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From: Don Pearce
Subject: Re: Acoustic Feedback reduction
Date: Sun, 12 Jan 2003 22:29:42 +0000
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On Mon, 13 Jan 2003 00:20:58 +1100, "Phil Allison"
>"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.
Not in the context - both do indeed necessitate a frequency shift.
>> 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.
Depends on the frequency. At low frequency a few Hz shift could
represent a semitone - decidedly detectable.
>> 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.
> .................. Phil
Shame you decided to start out by being rude. Especially as in the
rest of the post you haven't really contradicted anything I said -
merely expanded on it. There are a few areas of specific error in your
last section, though. The main one is your confusion between
reverberation and standing waves. They are not the same thing and a
space can be highly reverberant with no standing waves whatever. And
of course your arbitrary limits of 4 to 20Hz have no meaning.
And of course there are resonant structures that affect howl frequency
besides the room. The loudspeakers and microphones are a mass of
resonant peaks themselves. Being able to shift the feedback tone off
the top of one of these peaks can give you an extra few dBs of margin
in the onset of feedback. As you say, though, when feedback does
eventually happen, the onset is far more severe, and has a much
sharper cutoff threshold when you turn the gain down.
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