Hi all,

I was dragged to IKEA yesterday, so thought I'd look for some cheap duvets to aid vocal overdubs. On the way round my Missus spotted some Cot Mattresses.... made of foam and with a zip on cover.

Label said 35kg/m3, and it's breathable (since kiddies will be sleeping on it) so I assume open cell, and about £30 for 3 section totalling about 2m x 70cm x 4cm ish.

Any use as mid/high treatment? I have a wooden summerhouse as a studio, so no real bass problems that I can detect, although the neighbours can

Zarf

What can I say? There's only one way to find out - but the test will cost you £30!

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www.the-byre.com No longer Forum Member

As The Byre points out, only one way to know for sure...

One thing to keep in mind, density is not the end all of acoustic properties.

I'll test them for £200. You provide the test materials!

P-X

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My Home Studio Build Thread

Good point. Trouble is, without anything to compare it to I'm not sure how to test! Anyone done a comparison of foam/rockwool versus thick duvets? Also, any thoughts on how duvet density affects absorbtion? Honestly, not kidding. I mean, SoS regularly suggests using duvets for "instant vocal booth", so does it really make a difference?

Quote Zarf:

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Good point. Trouble is, without anything to compare it to I'm not sure how to test! Anyone done a comparison of foam/rockwool versus thick duvets? Also, any thoughts on how duvet density affects absorbtion? Honestly, not kidding. I mean, SoS regularly suggests using duvets for "instant vocal booth", so does it really make a difference?

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Well - other than "I put it in my rom and heard a difference" (Which for the price quoted by Byre is what he's telling you) there would be labtesting - which is obviously out of the question.

So the way it works is this (which also means I happen to agree with Byre)- if you want to take the chance to loose your money - buy it and give it a shot -

If it works - you're ahead - if it doesn't - you're out the cost of it all - which - seeing as you have another use for it - sounds like it wouldn't really be anything.

I doubt very much if anyone is ready to go out and purchase these to take them to a testing lab and have them tested.

Sincerely,

Rod

I don't think I was suggesting a lab test. I was asking two questions:

1. Is 35kg/m3 open cell foam sufficiently dense IN THEORY to be worth trying, compared to 45kg/m3 mineral wool?

2. Does the density of a duvet make a difference in anybody's opinion, and would SoS thing about doing some field testing next time they do a Studio makeover? Or how about coming and doing mine?

And for what little it's worth, I don't have another use for it (did I say I did?). My kids have been out of cots for some time now!

Zarf

it will help however

a few points to look out for

The foam will almost definatley be a high resillience polyurethane foam, the cell structure will be large and irregular as opposed to acoustic foams which have a very small regular cell structure,

the density will be quite high because of the fire retardent additives in the foam

i presume the colour is white, this is the worst colour for uv sensitivity, the foam will soon go yellow and look dirty, its not that the foam is dirty its just that foam is light sensitive

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www.acoustic-foam.co.uk

Thanks, that's interesting. Yes, it's white. The main reason it seems low-ish cost is that there's a fitted cotton zip cover, which could be dyed. Therefore it won't matter if the foam yellows.

What difference is the cell structure likely to make?

Zarf

I have to rely on advice from the foam manufacturers, acoustic foams are usually polyesters with a very fine regular cell structure, i presume this is for a reason i did read on one acoustic foam specification the following

Compared with polyether foams of the same density , acoustic polyester foams have the following characteristics:
Regular cell structure and accurate control of the cell size
Higher air flow resistance, and therefore better acoustical absorption.

Auralex state this on their website : there are very dramatic differences in cell structure and density between acoustic foam and the thousands of other types we could manufacture. (This is why you can't just run down to the local SuperMart and buy mattress pads with which to acoustically treat your studio.)

all in all it looks like it will work although not as well as specialist foams . id be more worried about the acoustic tranparency of the cloth affecting things rather than the foam though

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www.acoustic-foam.co.uk

Not that it's a big deal, but the Auralex quote seems to contradict other "received wisdom" that open cell foam is needed for acoustic absorption. If higher air flow resistance gives better acoustical absorption, infinite air flow resistance gives infinite absorption, which is demonstrably untrue. By definition then, at least SOME air flow is required, somewhere between no resistance (i.e. open air) and infinite resistance (solid stuff or closed cell foam).

Which I guess is where the manufacturers make their money.

Back to Rockwool then.

Zarf

Hi Zarf, you say that 'If higher air flow resistance gives better acoustical absorption, infinite air flow resistance gives infinite absorption, which is demonstrably untrue'.

Bit of confusion here (not necessarily from yourself). We're after sound wave flow resistance, not air flow resistance. As we know, sound waves are oscillations in air - we want the energy removed from the vibrating wave, not (necessarily) from the airflow. We can certainly have a high airflow with no sound. In practice, though, they're often the same thing. Acoustic foam and other materials work by the resistance to airflow created at the foam surface/airflow interface. When reflecting from the sound insulation interface, for an ideal acoustic insulator there would be no AC component in the reflected airflow. This could be possible via a very long U-shaped transmission line filled with appropriate absorption material.
By the way, I think the ikea mattress would work quite well.

Quote BillPetrie:

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Bit of confusion here (not necessarily from yourself). We're after sound wave flow resistance, not air flow resistance. As we know, sound waves are oscillations in air - we want the energy removed from the vibrating wave, not (necessarily) from the airflow.

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Without the gas flow resistance there isn't wave flow resistance. The wave causes oscilating air particles with the highest velocity at the crossover between over and underpressure of the wave.
The gasflow (air flow) resistance of the material causes this absorption (friction of moving air particles versus cell boundaries = kinetic energy transformed into heat).

Bill how do you unlink both?
The wave exists by what it does with the air. Hence one damps the movement of the air particles to get rid of, or diminish the energy content of the wave.

Too high gasflow resistance and entrance impandance causes reflection.

Zarf's conclusion or feeling that there is somewhere a compromise between too low and too high gas flow resistance is correct. But without checking I doubt that Auralex's site should tell something contradictory.

Luxs comment about the polyester is quiet correct. And this cheaper matress foam is standard polyether, not because it's cheaper, but its elasticity, aging , breathing and moisture resistance properties are well adjusted to this matress use (for cheaper matresses)and hence for this purpose better than acoustic foam made from standard polyester.

With 35 kg/m3 its a good density but it will give lower absorption in the lows and low mids than polyester acoustic foam.
However sometimes the fabric around it can improve on that somehow. I expect it to be acoustic transparent since it should be as standard matress function.

But as other said: It's all a bit gambling.
Standard one could expect it to be worse than acoustic foam, since the low gasflow resistance of the matress is functional while less good for acoustic absorption.
Hence the design goals are different.
One should have a less comfortable, or less good matress when filled with more expensive acoustic foam.


Best regards
Eric