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Do porous absorber bass traps benefit from rigid walls?
Old 23rd October 2014
  #1
Here for the gear
 
🎧 5 years
Do porous absorber bass traps benefit from rigid walls?

Hello to the Gearslutz community. I am very interested in the difference in performance between a porous absorber bass trap that is enclosed in a drywall box, versus one that has the same volume of fiberglass, but no rigid boundaries.

Some background:

I am in the process of putting together a small (approx. 140 SF) studio, with stereo monitoring. The studio space is on the top floor of a house, and is surrounded by attic space. I am lucky in that I can borrow 96 cubic feet of space from this attic cavity to create a bass trap at the back of the studio. See the attached diagrams. The yellow area is the bass trap. This will be a porous absorber trap, filled with low density glass fibre insulation.

My default plan is to frame in the bass trap area with drywall – i.e. create a drywall box within the attic cavity to contain the bass trap. The interface between the bass trap and the studio will be covered in fabric and wood slats, to reflect some mids and highs back into the room.

But I am questioning whether I actually need to frame in my bass trap cavity with drywall. It would be less work if I could simply stack the insulation in the attic cavity, with no rigid boundary between the stack of insulation and the rest of the attic cavity. But would it be less effective?

So here is my question – what would be the ramifications on the performance of my bass trap if I did not frame it in with drywall? What if I filled the bass trap area with fiber glass, but left this stack of fiber glass open to the attic cavity? Keeping in mind that low frequency energy passing through the bass trap will dissipate into the attic cavity, which is approx. 40 feet long and itself lined with low density insulation – a de facto bass trap.

I’ve spent many hours reading back through the posts in this forum, and have managed to drag myself up the steep small studio acoustics learning curve, assisted by some helpful advice from OpusOfTrolls. This is one of my last remaining questions. Construction on the bass trap starts this weekend, so any and all input would be very welcome.

Cheers,

George

Do porous absorber bass traps benefit from rigid walls?-rear-bass-trap-01.jpg

Do porous absorber bass traps benefit from rigid walls?-rear-bass-trap-02.jpg
Old 23rd October 2014
  #2
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akebrake's Avatar
 
🎧 10 years
Isolation

Hi George!

Interesting question!

Usually one needs isolation to the surroundings. Noise in/out ( traffic, wind, rain, kick drums in the night, sleeping baby etc.) What's your situation?

Quote:
This is one of my last remaining questions
Are you sure?

Cheers
Old 24th October 2014
  #3
Gear Maniac
 
🎧 10 years
I'd say leave the drywall out - bass just passes through it anyway. Leave the attic space and its insulation to swallow the LF energy.

You've a much easier environment to treat than those of us in basements or with concrete/masonry walls. Should sound lovely.
Old 24th October 2014
  #4
Here for the gear
 
🎧 5 years
Isolation is not something I'm even trying for. The room will be primarily mixing, electronic composition, with an occasional live voice / acoustic instrument recording in the room itself. My only criteria is acoustic isolation.

The fact that the drywall would not be stopping much bass anyway is what got me thinking about whether I needed to box the bass trap in. I am lucky in that room. With no treatment of any kind, the bass just disappears out the walls. The reverb tail is bright and short - again before treatment. My biggest challenge will be to overcome the small size. After I take care of the bass and broadband absorbers, I am putting some energy into building three skyline diffusors for the rear surfaces.

Thanks for the input. At this point I am being pulled towards not enclosing the bass trap in drywall. Can any of you acoustically insightful readers give me a strong reason to go the drywall route?

Cheers,

George
Old 24th October 2014
  #5
Gear Guru
 
1 Review written
🎧 15 years
Why

Hi George, what possible advantage do you think might result from containing a bass trap in drywall as opposed to the more usual say fabric?
DD
Old 6th November 2014
  #6
Here for the gear
 
🎧 5 years
Hi. Sorry to take so long to respond - it has been a crazy couple of weeks. I've got some news / results of my bass trap build, but first I'll respond to DanDan's question.

