Bass trapping

[archive]

From: Jon Best <jrbest@e...>
Date: Tue Feb 6, 2001 9:15 pm
Subject: Bass trapping

Hey- here's a question. I have some modal action at around 62Hz (the presence
of
which I predicted, just not the severity). If you look at the shape of my
control
room on the studiotips website, you can see I've got a sort of foreshortened
cathedral
ceiling. After putting a tone generator through the speakers and waving an
omni C451
around the studio on a long pole, I've figured out that the modes are pretty
extreme
along the ridge of the ceiling, making that a good place for bass trapping (I'd
catch
a much smaller 100ish bump in the ridge front and back corners as well, as a
bonus).
I don't think I really need tuned trapping, although I'm certainly open to
suggestions. I'm looking at a pretty deep peak/notch (12-18 dB, as far as I
can tell,
depending on location) that's probably 3 dB down at 58-60 and 65-67. I've also
got
some light general boominess, which I expected- I have done minimal bass
trapping so
far. So, any suggestions? I am thinking of either a bunch of hanging tubetrap
style
bits along the ridge (how big to be effective at 60?) or just dropping a curved
ceiling of 703/compressed R19 about 4' wide and 1 1/2' deep at the peak.
Thanks!

--
Jon Best
Muddy Creek Audio

“In the absence of talent, imperfection becomes intolerable.....”

-Richard Hulse, on the Mastering web board

[archive]

From: SRF7@a...
Date: Tue Feb 6, 2001 10:40 pm
Subject: Re: [acoustics] Bass trapping

In a message dated 2/6/01 4:11:44 PM Eastern Standard Time, jrbest@e...
writes:

> Hey- here's a question. I have some modal action at around 62Hz (the
> presence of
> which I predicted, just not the severity). If you look at the shape of my
> control
> room on the studiotips website, you can see I've got a sort of
> foreshortened cathedral
> ceiling. After putting a tone generator through the speakers and waving an
> omni C451
> around the studio on a long pole, I've figured out that the modes are
> pretty extreme
> along the ridge of the ceiling, making that a good place for bass trapping
> (I'd catch
> a much smaller 100ish bump in the ridge front and back corners as well, as
> a bonus).
> I don't think I really need tuned trapping, although I'm certainly open to
> suggestions. I'm looking at a pretty deep peak/notch (12-18 dB, as far as
> I can tell,
> depending on location) that's probably 3 dB down at 58-60 and 65-67. I've
> also got
> some light general boominess, which I expected- I have done minimal bass
> trapping so
> far. So, any suggestions? I am thinking of either a bunch of hanging
> tubetrap style
> bits along the ridge (how big to be effective at 60?) or just dropping a
> curved
> ceiling of 703/compressed R19 about 4' wide and 1 1/2' deep at the peak.
> Thanks!

Maybe deflection (down and to the rear ... sort of like JFK's head in Dallas
... 'cept different) would reduce the amount of required trapping along that
ceiling ridge. From your sketch it looks like you have some devices at the
rear wall .. looks like a bookcase flanked with panels. Fill the bookcase
with bowling trophies (or replace it with a floor to ceiling low end
diffuser) and mount/hang some slanted panels along the ceiling to direct the
sound away from the mix / producer's desk positions and back to the rear wall
/ bowling trophy collection / diffusion array. Maybe hang angled panels that
have cloth covered 703 glued to the back side ... start with a wide sharply
angled one over your desk, and then run a few down the ridge .. reducing the
angle as you go ... that way you could kill the mode without deadening the
room too much.

My $0.02 is your C-room is big enough to get by very little trapping if any
... find a away to bounce the sound away from the mix position and then
randomize it, this will reduce the need for traps.

