Mic Basics, Part 1
What you know about mics could change everything...
Microphones are the most important tools used in live sound. Just as a carpenter must know how to use power tools to accomplish a certain task,musicians and engineers need to know how to effectively choose and use the right microphone in order to get predictable, consistent results.
Here is the first in a three part series on microphones. In this section we'll examine how microphones operate. In the next two, we’ll learn how mics pick up sound from different directions and then take a look at which mics are suitable for use on specific instruments.
Let’s take a basic look at how a mic operates. A microphone is a transducer, a device that converts energy from one form into another. A mic changes sound vibrations moving through the air into electricity that then can be manipulated or sent into recording consoles or tape machines. Let's find out exactly how a microphone does this.
The most common types of mics are known as dynamic and condenser. Dynamic mics come in two flavors: moving coil or ribbon. Dynamic microphones generally do not require a battery or any kind of power to operate, whereas condenser mics generally do require a battery or 'phantom power' (there are exceptions which we’ll discuss later).
So how does a mic convert sound waves into electricity? Take a look at figure one.
It shows the inner workings of a moving coil mic, and we can see several basic parts: the diaphragm (a) is a flat piece of plastic (much like a drum head) that "catches" the sound waves. Attached to the back of the diaphragm is a round tube with wire wrapped around it (b). This is called the coil. The whole assembly is suspended within a magnet (c). When a sound wave hits the diaphragm, the diaphragm and the coil move. One of the basic laws of physics states that a wire or coil of wire moving in a magnetic field will generate a voltage (electricity) in that wire. This is called Electromagnetic Induction and it is what causes the sound wave to be changed into electricity. This electricity comes out of the output wires (d) and it just so happens to be a very good replica of the original sound wave. How convenient!! Common examples of moving coil microphones include the Shure SM57, Heil PR20 and Audix OM5.
Moving coil mics have several strengths. They are very rugged. If you drop a moving coil mic chances are good that it will still work. Moving coil mics are cost effective, have a long lifespan and deliver reasonably good sound without breaking your bank account. As with all things, the better sounding moving coil mics are usually more expensive.
On the down side, moving coil mics are not the most sensitive mics. They can miss some of the high frequencies of an instrument (possibly sounding dull) and since the diaphragm and coil assembly is relatively heavy they are not the best at capturing very soft sounds. Keep in mind that these same qualities can translate to better sound on stages where loud instruments “bleed” into a vocal microphone.
What, You Want That Wrapped In A Ribbon?
A second type of dynamic microphone is the ribbon mic. The element of a ribbon mic is shown in figure two.
Notice that there is still a diaphragm (a) and a magnet (b), but there is no coil. This time the diaphragm is a thin ribbon of metal such as Beryllium or Titanium, pleated or folded for strength. This ribbon is about ten times the thickness of the foil used on a gum wrapper and it also acts as our wire. When a sound wave hits the ribbon it moves, again creating Electromagnetic Induction. The sound wave, now converted into electricity, appears at the ends of the output wires (c) attached to the ribbon.
Since a ribbon is very thin and lightweight it reacts much more quickly than a moving coil, giving it a faster “transient” response (transients are sounds with a very quick attack, such as a tambourine). This makes ribbons suitable things like horns, percussion and vocals.
Ribbon mics tend to be delicate, so we’d strongly advise against placing one in a kick drum—the ribbon will stretch out of shape causing audible problems or (at worst) the blast of air pressure will tear the ribbon, making the mic useless.
Ribbon mics also have notoriously low output level so you need to plug them into a mixing console that is capable of supplying a lot of gain without adding noise. For that reason we are seeing modern ribbon mics with built-in preamps to add some gain before the signal travels down the mic cable.
In the past we’d say “never apply phantom power to a ribbon mic (more on that in a second) but most modern ribbon mics are protected from phantom power and in fact may require it for the built-in preamp. Having said that, be very careful about ensuring that phantom power is off when plugging in your vintage RCA 77DX.
Until a few years ago you never saw ribbon mics on stage because of durability issues and the fact that they tend to be expensive. That has now changed with companies like Royer Labs, sE Electronics and Audio-Technica manufacturing ribbons mics that are road-worthy.
Finally we have the condenser microphone shown in figure three.
A condenser mic operates on a totally different principle without a magnet. Notice that there are two plates (a and b) with a space between them. The plates are either metal or plastic treated with a metallic coating. One plate is fixed and the other moves. The movable plate (a) is the diaphragm. Together these plates form a condenser or capacitor — a device that temporarily stores electricity (sort of like a battery).
When there is no sound wave hitting the diaphragm, the two plates hold a certain amount of voltage, which is drawn from either a battery in the mic or from the mixing console.
When a sound wave hits the diaphragm, the diaphragm moves back and forth and this causes an extremely small change in the amount of voltage the condenser stores. It is this miniscule change of voltage that represents our sound wave and it must be amplified.
To send it down a cable to a mixing board without amplification would result in the tiny signal becoming overwhelmed with noise. So built into every condenser mic is a miniature amplifier which boosts the signal before it goes down the mic cable. This amplifier requires power to work and it comes from either the built-in battery or from something known as phantom power. Phantom power is 48 volts of DC electricity supplied by a mixing console for the purpose of powering condenser mics. It is called "phantom" because we cannot see anything more than an ordinary mic cable connecting the microphone with the mixing console.
In the old days condenser mics used big power supply boxes that you’d trip over.
[Without getting overly technical here’s a bit of practical advice. Phantom power is “specified” as 48 volts DC. Not all mixing consoles actually produce 48 volts. Some mixing consoles deliver 18 or 24-volt phantom power, in which case most mics will work but they will be a bit noisier than if they received 48 volts. For example a Shure SM81 (a very popular condenser mic for high hat and cymbals) will run on anything from 9 to 52 volt phantom power but other microphones (Neumann U87, Earthworks TC25, Audio-Technica AT4050 and Audix SCX25) will require 48 volts. This is a good fact to remember so it won't bite you on the rear end sometime down the line.]
Condenser mics are sensitive and typically have a very extended high frequency response so they capture more detail than most moving coil or ribbon mics. This makes them ideal for use on lead vocal, cymbals, acoustic guitar and just about anything else you might want to record — but keep in mind that this ability to capture detail, plus the tendency for condenser mics to be less directional can present problems on stage.
Condenser mics come in a variety of styles and sizes from small clip-ons to hand held to big shock-mounted monsters (at prices approaching that of a new car). Condenser mics tend to be more rugged than ribbon mics so you can safely use them on loud, rude instruments like kick drums or guitar amps. Most recording studios have at least a few condenser mics on hand but for live sound situations, moving coil mics are most common.
That covers the three main types of microphone design. Next time we will examine how microphones pick up sound from different directions.
Also check out some videos, Rev Bill created to help explain the differences: