[Pumblechook (OP)]

It is possible to have acoustic waves to around 3 GHz in some media but maybe that is a restriction of the transducers and not the media?

Does air have a cut-off?

[Soul Surfer]

The attenuation of air rises rapidly above the highest audio frequencies we can hear and is getting quite high around 40KHz.   It varies a lot dependant on the amount of water vapour in the air and this accounts for the silence of fog.  the reason is that the mean free path and time of the molecules starts to become significant.  Very high frequency bat signals are only used for short range interception of insects the lower ultrasonic frequencies are used for longer range but still relatively short ranges.

It is interesting to note from the point of view of hi fi fanatics that the attenuation in the air at 20Khz can be a few dB on damp days and to get the best results you should make sure that the air in your room is thoroughly dehumidified!   []

[Pumblechook (OP)]

I found this.

http://www.kayelaby.npl.co.uk/general_physics/2_4/2_4_1.html

Interesting that for frequencies below 16 KHz attenuation fails with humidity but above 16 KHz it rises and by 100kHz it is a massive rise.