Date: Fri Feb 9, 2001 4:45 am
Subject: Re: [acoustics] Digest Number 302
In a message dated 2/7/01 10:25:45 PM Eastern Standard Time, jrbest@e...
> Hm. So 703-backed clouds are going to trap/diffuse/affect low end that
> much? I guess
> I've heard/maybe read that, but it hasn't sunk in as well.
From your first post it sounds like you have a positive node at about 62Hz in
your ceiling ridge ... I assume you went looking for this node becuse you
have an overlong reverb time in this range, and its causing boominess at the
My suggestion is to diminish the node by creating a series of variable depth
cavities (the area behind the panels) lined with 703 (the 703 panel backing)
placed along the mode path. Alternitively you could mount cloth covered 703
along the twosides of the ridge, and just hang painted or stained panels
below them. You could build one long cavity with a fiberglass filling ...and
therefore one long bass trap, but I suggest you instead create a number of
smaller ones by putting in multiple panels and use it as an opportunity to
also steer reflections off the ceiling to the back of the room (this is a HF
thing not related to the 62 Hz problem but as long as you are up there, why
I like this approach because it will disrupt the waves travel to and through
the location of the positive node, provide LF absorbtion, and also, reduce
the 1st reflection content of the mix position and the producer's desk
Also I suggested you hang the panels vs. rigid mount them, because if free to
vibrate they will act more effectively as panel absorbers (they wiggle a bit
in response to sound) which makes for even more LF absorbing, but will be
reflective to mids and HF stuff and therefore preserve the reverb energy in
I'd try 1/2" plywood panels because 1/2" plywood over a cavity 6"s deep
resonates at about 60 Hz .... being hung at angles the depth will vary and so
will the center freq. of resonance, but if you make them about 2'x2' you'll
be in the ball park (for average depth = 6").
> trapping- the big weird triangles above my
> angled front wall sections are filled with
> loose fiberglass and covered with thin Auralex. Those angled walls have 4"
> of 703 on
> them, also covered with Auralex, so visually the angled bits go floor to
> ceiling. At any
> rate, that is probably enough room damping for me, so I am really open to
> bass er
> management that doesn't damp too much.
Yeah sounds like you got a good bit already ... what is your RT60?
> could I use some sort of
> > like you
> suggested, and expect any positive results?
Not I think on LF ... not unless you filled it with fiberglass ... and that
would eat up reverb energy across the band.
> diffusion suggestions, doesn't a
> trap/deflector/diffusor need to be about a quarter
> wavelength deep to affect frequencies?
I figure the pegboard resonates in a range running from about 100 to 200 Hz
... the interior is variable depth Helmholtz cavity working from about 300 to
600 Hz .. keep in mind these are bell curves so there is undershoot,
overshoot and overlap. As to angle of reflection it drops about 1" per foot
(actually twice that becuase there are two of them opposite each other) which
is plenty to disturb the flutter between the two side walls across the band.
As to quarter waves being the limit for LF effectiveness, there are a lot of
factors in play. For hanging carpet in front of a wall that works fine, but
for instance in a pegboard absorber its also a matter of the perforation %
and panel thickness.
For diffusers, I think LF limit is the 1/2 wave, BUT diffuser depth is not
its actual depth of the deepest well, but rather the depth of the modulus
used to calculate the diffuser. For a primitive root mod 7, g=2 diffuser the
maximum well depth to modulus ratio is 4/7ths or 0.57 ... assuming inches is
your units, the primitve root sequence is 2, 4, 1, 2, 4, 1, 2 and thus your
deepest well is only 4" deep (1/2 wave to 1695 Hz), but the LF limit is the
derived from the mod depth or 7" (1/2 wave to 969 Hz).
That said even using a modulo 5, n^2+n sequence (with maximum well depth to
modulus ratio of 2/5ths or 40%, the lowest I ever found) you'd need a
diffuser 44"s deep to get down to 62 Hz).
> many, many feet to get any
> diffusion in the real low end.
Malcolm told me: "if you have LF problems ... slap a poly in every corner
... works like a champ". But it doesn't look like you have any corners left
Scott R. Foster