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](https://i.stack.imgur.com/6Oz4r.png)](https://i.stack.imgur.com/6Oz4r.png)

This is somewhat opposite to [this](https://engineering.stackexchange.com/questions/7520/are-there-any-materials-known-to-be-sound-insulating-but-thermally-conductive) question.