The Feeding and Care of RF Microphones


There used to be a time, long long ago (okay, not that long ago), when stage performers were trained to project their voices. The trend at the same time in theatre history included an acoustic orchestra, talented orchestrators and music directors, who knew how to manipulate the orchestra around the voices on stage such that dialogue and lyrics were not lost amongst the mess of instruments playing in the pit. Audience members were not accustomed to the loud instruments that rock-and-roll music would introduce, and most of them didn't have home hi-fi systems capable of producing loud sound pressure levels beyond what was needed.

Unfortunately, that time has passed.

The audience who actually knows how to listen is diminishing rapidly, and is replaced by a somewhat boorish, lazy, and selfish audience, who are used to Dolby Surround, The Matrix on their own home-theatre system with 7,000 watts of audio power, Metallica and Rammstein playing 140 dB SPL live, etc., etc., etc. You get the picture. They want remote-control comfort. "To hell with nuances and naturalism!" they cry. "I want every sibilant monosyllable that Keanu Reeves is capable of uttering shoved directly into my inner ear! And I want more drums! I can't feel the drums!"

At the same time, the art of the musical has dwindled. Gone are the happy-go-lucky musicals of Rogers and Hammerstein or Cole Porter; they've been replaced by stage adaptations of movies we enjoyed in the 1960s, 1970s and 1980s. With this change came a different type of musical: the rock musical. The first rock musicals, such as Hair and Jesus Christ Superstar, took an existing format, the musical, and added to it the rock-and-roll sound (hence the name, dumbass) [editor note: remove "dumbass" from previous sentence], which meant electric guitars, synthesizers, and drums, drums, drums. Orchestra pits were becoming more band-like than orchestra-like, and they were getting louder. To alleviate problems of hearing the lyrics over the music, dreaded sound amplification was utilized. In the early days of sound reinforcement, area microphones covering different sections of the stage were used to amplify dialogue and lyrics, but this clearly wasn't sufficient to compete with the increasingly loud music emanating from the orchestra pit. Besides, people wanted it loud.

Even more disturbing was the trend of acting programs preparing their students not for stage work, but for film and television work, in which a boom microphone is suspended just out of the camera shot. Why project? It takes work, training, and care. Why not just speak loudly enough for the camera? Unfortunately, this trend has produced performers who are incapable or poorly trained in the art of projecting.

All of these factors have contributed to the use of wireless microphones in theatrical sound reinforcement. Awireless microphone is, as the name implies, a system utilizing a small microphone capsule wired to a small transmitter, usually about the size of a pack of cigarettes (note to the young people: "cigarettes" are these things that people used to smoke before non-smokers whined so much about secondhand smoke; they were usually available twenty to a box, called a "pack," which was about the size of a small wallet), which converts the audio signal from the microphone into radio waves, which are then picked up by antennae connected to a receiver, which sits offstage and reconverts the radio waves into an audio signal that iss then manipulated by the sound system. As the technology has improved, the microphone capsules have become smaller, allowing the capsules themselves to be hidden on a performer. Early sound designers clipped large-ish microphone capsules to clothing, which presented a whole range of problems due to clothing noise and frequency response. I think it was Abe Jacob or Otts Munderloh who put the first Sony ECM-66 microphone onto a performer's hairline in order to hide the microphone, as well as improve the frequency response of the microphone. The transmitters haven't changed much in size but have changed greatly in electronic technology. One of these days a manufacturer will address the size issue, we hope. Maybe. There are also wireless handheld microphones, which are, as the name implies, a handheld vocal microphone in which the transmitter electronics are housed. This arrangement provided singers with the freedom to roam around stage without entangling themselves in the microphone cable, which just looks silly and unprofessional.


"There is nothing in the world worse than a cheap wireless system," Hosch says. "It's scary when your future is hanging on a $ 1.98 battery. Anytime you can, you should get someone to use a wired mic over a wireless system." Scheirman adds, "and just remember that even the best wireless microphone system, a $10,000 system, is almost as good as a mic cable." (From Theatre Crafts International, May 1993, page 25). See the "Crash Course Guide in Sound Reinforcement" to persuade yourself not to use wireless microphone systems.


