# Diaphragm on absprbers?



## CAZABON (Apr 11, 2012)

So I've been doing some upgrades to my home studio over the past week or so. I don't have the REQ measurements handy at the minute but the upper end (300Hz>) of the frequency spectrum has greatly improved both in relative level and in decay times. However, the low end still displays an undesirable level of variance, with large peaks and troughs located between the 20-300Hz range. I'm aware that my desktop has a large part to play in some of these low end irregularities and I'm considering building a 4" panel to cover the work surface in an effort to counteract this. That said, there are some issues that are not related. After discussing the issues with two more experienced engineers, they both hypothesised that my bass traps "don't extend low enough" and suggested installing a diaphragm on each. One suggested using a very thin profile stainless steel while the other suggested heavy duty rubber. I can understand the principal, the diaphragm resonates with the low end frequencies, removing some of the sound wave's energy but what I don't understand is how to know what materials resonate at what frequencies? I also assume that the diaphragm is located at the rear of the absorber to prevent it from reflecting higher frequencies before the absorbent material gets a chance to work its magic. Any enlightenment on materials to use, how to use them, etc., would be greatly appreciated! If necessary, I'll be able to provide specific peak and trough frequencies when I have my REQ measurements at hand. Thanks in advance!


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## bpape (Sep 14, 2006)

The membrane would go on the front and reflect mids and highs based on the mass. Limp vs stiff membranes will react differently. Stiff ones will have a peak of resonance where the limp ones tend to be more even.

You can do whatever you want in terms of a membrane but if the underlying absorber is not thick enough, the amount of help it will give will be limited. 

We would really need to see what's happening to make recommendations.

Also, you would need to address the overall decay time in the space at the same time. It's just as important as frequency response.

Bryan


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## CAZABON (Apr 11, 2012)

View attachment 45028
Thanks for your input Bryan, much appreciated.  This is kind of tied in with a thread I posted last year, so I'm not sure whether to continue on here or move this back to my old thread? http://www.hometheatershack.com/for...room-achieve-suitable-mixing-environment.html

Either way, I'll just give some background details on what I'm working with/against: The room was far from acoustically ideal to begin with, it's basically an 8'x8'x8' cube! Due to space restrictions, I'm forced to sit dead centre in the room. I began treatment by installing two 4'x2'x4" cloud absorbers (with a 4" gap) above the listening position, two 4'x2'x4" panels on each wall to left and right of listening position. Panels on left wall also have a 4" gap but I ran out of materials to do the same on the right wall :/. I mounted two 4'x2'x6" panels (with a 6" gap) across the rear wall directly behind the listening position. 

My most recent upgrades included two 4'x2'x4" absorbers which straddle across the rear corners, a 4'x2'x4" absorber mounted horizontally across the front wall and four 2'x2'x4" absorbers on the walls to the rear and sides of my monitors (these are the ones I was thinking of installing diaphragms on). I also placed a 4'x2'x4" absorber under the desk which oddly had a positive (yet small) affect on both the 62Hz peak and the 78Hz null. I tooled around with different positioning/absorber thicknesses etc. on a trial and error basis,
but each addition seems to have yielded some sort of positive affect on both the frequency response and decay times. Pretty much everything above 200Hz (with the exception of 350Hz @ 160ms) has an average decay time of between 90-120ms. Not sure if that's good or not but any enlightenment would be appreciated!?  The main issues for me lie below 150Hz and also between 6000Hz-12000Hz. Bearing in mind that my monitors are bass limited at 65Hz so I assume I should see some sort of roll off at that cutoff point. However, the low end peaks and troughs are 25dB apart in some cases so I was wondering if it might be possible to tame these rogue frequencies and decay times with the addition of a diaphragm on the absorbers surrounding my monitors or indeed any other suggestions! Any enlightenment on the 6-12k region would also be a great help. Thanks in advance for your feedback! 

Rockwool density 3lb/cu ft across all absorbers.
Green graphs represent untreated room.
Pink graphs represent treated room as described above.


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## bpape (Sep 14, 2006)

Unfortunately, 4" isn't going to do the trick for the variations down low, especially the ones that are flat on the wall. You need to do the other side too and I would consider 6" though 4" spaced 4" would work pretty well. You just need this all behind you and beside you. 

Something else is going on in your measurements. The null at 80 (likely off the rear wall) is in the same place but the peak moved 10Hz between measurements. The treatment would not have done that.


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## CAZABON (Apr 11, 2012)

Thanks Bryan. I'll add 4" spacers to the side wall panels today and Ill see what I can do with regards to spacing the panels to the rear and sides of the monitors. I've already constructed the panels out of 4" timber so perhaps I can manage 4"absorption with 4" spacing. 
I'm pretty sure that the null has to do with boundary issues from either the surrounding walls or the desktop surface as I remember accidentally knocking into the mic stand before that measurement, so it probably moved. I was considering a 4" absorber faced off with steel mesh & fabric to cover the desktop surface but as you've just clarified, it won't have any impact on the low end of the frequency spectrum. If the 4" absorber w/ 4" spacing has negligible effect to rear and sides of the monitors, I'll look into constructing 6" absorbers faced with a rubber diaphragm to replace them. 

Also, I've come across a second hand KRK 10s subwoofer for very reasonable money, but with such a small room (40" x 40" x 40"), and only a limited number of places to position the sub (most likely under the desk), I'm concerned that it might introduce more problems than it could potentially rectify. And enlightenment would be greatly appreciated.


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