Perspectives of a Writer and Musician

Issues related to writing, publishing and playing jazz music: One man's muse.
by Al Stevens

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Location: Florida, United States

Sunday, May 21, 2006

Audio Feedback

Feedback is an audio phenomenon that occurs when you point a microphone at a loudspeaker and turn on the amplifier (assuming they are connected, of course).

Here's what happens. The room is quiet. Then almost immediately something makes a sound, even a tiny sound. The air conditioner cuts in, someone taps their foot, a car goes by outside, a mouse burps, or whatever. The microphone picks up that tiny sound and sends it to the amplifier. The amplifier amplifies the sound and sends it to the loudspeaker. The loudspeaker produces the sound louder than the original. Now the loop begins.

The microphone picks up the louder sound produced by the loudspeaker, combines it with the earlier sound which hasn't faded out, and sends it to the amplifier, which amplifies and sends it to the loudspeaker. The microphone picks up this louder combined sound, the loop iterates endlessly, and in a matter of milliseconds a high-pitched squeal resonates throughout the room as all the components in the system are being driven at their upper limits. And all the dogs in the neighborhood go nuts.

You might wonder why that doesn't happen every time an active microphone is connected to a powered-up amplifier. Most public address microphones are directional, which is to say they pick up sound that originates only from the direction in which they point. If the microphone is behind or pointed away from the loudspeaker, or if something is between the loudspeaker and the microphone, you get feedback only if you turn the microphone's gain control on the amplifier way above its normal operating limit causing it to pick up more than it should.

Two ways to prevent feedback are to place the loudspeakers ahead of the microphones or to keep something—a warm breathing body, for example—between the microphone and the loudspeaker.

Loudspeaker placement is the ideal solution, particularly if you like to move around onstage. But it introduces another problem in that you probably can't properly hear your own performance if you are behind the loudspeaker. This problem can be solved by the use of monitor speakers, which is another discussion.

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