A wireless microphone system (also termed "radio microphone" or "rf microphone") is essentially a miniaturized version of an FM radio broadcasting system. In a commercial radio system, a disc-jockey speaks into a microphone that is connected to a high-power transmitter. The transmitted radio waves are picked up by an FM receiver and converted into the audio signal, heard through a loudspeaker. Handheld wireless systems are commonly found in rock-and-roll applications, where singers like to wander around the stage making lewd motions with their free hand whist lying on their back. They are also used extensively in television production, where talk-show hosts can roam the audience with impunity. The transmitter electronics and battery by which the transmitter and microphone receive their power are built into the casing of the microphone. Lavalier wireless systems, so called because the original location for the microphone capsule was on the lavalier of the performer, operate in much the same way, but a wire connects the microphone capsule to the bodypack. In theatre sound, we hide both the bodypack and microphone capsule as best we can, but in television production they seem to care less- watch MTV sometime and you'll see the bodypacks and the capsule clipped to the clothing. In film production, wireless lavalier microphones can be hidden in props and on set-pieces, and provide an easy way of obtaining usable sound tracks on the first take. Some wireless transmitters can also accept high-impedance line-level inputs, such as electric guitars, so guitarists are free to stroll around the stage, kicking fellow members of their band.


As the wireless microphone system is a miniature FM broadcasting system, it operates within a given band of radio frequencies. The first wireless microphone systems utilized the FM broadcast band and the lower VHF television channels, 2 - 6. These frequencies were plagued with interference from local stations and did not offer the bandwidth necessary for very high quality transmission. In the 1970s, the Federal Communications Commission (FCC), a branch of the US government which regulates the telecommunications and broadcast industries, authorized the use of the higher VHF television channels 7 - 13 for wireless microphones. Analog television broadcast on a particular channel uses only a section of that channel's frequency band, so wireless microphones can be shoved into open spaces between the actual broadcast frequencies, and many wireless systems can be crammed into one open television channel. Other countries allotted similar frequency bands for use with wireless microphones. The VHF band provided some flexibility, but was still limited by some persistent interference, especially in big cities. A chunk of UHF frequencies around channels 64 - 74. The UHF frequency band is less occupied by television broadcast channels than the VHF band, but users are allowed to transmit at powers higher than that of the VHF band, which increases the actual operational range between transmitter and receiver (this is mainly because more power is needed to broadcast a higher-frequency radio signal over a given amount of space). Unfortunately, higher power requires more current, reducing battery life or requiring a larger battery supply. Manufacturers generally use the UHF band between 614 - 806 MHz, which corresponds to channels 38 - 70. As we enter the twenty-first century, audio professionals are somewhat worried about the effect of digital television broadcasts. Whereas analog television broadcasts do not occupy an entire channel of bandwidth, new digital television broadcasts take the entire channel and spit out digital noise, which sounds an awful like white noise. As more and more networks go on-line with digital television, frequencies are becoming scarce. It is still possible, though, to operate a multiple-unit wireless system totalling 60 UHF microphones in one location.

What is the breakdown for number of units in a given channel? Mathematically, a channel is 6 MHz wide. Depending on the manufacturer and licensing agreements, approximately nine wireless frequencies can be obtained in a band 1 MHz wide, and thus mathematically fifty-four systems can be crammed into one television channel. However, this is not obtainable in real-life situations due to intermodulation and other types of interference. Intermodulation is a type of interference in which a receiver picks up two dissimilar frequencies that interact within the receiver's electronics to produce sum and difference frequencies, including harmonics of these frequencies, which also results in a whilsting noise. Most frequency coordination software concentrate on eliminating third-order and fifth-order harmonics. It is thus absolutely essential that frequencies used in a given location are properly coordinated, taking into account nearby RF transmissions as well as interaction within a given system.

Although manufacturers provide wireless equipment with only 0.125 MHz between adjacent frequencies, most frequency coordinators will place adjacent frequencies at least 0.500 MHz apart. RF signals at nearly identical frequencies can interfere with each other; this interference is called heterodyning, which results in a "whistling" noise. Wireless microphone systems are not the only generators of RF signals-- digital musical equipment, computers, neon and fluorescent lighting, walkie-talkies, in-ear-monitors-- the list goes on and on. The Times Square area in New York represents what most consider the worst-case scenario: almost every theatre in the area uses wireless audio systems, walkie-talkies, wireless intercom systems in a city where the broadcast bands are already inundated with analog and digital television signals.

No matter what the frequency, most wireless systems utilize a frequency-modulation scheme: an audio signal is fed into a transmitter which modulates, or changes, a carrier frequency above and below the center frequency that corresponds to the audio signal. The receiver is also tuned to the center carrier frequency, and extracts the modulated information from the radio signal, resulting in the original audio signal. In order to generate the carrier frequency, most wireless systems take a crystal oscillator (a naturally-occuring element which oscillates at a given frequency) and multiplies that crystal frequency until the desired carrier frequency is achieved. While this scheme works well in most applications, the crystals and corresponding multipliers are all subject to physical shock and their tuning may vary over time. Newer technology called frequency synthesis uses electronic oscillators to generate the carrier frequency directly. Since they are electronically controlled, it is possible to manufacture frequency-agile equipment very effectively, allowing the operator to pick a range of frequencies from a group within a channel, which promotes flexibility in dense RF situations.


Early wireless microphone systems utilized one broadcast antenna on the transmitter, and one antenna at the receiving end. As the transmitter traveled farther and farther away from the receiving antenna, the radio signal would diminish, resulting in dropouts (lack of audio signal at the receiver) or noise (interference) or distortion. This system was also prone to multipath cancellation, a situation in which a transmitted radio signal reflects off a surface and is combined with the original signal, creating phase problems in the radio signal, resulting in dropouts in given areas. In theory, we can just about map out physical areas of multipath cancellation zones, which depend on the position of the transmitter, receiver, and any reflective surfaces as a function of frequency. In a stage situation finding the particular multipath cancellation zone is more difficult: the receiver is generally static, but any reflective surfaces may come in the form of movable set-pieces; the transmitter, too, is attached to a human which also moves around, which causes the cancellation zones to move around as well. Since VHF frequencies are lower, their broadcast wavelength is longer, which essentially creates an easier situation in which multipath cancellation can occur. UHF dropout zones are much shorter, and thus the dropouts are heard as a "fluttering" sound.

In addition to using a UHF-based system, multipath cancellation was partially alleviated by the introduction of diversity wireless systems, which resulted in greater system reliability. A diversity system utilizes not one but two receiving antennae; simply put, the receiver constantly monitors the output of both antennae and will use the antenna with the stronger radio-frequency signal. Most modern wireless systems also offer a selection of frequencies within a given channel to provide maximum flexibility; if one frequency doesn't work, try another one. More complicatedly, if the two antennae are placed at least a quarter of a wavelength apart, it is almost impossible that both antennae will fall within a cancellation zone at the same time (multipath cancellation). There are several types of diversity receivers:

It is important to note that in all cases, it is necessary that the two antennae produce two different, uncorrelated signals; ostensibly, the two diversity-reception antennae should not share the same physical space-- the antennae must be at least one-half wavelength apart, and it is usually found that separating the receiving antennae are located in completely different areas.

