Waterproofing a small microphone

[ollihd]

Hi everyone,

I'm trying to completely waterproof a shot timer (used in shooting sports to register shots). The small mic and speaker have an opening to them (of course) and I was wondering would there be a material that I could use to cover that opening, but still let sound through? Everything else I can just coat with an electronics "waterproofing" spray.

[Dubbie]

Does it have to be submerged or just splash proof? Will make a difference to approach taken.

[ollihd]

Only splash/normal rain would be enough. Won't be submerged.

[soldar]

A plastic bag?

[rdl]

Gore-Tex or maybe some other fine woven water repellent material.

[Gyro]

A membrane of cling-film (food wrap) is probably the most acoustically transparent and least susceptible to wind noise if you can provide suitable mechanical (and UV?) protection.

[frozenfrogz]

I guess you can just dip it in latex / rubber coating. Since you do not seem to require a specific frequency response and the to be expected sound level is pretty high I do not see any problems there (unless you have more specific needs than just log the impulse event).
When in doubt use a capsule with a bigger diaphragm.

[ogden]

Your application is about pressure wave, not Hi-Fi sound. Could be so that any venting membrane will do:

https://www.electronicsweekly.com/news/products/emech-enclosures/membrane-passes-sound-blocks-water-portables-2015-08/
https://www.gore.com/products/gore-protective-vents-for-telecommunication-systems

Those guys even have waterproof hearing device filter:

https://www.donaldson.com/en-us/venting/all-products/hearing-device-filters/

[frozenfrogz]

Rubber balllon or condom should also do a good job.

[Zero999]

Rubber balllon or condom should also do a good job.

Nitrile rubber would be even better than latex, as it's less sensitive to UV.

[Bicurico]

Your question automatically reminded me of "Last chance to see" by Douglas Adams: https://en.wikipedia.org/wiki/Last_Chance_to_See

This is a book about him visiting remote locations to see species that are on the verge of extinction.

One of the passage is located at the Yangtze river in China, where he seeks the rare river dolphin. These animals suffer from the noise pollution in the river, caused by all the boats and ships. In order to grasp this noise pollution he and his mate wanted to record the sound under water and hence needed to protect their microphone. The hilarious passage is him explaining to a chinese girls in a store, by means of hand signals, that he needs a condom to record sound under water...

Even better than reading the book is to listen to a public reading he did in Germany (he is/was really popular in Germany).

Conclusion: you can take a condom and put your mic inside...

Regards,
Vitor

[StillTrying]

Unless long range is needed I'd have thought just having the mic on the PCB inside the box would be enough to detect low frequency pressure waves.

[Gyro]

Rubber balllon or condom should also do a good job.

Nitrile rubber would be even better than latex, as it's less sensitive to UV.

Is anyone going to admit to having carried out extensive outdoor UV sensitivity testing of condoms? 

[golden_labels]

OK, who is going to ask Durex about the frequency response of their products?

If it doesn’t need to last very long (a month) or be exposed to high temperatures, a thin plastic bag from a shop may be a cheaper alternative to a condom. Unlike the alternative solution it has known durability: there is a lot of sources that discuss PLA degradation.

But I see a problem with all proposed solutions which are based on wrapping the mic in some kind of a bag-like thing: wind. If the cover is loose, the wind will make it produce sound. Putting it under tension would greatly reduce the effect, but would also cause distortion to the sound. You could put a muff on the whole device, but those can cost $10+ even if imported from China and I am guessing you are seeking a cheap solution.

Note: I am not stating that the idea others proposed is wrong. I’m just noticing a potential issue that has to be addressed.