December 03, 2012, by Bruce Bartlett
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.
Mic’ing an instrument can have a dramatic impact on its tone quality as heard through a sound reinforcement system (or studio monitors for that matter). There can be a bassy or honky tone that mars the signature.
Moving the mic changes the sound. A musical instrument radiates a different tone quality (spectrum) in each direction. Also, each part of the instrument produces a different tone quality.
It pays to experiment with all sorts of mic positions until you find a sound you like. There is no one right way. You also place them to minimize feedback, and those two requirements often conflict with each other.
To determine some useful mic locations on an acoustic guitar, I ran extensive measurements of the spectrum of an acoustic guitar as picked up in various mic locations.
The spectrum of a musical instrument is its output level versus frequency – its fundamental frequencies and harmonics, and their relative levels.
To establish a natural-sounding reference, I recorded an acoustic guitar with a lab-reference omnidirectional mic one foot away (ACO Pacific 7062PH). This mic has a ruler-flat frequency response. At the same time, I recorded with a flat-response mini omni mic on the guitar. I placed the mini mic in various locations, and measured the guitar’s spectrum (fundamental and harmonic frequencies).
Finally, I compared the reference spectrum at 1 foot to that of the mini mic on the instrument.
The results show the differences in bass, midrange and treble between the reference mic one foot away and the mini mic on the guitar. They describe the timbre or tone quality picked up in various mic placements. I also ran listening tests with 10 trained listeners and musicians. They described how each mic placement sounded compared to the reference.
The results shown here are for one guitar. And because every guitar is different, these measurements are meant to indicate general trends only.
Figure 1: Above, the reference mic spectrum and the close-mic’d spectrum. Below, the difference between the two curves. (click to enlarge)
In Figure 1, the mic is near the low E string, halfway between the sound hole and the bridge. Its spectrum is a fairly good match to the reference, so its sound is natural.
In the top graph, I overlaid the reference mic spectrum and the close-mic’d spectrum on the same graph, so you can see in how mic placement affects the spectrum, which affects the timbre you hear. The red curve is the flat-response reference mic one foot out front, and the yellow curve is the mini mic. Each horizontal line is + or - 10 dB relative to the adjacent line.
The lower graph in Figure 1 shows the difference between the two curves, or the difference between the reference spectrum and the close-mic’ed spectrum, for the same placement. The differences between the two are slight, so the close-mic’d sound in that position is natural or hi-fi.
All of the following graphs (Figures 2-6, below) show the difference between the reference spectrum and the close-mic’d spectrum. In other words, the graphs show how the sound changes when you move the mic from one foot out front on to the instrument’s surface, in various locations.
A note about Figure 6 – to get the acoustic guitar to sound natural when mic’d in that location, apply a complementary EQ curve: roll off the lows (about -10 dB at 100 Hz) and dip the upper midrange (about -6 dB at 6 kHz.). Or use a mini mic with that frequency response.
Again, every instrument is different, so these measurements are meant to show general trends.
Figure 2: The mic is on the guitar body halfway between the bridge and the edge of the guitar. The sound is midrangey –thin or weak in the bass, and diminished in the treble, as shown by the reduced output below 300 Hz and above 1 kHz.
Figure 3: The mic is one inch inside the soundhole, about a half-inch below the strings. The sound is bassy because the low frequencies below 200 Hz are emphasized in the sound hole. Also, the mids and highs are reduced, giving a slightly dull or dark sound.
Figure 4: The mic was moved next to bottom edge of sound hole. The sound is bassy due to the emphasis below 200 Hz. Also, it is lacking in presence due to the weaker output around 1 kHz and 4 kHz.
Figure 5: The mic is under strings between sound hole and bridge. Sounds fairly similar to reference but with reduced mids and highs.
Figure 6: The mic is just inside the sound hole, mic touching the underside of the front surface, at 8 o’clock relative to the fingerboard. The sound emphasizes the bass below 200 Hz and emphasizes the treble above 4 kHz. The guitar picked up here is relatively loud, so the gain-before-feedback is good..
Bruce Bartlett is a microphone engineer (www.bartlettmics.com), recording engineer, live sound engineer, and audio journalist. His latest book is “Practical Recording Techniques 6th Edition.”