Adventures In Audio
I am building a studio. Where should I put the absorption?

David Mellor

David Mellor is CEO and Course Director of Audio Masterclass. David has designed courses in audio education and training since 1986 and is the publisher and principal writer of Adventures In Audio.

Saturday October 8, 2005
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Question from an Audio Masterclass visitor...

Dear David,

I am in the process of building a studio in my home. I don't know where to place the absorption in the room because of its shape. The dimensions are 12'W x 18'L x 20'H. The ceiling is 20' high on one side of the room, and 8' at the other. It's basically a giant wedge. Here's a rough draft of the room. There is a wall with an 8'W x 7'H opening where the floor is recessed.

Thanks,
Jason Pancoast

David Mellor responds...

Your ceiling is twenty feet high? Wow indeed ! But only eight feet high on the other side of the room? That's one strange room you have. I wish you had sent a photograph.

Having a sloping ceiling is good. Place the monitors at the low end of the room and sound will be reflected from the ceiling towards the rear of the room. The room isn't all that long so I would place a lot of absorption at the rear so it doesn't reflect back to you. That should give you a nice deep sweet spot.

The other places where absorption is useful is on the side walls halfway between the monitors and the mix position. This is because there will otherwise be reflections that will create a confused sound at the mix position.

Other than that, you will need to deal with standing waves. You could calculate the frequencies of the standing waves due to the 12' and 18' dimensions. Oh heck, I'll do it for you...

31, 62, 94, 125, 156, 188, 219, 251, 282 and further multiples of 31.388 Hz

47, 94, 141, 188, 235, 282, 329, 376, 423 and further multiples of 47.08 Hz

There will be others, but these are the main ones. Notice that 188 Hz and 282 Hz appear twice - these will very likely be the worst offenders.

You could further calculate the dimensions of a membrane absorber. I'll leave you to do that otherwise we might get confused between European millimeters and US inches :-)

For example though, to tackle the 188 Hz standing wave, you could make a panel absorber with thin plywood of density around 2 kg/m2 and space it from the wall by 5 cm. Enclose it all around the edges. The greater the area of such treatment, the more effective it will be.

However, many designers opt to provide general low frequency absorption with a membrane absorber. Look in your local builders' merchant for roofing materials. What you need is a flexible membrane, as thick and heavy as possible. Space this from the wall by as much distance as you can allow up to around 20 cm, support and seal it all around the edges, and fill the void with mineral wool. This will suck up low frequency energy like a leech.

The optimum positioning for absorption of low frequency energy due to standing waves is in the corners of the room as this is where the pressure due to standing waves is at its peak.

Let us know how you get on.

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