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Natural Science Forum / Physics / Acoustics / October 2004foam plastic as sound isolation barrier?
A guy who I suppose ought to know (he spent 30 years in the industrial plastics industry) told me that expanded polyesterene or polystyrene foam sheets...IOW, those foam insulation boards made out of the same stuff as beer coolers, like Styrofoam...make an effective sound isolation barrier when used in a layered wall or floor construction. He claimed this is because the non-linear (random?) shape of the expanded foam cells provided an indirect transmission path, and sound energy was dissipated as it passed through this maze-like medium. Basically he said foamboard could be used in lieu of Homosote or Celotex or other "soundboard" materials. Anyone have any experience or data that might corroborate this claim? > He claimed this is because the non-linear (random?) shape of the > expanded foam cells provided an indirect transmission path, and sound > energy was dissipated as it passed through this maze-like medium. The "random transmission path" is only a valid argument if there are air spaces between the foam granules. In my experience, styrofoam and its variants tend to be solid. Therefore (and I am taking an educated guess here), the foam will be fairly reflective - especially at higher frequencies. If the foam your friend is taking about does actually has an open celled structure (I.E. air could be forced through it under pressure), then his statement is probably correct. However, it must be said that the commonest materials to use as absorbers are fibre based products whose absorptive properties have been studied in detail and are well known (E.G. Delaney & Bazley) Regards Chris W  SignatureThe voice of ignorance speaks loud and long, but the words of the wise are quiet and few. -- > He claimed this is because the non-linear (random?) shape of the > expanded foam cells provided an indirect transmission path, and sound > energy was dissipated as it passed through this maze-like medium. > Basically he said foamboard could be used in lieu of Homosote or > Celotex or other "soundboard" materials. Garbage. I once had a fellow call and complain about hearing the trickle sounds from his flushed toilet while he was standing in his yard outside his new frame house. The adjacent wall of that house had styrofoam insulation. Ang. C. "Soft" "plastic" foam with open "communicating" pores can be great sound absorbers and can help sound insulation when placed between two solid plates. "Stiff" "plastic" foam with closed pores are poor sound absorbers and can completely destroy sound insulation. Alone they both have poor sound insulation properties. And then there may be a fire and toxic smoke hazard.... Asbjørn >A guy who I suppose ought to know (he spent 30 years in the industrial > plastics industry) told me that expanded polyesterene or polystyrene [quoted text clipped - 9 lines] > > Anyone have any experience or data that might corroborate this claim? My experience with this type foam is that it is only a little better than an empty cavity, and then only at what would be the mass-air-mass resonance of the empty cavity. It could create a new mass-foam-mass resonance frequency at which it would be weak. It is not an effective sound absorber. >A guy who I suppose ought to know (he spent 30 years in the industrial > plastics industry) told me that expanded polyesterene or polystyrene [quoted text clipped - 9 lines] > > Anyone have any experience or data that might corroborate this claim? I have heard similar claims from insulation installers who have confused thermal insulation with acoustic insulation. One product that I am aware of is used for cool room construction and consists of a sandwich panel with a 0.4mm (?)aluminium skin on either side of a polystyrene core. At 45mm thick it has a STC of 24dB and a text book undamped coincidence dip at about 1,250Hz. At 150mm thick it has a STC of 19dB and again a text book undamped coincidence dip at about 600 Hz. A client once used this material to construct an instruction room in a factory noise environment before calling me for help. The narrow band pass filter effect had to be heard to be believed and it was the most expensive "storeroom" that they ever constructed. As a then very young engineer it taught me to be very wary until you see the "black & white" of the laboratory test results. > A guy who I suppose ought to know (he spent 30 years in the industrial > plastics industry) told me that expanded polyesterene or polystyrene [quoted text clipped - 9 lines] > > Anyone have any experience or data that might corroborate this claim? > I have heard similar claims from insulation installers who have confused thermal > insulation with acoustic insulation. [quoted text clipped - 26 lines] > > > > Anyone have any experience or data that might corroborate this claim? hi, i seem to be having some trouble posting, this is a test > I have heard similar claims from insulation installers who have confused thermal > insulation with acoustic insulation. [quoted text clipped - 26 lines] > > > > Anyone have any experience or data that might corroborate this claim? perhaps related: i have been making a modest survey of resonant characteristics of walls found in normal homes (regular walls, not walls built for isolation purposes). just taking impulse responses w/an impact hammer(s), no airborne stimulation save a half-dozen or so cases to check if the behavior stimulated by the hammer is analagous to that stimulated by airborne sound. when possible (meaning when a discrete mode can be clearly observed, which isn't always the case), i have tried to estimate the damping present in the walls. to end the rambling intro and get to a point, the best-damped wall i have encountered was one where a gentleman had desired additional thermal insulation, and so added a layer of styrofoam (closed cell, rigid or semi-rigid foam, he couldn't really describe it (grades on the market vary considerably in physical properties)) and another layer of drywall over that. attempt to reproduce improved damping in a wall with styrene-foam sandwich are pending. > One product that I am aware of is used for cool room construction and consists > of a sandwich panel with a 0.4mm (?)aluminium skin on either side of a > polystyrene core. At 45mm thick it has a STC of 24dB and a text book undamped > coincidence dip at about 1,250Hz. At 150mm thick it has a STC of 19dB and again Brian I have also used this sandwich panel to build an additional wall for a carpark exhaust fan. In this case I only needed to reduce the noise level by about 15 dB(A). You are correct about the coincidence dip but on this application it worked a treat. The wall was built within a day including fitting the access panels and reduced the noise down to the criteria. It can be a very useful material when only modest noise reductions are required. Regards, Tom Harper Roscoe - have a look here for the types of materials that are used in practice to provide insualtion in drywall/floor buildups http://www.acoustic.co.uk/H&H/bmats.htm. You'll notice that the only 'foam' based products tend to be open cell melamine based foam, which is reasonably compressible. Al the others are variants of mineral/glass fibre. Adrian |
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