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Is there a solid material with both a low acoustic impedance (specifically, an acoustic impedance as close as possible to that of the air) as well as low attenuation? In other words, is there a material that allows a substantial portion of an incident acoustic wave traveling through air to enter into it without reflection, and also allows that acoustic wave to pass through it without substantial attenuation?

For example, porous materials like foams or aerogels generally have a low acoustic impedance, but unfortunately they tend to have a high attenuation as well. Is it possible to have "the best of both worlds"?

I am studying the formation of acoustic bandgaps in materials with periodic density variations, and it would be convenient to have a material with those properties.

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  • $\begingroup$ Low impedance does not mean that waves enter easily. Reflections at an interface are suppressed when both materials at the interface have the same impedance whether it's high or low. Are you asking for more impedance because you're considering the case of reflections as the sound travels from some particular material into the new one? $\endgroup$
    – DanielSank
    Commented Aug 16, 2016 at 16:41
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    $\begingroup$ That is an omission on my part, sorry. I am considering acoustic waves in the air, and thus I want the acoustic impedance of this material to be as close as possible to that of the air (which is very low). $\endgroup$ Commented Aug 16, 2016 at 16:44
  • $\begingroup$ Definitely update the post to make that clear. It's a good question. It would also be interesting to add a note describing why you're looking for such a material. $\endgroup$
    – DanielSank
    Commented Aug 16, 2016 at 16:58
  • $\begingroup$ Are you looking for substance that has low impedance mismatch and low attenuation across all frequencies or would it be acceptable to suggest materials that have those qualities for a specific frequency or band? $\endgroup$ Commented Nov 18, 2017 at 18:55
  • $\begingroup$ This is an old question but the frequencies I was working with at the time went up to 10 kHz. $\endgroup$ Commented Nov 20, 2017 at 3:41

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there are materials that do this, but they are not solids: the most common is open-celled elastic rubber foam. This material is used to make windscreens for microphones, to keep the wind from creating its own noise as it flows around the external housing of a microphone. here the idea is to get as much of the sound wave to penetrate the foam which then conducts it with minimal losses to the microphone element on the other side of the sheet of foam. My Shure PE54D-CN has one of these on it and it works quite well.

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