AS THE SMALLEST CONDENSERS around, lavaliers are turning up anyplace there's a need for tiny wireless microphones. Whether hidden in wigs in Broadway productions, clipped to a pastor's suit, or peeking out of a book of matches on a Hollywood set, lavaliers play a key role in capturing sound without drawing any attention to themselves.
This month, I spoke with Gary Stanfill of Colmar Systems. For 18 years, Stanfill served as president and general manager of Vega, a leading manufacturer of wireless microphone systems. Currently, he serves as an industry consultant and contributes regularly to ProSoundWeb's Install Sound section.
SRO: Are there any trends you've seen in lavalier design?
Gary Stanfill: Clearly, the trend has been to go smaller. This is especially important in stage productions, where the demands of costuming have been the invisible microphone. When I first came into the wireless business, Sony pretty much owned the high-end market for lavaliers. However, they were slow to come out with a smaller microphone, and a number of companies stepped in to fill the demand. As a result you see quite a few lavaliers on the market, from the fairly expensive Sennheiser to a wide variety manufactured in China. And an omnidirectional lavalier is not a tough design proposition if it's not required to be extremely small. Most manufacturers can make a perfectly decent lavalier for everything but the very high end of the market.
Also, back in the day, a lot of lavalier microphones were moisture-sensitive. So it wasn't uncommon for them to be destroyed by perspiration. That's been gradually erased although there are still a fair number of expensive ones on the market that are not moisture resistant. So you may need to consider the moisture resistance of a microphone equally or above sound quality.
SRO: Doesn't foam protection help address that?
Stanfill: Not if it gets drenched. For the longest time, Sennheiser has been the microphone of choice for Broad-way. But for several years, they weren't moisture-proof. So it wasn't uncommon to lose a couple of $250 microphones every performance. It reached the point where they used to bring in bottles of ultra-dry nitrogen and carefully blow out the microphone with it to drive off the moisture. Strange as it may seem, they used to dial up the local gas supply company to rent nitrogen bottles so they could dry out the microphones as a money-saving measure. [Editor's Note: Sennheiser now has several moisture-proof lavalier mics for such applications.]
SRO: I imagine physical frailty is an issue, too.
Stanfill: The weakest area is where the cable exits the microphone body. Some of the slightly larger microphones have addressed that by allowing users to re-terminate or at least replace the cable. The greater wear appears at the connector going into the wireless transmitter. And, for the most part, that's reparable. Generally speaking, as they get near to failure, you get various strange noises. Inexperienced sound techs are often baffled by what's causing that. When the braid on the wire starts to go, you get pops, crackles, and changes in levels. It takes some experience to recognize that's what happens. Once you know how to check it, which is simply to listen to the receiver and flex the wire around the strain-relief area, then you can isolate the problem. Most manufacturers of wireless allow you to re-terminate until the cable gets too short. There are microphones that are robust enough that they'll last up to the point where you chop until it gets too short.
SRO: Noisy wireless systems are definitely an issue. You've written that since RF energy can couple into the microphone cable and affect the preamp circuitry, the use of a 220 pfd capacitor between the ground and the mic signal can help deal with some of the RF interference that you may encounter.
Stanfill: Well, everybody asks why you can't put the capacitor in a transmitter and all the manufacturers usually try. But, the RF shielding on the cable is generally not that good and the external length of the lead between the connector and the transmitter's circuit board is such that it's not as effective as putting one inside the connector. Most wireless transmitters have at least 15mW to 20mW of power, and most lavalier microphones require a bypass capacitor. Normally, the manufacturer will provide that when they provide you a pre-terminated mic, but you have to put it in if you do your own termination.
In a wireless transmitter, the “antenna” is really only half the antenna. And the antenna has two parts: a radiating element and a counterpoise. In effect, no matter what you do, the microphone cable becomes part of the antenna and the RF fields on it are generally not a lot less than they are on the antenna. Since the active element in a lavalier microphone is a FET, then the RF energy can change the bias of the FET giving you distortion or a weak signal. And the distortion varies with cable flexing due to the amount of RF being coupled. So now it's pretty much standard practice to insert a capacitor.
SRO: What about a tying a knot in the cable to reduce the noise level?
Stanfill: To make some sort of a coil on a cable to reduce the RF effectively makes inductance. It generally will reduce the amount of RF that's traveling down the mic cable. The disadvantage is that the knot will typically exceed the minimum bend radius and you'll get a failure at the knot. You can buy ferrite beads that will slip over the cable that are a half-inch or so long and not too much larger than the cable's outside diameter. Those are much more effective than using a knot.
My experience is that if you put the right capacitor inside the connector, it will do more good than any of the other fixes. Now, there are some lavalier microphones that don't work with any medium- to high-power wireless transmitter because the amplifier is not shielded in any way and is affected by practically anything including your local TV station, not just the wireless transmitter. If you're next door to an AM, FM, or TV station, problems can still happen even if the preamplifier in the lavalier is shielded.
Now, they are designing integrated circuits to be used in preamps instead of a FET and they claim lower noise. It's difficult for me to imagine making the microphone much smaller because of the acoustic reality. Aside from replacing one type of preamp with another, it's not too clear what they can do. It's a very mature product and absent some totally new technique for the transducer, I don't see that there's going to be a lot of evolution in the near term.
SRO: So they won't be able to go much smaller than they already have?
Stanfill: That's correct because, acoustically, the microphone isn't going to work anymore. Lavaliers are already at a point where they're greatly susceptible to ultrasonic energy. Because of the small size, their sensitivity tends to rise in the above audible range. In some cases, you can find that the increased sensitivity causes the preamplifier or mic preamp to saturate with sounds you can't hear. Unless something is done in the preamplifier to correct this, it can cause serious problems.
Alex Artaud is a musician and engineer living in Oakland, Calif.