The potential advantage I could imagine for having rigid drywall boundaries for the bass trap would be to prevent low frequencies from wrapping around the layers of fiber glass, rather than moving through it. My bass trap is greater than 42 inches at its widest. After fairly exhaustive reading of this forum and others, it seems best practise with low density porous absorbers is to layer the insulation with gaps inbetween. So I have three rows of low density fiber glass (11 inches wide), with each layer separated by a gap of several inches (see pictures below). My concern with not having a rigid boundary for the bass trap would be that sound waves of sufficient length would simply wrap around the layers of fiberglass, instead of dissipating through them.

Now, keep in mind that my instincts tell me this is not a problem. But instincts and the science of audio are often a very uncomfortable fit, so I posted on this forum to double check.

As it turns out, I built the bass trap without rigid boundaries. I've got a killer modal null in my room around 88 Hertz. The bass trap did some good, but I've still got a long road ahead of me. I've attached a series of three spectrograms that illustrates the impact of the bass trap on the room. The first spectrogram was measured in the untreated room. The second spectrogram is measured with a partially completed bass trap and a few broadband absorbers installed. The third spectrogram is measured after the bass trap is finished, and all first reflection absorbers are in place, including a 24 SF cloud centered over the listening position. Depressing how much of a problem that 88 Hertz null still is.

George

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Do porous absorber bass traps benefit from rigid walls?-tale-3-spectrograms-01.jpg

Do porous absorber bass traps benefit from rigid walls?-tale-3-spectrograms-02.jpg

Do porous absorber bass traps benefit from rigid walls?-tale-3-spectrograms-03.jpg
Old 7th November 2014 | Show parent
  #7
Lives for gear
 
🎧 10 years
Quote:
Originally Posted by Tweeterdude ➡️

I've got a killer modal null in my room around 88 Hertz.
Are you sure it is modal? It may be, in which case which mode(?) and which surfaces are therefore associated?

Or, it may be speaker boundary interference related, which can cause very deep nulls. In your case for a first order null at 88Hz that would equate to a time difference between direct and reflected sound of approx 5.68 msec (or 1.95m difference)from a "hard" surface somewhere in your room?

My point being that for modal issues the peaks and nulls are room related, and a microphone (or your ears) should be able to map out the peaks and nulls. For, SBIR issues the problem is source/listener related within the bounds of reflective surfaces in your room. The issue is are you treating the correct surfaces for the problem you are trying to solve?
Old 7th November 2014
  #8
Gear Guru
 
1 Review written
🎧 15 years
Blocked

Sorry tweet I don't understand. Are you suggested blocking the exposed surface area of the bass trap by installing plasterboard. This would create a panel trap with I reckon very limited effect. An open fibre trap is quite predictable.
Quote:
it seems best practise with low density porous absorbers is to layer the insulation with gaps inbetween
Again sorry, but I have never come across that one. Fibre is better at absorbing than air is. Gaps are not better than fibre.
DD
Old 7th November 2014
  #9
Here for the gear
 
🎧 5 years
Hi Icecube1. No, I'm not sure the killer 88 Hz null is modal. I'm learning the ropes of small room acoustics as I go. I think it is interesting that this deep null at 88 Hz is fairly universal around the room. I played periodic pink noise, and explored the room with an RTA. For some modes, I saw extreme swings, as I expected. For example, the frequency profile around 300 Hz swings between a deep null and a pronounced boost (over a 30 db swing), depending on mic location. However, the 88 Hz null persists virtually everywhere - certainly everywhere that would be feasible as a primary listening location.

This weekend I will be converting another 80 SF of wall space to broadband absorption. This will include the last possible surface that could be causing a direct reflection. I'll post the results, and if this latest move doesn't significantly impact the 88 Hz null, I will be in need of advice.

Here is something interesting. By creating the bass trap at the back of the room, I effectively extended the length of the room by between 1.5 and 4 feet. The measurements post-base trap show that the frequency of the peak null was lowered slightly. Can broadband absorption lower the center frequency of a null, or is the cause of this shift in frequency attributable to the lengthening of the room?
Old 7th November 2014
  #10
Here for the gear
 
🎧 5 years
Hi DanDan. I'll attempt to better explain the bass trap. The bass trap is located in what was formerly the attic cavity, which was separated from the room by 3 inches of Roxul and 1/2 drywall. The boundary between the studio and attic cavity (i.e. where I removed the drywall) will be covered by A) vapor barrier, B) burlap, and C) slats designed in a QRD configuration.

