# Low frequency absorption in a closet/pantry



## MarzZ (Dec 31, 2009)

Hi,

I have been researching how to reduce my decay time of the low frequencies( 100hz down to 30hz) in my room. I know these frequencies are VERY hard to tame because of size and all that. I have read master handbook of acoustics etc etc, plenty of mfg's education articles and videos, but nothing really specifically deals with the 30hz to 100hz range, except maybe some narrow band absorption via Hemholtz devices. 

I have attached my room layout to show what I am dealing with. I can post a waterfall or RT60 later from REW. I am using a custom calibrated ECM8000 measurement mic which has been properly calibrated with it's preamp etc. I get decent decay times (450ms) from 100hz on up, evenly. But from 100hz down to 10hz it obviously lags on a bit longer. I am guessing probably over 1000ms, which is the extent of the waterfall window.

The ¼ wave of 20hz is approximately 14 ft, which happens to also be located where a closet and pantry are (see crude illustration). I played 20, 25, 30, and 50hz sine tones and noticed that there was a very high buildup in these exact areas, more so then the back wall corner. I believe this is because the velocity of the wave in this area is at its peak there in the coat closet and pantry. Could I simply fill some of the cavity with some 8lbs density rockwool to help the decay time of the mentioned frequencies? Given that the closet size is 2' x 2.5' x 9', how much and how thick would be effective for these frequencies? 

Thanks!


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## bpape (Sep 14, 2006)

You can certainly try that and it will help to a certain extent. 30Hz will be almost impossible to deal with - even with a Helmholz resonator. The problem is that at those low frequencies, tuned absorbers tend to emit their own problems as much as fixing other ones. I'd recommend investigating air tight, membrane type absorbers with wood faces. The depth of the cavity and the mass of the front membrane will control the tuning. They're good for around 1.5 octaves.

You can also have some impact down into the 50's with more standard broadband absorbers that are on the order of 6-8" thick - much thinner than one would think via theory, but it does work. Below 50Hz, it's membrane time and getting out of the way of the resonances.

Also remember that if you put your sub in a null of the frequency that is the problem, it has a much harder time exciting that mode and the resulting ringing.

Bryan


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## MarzZ (Dec 31, 2009)

Thanks!

I am happy with my frequency response from 10hz to 50hz. It is very flat. I am just wondering if there is anything I can do to reduce the decay time of 100hz to say 50hz by about 200ms? I would like to hear the difference of "tightness" in that region. 

I should note that it has been said by ASC of tubetraps, that at 100hz below, longer decay time can be a preference to those who enjoy rock music. I would think though that "tighter" is always better. In any case, it is just an experiment and I am really curious to see if anything can be done effectively.

Regarding a membrane absorber; are there any online calculators out there?


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## bpape (Sep 14, 2006)

Try this

http://www.customaudiodesigns.co.uk/articles/membranes.html

From 50-100, you'd be very surprised at what an 8" thick panel will do. Add FSK facing to minimize upper mid and high frequency absorption.

Bryan


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## MarzZ (Dec 31, 2009)

It's going inside a closet so HF absorption will not be a problem. Would you recommending a panel/box type trap to design inside the closet? There is a 3ft high cavity about 2.5 ft wide and deep in both the closet and pantry above the highest shelves. Do you think those dimensions are sufficient to design something to be effective at those frequencies given the fact that it is also in a high velocity zone (midway into the room)? I am going to just try stuffing it with Roxul 8lbs density tomorrow and see if there is a difference. I'd like to know before hand before I cut up a bunch of panels though. Your advice is appreciated.


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## MarzZ (Dec 31, 2009)

... bump so I can get over the 5 post hurdle.


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## bpape (Sep 14, 2006)

If you're counting on a door to stop HF absorption, it will also severely limit the effectiveness in the bass region.

Bryan


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## MarzZ (Dec 31, 2009)

I see your point, but doesn't it depend on the thickness and material of the door? In this case, its merely a cheap plastic hollow closet door. I doubt it will effect a 50hz and below wave, but I could be wrong.


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## bpape (Sep 14, 2006)

It will have some impact but some will go through. 

