Acoustically Transparent Epoxy

Jesse Turner

@jcturn3
| He/Him

Colorado State University

I am a graduate student at Colorado State University working to develop novel acoustic technology for remotely monitoring wildlife.

Groups

Hello all,

I'm developing an animal-borne passive acoustic monitoring system and plan to pot the internal electronics in the housing with epoxy to waterproof the system. We're looking into both ECM and MEMS for the microphone. However, a MEMS becomes difficult when potting in epoxy because it would be directly mounted to the PCB and therefore the measured sound would attenuate through the epoxy. It's not a problem with ECM because the mic is not board mounted and can be external. I'm wondering if anyone has any thoughts on acoustically transparent epoxy that could be used for potting a PCB with a MEMS mic with no (or little) sound attenuation? I've made hydrophones with the same concept, by using an epoxy that was acoustically transparent in seawater, but not for airborne acoustics.

Cheers,

Jesse

Brett Muir

@BrettMargoSupplies

R & D Tech | Industrial Designer | Wildlife Management Technology

26 April 2024 4:27pm

Sounds tricky, Aerogel? haha.

Could maybe get away with using conformal coating and an acoustic membrane similar to what a cellphone mic uses.

Or locate the mems mic near the edge of the pcb and mask off that area when potting.

Robin Sandfort

@capreolus | he/him

Capreolus e.U.

wildlife biologist with capreolus.at

29 April 2024 9:19am

Sounds interesting.
We all used acoustic membranes.
Just like @BrettMargoSupplies suggested.
A friend of mine had really bad results with animal borne
mics where the pcb was potted and the mic was just covered by a thin layer of epoxy where it hit the surface.
That was not working good!

Greetings from thhe woods,
Robin

Jesse Turner

29 April 2024 9:19am

Thanks for the reply. I'm curious to hear more about your friend's issues with this set up!

Matthew Duggan

29 April 2024 9:19am

Same issues here. A MEMS is a great idea to pot, but you really need a piezoelectric element for this to work and not a MEMS based on capacitance (btw they're all capacitance, except for one now discontinued...). It was originally made by Vesper, but the company was bought out last year and the MEMS is no longer made.

This is because you're no longer really doing a typical microphone, this would be a contact type hydrophone. For waterproofing, you can actually get a waterproof MEMS. As long as your not submerging this for an extended period, it should do the job. Be sure to keep the cable short between the PCB and the mic as you'll get noise as I've experienced.

For generally answering your question on the "best" epoxy to with sound transparency, in general the harder the material the lower the acoustic impedance. I use Epotec 301 resin with a hardness of 85. Your shape will also influence the resonance frequencies, meaning the flat frequency response will now be distorted and you'll probably have distorted audio. .

Patrick Chwalek

29 April 2024 9:19am

You generally don't want to pot MEMS microphones since they're designed to pick up on air pressure changes and adding any material in front of the microphone just introduces another transition layer where pressure waves need to propagate through. Also, potting the MEMS microphone can be tricky since if you get any material in the port, you could damage the microphone or drastically reduce its performance. If you want to seal something with epoxy, take a look at contact microphones. Higher frequencies will be attenuated but depending on the application, it could work.

There are companies, however, that design fabrics that are waterproof/resistant but have a relatively low acoustic impedance. SAATI has a variety of samples that you can request and GORE makes Acoustic Vents that could work. You can design a mechanical housing around your MEMS microphone with small perforations that are covered by one of these materials. I did this for one of my latest projects and it holds up just fine in heavy rain conditions.

Harold Tay

@htarold

Worked as a mechanical engineer for a defence co, then software engineer, then for a research lab specialising in underwater robotics.

3 May 2024 5:34pm

Hi Jesse,

For a material to be acoustically transparent (in air), the speed of sound in the material times its density must match that of air. Realistically, any solid material will have a greater density than air, and a higher speed of sound to boot, so I'm afraid there's no way to match it to air. Sorry.