When I refer to rigid boundaries, I am referring to encasing the rest of the bass trap in drywall. As it stands, the bass trap was built by simply layering fiber glass in the attic cavity. The alternate would have been to first build a wall within the attic cavity out of drywall, enclosing a space of the desired volume, and then fill this space with drywall.

Because I built the bass trap without an external rigid boundary, my studio room is effectively coupled to the entire attic cavity by a 40 SF opening, which is stuffed with low density insulation.

As to the recommendation to leave air gaps in large volume porous absorbers, I can't recall where I ran across it, but I read it several times here in the Gearslutz forum. I will try and track down where.
Old 8th November 2014 | Show parent
  #11
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🎧 10 years
Quote:
Originally Posted by Tweeterdude ➡️
However, the 88 Hz null persists virtually everywhere - certainly everywhere that would be feasible as a primary listening location.
If it is modal then you should also expect to find the associated peak as well? Are you measuring at the same "height" around your room as you test? The first axial height mode in home set ups (with relatively low ceilings) often means that you are sitting almost in the null. If you play a pure 88Hz tone through one loudspeaker at a time do you, for instance, get a peak if you listen at floor or ceiling level?
Old 8th November 2014
  #12
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🎧 5 years
Tweeterdude, I've seen the lowering of room mode null and peak frequences in my room as treatment was added.

One possible explanation-- Sound propagates slower in loose fiber than in air. So in addition to attenuating the audio energy build-up in the room, the fiber delays whatever percent of the signal that doesn't get absorbed and bounces back. So the audio behaves like the room got bigger, because the time constants got stretched bigger.

Someone posted a calculator that might make it possible to guestimate the acoustic increase in room size, haven't a clue about its accuracy or applicability-- mh-audio.nl - Home

For instance if you added a foot of absorption, and can calc the audio propagation speed of that foot of absorption-- If audio moves thru that foot of absorption at 1/3 the speed of sound in air, then that section of the room got 2 feet bigger?

I wonder if the propagation slowdown might be frequency specific. If highs propagate thru fiber slower than lows or whatever, then the effective room size would be different according to frequency?

In electronic networks the group delay tends to be frequency specific, and acoustic materials can act analogously to inductors or capacitors, depending on config. So the "acoustic network" might show group delays separate from whatever delays are added by the slowdown in propagation speed. Maybe both effects would be the same thing, but maybe they are two not-entirely-related effects. Dunno. Would be interesting to read an expert explanation.

In dsp, delays are used to emulate inductors and capacitors, so maybe the medium's propagation delay is at the heart of freq specific group delay as well.
Old 8th November 2014
  #13
Lives for gear
 
🎧 5 years
Now having some measurements to look at, I can say your trap is working very well so far. Nulls are one of the most difficult frequency abberations to correct, as they have two things working for them:
1) Room Geometry causing out of phase reflection
2) Less desireable audial effect of having "nothing" vs having "something"

I would not be hasty to think the null is non-modal, by the lack of decay in the sonogram.

The ceiling in your room has an advantage compared to rectangular rooms, as far as I can see it, that waves emanating from the speakers will be reflected like an arc below it, and hopefully ending up inside the bass trap. The floor/ceiling resonance of the room should be diminished by this. But resonance is not the only problem, there is also reflection which is non-modal. The ceiling is only helping a little bit overall.

The side walls of your room are parallel, and they will be the biggest contribution to modes. The best way to test this is to move the speakers in and out from the centerline of horizontal symmetry. As you move them together and apart, you will see a wide variation in the locations and peaks and nulls. If this starts to move 88hz around, you have found a part of your answer, and you will need bass traps effective at 88hz on the side walls, and most likely in other areas as well.

As for slats, I didn't realize the couch would be in front of the trap, so as for their diffusing effectiveness, not so good, but a low perforation slatting to cover the trap will help improve bass and lowmid absorption. You will have to look for alternative locations to apply diffusers. The obvious candidate is the attic slope of the rear wall, above the couch. What diffuser to use might be somewhat subjective, but there should be something to help what small possibility of reverberance a room of this size could have. At best, you can expect to clear up the audibility of any discrete echoes that may interfere with the sound of the speakers, and at the time give the room some bite so as to make the speakers appear more neutral. Diffusers on the sides of the room are highly recommended.

