Contributors: Gino Sigismondi and Doug Totel
The hum. The buzz. The crackle. Or no sound at all. While we’re all familiar with the unwanted sound of, let’s say, guitar amp feedback, there are other issues that are a little trickier to identify and eliminate.
This time around, you’re in complete control. You can actually choose to hear some of the most annoyingly common, along with helpful remedies suggested by Shure experts.
60 Cycle Hum
Ground loops in the 60Hz AC signal
Question: “In one small area of my studio near my mains circuit breaker/electrical panel, I have some type of electrical interference and all my microphones except the Neumann mics have hum when turned in a certain direction. If I turn the mics 90 degrees, the hum disappears.
The Neumann mics (U87s and U64s) have no hum in any direction and work fine. So, before I write to the other mic manufacturers, I want to ask you if you use a special kind of shielding in your mics that prevent this type of hum from electrical interference.”
Answer: The likely source of the problem is a hum field in your studio caused by the AC power lines.
Any type of dynamic mic, like the Shure SM57 or SM58®, contains a voice coil that is very susceptible to picking up the hum field. Nothing can be added to these mics to control this.
Some dynamic mics like the Shure SM7B contain an internal hum-bucking coil that reduces hum pick-up. Reduces, not eliminates. Other dynamic mics, like the Shure Beta 58A, have more effective shielding around the voice coil that also reduces hum. ALL dynamic mics will pick up this hum field to some extent.
Condenser mics, like the Neumann’s you have or the Shure KSM mics do not have voice coils. These mics will perform much better in a hum field. But many of these mics have output transformers that, like a voice coil, will also pick up the hum.
If the hum field cannot be eliminated, stick with high quality condenser mics with transformerless outputs. Shure KSM mics are condenser mics with transformerless outputs.
Sibilant sound of “SH” between two active microphones.
Question: “How do the theatre guys do it? I frequently have actors with lavs in their hairline or over their ears talk while hugging. I get a comb filter-like sound when they do that. Often I do not have time to reduce the volume to one of the mics to reduce the comb filter distortion. I know some people route the lavs to different speakers, but when you have many scenes like that with many lavs, the routing can be quite a headache.”
There are basically 3 methods:
• Reduce the volume of one of the microphones.
• Route the microphones to different loudspeakers.
• Use an automatic mixer inserted into your main console so that the automixer can turn the microphones on and off as they are being used. Essentially, this is the same as #1, but it is much easier.
Question: “Where can I get information on how to properly mic a podium with two Microflex Gooseneck mics? I want to avoid comb filtering.”
• If you want both mics activated at the same time, position the mic heads in the center of the podium, one on top of the other.
• If you want the mics on either corner of the podium and the heads widely separated, connect the mics to the Shure SCM410 automatic mixer, which will activate one or the other, depending on which mic is picking up the better signal. In this case, DO NOT manually activate both mics at the same time or the audio quality will be poor due to acoustical comb filtering.
• You can also position one mic directly in the center of the podium and forget about a second mic.
Musical passage experiencing multipath dropout
Question: “I just bought an in-ear monitoring system, but sometimes the signal cuts out, then it comes back in. Is that to be expected?”
Answer: You probably have a single antenna wireless systems receiver.
As with any wireless system, whether it is a microphone or personal monitor, a single antenna system will be subject to occasional dropouts. Diversity receivers used with wireless microphones overcome this problem by using two antennas, or even two entirely separate receiver sections to prevent dropouts. Consider using a diversity receiver like the P10R, which will greatly reduce the likelihood of dropout when using a good frequency.
Transmitter/receiver line of sight
Question: “How can I avoid dropouts with my wireless mic system?”
Answer: Professional wireless microphones operate in the same portions of the frequency band used by broadcast television channels. Many wireless systems offer a scan feature that will allow you to find open frequencies.
