Acoustically transparent Windows

Up to Velocimeters - single point

Acoustically transparent Windows

Posted by Paul at August 26. 2004
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We are interested in making an acoustic window for a 10MHz system. Do you have any advice as to choice of materials for such a window?


This is a question we occasionally recieve, we have asked the people we know are using such materials to comment, but if we've missed you, please add a comment about good or bad materials you have tried.

Here's one reply I have received.
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We are using Mylar sheets of about 0.003 mm thickness as a window. For our application in the range of 1 to 2 MHz, its thickness is short compared to the wavelength. Thus it does not severely change the sound path characteristics. Mylar is not 100% acoustically transparent (you still get a fixed echo in profiling which is actually a good check on the performance of the system) but sufficiently so as to not affect our measurements. We have also tried to work through plastic (PVC) and it looks like it works. However you have to be careful:

There may be a problem with thicker materials for sound beams (such as the return beams of the NDV) which pass through the material at an angle other than 90 degrees. For thicker materials even at 90 degrees there may be also another problem if the speed of sound in the material is much different from that in water. Both of these "problems" will affect the travel time and thus falsify your results particularly in profiling. What you try to find in a window material is that the mathematical product of rho*c is close to that of water (rho=density of the material; c= sound speed inside the material). You can of course compensate but that means changing certain parameters in your system.



Looking forward to hear other comments!
Paul

Re: Acoustically transparent Windows

Posted by Evelyn Delacruz at May 08. 2019
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