which material allows ultrasonic waves to pass through it?

Question

I am currently working on this project for my internship where i need to study the flow of a fluid that flows on a curvy rotating surface. Since this surface is rotating, to study the velocity I could not install a pitot tube so I thought of using a ultrasonic flow meter. However my surface is made of PLA, which I believe reflects ultrasonic waves. I can change the material of it, but I would like to know which material allows ultrasonic flow waves to travel through with minimal absorption to run my experiment. Thank you for your help.

Answer 1

Acoustic Damping

Instead of allowing the ultrasonic wave to "pass through", I think you want to absorb it so it doesn't ever reflect back to the flow's ultrasonic sensor.

It should be fairly cheap and easy to use an acoustic damper such as this (made for human hearing frequencies) - but you'd want to find one that works for damping the frequencies of your ultrasonicator, which should be >20kHz.

This page suggests that any standard acoustic damping material might work, as long as it's thicker than ~1-2cm:

20KHz has a wavelength of 1.7cm. A basic rule states that to completely absorb (stop) a soundwave, an obstacle must be greater in size, or thicker, than the sound's wavelength. When an object is large in relation to a sound waves wavelength, the sound will be partially reflected and/or partially absorbed. For example, high frequency soundwaves at 20kHz (with a wavelength of 1.7cm) will be easily reflected or absorbed by walls and ceilings because they will be thicker than the sounds wavelength.

Calculate the wavelength of the ultrasonic frequencies you are using, and you might find you need a pretty thin acoustic absorbing layer.

Impedance Matching

However, to answer your question about allowing the US to "pass through", Impedance matching will allow the ultrasonic sound waves to pass through and into the PLA. However they may reflect off the back surface of the PLA and still interfere with your measurement - and then the anti-reflection layer will help those reflected waves pass through and hit your sensor.

If you can find the acoustic impedance of the PLA at the ultrasonic wavelength, you can possibly coat the PLA in an anti-reflection layer for the ultrasonic frequencies used. (If you could specify the frequencies used in your ultrasonic flow meter that would help.)

Here is an example of a few materials used to cancel ultrasonic reflected-waves. PLA may have similar acoustic impedance to one of the plastics mentioned. Usually "impedance matching" means you'd make a layer 1/4 the wavelength of the sound, with "impedance" in between that of your PLA and water. (although this particular paper doesn't appear to do that).