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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.

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  • $\begingroup$ As sound is the transmission of vibrations in air and thermal insulation is partially the removal/blocking of heat via air, this goal is in contradiction. I think finding that which you seek will be challenging, bordering on impossible. $\endgroup$ – fred_dot_u Feb 11 at 16:08
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    $\begingroup$ Ceramics will be your only hope . $\endgroup$ – William Hird Feb 11 at 17:44
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    $\begingroup$ Could you expand? Any kind of ceramics in particular that you're thinking of? I should add that I'm particularly interested in low frequency sound transmission only (50Hz - 350 Hz); I'll add this clarification to the main question. $\endgroup$ – Poompil Feb 12 at 4:29
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    $\begingroup$ Where and how is the material to be applied? Should it wrap a (metal) pipe that carries the exhaust? Or should it be the pipe that carries the exhaust? $\endgroup$ – Jeffrey J Weimer Feb 12 at 16:07
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    $\begingroup$ I've added an image to my post which should clarify this. $\endgroup$ – Poompil Feb 12 at 16:28
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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 .

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  • $\begingroup$ Could you please expand on why the suggested method above is inefficient? Is the inefficiency due to geometrical reasons? Or just because I'm trying to solve the whole problem using active elements? The passive impedance elements (traditional muffler) as you're suggested actually do exist in the complete design, but they have to be downstream of the active system due to other design constraints (happy to expand on these reasons if you're interested). $\endgroup$ – Poompil Feb 18 at 15:57
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    $\begingroup$ Because if you muffle the sound passively first, then the SPL of your loudspeaker can be drastically reduced = smaller , more compact transducer, opens up the possibility of using silicon piezoelectric or silicon membrane transducers as your active sound cancellation devices, these have a very high temperature rating. $\endgroup$ – William Hird Feb 19 at 18:18
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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.

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  • $\begingroup$ Yes, I came across this when the paper was first published. It seems interesting for sure, but the manufacturing complexity & the fact that it is really tuned to one frequency alone makes it slightly less appealing. The fundamental frequency for engines that I deal with are about 12.5 Hz - 15 Hz. So you have about 20 harmonics + fundamental that are quite loud in the low-frequency range you need to deal with. $\endgroup$ – Poompil Feb 18 at 15:41
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    $\begingroup$ Well if you are really smart, you can take their basic design and apply fractal waveguide principals to make the muffler broadband :-) $\endgroup$ – William Hird Feb 19 at 18:20
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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.

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  • $\begingroup$ If I'm understanding you correctly, you're suggesting using a thin gauge section of the main exhaust duct as the sound radiating surface & insulation barrier. However, your statement needs to be qualified: "The thin pipe wall should vibrate just fine with your speaker." The ability of a material to transmit sound through it (or more accurately, NOT transmit sound through it) is given by it's transmission loss (TL) curve. The inherent stiffness of a duct leads to relatively high TL at low frequencies; see here. $\endgroup$ – Poompil Feb 18 at 15:51

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