By: Shure Notes Editors, Contributor: Tim Vear, Senior Applications Engineer
Readers of Shure Notes have enthusiastically embraced our annual review of audio myths, which Tim Vear shatters (or not). Here he delivers the answers to some of the most recent questions that originate from Shure customers who call Tim and his fellow Apps Experts for help.
Myth: Loud sounds can blow out a microphone
Generally it is not at all likely that a loud sound will damage a microphone. A dynamic microphone can handle levels above 150dB SPL. That’s not a volume that can be achieved short of shoving a microphone into the exhaust of a jet engine. It is almost impossible to physically damage a dynamic microphone at any achievable sound level.
Classical ribbon microphones that use an aluminum ribbon can be damaged not by sound pressure but by a puff of air. A blast of air could physically push the ribbon out of its magnetic gap far enough to stretch it. (It’s worth noting that Shure’s ribbon mics use a material – Roswellite® – that can withstand air pressure making them about as durable as any other Shure microphone.)
Condenser microphones can experience distortion if the sound pressure level exceeds the rated capability of the mic. But the distortion is coming from the electronics of the microphone and not the capsule itself. The electronics are essentially a fixed gain amplifier. Any amplifier has a maximum output level that it can deliver before it distorts. So if you increase the input level to the amplifier beyond what the max should be, it will drive the amplifier to distortion. It’s the pre-amp in the condenser microphone that’s distorting.
Many condenser mics feature a switchable attenuator that allows the mic to handle higher sound levels.
Myth: A wireless microphone can’t cause feedback.
We get this question every once in a while from someone who may be using a wireless mic either for the first time or in a new way. They’re surprised to experience feedback when using a wireless mic in front of loudspeakers.
They believe that if they eliminate the wire, they’re eliminating the feedback loop. The fact is, the cable is just being replaced by a radio wave, so acoustically; the system behaves exactly like a wired system.
It may be triggered by someone who is accustomed to using a wired mic that’s tethered to a mic stand or pulpit. A wireless mic gives the user the ability to walk around in the audience or among the congregants and potentially in front of the house speakers. In that case, the system may very well cause feedback.
If the user is too close to the speakers or the gain is turned up too high, feedback will occur in both wired and wireless systems.
Myth: Digital wireless microphone systems are interference free.
Some people believe that the error correction capabilities of digital systems can compensate for extreme radio interference. But the fact is that any radio microphone or wireless device can experience interference because there is no frequency band that is reserved specifically for an individual’s wireless microphone. At minimum, there are other wireless microphones operating in the same frequency range and if they happen to collide, interference will occur.
Each frequency band has its own unique interference sources – it could be television stations in the UHF band or baby monitors in the 900 MHz band, WiFi hotspots in the 2.4 GHz band – but all frequency ranges are shared by different users that can interfere with one another.
Whether the transmission is digital, analog or some unknown modulation that hasn’t been discovered yet, if it depends on a radio signal, it is subject to interference.
Myth: It’s OK to use just one earphone for in-ear monitoring.
There’s a functional reason and there’s a health reason.
Functional: With only one earphone in place, you have only the possibility of monophonic sound. Whatever the mix is that you’re listening to; you’ll only hear it in mono, which removes all of the spatial cues that you get from a stereo mix. A stereo mix makes it not only more pleasant to listen, but also enables you to pick out the parts that you need more easily by virtue of their pan location, not just their level.
Health: If both ears are exposed to an open sound field, the perceived level of sound at each ear will be equal. However, if one ear is blocked from the outside sound but presented with that same sound via an earphone while the other ear is still exposed to the open sound, the relative level perception changes. It is found that in order for the sound levels to appear equal, the level of the earphone sound must be about 6dB louder than the level of sound at the open ear. This is called the “occluded ear effect.”
Here’s an example: Let’s say the left ear is being exposed directly to ambient sound and the listener wants to match the sound level that’s coming through the earphone in the right ear (the occluded ear) so that the volume appears roughly equal. By measuring sound levels at the eardrum, studies have shown that the listener will increase the earphone level at least 6dB above the open ear level to perceive an equal level. So in a loud environment where the open ear may be subject to 120dB SPL, the earphone may be cranked up to 126dB SPL just to compete. Over time, this increases the risk of noise-induced hearing loss.
It’s not a huge problem in a quiet environment – a jazz band or maybe a theatrical application – but for a heavy metal, rock and roll or even a loud country band – it’s a serious consideration.
Myth: Phantom voltage is the same thing as bias voltage.
Phantom power is a dc voltage (11 – 48 volts) that powers the active circuitry of a condenser microphone. Phantom power is normally supplied by the microphone mixer, but may also be supplied by a separate phantom power supply. Phantom requires a balanced circuit in which XLR pins 2 and 3 carry the same dc voltage relative to pin 1. So if a mixer supplies 48 volts of phantom, XLR pins 2 and 3 of the microphone cable each carry 48 volts dc relative to pin 1. Of course, the mic cable carries the audio signal as well as the phantom voltage.
Bias is a dc voltage (1.5 – 9 volts typically) that is provided on a single conductor. Unlike phantom power, bias does not require a balanced circuit. Bias supplies power to a single Junction Field Effect Transistor (JFET) connected to the output of an electret condenser mic element. The JFET acts as an impedance converter, which is a necessity in any microphone design that uses a condenser element. A condenser element has a high output impedance (>1,000,000 ohms). The JFET lowers the output impedance of the condenser element to about 1000 ohms to feed the signal to the rest of the microphone active circuitry. Typically, the JFET impedance converter is placed very close to the condenser element to prevent the loss of high frequencies that would occur from even a short length of high impedance circuitry.
How are phantom power and bias voltage related? In a full-size condenser microphone, phantom power operates the active electronics in the body of the microphone – and those electronics have the function of converting the unbalanced signal from the condenser element to a balanced output signal. The electronics may also provide some equalization and possibly some amplification. Finally, those same electronics provide the bias voltage that powers the JFET impedance converter connected to the actual condenser element.
Since the JFET impedance converter allows some length of connecting cable without high frequency loss, it is possible to separate the main condenser electronics from the actual microphone element. This is the form used for condenser microphone designs in which the microphone element is connected to the main electronics assembly by a miniature cable up to perhaps 30 feet in length. This cable conducts bias voltage from the main electronics to the JFET attached to the condenser mic element.
In some condenser microphones, the bias voltage must be supplied on the same conductor as the audio. Condenser elements with a built in JFET use this configuration and employ a single conductor, shielded cable. Other designs utilize separate conductors for bias and for audio. Consult the manufacturer’s data sheet to find out the exact wiring configuration for a specific microphone.
It’s important to understand the phantom vs. bias distinction when using wired microphones with a wireless transmitter. A typical wireless bodypack transmitter provides bias voltage on its input connector. Any condenser microphone element with an integrated JFET can be plugged into such a transmitter. This includes most condenser lapel mics, headset mics, as well as boundary mics and miniature gooseneck mics that have removable electronics assemblies: these microphones only require bias voltage provided by the wireless transmitter.
But you cannot plug in a Beta 87 or a KSM32 or some other full size microphone that needs phantom power into a device that only provides bias voltage. Those microphones can only be used with mixers or other devices that provide phantom power.
Here’s what you need to remember: A condenser microphone that requires phantom power will not work with an input that only supplies bias voltage. Similarly, a condenser microphone that requires bias voltage will not work with a standard phantom power mixer input.
Know of any other audio myths that you want busted? Leave a comment/suggestion and we may include it in a future blog post!