For best results, the receiver’s antennas need to be located in clear line-of-sight to the microphone transmitters. The receivers can then be located wherever convenient. Putting the receiving antennas in a closet or behind a wall will usually result in reduced operating range or an increased chance of momentary lapses in the signal.
Cell Phone, Mobile PDA, and Wireless Devices near computer speakers
Question: “Please provide frequency information about smartphones and other wireless communication devices. I have encountered situations where such devices created interference in the sound system components: microphones (wired and wireless), cable snakes, mixers, and other pro audio equipment. Nothing seems immune!”
Answer: The actual frequencies used by smartphones and cellular-enabled tablets vary by country and by carrier. In the United States, there are two major frequency ranges in use:
• 698-894 MHz (700, 800, and 850 MHz Bands)
• 1710-2155 MHz (PCS and AWS Bands)
Smartphones and tablets have variable RF power output. The serving carrier’s base station tells the device to increase or decrease its power depending on how strong the device’s signal is. If the user is inside a meeting room in a large building like a hotel or convention center, the device’s signal will be weaker at the cell site antenna. In those situations, the device might be operating (exchanging data traffic) at higher power levels, even if no voice call is in progress. In the meeting room, the ambient RF noise floor may be elevated with dozens, or even hundreds, of smartphones and tablets operating at the same time. This may result in interference or reception problems for wireless microphone systems.
Interference to audio equipment from cellphone devices and the wireless infrastructure generally falls into two categories:
1. RF signals that get into audio circuits and are detected (demodulated) similar to an AM radio.
Certain cellular carriers used a radio protocol known as GSM-TDMA. This mobile device to cell-site radio interface used a supervisory signaling scheme that was based on data being sent via a series of RF pulses with a repetition rate of 217 Hz. This “rep rate” conveniently falls in the audio range, and typically was very strong. The handshaking signals from the mobile device were easily demodulated in audio circuits as a very recognizable blap-blap-blap sound.
Solution: The only mitigation for this type of audio interference was to power down the mobile device or to place the mobile device at least five feet (and sometimes more) away from the audio device. Fortunately, the GSM-TDMA protocol is now essentially obsolete, being replaced with newer higher capacity protocols that do not result in interference to audio circuits.
2. RF interference to RF circuits, such as receiver front-ends, transmitter output stages, and active antenna systems.
Individual cellular devices generally do not interfere with wireless microphone systems. Wireless microphones and cellular systems operate on completely different frequency ranges. But the proximity of the 470 – 698 UHF-TV band where wireless microphones operate and the cellular 700, 800, and 850 MHz bands can be problematic in some situations.
If a large number of cellular devices are carried by people in a single venue, the cumulative RF energy from many devices trying to communicate back to a cell-site simultaneously can overload wireless microphone active antenna devices as well as receiver front-end circuitry. The result can be the inability to receive a wireless microphone signal, or mysterious signal dropouts, and in some cases receiving undesirable noises.
Solution: Mitigation in these cases usually involves improving the antenna design. For instance, relocating antennas to a more favorable position near the stage, removing active circuitry in antenna systems, implementing bandpass filters, and in some cases using attenuators to reduce receiver overload problems may resolve reception problems.
TV Audio Interference
Dropouts resulting from operating a wireless system on the audio carrier of an active analog TV station. Note: Audio is sample of interference from an analog station, not as common in today’s post-DTV era.)
Question: “I know that wireless microphones use the same frequencies as television stations, but where can I find out what stations are in my city?”
Answer: Professional wireless microphones do use the same frequencies as broadcast television stations. For example, a city may have television channels 23 and 25 on air. So, wireless microphones cannot be used on those frequencies. You would use wireless microphones on television channels 22 and 24 to avoid the active TV channels.
Every city has a different set of occupied TV stations. Most pro audio companies, including Shure, have online ‘wireless frequency finders’ that are dynamic in nature, responding to FCC changes. They’re not perfect, but they’re one place to start. For general guidance, enter your zip code and wireless system here.
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