Bryan


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## MarzZ (Dec 31, 2009)

Waterfall graph with nothing in closet:


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## MarzZ (Dec 31, 2009)

Waterfall with 4in Roxul 8lbs density absorber 2x4' in closet:


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## MarzZ (Dec 31, 2009)

Waterfall with scrap ¼ fiberboard appx 2x4ft hanging in closet, no absorption.


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## MarzZ (Dec 31, 2009)

Waterfall with scrap ¼ fiberboard appx 2x4ft hanging in closet with 4" 2x4ft absorption.


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## bpape (Sep 14, 2006)

The question now becomes whether you're actually getting any benefit or whether you're simply damping the resonances in the closet itself. Try running one with nothing in the closet and the door open to see what that's doing by itself.

Bryan


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## MarzZ (Dec 31, 2009)

Here you go: Door fully open, nothing inside:


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## bpape (Sep 14, 2006)

:scratchhead:


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## MarzZ (Dec 31, 2009)

Here's one with the door open and 2 4in panels 8ft high inside the closet. The panels are removed there default location along the side of LP. I need more panels!:crying:

This is very educational so far, thanks!


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## bpape (Sep 14, 2006)

That's kind of what I figured. The one with the door open is as good or better than any of the ones with it closed and things inside. The resonance of the closet cavity itself is causing more issues than what it's worth and they're bleeding out into the room via the thin door.

Bryan


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## MarzZ (Dec 31, 2009)

bpape said:


> That's kind of what I figured. The one with the door open is as good or better than any of the ones with it closed and things inside. The resonance of the closet cavity itself is causing more issues than what it's worth and they're bleeding out into the room via the thin door.
> 
> Bryan


Okay, awesome! So the simple solution: remove door and add a drape as cover, no problem. However, I am willing to try a panel absorber as this does seem to be the ONLY (other than Hemholtz rez) way to tame the ultra low frequencies. Can you lead me in the right direction? I have a panel absorber calc, but I am not too familiar on it's usage. What material and where would be the best place to hang it? My guess is somewhere in the middle to back of room, no?

BTW, this is fun!!


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## bpape (Sep 14, 2006)

Well, it's not only that but also getting the panels in the right place if you're trying to address specific frequency issues rather than just decay time - which is OK too. Move the measurement mic forward and back a bit and see if the 50hz changes. If so, then the back will likely.

You can certainly stuff the closet with insulation and hang the curtain over it to add to the absorption. 

As for materials, use the calculator I linked to. Usually for the sealed panels, you'd just use 1x for the sides, MDF for the back and then plywood for the front membrane. Remember it's a MASS calculation. The lower you tune it, the less efficient per square foot it becomes. Also, as you change the size, the tuning will change slightly. 

Remember when building this that the enclosure has to be 100% air tight. Caulk everything. Also, remember that you can't have anything touching the front membrane except where it's attached to the frame. If you do, it's just like damping a bell that's ringing by grabbing it with your hand.

Bryan


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## Laycoc_Audio (Feb 28, 2012)

You say the frequencies you're trying to tame are loudest at the point of this closet that you want to fill, meaning you have an area of maximum sound pressure there. 

Is it not true that treatment would have the least effect at the point of maximum sound pressure and would work best spaced away from the closet at the point of a node where there would be maximum particle velocity?

The more particle velocity at the point of treatment - the more effective the treatment should become, right? 

From what I've read this is due to energy having travel through interstices (all the little air holes) in a porous or fibrous material and losing energy as frictional loss. Right?

In the same way, a membrane absorber would find most movement at this point due to particle velocity and decreased pressure (as opposed to high pressure at the anti-node), which when combined with the porous material as a dampening device would prove most effective.

I just came across this while researching for an acoustics assignment so please correct me if I'm getting it wrong! :dontknow:

Edit: Ok so perhaps what I was talking about there was for higher frequency absorption where frictional loss in porous absorbers needs max particle velocity? From doing a bit more reading I see that bass traps and resonators (such as a helmholtz) focusing on lower frequencies need to be placed in an area with maximum sound pressure?

Sorry so many questions, I've only gone and confused myself!

Edit (again): Just thought; that's probably specific to Helmholtz where pressure needs to be different outside of the opening to induce the sympathetic vibrations within the Helmholtz through inducing a vacuum effect. Sorry!


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