>Subject: RE: #8743 Noise Cancelling Mics
>Date: Fri, 25 Jan 2008 15:21:01 +0800
>Organization: TEAM/CCG
>Thread-Index: AchfGMRSYhhE6lGMRtGD0KvdHhzGbwABWRjw
>In-Reply-To: 



The answer I'm afraid is that noise cancelling mics are liable to work quite
unreliably in the situation you describe.

First there are two kinds of "noise-cancelling."
1) Cut off any sound when it is below a certain threshold. Otherwise let it
all through. As you can see, this won't help you if there's too much ambient
noise. A variation is to cut off all sound below a certain threshold all the
time. This is like the "squelch" on a cb radio. It greatly lowers the sound
quality of recorded voices because it eliminates attacks and fades and
resonant frequencies.
2) Measure steady sounds and actively generate the "negative" of this sound.
Recorded (or played) together, the two will (mostly) cancel each other out.
This works best for low frequency noises that are steady, like jet engines,
road noise on the highway, and transformer (fluorescent light) hums. It
won't impact the sound of other voices because they are higher in frequency
and vary too much. If you had a big enough crowd that there was a general
"murmur from the crowd" it might attenuate it somewhat.

The best solution for rows of people using mics is to use low-sensitivity
(dynamic) mics very close to each person's mouth, and this can also be aided
by using "windscreens" on the mics. Picture the tv reporter live on the
scene with a big, foam covered mic right in front of his/her mouth and an
earpiece in. An additional help is to install partial barriers around each
work station, preferably with sound-absorbent covering, so that people are
speaking into a deadening semi-box. Picture the "rows of telephone operators
waiting to take your calls."


Dr. Steve and Laura Spinella, Sarah, Joey, Robby
Da Yi Street, Lane 29, #18, 2F-1, Taichung 40454, TAIWAN
011.886.4.2236.6145, of 4.2236.1901, fx 4.2236.2109, cell 9.2894.0514
USA: 9685 Otero Ave, Colorado Springs, CO 80920, 719.528.1702, cell
719.360.6485
TEAM, PO Box 969, Wheaton, IL 60189, 800.343.3144 <www.team.org.tw/ccg>,
<http://groups.google.com/group/TEAMspinella/>, 
<[log in to unmask]>, <[log in to unmask]>

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