AudioMoth Housing

Fantastic! Thank you Alex.

HI, The 15mm includes the depth of the batteries inside the holder. There are some more detailed photos on the GroupGets site here - https://groupgets.com/manufacturers/open-acoustic-devices/products/audiomoth.

Attempting to order waterproof junction boxes for these, but have a sizing question. Do the batteries increase the depth of the device substantially or are they seated inside the board (mostly included in the 15mm depth measurement)? If ever updating the page, it would be really helpful to see the back of the device on the specs page ("Audiomoth" tab). Thank you.

I did a lot of work on high frequency capacitance microphones back in the 70's. It is quite difficult to provide protection without affecting sensitivity and frequency response and that is basically just the price you have to pay. Especially at the higher frequencies where dimensions are becoming comparable with a wavelength (3mm at 100kHz ish). Membranes will reduce the sensitivity and reduce the top end frequency response - we had to use diaphragms of 3.5um Mylar to get over 1000kHz. Grids of any sort tend to do horrid things to the overall frequency response adding sharp peaks and troughs throughout the range. Probably not such a problem is you are just interested in identification unless you get a horseshoe bat just at an anti-resonant frequency, but you might get some funny effects in spectrograms and power spectra. Mainly it is important to be aware of the possible effects and take them into account when analysing recordings. For high frequency tropical species it might be best remove any protection unless it is explicitly required due to weather conditions. The MEMS microphone is quite well characterised in the Knowles Application Note on ultrasonic microphones, and there is another interesting technical document on the effect of different enclosures and ports on frequency response and sensitivity.

Hi Jez, These are the ones we've tried out. https://voir.en.alibaba.com/product/60595361396-801219240/Adhesive_vents_sticker_e_PTFE_acoustic_membrane_for_microphone_speaker_smart_watch_and_so_on_portable_products_.html?spm=a2700.8304367.prewdfa4cf.16.4d342e2cA11Vdx

Thanks,

Andy

Hi Andy, I was wondering if you have a link for the stickers you used over the holes drilled in your housings for the Belize project? Alasdair mentioned Goretex stickers when I met him but I've been unable to find anything online matching that description. If there's already something tried and tested I'd be keen to replicate it. Thanks.

I used an old margerine pot, wrapped in parcel tape! Worked fine, and there were some fairly heavy downpours last week. For a membrane I used cling film...having punched a small hole in the case itself.

Membranes: Reading the critique of the MEMS microphone (admittedly written by the makers of the competitor piezo design), it may be inviting trouble to operate them without a waterproofing membrane. Might an inflated ziplock bag with an packet of dessicant make a suitable inner container? Purists might even like to pop an oxygen-absorber packet in as well. An easy way to get (very close to) a dry Nitrogen fill). It will be very interesting to characterise the response of the AudioMoth with swept frequency and transient impulses, in various types of enclosure and membrane.

Enclosures: As 'total waterproofing is near-impossible (for those of us without serious engineering shops)', I'm presently inclined to separate the mechanical protection and mounting, from the moisture ingress function (above). Perhaps use a standard electrical enclosure, or that hardy standby, plastic plumbing. I'll be keeping the size quite small to avoid unwanted echo & resonance, especially at the highest frequencies, and may insert some acoustic absorbent foam. For the aperture, we have a product in Australia called 'Termi-Mesh', a lightweight woven stainless-steel fine mesh, that I have fused to plastic pipe for other projects.

Pensively waiting for the Group-Gets announcement ...

Greg Hall

Thanks Greg for all the info. Although the data sheet does say 10kHz, being analog the microphone is very versatile and can capture sound over 100kHz, the frequency response does vary across the range though.

We are experimenting with different housing options at the moment, such as a using layers of sandwiched acrylic and also looking at custom injection moulded cases.

Anthony, great that you found a box for your AudioMoth. What box and membrane did you use?

Thanks,

Andy

Anthony, the guys at the ARBIMON project solved the problem of automatic species identification from audio recordings. Maybe you can find some useful info on how to approach this topic in their publications ( https://www.sieve-analytics.com/publications ). I'd be interested in your tools, since i don't have the funding to use the ARBIBON web-application service. :(

Cheers!

Thanks for this. That' helpful. I seemed to have had more success by hosuing the AudioMoth in a small box that is water tight, and drilling a small hole that is covered by a membrane. I agree, I dont think this will last long in a hostile environment. I will post some sounds - first I need to write some tooling to help look for something interesting in several thousand samples! I'm a bit rusty on my FFTs, but I'll post my code to github if I can get something working. If anyone has anything that will do this already, I'd be interested in finding out more? Cheers.

In my experience, it's all but impossible to keep condensing moisture out of equipment, especially in tropical forests. Better to provide very good protection against rain and splashes, and a generous area of finely-screened ventilation below, so the inevitable accumulation of dew can be evaporated away when the day warms up.

Probably needless to say, the PCB must be well lacquered, and all external contacts well lubricated to displace corrosion-causing moisture and electrolysis. (I like lanoline-containing sprays, they are slow to evaporate)

The area of (stainless-steel) wire mesh will also provide an ample area for the ambient sounds to get to the microphone, rather than depending on a tiny hole with an occluding membrane.

The microphone specified for 'our' device on page 6 of http://datasheet.octopart.com/SPM0408LE5H-TB-Knowles-Acoustics-datasheet-14433657.pdf is a bit vague about environmental tolerance, although page 3 of http://datasheet.octopart.com/SPM0408LE5H-TB-Knowles-Acoustics-datasheet-10100519.pdf shows it to be a tough little beast.

Incidentally, it seems to be only specified up to 10kHz. The ultrasonic sensor on page 3 of the first catalogue is charted up to 100kHz, but its response does change quite a bit over that range. It would be very helpful to see a sweep plotted up to 250kHz.

All that said, I like hundreds of others, are champing at the bit to get experimenting with our AudioMoths!

Regards

Greg Hall

PS: These piezoelectric microphones may be worth paying attention to: http://www.tomshardware.com/news/vesper-vm101-piezoelectric-mems-microphone,29187.html & http://www.edn.com/design/analog/4457396/3/SmartEverything-and-the-rise-of-the-microphone-array

Would be great if you could share your recordings somewhere so others can take a look.

Yes, we've used the plastic bags for short deployments in the UK but wouldn't recommend for longer term deployments in more extreme locations. In Belize we deployed them inside off-the-shelf electronics enclosures with a small hole drilled thorugh the front and a self-adhesive waterproof membrane stuck on the front. We're currently trying out some custom cases made out of laser cut acrylic at the moment which should work long term - they'd be £2-£5 to manufacture and assemble. Also looking into an injection moulded housing but that has an initial outlay for the tooling. We're keen to see what housings the community can come up with.