MEMS microphone environmental protection - Page 1
Forum section: electronics / projects

author: davegravy
author_rank: Regular Contributor
timestamp: May 12, 2022, 01:57:17 pm
content: I'll have a MEMS microphone in an outdoor environment (no submersion) and I need to make sure it stands up to environmental dust, rain. Several MEMS application notes refer to ePTFE membranes providing this protection over the acoustic port, but I'm having trouble finding tangible products to accomplish this.I think IP54 or 64 should suffice in terms of ingress - no submersion. Minimally impacting the acoustic performance is critical, namely frequency response linearity and overall sensitivity. I don't exactly have a spec yet for this since I can do some level of DSP equalization but this has its practical limits.At this point I'm mainly lost in terms of the options available to me.Anapplication note by InvenSensementions the use of a thin washer as a spacer, covered with a piece of "0.01 inch thick, 35A durometer silicone rubber". While I can easily get these materials I assume I'd have to cut the silicone and adhere to the spacer and then the ultimate MEMS PCB - a level of precision fabrication I'm not sure I'm up for.GORE hasproductsthat look to be intended for this, is pre-cut to size and has adhesive backing ready to go, however their minimum order is 5k units and I'll be making at most 50-100.I haven't found another "off-the-shelf" product like the GORE vents, which surprises me given the ubiquity of MEMS microphones in consumer electronics today. Am I missing something?

author: themadhippy
author_rank: Super Contributor
timestamp: May 12, 2022, 02:16:50 pm
content: Try a condom,un lubed of coarse.Used em often on lav mics to stop them getting covered in sweat and make up.It can get a bit awkward when your partner finds a receipt for 2 dozen durex in your pocket,but thats a story for my memoirs

author: davegravy
author_rank: Regular Contributor
timestamp: May 12, 2022, 03:00:05 pm
content: I imagine that was a funny conversation.I'm looking for something a bit more form-fitting however, I don't think they make condoms that small ( but I'm constantly surprised by that industry )

author: davegravy
author_rank: Regular Contributor
timestamp: May 12, 2022, 04:29:58 pm
content: To add a visual to what I'm trying to accomplish, see attached.Does someone make something like a 1mm ID thin double-sided adhesive film donut that's precision cut? I could top this with a thin silicone sheet (that's less precisely cut).

author: RoGeorge
author_rank: Super Contributor
timestamp: May 12, 2022, 04:42:53 pm
content: The one in your picture seems already protected by a membrane, the translucent whitish film that covers the hole.Uncovered would show an empty hole, like a pit with the silicon die at its the bottom. For example, the small metallic nail marked in the pic is just an empty tube, and when looking with a microscope the MEMS die can be seen on the bottom, sitting in free air.

author: davegravy
author_rank: Regular Contributor
timestamp: May 12, 2022, 04:47:55 pm
content: Quote from: RoGeorge on May 12, 2022, 04:42:53 pmYes in the photo it is covered. Sorry, to be more clear the photo is of a product similar to the one I'm designing, and I'm wondering how to accomplish this.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 12, 2022, 04:57:27 pm
content: The GORE protections are well known indeed. I'm not surprised they are hard to get in small quantities.I would suggest thinking about maintenance. Any protection will eventually get clogged and will need replacement. Do you have any plans for this? Or how long is your product supposed to "last" in the field?So, if you include means of replacing the protections (ideally without having to entirely take the product apart), I would additionally suggest an approach similar to what is used in hearing aids. Which leads me to the suggestion of using protections made for hearing aids, which you should be able to buy in relatively low quantities through specialised distributors.(Example of those:https://www.hearingcarecentre.co.uk/shop/wax-guards-and-filters/phonak-wind-weather-microphone-protector-filters)

author: RoGeorge
author_rank: Super Contributor
timestamp: May 12, 2022, 05:09:46 pm
content: Don't know where to buy them, found an application note, AN-1124 from InvenSense) naming the cover membrane material, maybe you can find a manufacturer by googling more those terms:Quotehttps://www.digikey.gr/Site/Global/Layouts/DownloadPdf.ashx?pdfUrl=927C1DEE291644B580A5E0747C87C5AFSome MEMS mics are advertised for working in harsh environments, and sealed from factory:Quotehttps://www.designworldonline.com/how-can-a-mems-microphone-be-used-in-harsh-environments/LATER EDIT:For low production volume, can be 3D-printed, see the attachment.

author: davegravy
author_rank: Regular Contributor
timestamp: May 12, 2022, 05:18:12 pm
content: Quote from: SiliconWizard on May 12, 2022, 04:57:27 pmI certainly am familiar with the "clogging" you mention with my cell phone but for this application it won't be getting handled much by people and it'll have a decent size foam windscreen around it which I'd think gives extra protection. I would expect a few years operation before failing calibration but that's not based on any data or experience.I don't really know the sensitivity of this kind of protection to clogging - maybe it only takes a few particles of dust on the protection to throw the frequency response off but I'd expect that's more the case with an unprotected mic diaphragm.I'm also considering making the mic capsule (outer grill, MEMS sensor on PCB, a few SMD caps/resistors) modular and replaceable so it could be inexpensively swapped. Then it's just a matter of detecting failure and servicing it.I'll have to look into this hearing aid protection. Looks like there must be a standard aperture geometry for hearing aid microphones or something, so I'd need to design around this.

author: davegravy
author_rank: Regular Contributor
timestamp: May 12, 2022, 08:09:49 pm
content: Quote from: RoGeorge on May 12, 2022, 05:09:46 pmThe products to make this are readily available from McMaster-carr but I think would involve custom fabrication:* cutting the silicone sheeting* adhering the sheet to a small thin washer/spacer (ID ~ 1mm)* adhering the above to the pcbAll while needing be very precise - too much adhesive means some gets in the MEMS cavity and fouls the device, too little adhesive or applied unevenly mean the protection is compromised.It would effectively be manufacturing my own GORE-like product I think, which strikes me as too challenging. Or do PCBA houses handle stuff like this?Quote from: RoGeorge on May 12, 2022, 05:09:46 pmHmm, that's an enticing sensor. I'm wanting a digital sensor, and they have a digital version of this but seems both are unavailable.Quote from: RoGeorge on May 12, 2022, 05:09:46 pmThis is very cool, and similar to my application. Although, the frequency response they achieved was not particularly impressive. 0.1mm is very thin for a 3d print!

author: davegravy
author_rank: Regular Contributor
timestamp: May 12, 2022, 08:46:54 pm
content: Not sure the acoustic performance (I could test), and a bit big for how I was thinking of using them.https://www.mcmaster.com/vents/porous-ptfe-adhesive-back-discs/