shavlogo2.gif (3085 bytes)Microphones for Stage Use

8/04 - Larry Mundy - http://www.colomar.com/Shavano/stagemikes.html


Note: Larry Mundy is the author of Design and Build Your Own Live-Sound Speakers and an occasional contributor to this site. 


If you are a vocalist, a microphone is your primary tool.  Everyone knows what a microphone is, although there is a confusing variety of types and subtypes – dynamic, condenser or ribbon, omnidirectional or unidirectional, and so forth.  They all work generally the same way.  The sound pushes around a little diaphragm inside the microphone, and that movement is then translated into tiny electrical currents.  The key word here is “tiny;” these currents are very weak and require a huge amount of amplification in the signal chain – much more than many other signal sources.  For this reason, their signals require special “preamplification” treatment later in the signal chain, and special inputs (“mic preamps”) in your mixer or PA amplifier to accommodate them.  That’s why microphones plugged into the input jack of an instrument amp sound so thoroughly terrible.

Microphones are also among the most low-fidelity components in the signal chain, meaning they can add the most “coloration” to the sound by reason of inaccurate frequency response.  In this respect also they are like speakers – since both have moving, mechanical components (the microphone diaphragm, the speaker cone), they are the most subject to abuse, sensitivity to temperature and humidity, and capable of simple mechanical failure.

So for live performance, should you buy the most expensive microphone, with the most uniform frequency response, you can find?  No.  Those microphones are for the temperature-controlled environment of recording studios, where they are never knocked around, are protected from theft and where their output will be recorded for critical listening, over and over.  For live performances, I’d advise you get something a lot cheaper and tougher, even if it doesn’t convey your sound with absolute perfection.

Dynamic and Condenser Microphones

First let’s talk about designs and types of microphones.  All of them translate sound vibrations into current, but they can differ in how they do that.  One point of difference is in the diaphragm, the little eardrum-like part that “hears” the sounds you make.  A “dynamic” microphone couples that diaphragm to a coil of fine wire that moves back and forth relative to a fixed magnet.  As the coil moves through the magnetic field, a small current corresponding to the sound input is “induced” in the coil wire, producing a weak AC current that constitutes the signal.  A dynamic microphone is truly like a loudspeaker in reverse.  A “condenser” microphone uses a source of power – either an internal battery a or current called “phantom power” flowing to the microphone from your mixer or amp through the microphone cable – to “charge” a flexible, metal-plated diaphragm mounted very close to a conductive metal plate.  As the diaphragm moves, it creates a varying AC current in the conductive plate, resulting in a signal.

Generally speaking, the diaphragms of “condenser” microphones are lighter and move more freely because they don’t have the mass of a coil of wire attached to them.  As a result, condenser microphones tend to react more quickly to quick changes in sound levels (“transient” sounds) and have wider and “flatter” frequency response, particularly at high frequencies.  At the same time, with their thinner, lighter diaphragms, condenser microphones tend to be more fragile and expensive.  “Dynamic” microphones are simpler because they require no external or battery power, and are generally less expensive and more rugged.  There is also a type of microphone called a “crystal” mike in which sounds excite a piezoelectric crystal and produce a signal directly.  These are the cheapest, worst-sounding microphones you can buy, and so are becoming extremely rare outside the realm of low-fidelity, “voice-grade” equipment like telephones and kid’s toys.

Directionality

Microphones are also designed for varying degrees of directionality.  “Unidirectional” means that their physical construction is designed to pick up sounds from in front of them, and designed to reject sounds from the sides and back (you may also hear the term “cardioid,” which means heart-shaped, since the “pickup pattern” of a unidirectional microphone, seen from above, looks like a stylized Valentine’s Day heart with the point directed toward the sound source).  An “omnidirectional” model is designed to pick up sounds from all directions, and would be useful in a conference call with people seated all around a table.   For most live-sound use, you don’t want an omnidirectional model, which will happily pick up audience sounds and even “recycle” the sounds from your speakers, making it more likely you’ll experience the dreaded howl of feedback. 

So back to the initial question: which kind should I buy?  If you are only reproducing the human voice, for vocals, DJ patter or speech, or instruments without significant high-or low-frequency sounds, the standard stage microphone is the “unidirectional dynamic” type.  Within this class there are “vocal” microphones (typified by the Shure model SM58 and its hordes of imitators) and “instrument” microphones such as the Shure SM57.  A “vocal microphone” is simply a microphone with a slightly exaggerated response in the midrange frequencies, where the bulk of human speaking and singing voices lie.  An “instrument mike” usually has flatter frequency response to better reproduce the range of an acoustic instrument.  Vocal mikes are good for vocals; instrument mikes can be used for either vocals or instruments although they will not have the “punch” and intelligibility in the vocal range of a “vocal” mike.  You can buy perfectly suitable microphones of either type for under $100 U.S., and sometimes for much less.  As with most sound equipment, however, a very inexpensive microphone is likely to have design or quality compromises that aren't good for sound or reliability.

