I've just been asked if I know anything about audio feedback theory. Specificaly, when you get a howl-round from mic/speaker is the frequency of the hum related to the distance between mic and speaker?
I thought you chaps would like to read and learn from my considered reply:
Feedback is the propergation of the standing waves generated when a sinusoidal wave front meets or exceeds the terminal frequency that's present in the original. Simply put this expands on the Rodriguez Theory that decrees that the leading edge of the wave causes the blockiness in the higher Mhz range, and this in turn can be deteted
using a Boell and Calleger meter callibrated in dBs rather than Hz.
My assumptions only hold good of course if the feedback travels along the hypotenuse of the triangle formed by the aforementioned standing wave and the relevant transducer that originally emitted the frequency,
though Fritzmeir would counteract this with the well known Nyquist Nebulizer that impedes the peak levels at half term stabilisation in the receiver.
I hope this clarifies the situation.
tomthinkalott.
I don't know why you bothered Tom. We all knew that already! ;)
Or put another way
There was an old man of Bengal
Who invented a mathematical ball
Circumference times weight
Minus height over eight
Equals twice the square root of **** ***
[This message has been edited by John Farrar (edited 01 June 2001).]
I have a theory about Dinosaurs - it's much, much, simpler...and what's more...it is mine.
Ann Elk [Miss]
Yes....yes indeed..........
You should write a book on it, Tom. Call it something like "Half Term Theory of The Nyquist Nebulizer".
And Chirpy, I wish you'd stop going on about your bloody Dynosaurs theory, it was disproved years ago. How could it be true seeing as the half life of Carbon 57.2 is only two weeks? And that was a week ago.
Well Gladders, let me tell you that I've just watched this brilliant film by Walt Disney (that's what I've been doing for the last hour-and-a-half)
Apparantly, all Dinosaurs came from Buena Vista.
Oh yes...and I forgot to mention...they made lot's of Carbon copies - Whether they were 57.2 or not...no-one seems to know.
Chirpy 
Very good Tom, you're showing all the signs of starting to understand it
The frequency at which you first hear the onset of howl-round is the frequency at which the marginally unstable system has highest gain. That's usually a function of the distance between two opposing walls with high reflectivity, and so you can work it out from the speed of sound (about 700mph). For a reason that I can't remember, we're talking about Eigen-tones, which form Eigen-vectors when you plot the stablility criteria for the room. Never mind.
The exact frequency at which it will hoot isn't necessarily the fundamental frequency derived from that calculation, because it will always be a low frequency, often below hearing limits, so we're always talking of harmonics of those frequencies, and that's where it gets confusing, because we need to know the frequency response of the mic, of the amplifier system, and of the loudspeaker. Not just the lab responses, but the resonses in the particular location (e.g. speakers make more bass in a corner). Plot all that together, and you'll find a frequency at which there's a peak in the gain that will make it hoot if it coinciudes with one of the room harmonics.
There's also the Helmholtz criterion to worry about, the volumetric resonance of the room. Modelled as a resonant cicuit, the whole room will honk like a bell (like blowing across the top of a bottle).
I could go on, but I prefer your explanation, excpet that it doesn't depend on the distance between speaker and mic.
[This message has been edited by Alan Roberts at work (edited 01 June 2001).]
it is also a well known fact that eating a realy good hot curry and then drinking copious amounts of beer will also produce the above mentioned honking like a bell and low frequency hoots!!
You'll be pleased to know that both Tom and Alan have been nominated for the much coveted 'Plain English' award.
Also entered in the 'Two-year-old and under' category, is Bananaconda for his 'easy to understand' wording found in the opening topic of:
http://www.dvdoctor.net/cgi-bin/ubb/Forum29/HTML/000005.html
Chirpy.
Speaking for myself (and, I hope, for Tom as well), I'm honoured to be nominated for such a prestigious award. And for your continuing support, I shall wear it always.
I'd also like to thank the entire cast and crew, the producer, my mother and priest and I'd like to work with children in a poor country with flies and that.
Ooh shucks!!!
Sorry folks...The award went to:
Bananaconda
"Boo...Hiss...Fix..."
And you can't get much plainer than that!