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From: "Phil Allison"
References: <email@example.com> <96dU9.21919$jM5.firstname.lastname@example.org> <email@example.com>
Subject: Re: Acoustic Feedback reduction
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Mon, 13 Jan 2003 00:20:58 +1100
NNTP-Posting-Date: Mon, 13 Jan 2003 00:10:17 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Don Pearce" wrote in message
> On Sun, 12 Jan 2003 23:33:10 +1100, "Phil Allison"
> > The number of persons I have had to FIGHT with over this simple
> >terminology issue is appalling - they all think that frequency and pitch
> >mean the same thing.
> > Pitch shifting is of course of NO use in feedback supression.
> > ................... Phil
> First - I am not looking for a fight.
** But I am gonna give you one Mr Expert.
> But - the only difference between pitch shifting and frequency
> shifting is that in pitch shifting, each frequency is shifted by a
> certain percentage of itself, while in frequency shifting each
> frequency is shifted by a given number of Hz.
** THAT is one HELL of a difference.
> The two are different in one important regard - in pitch shifting the
> harmonic relationships are preserved and the result remains musical.
> In frequency shifting this does not happen, and if the shift is
> carried too far, the result is very unmusical.
** A few Hz upshift has no detectable impact on most ears.
> Both, however, involve changes in frequency, and as such are of value
> in reducing the possibility of feedback, which is at its most
> troublesome when frequency is maintained around the feedback loop.
** Neither affects the feedback threshold of a PA system if there is no
reverberation in the room. All that using them will do is change the fixed
pitch howl into a warble or whoop. For positive feedback to become
unstable does not in fact involve a fixed frequency - merely the same
*polarity* at the mic so signals increase in magnitude when summed.
The reason for the very precise pitch of typical acoustic feedback is
that a dominant standing wave in the room is being excited. These have
bandwidths of only 1 Hz or less.
The unique characteristc of reveberant room is that it is full of
standing wave response peaks seperated by a fixed number of Hz - between 4
and 20 - right up and down the audio band, depending on the reverberation
time of the room. Naturally there are dips in between each pair of peaks.
If you shift the frequency of a signal comimg from a PA system in a
room by the half the interval between the adjacent standing wave peaks then
the tendency to break into feedback howl is reduced. The result is as if
all peaks and dips have been averaged out and the room has a very smooth
An extra benefit is the increased stability near to feedback
conditions and the slight warble that warns the system operator that the
limit has been reached. There is no more of that slowly increasing scream
that also slowly dies away when the gain is cut. It all becomes instant.
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