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Room resonance in live performance -Guy Macon

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Guy Macon

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May 4, 2008, 12:18:00 PM5/4/08
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N_Cook wrote:

>Well I assume that is what it is. Scenario musician/s in a pub, ie non-ideal
>environment. , so hard wall surfaces, using a pair of speakers on tripod
>stands so about head height. Fine at low sound level, but sometime into the
>set they turn the level up.
>I'm guessing, that at some frequency about 2KHz, the room starts ringing,
>not microphony/ howl-round as its quite stable in level, whenever that pitch
>is being emitted. What is this effect called ?

Any chance that you are really bad at estimating frequency? A rough
rule of thumb to remember is that sound in air travels at about 1000
feet per second and the wavelength of a 1 KHz tone is about one foot.
So the wavelength at 2KHz about half a foot. Room resonance tends
to occur at much lower frequencies than that.

>Again assuming this is associated with the separation between the wall in
>front of the speakers reflecting back to the wall behind the speakers, is
>there a way of calculating what this frequency would be for a given room
>dimension and relative speaker position.

There is another rule of thumb that helps when making such estimates.
Imagine that all of the walls (including ceiling and floor if they
are reflective) are mirrors, picture in your mind the resulting picture
(a grid array of identical speakers, half pointing backwards), imagine
setting up a real grid in an open space, then figure out what that
array would sound like as the sound from each speaker either cancels
or adds to the sound from the others. Read this article to see
how it is done:

[http://mixguides.com/studiodesign/basics/understanding-phase-sound-1001/ ]

>And would it be possible to make up some device to ping short but
>high level bursts of various frequencies, in that range, via the
>PA and speakers , during sound-check, to determine what this
>frequency is and then ideally notch-out but more likely attenuate
>using the graphic on the mixer ?

Standard stuff that is done all the time when setting up PA systems.
The bursts aren't the best signal though; pink noise is. Do a web
search on "real time analyzer" to see how it is done:

[ http://www.google.com/search?q=real+time+analyzer ]

That being said, you will get far better results by applying a
solution that is acoustic rather than electronic. Here is a
good place to start learning more about this:

[ http://www.audioholics.com/education/acoustics-principles ]


--
Guy Macon <http://www.guymacon.com/> Guy Macon <http://www.guymacon.com/>
Guy Macon <http://www.guymacon.com/> Guy Macon <http://www.guymacon.com/>
Guy Macon <http://www.guymacon.com/> Guy Macon <http://www.guymacon.com/>
Guy Macon <http://www.guymacon.com/> Guy Macon <http://www.guymacon.com/>


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