I had a similar problem running between my side walls at the producer's desk.
I built a frame of 2x4 on the two large bare spots on the sidewalls beside
the mix position ... where the speakers hit on their 1st reflection to the
producer's desk (a sofa and coffee table). The frame is made of 2x4's flat
to the wall making two boxes 4' tall and 2' wide ... but the front edge has
an added 2x4 on edge, the middle an added 2x4 on flat (the rear edge is just
a single 2x4 flat to the wall). I filled the boxes with a layer of
fiberglass batting and then covered it the whole thing with black burlap and
then with a 4'x4' sheet of black painted pegboard mounted on rubber grommets.
The result was a high end deflection panel aimed at the rear of the room
(Hi's and Mids), which can vibrate feely on the grommets (Ultra-Lows, panel
absorber), which acts as a bass trap (Lo-Mid and Low Helmholtz absorber),
which I can hang cables on .... while it does a bit of trapping, it also
deflects a good deal of sound backwards toward a low end diffuser and lessens
the side wall mode (which was causing problems).

[archive]

From: Jon Best <jrbest@e...>
Date: Thu Feb 8, 2001 3:25 am
Subject: Re: [acoustics] Digest Number 302

> In a message dated 2/6/01 4:11:44 PM Eastern Standard Time, jrbest@e...
> writes:
>
> > Hey- here's a question. I have some modal action at around 62Hz

<snip>

> Maybe deflection (down and to the rear ... sort of like JFK's head in Dallas
> ... 'cept different) would reduce the amount of required trapping along that
> ceiling ridge. From your sketch it looks like you have some devices at the
> rear wall .. looks like a bookcase flanked with panels. Fill the bookcase
> with bowling trophies (or replace it with a floor to ceiling low end
> diffuser) and mount/hang some slanted panels along the ceiling to direct the
> sound away from the mix / producer's desk positions and back to the rear wall
> / bowling trophy collection / diffusion array. Maybe hang angled panels that
> have cloth covered 703 glued to the back side ... start with a wide sharply
> angled one over your desk, and then run a few down the ridge .. reducing the
> angle as you go ... that way you could kill the mode without deadening the
> room too much.

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. I do actually have
some bass
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. There was talk recently about using a
parachute-
could I use some sort of thin nylon cloth or something to 'roof off' the peak,
or to hang
like you suggested, and expect any positive results? As far as your sidewall
and
diffusion suggestions, doesn't a trap/deflector/diffusor need to be about a
quarter
wavelength deep to affect frequencies? Malcolm Chrissholm seemed adamant about
needing
many, many feet to get any diffusion in the real low end.

Thanks!

>
>
> My $0.02 is your C-room is big enough to get by very little trapping if any
> ... find a away to bounce the sound away from the mix position and then
> randomize it, this will reduce the need for traps.
>
> I had a similar problem running between my side walls at the producer's desk.
> I built a frame of 2x4 on the two large bare spots on the sidewalls beside
> the mix position ... where the speakers hit on their 1st reflection to the
> producer's desk (a sofa and coffee table). The frame is made of 2x4's flat
> to the wall making two boxes 4' tall and 2' wide ... but the front edge has
> an added 2x4 on edge, the middle an added 2x4 on flat (the rear edge is just
> a single 2x4 flat to the wall). I filled the boxes with a layer of
> fiberglass batting and then covered it the whole thing with black burlap and
> then with a 4'x4' sheet of black painted pegboard mounted on rubber grommets.
> The result was a high end deflection panel aimed at the rear of the room
> (Hi's and Mids), which can vibrate feely on the grommets (Ultra-Lows, panel
> absorber), which acts as a bass trap (Lo-Mid and Low Helmholtz absorber),
> which I can hang cables on .... while it does a bit of trapping, it also
> deflects a good deal of sound backwards toward a low end diffuser and lessens
> the side wall mode (which was causing problems).
>
> [Non-text portions of this message have been removed]
>
> ________________________________________________________________________
> ________________________________________________________________________

--
Jon Best
Muddy Creek Audio

[archive]

From: SRF7@a...
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...
writes:

> 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
mix position.

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
not?).

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
locations.

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
higher bands.

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.
>
> Thanks!

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
;-]

Good Luck

Scott R. Foster