Developing and Testing a Waterproof Enclosure for the AudioMoth Acoustic Logger - Part 1

4 Nov 2019

Thanks and acknowledgments...

 

We would like to extend our thanks to Mr Andy Avery of Somerset Bat Group, who originally posted the group’s experience with this type of enclosure on the AudioMoth support forum. Andy’s post on the forum can be viewed by clicking on the following link

Background to the AudioMoth full spectrum acoustic logger...

 

AudioMoth is a low-cost, full spectrum acoustic logger, which can record uncompressed audio at a range of frequencies, from audible through to ultrasonic, at rates up to 384,000 samples per second. It is for this reason that it has become a popular low-cost option for recording bat echolocation and social calls in the UK. In comparison to other available full spectrum bat detectors, the AudioMoth is tiny, measuring just 6cm x 5cm x 2cm. The detector’s build is relatively simple - it features a circuit board mounted and secured on to a 3-cell AA battery enclosure. However, there is a trade-off in exchange for its low price, size and simple construction - it is not waterproof, and therefore in the absence of any type of additional enclosure would quickly yield to inclement weather. 

 

It is due to it’s inability to repel water that numerous professional and amateur researchers alike have sought to find a low-cost solution to safely deploying the device in the field for periods of time without worrying about poor weather conditions. Solutions have ranged from simple plastic zippered bags, as supplied with the device, through to the modification and use of pre-made electronic enclosures. Having successfully used the supplied plastic zipper bag since we bought our devices last year, albeit in mostly dry conditions, we wanted to explore a more permanent and reliable solution to deploying our devices for longer periods of time within a range of Scottish weather conditions. 

 

Modifying a waterproof electronic enclosure...

 

On the basis of Andy’s blog from September, we ordered two waterproof plastic electronic instrument enclosure cases from Ebay, which took just over a week to arrive from China - a link to the Ebay listing in provided above in Andy’s blog. The boxes come pre-sealed, so you will need a screwdriver to open them. Once open, you will find the remaining two screws to secure the lid and a silicon seal, which you will need to use to create the seal between the housing of the enclosure and the lid - this actually fits into the under-edge of the lid, just remember not to over-stretch it when you run it along the groove in the underside of the lid. 

 

It wasn't until we got to the next stage that we hit a slight snag… Because we have the first generation AudioMoths, which house the switch and microUSB card proud of the circuit board, the detector wouldn’t sit within the lid as per Andy’s blog. However, all was not in vein, having measured the width of the box, we realised we could house the detector lower in the enclosure provided we removed six small sprews, which was achieved using a drill and a 6mm metal drill-bit, followed by a spot of chiselling and some light sanding to removed any roughness. Once we were finished, we secured the detector using two small wedges of foam to ensure there was sufficient space around the edge of the box to close the lid around the detector. We also enlarged the two ‘lugs’ on the sides of the case so they could accommodate one of our small 5mm python-locks - an essential bit of security for longer deployments in the field.

 

Once we had modified the internal layout of the box, we turned our attention to the lid and the hole needed to allow the echolocation calls to pass through the box in order for them to be recorded by the AudioMoth detector. We measured the distance of the microphone from the main part of the box and transcribed the location to the lid, which was then drilled using the same 6mm metal drill-bit.

 

We then put some thought in to how we would make the entire box waterproof without attenuating the echolocation calls. On this subject there has been much discussion on the AudioMoth support forum! Some researches have managed to order small quantities of professional acoustic cloth, while others, like Andy, have opted for metal mesh waterproofed with Nikwax.

 

Taking the above into account, we decided to test an alternative barrier comprising a small piece of ziplock sandwich bag, which is made from very thin strong clear and slightly stretchy plastic - ideal to handle a little bit of flex due to pressure differences once the box is sealed. We secured a small amount of the plastic over the pre-drilled hole and secured it in place with a simple patch of black duck tape.  Once the lid was finished, all that remained was to test any movement within the box, which was prevented using more foam, where required. 

 

The testing scenario...

 

We decided to field test the new enclosure shortly after we completed the modifications, owing to a relatively good run of weather - a rare thing for early October in Scotland! In order to provide an interesting comparison, we decided to compare our AudioMoth in its new enclosure against a first generation AudioMoth in a plastic zipper bag, in addition to a first generation Wildlife Acoustics Echo Meter Touch (EMT) full spectrum bat detector covered by an identical plastic zipper bag. We thought the inclusion of the EMT would lend an interesting perspective to the test given the attested high quality output of the device.  

 

Field test #1...

 

Our choice of location for the first field test was the River Kelvin, as it passes under Kelvindale Road Bridge, which is located five minutes from our office. The river below the bridge is regularly used by common pipistrelle and soprano pipistrelle, so we knew there was a good chance of encountering at least a few bats so late in the season.

 

Having spent a nervous hour on the bridge over-looking the river, where we hoped we would record sufficient echolocation calls to underpin the test, we sucessfully recorded low to moderate levels of soprano pipistrelle activity. However, after returning to the office and examining the resulting recording in Wildlife Acoustics Kaleidoscope software, it became clear that the 1cm gap between the microphone and the hole in the case was causing too much attenuation of the sound waves. In essence, the recordings from the AudioMoth were unexpectedly faint despite bats foraging within close proximity to the detector's microphone. In terms of future deployment, this could result in faint bat echolocation calls not being recorded, so clearly a little bit of re-design was in order! 

 

Before undertaking a re-design, we still wanted to quickly test the acoustic properties of our sandwich bag patch. Thankfully, an unfortunate incident from last year presented an ideal opportunity... Not realising just how fragile the on/off switch is on a first generation AudioMoth, we managed to break one last year during an initial deployment. We realised that with the on/off lever effectively removed, we could mount the AudioMoth detector directly into the lid with a little bit of clever wiggling thereby facilitating a rapid continuation of the field test - hurrah!!!

 

Field test #2...

 

Our choice of location for the second field test was the River Kelvin as it passes through Glasgow University's Vet School campus, which is located ten minutes from our office and, in addition to common pipistrelle and soprano pipistrelle, is also home to Daubenton’s bats.

 

In terms of the testing scenario outlined above, how did the AudioMoth in the enclosure compare against the AudioMoth and EMT in their respective zipper bags? Well it did not take too long to find out thanks to a very obliging Myotis bat, as can been seen in the three figures below, which were selected to cover the same period of time (19:35 hrs).

 

Fig. 1: Myotis echolocation pulses recorded using EMT full spectrum bat detector covered with a zipper storage bag

 

Fig. 2: Myotis echolocation pulses recorded using AudioMoth acoustic logger inside a zipper storage bag

 

Fig. 3: Myotis echolocation pulses recorded using an AudioMoth acoustic logger inside the plastic enclosure with the microphone hole covered with sandwich bag plastic

 

Although the structure of the Myotis echolocation calls are easily distinguished on all three spectrograms, what is evident from examining the power spectrum of each file (the red line across the top for those not familiar with Kaleidoscope) is the difference between the power (strength) of the full spectrum signal. Basically, the EMT recorded the strongest, clearest full spectrum echolocation calls, while the AudioMoth acoustic logger in the enclosure recorded the weakest calls. However, it should be born in mind that the first generation EMT detector does not have an adjustable gain and therefore the sensitivity of the microphone on this detector may have been set to a higher level than the two AudioMoth detectors, which were pre-set to a medium gain.

 

At present it is unclear whether the reduced signal strength recorded by the AudioMoth in the waterproof plastic electronic instrument enclosure was due to the gain on the acoustic logger being set too low or the diameter of the hole, through which the sound waves pass, was too small, effectively reducing the recording capability of the device. Only further testing will help answer these questions...

 

Modifications to the enclosure design and field test #3...

 

On the basis of the above tests, two issues were identified. Firstly, in order to accommodate a first generation Audiomoth, the location of the hole in the enclosure needs to be changed from the front (i.e. the lid) to the back of the box. Secondly, either the gain of the acoustic logger needs to be increased and/or the diameter of the hole in the case will need to be enlarged to maximise the devices recording capability.

 

Developing an Enclosure for the AudioMoth Acoustic Logger - Part 2

 

In next part of this series, we'll report on the success of the additional modifications to the waterproof plastic electronic instrument enclosure, in addition to the testing of two other acoustic barriers!!

This blog was written by Graham Sennhauser. Graham is TETRIX Ecology’s Principal Ecologist and is a licensed bat consultant/licensed bat worker. He would be more than happy to respond to any bat-related questions in connection with his blog or in connection with any other project. You can learn more about Graham's background via his profile page or contact him on the following email address: graham@TETRIXecology.com.

 

 

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