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Limp mass bass trap. Smelly rubber!

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Limp mass bass trap. Smelly rubber!

Postby forumuser935368 » Wed Apr 22, 2020 9:15 pm

Hi! I am about to finish a couple of simple bass traps based on heavy 2cm thick rubber mats (20 kg/sqm). The problem is a very distinct an unpleasant rubber odour (I have tried several tricks including hot soapy water, vinegar solutions and sun exposure, all with little success). I am now considering wrapping them up in thin plastic film in an attempt to conceal the smell. My question is; would this jeopardize the matt’s basstrapping properties. There will be absorbers for mid and high frequencies hanging in front of the mats (the mat will be hanging freely between the absorber and the wall), so I am not worried about any reflections from the plastic wrap, but my concern is that the tightly wrapped plastic somehow might hamper the mat in doing its thing as a bass trap? Any thoughts? Paul
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Re: Limp mass bass trap. Smelly rubber!

Postby Philbo King » Wed May 06, 2020 2:28 pm

If you apply the plastic using a spray adhesive and roll out all the air bubbles on the side facing the room, the effect on its acoustic properties should be negligible. The very slight additional mass will lower the resonant frequency of the trap a tiny amount.

As far as I know, limp membrane absorbers work best when bonded to an absorber on the wall side. I'm not sure a free hanging mat will do much. The gap between mat and wall will come into play I would think.
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Re: Limp mass bass trap. Smelly rubber!

Postby Sam Spoons » Thu May 07, 2020 1:58 pm

AIUI a limp mass absorber is, by definition, free hanging?
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Re: Limp mass bass trap. Smelly rubber!

Postby Hugh Robjohns » Thu May 07, 2020 3:25 pm

There are multiple ways to skin this particular feline.

The problem with bass absorption is that the familiar lossy absorption using mineral wools and foams relies on air velocity, which is at its maximum at a quarter-wavelength. For very low frequencies, that point would be a long way from the wall... so purely velocity-based treatments aren't very practical for LF control in most situations.

Instead, a more practical approach is to use a pressure-based absorber since pressure is always maximum at or near the wall.

Pressure-based absorbers can be thought of as resonators -- blowing across a coke bottle, etc... -- and the classic approach is to build a rigid sealed box with a thin flexible membrane on the front (thin plywood, typically). The changing air pressure inside and out from LF sound waves forces the diaphragm to move, and that absorbs the sound energy. As a sealed box resonates at a fixed frequency, its design can be tuned to suit the room's standing wave frequencies, while packing the box loosely with mineral wool broadens the tuning peak to extend the absorption over a (slightly) wider range of frequencies.

There's a lot on the physics and practical aspects of this kind of design in the BBC R&D archives.

Commercial bass traps that use stretched foil or plastic sheeting across dense mineral wool batts work in a broadly similar way....

The free-hanging MLV type of limp mass works more like a carpet hanging on a washing line. It's a similar idea to the sealed box, and relies on the membrane fitting the aperture pretty tightly so that the changing air pressure near the wall in front and behind tries to move the membrane, which absorbs the energy.

The analogy PW likes to use is imagine bouncing a tennis ball off a hard wall. The wall doesn't absorb much of the ball's energy, so the ball bounces back to you. Throw it at a carpet hanging on a washing line instead and, despite their relative masses, the carpet moves ever so slightly to absorb the ball's kinetic energy, and so the ball falls straight to the floor, rather than rebounding.
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Re: Limp mass bass trap. Smelly rubber!

Postby Philbo King » Fri May 15, 2020 5:22 am

Hugh - an excellent explanation.
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