understanding of the technical issues which must be dealt with in the design, installation, and use of a distance learning audio system. These may require more time or technical expertise than you can commit to the project. In any case, one of the following three options should help you to get a distance learning classroom installed at your institution:

Work with a qualified A/V consultant or sound system contractor. An acoustic or A/V consultant designs systems and writes what is known as a spec, or specification, of the equipment and room modifications required for the project. The consultant does not actually sell or install the equipment, however. A sound system contractor, working from the consultant’s design, sells, installs, and services the equipment. In some cases, the contractor both designs and installs the system. It is very important to work with firms that have experience with two-way interactive systems, such as teleconferencing or videoconferencing rooms or distance learning classrooms, because of the unique problems which arise in them as opposed to more typical public address systems found in churches or auditoria.

Purchase a pre-packaged system from a turnkey system provider. Companies which offer certain key elements of a distance learning system, such as fiber optic transmission equipment or service, sometimes “bundle” their own products or services together with those of companies that make the other equipment necessary (such as cameras, monitors, etc.) They are then able to market a complete distance learning system package for one price. In most cases, the actual installation of hardware is handled by a local sound system contractor.
Do it yourself. Institutions that have qualified technical people on staff with a knowledge of audio, video, and room acoustics, may be able to configure, install, and service a distance learning system without assistance. This may be a viable option if the system is a simple one – meaning that class sizes are small, echo is not a problem, and necessary modifications to the room acoustics are minimal.

FOR THOSE WHO WANT TO LEARN MORE

Additional sources of information

Architectural Acoustics by M. David Egan
(411 pages, hardcover; approximately $65.00) available from McGraw-Hill, Inc.
Complete review of audio theory, sound absorption and isolation, room acoustics, noise and vibration control, electronic sound systems, and more. Many useful examples, illustrations, tables, and equations.

Teleconferencing and Distance Learning by Patrick Portway and Carla Lane
(384 pages, softcover; approximately $50.00) available from Applied Business teleCommunications, San Ramon, CA 510-820-5563 tel/510-820-5894 fax
Collection of papers written by representatives from equipment manufacturers and industry associations. Chapters cover audio, echo cancellation, instructional design, transmission standards, training instructors, and more.

Audio Systems Design and Installation by Phillip Giddings
(574 pages, hardcover; approximately $60.00)
Comprehensive reference guide to audio system powering, grounding, wiring, and installation. Extensive discussion of equipment interconnection and noise problems.

Any Questions?
If you have questions about this document, contact support@shure.com.

Revision: 2/5/2007