From: "Phil Allison"
Subject: Re: Acoustic Feedback reduction
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Date: Sun, 12 Jan 2003 11:59:45 +1100
NNTP-Posting-Date: Sun, 12 Jan 2003 11:49:22 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"John Woodgate" wrote in message
> I read in sci.electronics.design that Phil Allison
> wrote (in ) about 'Acoustic
> Feedback reduction', on Sat, 11 Jan 2003:
> > If you mix
> >direct and shifted signals you get beats - loud and clear. If you
> >the shifted signal by even 3 mS the beats are destroyed. As the PA
> >are more than 1 metre away from the performer.......
> That's VERY interesting. At 100 Hz, 3 ms is a fraction of a period,
> whereas at 1 kHz it's 3 periods. Something that happens over that range
> is obviously a significant feature of human psychoacoustics.
> If we take, say, 150 Hz and 155 Hz present together, there is a
> cyclically-varying phase shift between them, corresponding to the 5 Hz
> beat frequency. Adding a 3.33 ms delay adds a 'd.c. component' to this
> cyclical variation, 0.5 of a cycle of the 150 Hz, 180 degrees at 150 Hz
> or 6 degrees at 5 Hz.
> I wonder why that destroys the perception of the beats.
** Do you have a copy of WW May 1974 ? There is a letter ( Howl
Supression) from M. Hartley Jones who described a 5 Hz frequency shifter in
the July 1973 edition. I repeated the experiment he describes with a delay
unit, shifter and headphones. Fascinating.
In the letter he says: "I found the absence of beat in the majority
of musical applications rather mystifying at first, but the answer was found
to lie in the delay between direct and amplified sound".