Membrane Abosrber Calculator (Excel)

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Re: Membrane Abosrber Calculator (Excel)

Postby Terry Montlick » Tue May 18, 2010 12:34 pm

I've used the same algorithm for years which Jonessy has in his Membrane Absorber Calculator. And yes, it really does work in real life. One trick is discovering how much area you need to cover, and this is I think best done through experimentation.

- Terry
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Re: Membrane Abosrber Calculator (Excel)

Postby whitesnake » Tue May 18, 2010 1:57 pm

Terry, have you got any pictures or drawings of your membrane absorbers?
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Re: Membrane Abosrber Calculator (Excel)

Postby jonessy » Wed May 19, 2010 5:48 am

How well this works in 'real life' is highly dependent on the Q of the device, which is essentially determined by the existence\non-existence and properties of porous material in the cavity.

The higher the Q, the more difficult it would be to 'hit the spot' as there is much more room for error (because the effective absorption bandwidth is narrower).
But we normally want a high Q device since:

a) If we wanted a resonant device with a wider bandwidth (lower Q), it would be smarter to use a helmholtz absorber - which regularly outperforms membranes for wide-band yet tuned absorption.
b) The higher the Q, the lower is alpha at the resonant frequency, which essentially means that our absorber is now less efficient.

Luckily, empirically fine-tuning the resonant frequency of a high-Q membrane absorber is not all that difficult.
In practice we have some stiffness which is function of the tightness of the screws at the edges/corners of the membrane vs. its area (as Terry suggested).
Changing either would cause a shift in resonant frequency.

I was taught a trick (I don't remember by whom now) which makes this task very simple.
Beg/borrow/steal from a friend a piezo-electric microphone and mount it on the membrane;
Connect the output to an RTA and play some excitation signal to the room.
You should be able to clearly see at which frequencies the device resonates.

Then, make some correction to the tightness of the mounts to adjust the predicted frequency to what happens in 'real world'.

- Jon.
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Re: Membrane Abosrber Calculator (Excel)

Postby bert stoltenborg » Wed May 19, 2010 9:35 am

You can also buy a cheap piezo tweeter and take out the ceramic element.
Glue it to an old creditcard, solder a wire to it and you have a multi million dollar Type I vibration meter.
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Re: Membrane Abosrber Calculator (Excel)

Postby Terry Montlick » Wed May 19, 2010 12:13 pm

I've built a number of these over the years, but sorry, no picts (so according to Scott, it didn't happen! :) ). I've used mostly mass-loaded vinyl as the membrane for bass absorbers. It's very heavy and has ideal "limp mass" properties. You just attach this to the front of a wood or particle board box.
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Re: Membrane Abosrber Calculator (Excel)

Postby whitesnake » Thu May 20, 2010 7:30 am

What material you used and how are they installed for yours membranes?

Can you give me more details( materials, size, distance from the wall)?

How I can measure their resonances ?
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Re: Membrane Abosrber Calculator (Excel)

Postby joemeekums » Wed Jun 16, 2010 4:40 pm

Hi everyone

Firstly thanks to Jonesy for the calc, very helpful.

I have some practical questions about building membrane absorbers

1. Judging from the calculator and the document that jonesy refers to, it seems that the length/width of the panel has very little to do with the design frequency. Can someone confirm that that is the case? If not, how can this be accounted for?

2. I've read in some places that a membrane absorber should be positioned on the wall to the side of the direction of the room mode, ie if tackling a front-to-back mode, position the membrane on a side wall. This would make sense to me if the length/width of a panel did dictate design frequncy, but not if otherwise. Again could someone confirm this theory?

3. Does anyone have a really useful list a-la Bob Golds of flow resistivity and speed of sound properties for choosing an appropriate porous backing?

Thanks very much in advance
Joe
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Re: Membrane Abosrber Calculator (Excel)

Postby Bob » Wed Jun 16, 2010 7:58 pm

joemeekums wrote:3. Does anyone have a really useful list a-la Bob Golds of flow resistivity and speed of sound properties for choosing an appropriate porous backing?


Fibertex 350 Rockwool 2.2 x 10^4 Rays/m
Fibertex 450 Rockwool 3.3 x 10^4 Rays/m
Fibertex 650 Rockwool, has an acoustic resistivity of about 50000 mks Rayls/m
Fibertex Hd Rockwool 7.0 * 10^4 mks Rayls/m
Fibertex Rockwool Building Blanket Plain 0.5 x 10^4 mks Rayls/m
6000 mks Rays/m is roughly the resistivity of a fluffy fiberglass batt.

OC 703 has a resistance of 600 mks Rayls/inch (about 23600 mks rayls / m)
OC 705 has a flow resistance of 770 mks Rayls/inch (about 30000 mks rayls / m)
Bradford Fibertyex 650 (same density as OC 705) is 1270 mks Rayls/inch (about 50000 mks rayls / m).

Air, on the other hand, has a characteristic resistance of 407 mks Rayls/m.

Glasswool Ceiling Batts 105mm 0.5 x 10^4 mks Rayls/m


(above from various posts, and http://www.bobgolds.com/AbsorptionCoefficients.htm
below from ir761)

glass fibre (G1) 89 mm batt 4800 mks rayls/m
glass fibre (G1) 65 mm batt 3600 mks rayls/m
glass fibre (G1) 150 mm batt 4300 mks rayls/m
glass fibre (G2) 89 mm batt 7900 mks rayls/m
mineral fibre (M1) 89 mm batt 12700 mks rayls/m
mineral fibre (M1) 65 mm batt 11400 mks rayls/m
mineral fibre (M2) 75 mm batt 16600 mks rayls/m
mineral fibre (M2) 40 mm batt 15000 mks rayls/m
mineral fibre (M3) 83 mm batt 58800 mks rayls/m
cellulose (C2) 90 mm blown 33000 mks rayls/m

Image
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Re: Membrane Abosrber Calculator (Excel)

Postby jonessy » Wed Jun 16, 2010 8:15 pm

1. Judging from the calculator and the document that jonesy refers to, it seems that the length/width of the panel has very little to do with the design frequency. Can someone confirm that that is the case? If not, how can this be accounted for?


First, this is a membrane calculator not a panel calculator.
Second, it is based on a purely theoretical model of an ideal membrane.

As such, you will not find 'real-world' implications, such as finite dimensions (which would require some modal consideration), mounting conditions, etc...

I think that most of this has been covered in this thread already... As I already mentioned, these devices have very empirical designs.
The calculator could just help you to figure out if you are in the 'ballpark'.

Cheers,
Jon.
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