The more absorption you can fit into the room the better, but only if you combine it with a strategy of diffusion, otherwise the room will isolate reflections to places they aren't useful.
Old 8th November 2014 | Show parent
  #14
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GIK Acoustics's Avatar
 
🎧 5 years
Quote:
Originally Posted by Tweeterdude ➡️
Hi Icecube1. No, I'm not sure the killer 88 Hz null is modal. I'm learning the ropes of small room acoustics as I go. I think it is interesting that this deep null at 88 Hz is fairly universal around the room. I played periodic pink noise, and explored the room with an RTA. For some modes, I saw extreme swings, as I expected. For example, the frequency profile around 300 Hz swings between a deep null and a pronounced boost (over a 30 db swing), depending on mic location. However, the 88 Hz null persists virtually everywhere - certainly everywhere that would be feasible as a primary listening location.

This is definitely a sign that it is not modal (EDIT: unless you only checked the same height everywhere in the room, in which case it could still possibly be a height mode), and almost entirely points to the speaker positioning being at fault here. Try testing while moving a single speaker at a time with relation to only one dimension (ie move it farther from the front wall, or farther from the sidewall, or farther from the floor, etc) to see if SBIR is causing any interaction with respect to a specific boundary. You might also investigate any speaker coupling.

FWIW I definitely would not recommend putting gaps between multiple sheets of insulation for an absorber. You will be causing impedance changes as sound enters and exits each piece of insulation, which will reduce the absorption.
Old 8th November 2014
  #15
Gear Guru
 
1 Review written
🎧 15 years
Air

IMO an open gap in the boundary shell is the best possible absorption. Rain and burglars kinda prevent that though.....
Sound does travel slower in fibre. I looked into this some time ago. It is about 1.2 to 1.4 times slower. So yes the room becomes acoustically larger and the mode frequency changes. So much for mode calculators eh?

Tweeter, the air gap and much else is covered well in the Q4Avare thread. Moving absorption away from a boundary can be beneficial. e.g. A suspended ceiling of thin acoustic tiles can work down to 50Hz. But a full fill of fibre up there, of appropriate lower density, will perform better.
Gaps within a fibre layer is just, well, lack of fibre.

DD
Old 9th November 2014
  #16
Lives for gear
 
🎧 5 years
It's worth testing a complete fill.
Also, you should fill the entire space surrounding (at best) the room shell with insulation, since the walls are thin.
Old 9th November 2014 | Show parent
  #17
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akebrake's Avatar
 
🎧 10 years
Phase shift upon reflection?

Quote:
Originally Posted by Tweeterdude ➡️
...Because I built the bass trap without an external rigid boundary, my studio room is effectively coupled to the entire attic cavity by a 40 SF opening, which is stuffed with low density insulation. ...
A room with an open back wall behaves differently than a room with a rigid back wall. Intuitively all length modes would disappear?

AFAIK they do not. The room now acts like a ¼ wave pipe which supports odd order harmonics. (Length wise) One end closed the other end open.

Open vs Closed pipes (Flutes vs Clarinets)

But as OP:s room is not totally ”open” in the rear it’s hard to guess the result.

My point is: Room modes will behave differently with "rigid bottom” in the rear trap compared to open access to the entire attic. (Complex impedance)

Only measurements will tell.
Old 9th November 2014 | Show parent
  #18
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🎧 10 years
Quote:
Originally Posted by akebrake ➡️

Only measurements will tell.
+1
Old 9th November 2014 | Show parent
  #19
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Jens Eklund's Avatar
 
🎧 10 years
Quote:
Originally Posted by GIK Acoustics ➡️
This is definitely a sign that it is not modal (EDIT: unless you only checked the same height everywhere in the room, in which case it could still possibly be a height mode), and almost entirely points to the speaker positioning being at fault here.
+1

Quote:
Originally Posted by Jens Eklund ➡️
3: A dip can be caused by SBIR, a mode (if source and/or receiver is in a node), or simply by the lack of modal support. The first two are quite easily dealt with but the last one is somewhat trickier. If you treat other modes surrounding this gap, the situation might improve since the Q of the other modes will be lower and hopefully fill the gap.
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