19 April 2012

How mic placement affects tone

This article is an extract of an article made by microphone designer and engineer Bruce Bartlett.

Your mic technique has a powerful effect on the sound of your recordings and your amplified sound through a PA system. In this article we’ll look at how mic choice and placement affect the sound you pick up from various acoustic instruments. To get a good sound, you need to start with an excellent mic and mount it on your instrument where it sounds good.

Which mic should I use
If you want an accurate or natural sound, first go for a microphone with a wide, flat frequency response. Such a mic reproduces the true timbre of the instrument -- its fundamental frequencies and harmonics, and how loud they are relative to each other.

The Effect of Close Miking
Miking an instrument up close, or on its surface, can color its tone quality as heard through a PA system. If you mike very close, you might hear a bassy or honky tone instead of a natural sound. Why? Most musical instruments are designed to sound best at a distance, at least 1 foot away. The sound of an instrument needs some space to develop. 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, it will pick up the sound of the loudspeaker accurately. But if you place the mic close to the woofer, the sound will be bassy. Similarly, if you mike close to an instrument, you emphasize the part of the instrument that the microphone is near. 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 100Hz. A microphone placed there hears this bassy resonance, giving a boomy timbre that does not exist at a greater miking distance.  To make the guitar sound more natural when miked close to the sound hole, you need to roll off the excess bass on your mixer, or use a mic with a bass rolloff in its frequency response.

Usually, you get a natural sound if you put the 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. For example, if the body of an acoustic guitar is 18 inches long, place the mic 18 inches away to get a natural tonal balance. If this sounds too distant or hollow, move in a little closer.

In PA situations, however, miking at that distance can cause feedback because you need to turn up the microphone quite a bit to hear it. The Spark Mic is meant to be mounted directly on the surface of the instrument, so it picks up a loud sound. Then you don't need to turn up the mic so much, and feedback is less likely to happen.

Where Should I Place the Mic?
Suppose you mount a mic on an instrument. If you move the mic left, right, up, or down, you change the tone quality. In one spot, the instrument might sound bassy; in another spot, it might sound natural, and so on. So, to find a good mic position, simply place the mic in different locations—and listen to the results—until you find one that sounds good to you.

Why does moving the mic change the tone quality? A musical instrument radiates a different tone quality 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 to place the mics because you place them to get the tonal balance you want.

FIDDLE. Mic floating between bridge and tailpiece. 
The sound is bright due to the emphasis above 3 kHz. The sound is also a little thin because of the reduced output below 1 kHz. This was measured with a flat-response Spark Mic. The Bartlett Fiddle Mic is rolled off at high frequencies to give a more natural sound in this miking position.

FIDDLE: Mic over low f-hole. The sound is warm due to the emphasis below 400 Hz. Although the highs around 10 kHz are emphasized with the flat-response mic used in this measurement, the rolled-off high end of the Bartlett Fiddle Mic compensates, creating a more natural sound.

Thanks to Bruce Bartlett for letting me use this article

click here to read the entire article in PDF

1 comment:

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