It’s important never to underestimate the importance of microphones in the audio signal chain, be it on a live stage or in a recording studio. With the obvious exception of electronic instruments that can be connected directly, every other audio signal starts with a microphone so it’s vital to get this critical first stage right because if not, no amount of subsequent signal processing is going to fix it later.
Understanding how microphones work can be really useful in informing our choices as well as dictating how we deploy them, so let’s go back to school and have a quick look at how the two most common types work.
Dynamic designs (Figure 1) are essentially a loudspeaker in reverse. Incoming sound waves move the diaphragm, which has a coil of wire attached, and as the coil moves over a magnet, the principle of electromagnetic induction produces a current that’s a direct representation of the original sound wave.
Due to the fact that the coil has mass a certain amount of energy is required to move the coil in the first place, it means dynamics don’t pick up weak signals as well. It also means that the sensitivity drops off quite quickly as you move away from the mic, and it has an impact on the range of frequencies it can capture. Higher frequency sounds, with their weaker waves, struggle to move the mechanism, resulting in a rolling off the top end producing a more rounded and “warm” sound.
The need to physically move the coil also means that dynamics have a subtle compressing effect on the signal, particularly the initial transient content – it can be quite similar to the compression inherent in the narrowing of the ear canal, which might explain why dynamics, despite their limitations in terms of frequency response, sound relatively natural to us.
They’re also usually capable of handling high sound pressure levels (SPLs), which makes them great for loud sources (such as drums and guitars). In addition, they can be incredibly tough and rugged making them very well suited to the demands of live stages. Finally, they’re relatively easy to mass produce, making them cheaper to buy.
Capacitor (more commonly called condenser) mics get their name from the fact that the conversion is done by two conducting plates which are spaced a small distance apart, thus forming a capacitor (Figure 2). The back plate is fixed and the front plate is the diaphragm, therefore when sound wave vibrations move the diaphragm it alters the capacitance, producing a signal that accurately represents the source audio. For this to work, there needs to be a voltage present to create a capacitance that can be altered by the incoming signal. This means that a power source is required for operation (be it phantom power or a battery).
As opposed to dynamics, the only moving part is the diaphragm (typically gold-coated plastic) that is relatively light, meaning it can be moved much more easily by weak signals, making these microphones more sensitive and better at capturing high frequencies. Capacitor mics tend to capture a clear, transparent sound with a more even frequency response however they’re not adept at handling high SPL signals.
While capacitors tend to produce a more accurate representation of the sound, their greater sensitivity can be problematic when it comes to maximizing the amount of gain before feedback. This explains why we tend to use more dynamics than capacitors in live sound; in a sense we’ve learned to harness and exploit the flaws of dynamics to enable us to get the sound we want while avoiding the specter of feedback.
The Right Direction
I haven’t really touched on another of the key characteristics – polar patterns – but because practically all the mics we use on stage are cardioid we just need to remember that they’re most sensitive at the front, about 6 dB less sensitive at the sides and least sensitive at the back. Therefore, a useful rule of thumb is to point the front at what you want to capture and, if necessary, arrange the back to point at something you don’t want to capture.
Once we have a large number of mics deployed on stage there’s one very important factor to always be mindful of: our old friend, phase. If the same audio signal is picked up by more than one mic with slightly different arrival times then when those signals are combined they will be out of phase with each other resulting in certain frequencies being cancelled while others are boosted (depending on the degree of phase shift).