It is also important to consider the physical distance of the antennae from the transmitters. Not all of the power transmitted will reach the receiving antennae; a transmitter located too far away from an antennae will promote dropouts and, if the receiver is not properly tuned, may make the system susceptible to outside interference. The performance of the wireless system will also be degraded by signal losses due to interfering objects between the transmitter and receiver, such as other equipment, set pieces, or even people. Be especially wary of high-current electrical components, such as dimmers, fluorescent ballast, Steadicams, smoke machines, and stage automation machinery. Many commonly-used materials on the theatrical stage can reflect radio signals, which can adversely affect the total power received at the antenna. It is common practice to mount the antennae as close to the stage as possible, and many shows have rigged their receiving antennae to proscenium booms or pit rails, although it is important to keep cable lengths between antennae and the actual receiver units as short as possible. The use of high-grade coaxial cable will also prevent severe attenuation due to cable length. In some situations, it is necessary to amplify the radio-frequency signal transmitted along the cable. Manufacturers produce in-line amplifiers for just this purpose: an amplifier is placed close to the receiving antenna and thus amplifies the "clean" signal from the antenna to drive the signal down the coaxial cable to the receivers, keeping the signal-to-noise ratio of the radio signal as high as possible. Some people recommend wide-band amplifiers, which provide gain over a large range of frequencies, and some people recommend narrow-band amplifiers, which are carefully tuned to amplify only the frequencies used by the wireless microphone system. It is important, though, to keep amplifier use to a minimum; remember that the amplifier is not "smart." It will amplify outside signals picked up by the antenna just as well as the intended signal from the wireless microphones.

One may note that given a multiple wireless system utilizing diversity reception that the number of antennae required for proper reception grows astronomically. In order to make life somewhat easier, most manufacturers provide some sort of antenna splitter and distributor, so only a few sets of antennae are required, depending on the number of receivers and the frequencies in question. These antennae feed the splitter/distributor, usually located at the receiver position; the unit then feeds each individual receiver. Some splitter/distributors include a gain stage properly tuned for the receiver frequencies in question in order to provide a little more RF signal gain.


Technology in the 1970s introduced the compander circuit, a signal processing circuit that manipulates the dynamic range of an audio signal in order to reduce noise or bandwidth. The audio signal is compressed before it is modulated into a broadcast signal, and at the receiving end, the broadcast signal is demodulated, and the resultant audio signal is re-expanded. The net result is the ability to "squeeze" more dynamic range into a given amount of frequency bandwidth, and a lower noise floor. Futher advances, such as better compander circuitry and better circuit design, improved the dynamic range and frequency response of wireless systems, and the newest generation of wireless microphone systems sound almost just as good as a conventional hard-wired microphone. The compander circuitry directly influences the signal-to-noise (S/N) ratio of the wireless systems. Unaltered RF transmissions are capable of achieving a signal-to-noise ratio of no more than about 65 dB. This is hardly tolerably by professional standards, so all wireless systems use companding to improve this ratio-- approaching 115 dB.

In addition, manufacturers produced better and better lavalier microphone capsules. Capsules became smaller in size, allowing them to be mounted to the head of the performer; some manufacturers produced interchangeable capsules, which provided for easy replacement. Manufacturers responded to the demand for microphones that were more moisture-resistant, and several popular manufacturers have recently released capsules touted as being much more resistant to moisture than previous models. Handheld microphones were usually available with an assortment of popular vocal microphone capsules, and some manufacturers of handheld transmitters provided an interchangeable capsule so the designer (or, in some cases, perfomer) could select his or her own favorite microphone capsule.

Remember that audio control of the microphone shall lie solely within the sound department. Most transmitters have both "power on/off" and "audio on/off" switches. These switches, it shall be told, shall never be touched. Tape over the switches if you feel nervous.


Lavalier microphones were originally intended to be worn around the neck, suspended on a loop, or clipped to clothing. Film and ENG (Electronic News Gathering) production still use use lavaliers in this fashion. Theatrical sound reinforcement has graduated, if you will, to mounting the lav to the performer him- or herself, whether over the ear, in the hair, or built into a wig. Some newer theatrical productions who don't mind seeing the microphone use a thin boom, mounted on the ear, which places the microphone capsule in the general area of the mouth, which provides for an increase in gain before feedback. Most lavalier microphones are omnidirectional, although cardioid capsules are available. Theatrical sound designers have found that an omnidirectional microphone mounted in the center of the forehead provides the most natural sound, requiring very litle equalization at the mixing desk. Here are some popular rigging techniques, with their respective pros and cons:


Transmitters can be hidden just about anywhere on a person. Ideally, for safety purposes, it would be best to weld the transmitters directly to the skin or surgically implant a housing for an RF transmitter, but no actor has yet volunteered for this treatment. Instead, the most common locations for a transmitter are the small of the back, the rear of the hip, the inside of the thigh, around the ankle, in a pants pocket, or simply belt-clipped to loose-fitting clothing. In most of these applications, it's convenient to a have a cloth pouch with a velcro tab in which to place the transmitter; the pouch is then thread with an 1.5" elastic strap which fits around the waist, leg, or chest. The pouch can also be built directly into a costume. When the performer remains clothed for the production, this is the easiest and most comfortable way in which to deal with the placement of the transmitter. Care should be taken to route the microphone cable from the head to the transmitter location. Surgical tape can facilitate this process by securing the cable to the performer's skin. Note that some persons have allergies to different types of adhesive used by surgical tape manufacturers. Consult early!

Avoid placing the transmitter directly against the performer's skin without a barrier to prevent moisture from entering the transmitter and damaging the fragile electronics. Most engineers use unlubricated condoms in which to stuff the transmitter before rigging the transmitter to a performer. Normal-sized condoms work just fine- stretch them out a bit before rolling them onto the transmitter. Other people use small sandwich bags in a similar fashion. At the top of the transmitter, where the connectors and switches are usually located, stuff the top of the bag/condom with cotton or other absorbent material, then tape the whole package shut, allowing only for the antenna and microphone wire to stick out. Some engineers use art-gum eraser to mold around the antenna and microphone connectors, providing a watertight seal around those sensitive areas. Taping down all switches on a transmitter before the transmitter is applied to the performer is also a good idea to prevent inadvertent or intentional problems.

But what about special issues, such as beach scenes or nude scenes? One popular place to house a transmitter is within a wig-- early consultation with the hair designer can facilitate this process immensely, and a pouch for the transmitter can be built directly into the wig. The microphone capsule is usually rigged after the wig cap is installed and sandwiched between the wig and the wig cap. This arrangement also obviates the need to worry about the microphone cable being seen running down the back of the neck. Topless scenes with no wigs can present a problem, but proper dressing and painting of the microphone cable can make it almost invisible to the audience. Be creative! It's a good thing.

Care should be taken in securing the antenna cable, usually in the form of a short whip, anywhere from four to six inches in length. The antenna should not be allowed to be curled, or tucked, in any form-- it must remain straight for optimum performance. The antenna can be oriented vertically, up and down, or horizontally; with some cheaper systems it may be necessary to match the angle of the receiving antenna to the transmitter antenna for optimum usable range. Avoid crossing the antenna and microphone lines, as this can adversely affect the transmission power and audio signal. Avoid running the antenna directly against the skin, as body moisture can absorb the outgoing radio signal. While this does not present a problem for the performer, it may reduce the range of the transmitter. (At the low frequencies at which wireless microphones operate, radio-frequency radiation should not be a concern for anyone. Even at the higher mobile telephone frequencies-- 900 Mhz, 1.5, 1.8. 1.9, 2.4 GHz-- there is no official proof that radiation from radio-frequency signals can adversely affect humans).

Install a fresh battery in the transmitter every time you use it. It sounds like a detail that should be obvious, but all too often radio mic problems boil down to a weak battery in the transmitter.

With proper care and feeding, your radio microphone system should provide you with a clean audio signal, free from distortion, heterodyning, and overload, the quality of which is just below that of a proper, wired microphone.

Return to the Sound Index.

Comments, Questions, and Additions should be addressed via e-mail to Kai Harada. Not responsible for typographical errors. - © 2000 Kai Harada. 07.06.2000

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