Does foam-lined deadsheet affect acoustic volume?

[AntM]

The product I'm referring to is this one:

Wilmslow Audio UK

It is probably antiphon LDA Sound absorbers take care of the noise - antiphon LA

The box I have made is very small (around 4 litres) so any added volume would be a problem. The bitumen backing is very thin (2mm) so I'm not worried about that, but I am concerned about the acoustic foam.

Thanks for your help.

[breez]

It is too thin to have an effect on the volume even in a small box like that.

[AntM]

I wonder what the maths looks like for the acoustic volume that the foam will occupy. If the foam is (relatively) acoustically transparent, I'm guessing that the acoustic volume it occupies will simply be the volume it occupies when compressed.

The question is: how do you tell how acoustically transparent the foam is?

[wintermute]

The effect will depend a lot on whether it is open or closed cell foam, as it states it is acoustic foam it is probably open cell, in which case it theoretically should add to the volume rather than subtract from it. The way to tell would be to take impedance measurements in box with and without the foam. If the resonance peak shifts up it is subtracting from your volume, if it moves down then it is adding to the volume, and if it stays the same it is not having an effect at all.

Tony.

[AntM]

Good idea, but I wonder if there is a simpler method of checking the type of foam without impedance measurements? I think that if it is open cell and all the cells connect to each other, it should absorb water, just like a sponge. This certainly seems to be the case with the sample I have tried...

[gyro]

A quicker (and less messy) way to tell is to put it to your mouth and try to blow through it.

[badman]

Originally Posted by wintermute

The effect will depend a lot on whether it is open or closed cell foam, as it states it is acoustic foam it is probably open cell, in which case it theoretically should add to the volume rather than subtract from it. The way to tell would be to take impedance measurements in box with and without the foam. If the resonance peak shifts up it is subtracting from your volume, if it moves down then it is adding to the volume, and if it stays the same it is not having an effect at all.

Tony.

Just a point of clarity:

It's not really adding to the volume, it's lowering the Q of the box spring. Hence, damping. This is why you don't really get the benefit with vented systems of more output at Fb, if it was acting as additional air mass, you'd have more output, rather than the less that occurs in-practice.

[wintermute]

Thanks badman, I probably should have said effectively rather than theoretically It's always a bit of a tricky point, how adding something that takes up space, has relatively the same effect as making a bigger box (in the case of a sealed enclosure) hence making the volume appear to be bigger than it really is. I have to admit I did think the OP had a sealed box, which now when I re-read the first post it is apparent this was an assumption.

Tony.