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What is better for absorbing low frequencies - carpet

What is better for absorbing low frequencies – carpet, or old car tires?

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Question from a Audio Masterclass visitor…

I have professional recording equipment but not a room that has been treated acoustically. In the future I'm planning to make a soundproof room. I have an unfinished basement (concrete), with studs at every 16″, and insulation packed in the studs, there is also insulation in the ceiling. I tried recording and the basement seems to have the best sound.

Would a carpet with rubber help with the low frequencies? Or tires in the corner of the room with padding in the center of the tires?

I also have a barrier which has air bubbles in the center, with plastic on both sides, and I wonder if I could use this to cover the walls, to help with acoustics.

Best regards,
Richard

David Mellor responds…

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I have to admit that I have never considered old tires as acoustic treatment. A strange omission for me, since many years ago when I was a student I once had a job delivering tires to garages, and picking up the old ones. One thing I remember is that old tires are always wet. One shower of rain and the water never disappears, until you pick them up of course when it goes all over you!. But I digress…

Let's take this situation stage by stage. The first thing to consider is whether the room is adequately soundproofed. This must be done before considering any acoustic treatment. If the walls and ceiling are concrete, then providing it is thick enough, that will be a good start.

If not, then mass will have to be added to the walls. Drywall (plasterboard) is good cheap mass, and easily erected. If you separate the drywall from the structural wall, you will gain an extra few dB for free. Absorbing material such as mineral wool in the gap will help too.

Adding mass to a ceiling is more difficult. It also depends on how much load the ceiling will bear. The ultimate solution is to fix in substantial new joists and apply the mass to those.

Don't forget you will need a soundproofed door too. I guess windows will be an irrelevance in a basement!

Once you have soundproofed the room, you are ready to start with the acoustic treatment. Soundproofing and acoustic treatment are not the same thing. I have said much about this elsewhere in the site already, so let me concentrate on the tires.

It is always most difficult to absorb low frequencies. Low frequencies need thick absorbers, or at least absorption that is well spaced from the wall. If space wasn't a problem, then yes, I don't see why old tires shouldn't be used. Pack them with mineral wool too and cover them with fabric, probably stretched on a frame of some kind, and I suspect you will achieve excellent low frequency absorption.

However, where space is a factor, membrane or panel absorbers are the thing to use. A thin wooden panel, or a flexible membrane (such as used in roof construction) is spaced 10 to 30 cm from the wall and sealed all around the edges. Mineral wool can be placed inside. If a wood panel is used, it should be drilled so high frequencies can penetrate, with anything between 5% to 20% coverage of holes.

Membrane absorbers can be tuned to absorb standing waves too.

High frequencies are easily absorbed. But don't forget you need a combination of absorption and diffusion. Diffusion is where a surface is reflective, but it is irregular and breaks up the reflections into many directions.

Regarding your 'air bubble' acoustic treatment. It's difficult to know without seeing it, but nothing beats mineral wool for high absorbency at low cost. Carpet is expensive in comparison. But if you have something to hand, and it absorbs sound, there is no reason why you shouldn't use it.

David Mellor

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David Mellor

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