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I'm wondering if there are materials good at transmitting sound but is thermally insulating? I'm trying to figure out a good material in an engine exhaust application which will pose a barrier to the high-temp exhaust flow, but allow the exhaust noise to pass through.

I'm particularly interested in low frequency sound only - 50 Hz to 350 Hz range.

The following cross-section image defines the application I have in mind a bit better. I'm trying to develop active noise control for engine exhaust noise. I'm hoping to use commercially available loudspeakers and so need to separate it from the main exhaust flow's heat & humidity. The material I'm looking for here is labelled "Material X". Sound (green arrows) will be transmitted through it, but the exhaust flow (white arrows) cannot move through.

problem definition

This is somewhat opposite to this question.

Rodrigo de Azevedo
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Poompil
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3 Answers3

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From the diagram you have provided us it appears to me that trying to "brute force" a noise cancellation scenario with a single loudspeaker at right angles to the flow of the exhaust gas is highly inefficient. If I may be so bold as to suggest a better way to approach the problem, you might want to consider using a sequence of passive impedance elements (expansion chambers) to muffle the sound first and apply active noise cancellation transducers to each chamber in the array, a divide and conquer approach so to speak. An excellent reference for designing passive muffler systems can be found in NASA Technical Note D-7309, " An Improved Method For Design Of Expansion Chamber Mufflers With Application To An Operational Helicopter ", Tony Parrott, author. By combining active and passive noise cancelling elements, you should be able to dramatically reduce the extreme thermal/sound transmitting materials requirements for your system .

William Hird
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In doing a quick scan of Google for your answer, I came across an article on a ring type device developed by scientists at Boston University that passes air but blocks sound in certain frequency ranges. Sounds like it could be a breakthrough for muffler design, go check it out.

William Hird
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AFAIK, sound transmission is a matter of the interface between materials, not just the materials themselves. Especially going from a light/low-density material to a heavier tends to reflect a lot.

So I'd leave out "Material X" and attach the duct directly to exhaust pipe. The thin pipe wall should vibrate just fine with your speaker. The duct itself and the air inside will be the main heat conductors. For the duct walls, you want a dense material - I suggest stainless steel or any other highly alloyed metal ("bas metals" are good at conducting heat, highly alloyed metals not).

Next step, you need to find the operating temperatures of your exhaust and the allowable temeprature of your speaker. You can calculate or approximate the k value for the duct and the air inside, then you know how much heat the speaker will have to radiate away (don't forget the heat the speaker produces, may be neglible).

Both thermal convection/conduction and accoustics are full of weird effects at interface and surprises. Expect to spend quite some time working with prototypes before you get it right.

mart
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