Microphone choice and technique have a powerful affect on the amplified sound of a reinforcement system.
The sound picked up might be “natural,” “boomy,” “thin,” “colored” and any other number of descriptive terms that are applied to a sonic signature.
It’s usually most desirable to attain “natural” sound. But what does that mean?
I define it as the tonal balance heard with our ears in front of a musical instrument, a few feet away. Some synonyms might be “accurate,” “realistic,” and “high fidelity.”
When listening through a system, if the reproduced instrument has a natural tonal balance, it should sound like the unamplified instrument, only louder.
Of course, we may not want to hear a natural tone, but rather, improve the tone of a tinny-sounding instrument, or tweak its EQ to prevent masking other instruments.
I’ve been doing quite a bit of work of late in evaluating miniature microphones placed on acoustic instruments and how they can affect the sonic signature. Let’s take a look at what I’ve found, using an acoustic guitar as the example.
To attain a natural sound, we might instinctively choose a mic with a wide, flat frequency response. When placed a few feet from an instrument, it can reproduce the true timbre of the instrument – the fundamental frequencies and harmonics, as well as how loud they are relative to each other.
As we’ll see, though, a flat-response mic may not provide a natural tone when it’s clipped to the surface of an instrument.
Most musical instruments are designed to sound best at a distance, at least one foot away. The sound needs some space to develop, so a mic placed a foot or two away tends to pick up a well-balanced, natural sound. That is, it picks up a blend of all the parts of the instrument that contribute to its character or timbre.
Think of a musical instrument as a loudspeaker with a woofer, midrange, and tweeter. If you place a mic a few feet away from a loudspeaker, it will pick up the sound accurately. But place the mic close to the woofer and the sound will have too much bass.
Placing a mic on an instrument does much the same thing – it emphasizes the part of the instrument that the mic is closest to. The tone quality picked up very close may not reflect the tone quality of the entire instrument.
Suppose you place a mic next to the sound hole of an acoustic guitar, which resonates around 80 to 100 Hz. The mic picks up this bassy resonance, giving a boomy timbre. To make the guitar sound more natural, you would roll off the excess bass via the system’s mixer, or use a mic with a bass roll-off in its frequency response.
Usually, a natural sound can be attained by placing a mic as far from the source as the source is big. That way, the mic picks up all the sound-radiating parts of the instrument about equally. If the body of an acoustic guitar is 18 inches long, place the mic 18 inches away. If this sounds too distant or hollow, move in a little closer.
In some live sound situations, however, mic’ing at that distance can cause feedback because it’s gain must be boosted a significant amount, and the mic may also pick other sources on stage, muddying the sound. That’s when we turn to miniature clip-on mics mounted directly on the surface of the instrument.