If you have a specialized need to reproduce high-frequency sounds like cymbals, or want the ultimate in flat frequency response for making recordings, a condenser microphone will be a better choice.   A condenser mike will also better reproduce the delicate sound of a fine string instrument.  Just be sure that it is capable of utilizing “phantom power”, and that whatever preamplifier or mixer it’s plugged into is capable of supplying it, because your sound should never depend on the condition and lifespan of a small internal battery.  Treat a condenser microphone gently because its internal parts are fragile, and take special care to avoid humidity and moisture, which is very hard on condenser units.  For these reasons, I don’t recommend condenser microphone designs even for fixed PA installations, because even if your equipment is permanently mounted and hidden, the microphones still get plugged, unplugged, moved, beaten, dropped and left in strange places.  It’s a tough life for them, and dynamic microphones are better equipped to survive it.

There is a variety of other types of specialized microphones.  Some use diaphragms (or “elements”) made out of exotic materials for optimum frequency response in the recording studio.  There are microphones with long, skinny “barrels” (“spot” microphones) that are extremely directional and meant to focus on a narrow sound source from a distance (i.e. out of TV-camera range or hidden in a surveillance van).  There are dynamic mikes with oversized diaphragms and heavy construction designed to withstand the low-frequency abuse of reproducing a kick drum up close, and underwater microphones for recording the squeals of humpback whales.  You’ll probably never have use for any of these, which is good because they can all get really expensive. 

Some microphones are made to be worn, so they move around with a speaker or performer.  Little “lavalier” microphones which clip to a lapel or other piece of clothing are convenient for public speakers but tend to be distressingly “low-fidelity” for any other use, partly because they’re aimed at your chin from below.  “Headset” microphones popularized by choreographed song-and-dance acts like Madonna are generally just small dynamic units mounted on some sort of scalp-hugging bracket, and work just like any other dynamic microphone.

Wireless microphones are simply ordinary microphones conected to a battery-powered transmitter; the sounds travel through the air to a nearby receiver where they are fed to your mixer or amplifier.  Wireless systems introduce a number of complexities and choices and need a separate article to explain, but the microphone part of these systems works exactly the same as a "corded" unit.

Impedance

All microphones have an impedance, based on their internal circuitry and how severely they “impede” the free flow of electrons through them.  High-impedance or “Hi-Z” microphones are usually cheap, and sound that way.  They have an internal impedance of thousands of ohms and don’t do well with long signal cables, losing high-frequency response with anything other than relatively short cable runs.  Low-impedance microphones (“Lo-Z,” usually 1,000 ohms or less, 600 is fairly standard) work better with long cables and are usually a sign of a higher-quality unit.  The equipment a microphone is plugged into generally expects that microphone to have a certain approximate impedance and works better when it does, and most quality PA equipment will expect a low-impedance microphone (if your microphone has a cable with three conductors rather than two, that’s a good sign it’s low-impedance).  If you buy a decent microphone for PA use, it’s almost certain to be a low-impedance model.

Usage Tips

On-off Switches. Some microphones come with on-off switches on the housing and some don’t.  My advice is to make sure the ones you use onstage have them.  Microphone feedback, which is causing your audience to wince in pain, is instantly solved by switching off the offending microphone until you can reset the levels, speaker or microphone placement, or whatever is causing it.  A seldom-used or unattended microphone onstage will pick up the sounds of instrument amps, etc., often in an unintended and unpleasant way.  If you have multiple microphones onstage, it’s much easier to test them individually if the others are switched off.  Use unswitched mikes only when you have a professional “sound man” setting up your equipment and mixing your performance.

Matching Sound Characteristics.  If you have multiple vocalists and thus multiple vocal microphones, try to make them all the same make and model.  This will simplify your life because while most good microphone preamps have separate tone controls for each microphone channel, it’s a hassle to adjust each one for a live performance and in the crush of set-up this frequently just doesn’t get done.  More often, if someone says the PA system needs “more highs” or “more bass,” someone just tweaks the whole system with the master EQ (equalization) or tone controls, and tonal differences between different makes and models of microphones are accentuated.

Wind Noise.  If you’re playing outdoors in breezy conditions, wind blowing across and through a microphone is amplified just like your voice or instrument would be.  You may not notice it in the heat of battle, but your audience is listening to something that sounds like a hurricane in the background.  You can help avoid this by using foam “windscreens” that slip over your microphones, or even the cut-off end of a sweatsock (hopefully not a dirty, stinky one).

Care and Feeding.  Microphones are delicate enough that they shouldn’t be thrown in your gear bag with all your cables, pedals, dirty underwear and collection of discarded song lists.  Buy or make a hard-sided protective case for all your microphones (including the extra one you haul around in case one dies), and line it with cut-out foam so your mics are insulated from shock while being hauled around.  And old thrift-shop briefcase and some cheap hobby foam will do the trick.

Not For Human Consumption.  While most microphones designed for live-sound use are pretty forgiving of loud input levels, every microphone has a point where high sound pressures will make it distort unpleasantly.  This happens most frequently with vocalists who feel the need to have the microphone halfway down their throats at all times, on the incorrect presumption this will make their vocals louder or better.  When your mouth is very close to the microphone, there is a slightly enhanced bass response due to something called the “proximity effect,” but for the most part “eating the mike” is just an indication of an inexperienced vocalist.  The distorted sonic result usually sounds horrible, with massive distortion and every “P” popping the speakers like elephant flatulence.  You can buy little “pop screens” which are essentially mesh filters too large to swallow, and which keep the vocalist a decent couple of inches from the mike; these are useful at practice to help cure bad habits but look really stupid onstage.  The better solution is simply to understand that if your PA system has adequate power, there is no reason to “eat” the mike.  Perhaps there is a good use for that dirty, stinky sweatsock after all!


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