Hi, I have just downloaded the files from my Audiomoth which was set to record overnight. It produced 1600 plus files - most of which are noise - although I did pick up pipistrelle signals in some of them. The Audiomoth was housed in a case I downloaded from the internet and 3D printed (see attached photo). It incorporates an audio vent I bought from China. The weather was rainy. I attach one of the few traces (from kaleidoscope) that actually detected a pipistrelle (see 55 to 90 kHz) but the trace also shows a repetitive background signal at 10 to 20 kHz that is a feature of all files. Any clues what this signal is caused by and how I can avoid in future? Or is it a case of wading through all 1600 files to find the ones that contain bat signals? Should I adjust the amplitude threshold? I read on the forum that sound triggering is being developed presumably for this reason. Many thanks in advance .... (PS - this is the first time I have used an AM and I am new to this bat monitoring lark)
Hi Richard, If you update the Config App here (https://www.openacousticdevices.info/applications) to the latest version you'll have all the options. All the firmware versions will work with all versions of the AudioMoth hardware. The filter is a first order Butterworth filter so the filter cut-off is not very sharp. Also bat calls tend to be quite loud and will often nearly saturate the microphone anyway, so you probably won't impact the low frequency bat calls too much. The key thing the filter does is remove all the very low <1kHz background noise which allows the threshold value to be set very low so you still trigger on quiet bat calls. Try 15kHz and let us know.
Alex
Hi Alex, My AMs are version 1.0.0. I have flashed them with the latest 1.6.0 firmware. However I don't get the option of "minimum trigger duration". Presumably this is because they are the older versions? Also I only have three tabs "Recording Settings", "Schedule" and "Advanced Settings" rather than the four that you have. Also won't setting a minimum frequency of 40kHz miss the calls of a number of UK bats that signal at around 20kHz (eg noctules, Leisler's Serotines and barbastelles). I think I will set the minimum to 15Hz in line with Simon's post.
Thanks @Alex Rogers for the handy explaination of some of these settings. I'm fairly new to the bat recording lark and I've been stumbling around in the dark a bit myself, so this is really useful.
I have managed to capture some bat calls (only really Common Pipistrelle, so far) - but once I've gotten the hang of the settings, I intend to make recordings away from the back garden.
I've been uploading my recordings to the BTO Acoustic Pipeline, for automated processing with some success - as hacking about with hours of recordings in Audacity is taking up way too much of my time.
The Acoustic Pipeline can identify 24 species of European bat, plus small mammals and some insects and can be found here
https://app.bto.org/acoustic-pipeline/public/login.jsp
Hi Richard, As Simon said the noise is due to the SD card writes. One of the trade-offs in AudioMoth is that the MEMS microphone is on the same board as the SD card and some SD cards generate mechanical noise that gets transmitted to the microphone. This is more apparent as the battery goes flat, when the AudioMoth is in a case, or with some types of SD cards. We find that 32GB SanDisk Extreme cards work really well. If you use amplitude threshold recordings you will effectively remove all the silent periods between bat calls which makes the files very small and easy to inspect. The length of these recordings are encoded into the file and the file can be expanded back to their original length, or chopped up into individual files using the new tools in the Config App.
We generally use a high-pass filter with the amplitude threshold recording, setting the filter frequency to 40kHz and using an amplitude threshold of 1024 which corresponds to 30dB or 3% of full amplitude. Bat calls are very loud so this effectively places the threshold just above the background noise level and still allows quiet bat calls to be detected. Setting the minimum trigger duration to 5 seconds will ensure that each bat pass is recorded in a continuous block.
With these settings you can normally make one hour recordings, and they will only actually contain the periods when bats were passing by.
The application note from last year - https://github.com/OpenAcousticDevices/Application-Notes/blob/master/Using_AudioMoth_with_Filtering_and_Amplitude_Threshold_Recording.pdf - shows some examples of using amplitude threshold recordings. We'll update this with a description of the new tools in the Config App which give more options in expanding and chopping up the files that are generated. Alex
The constant ticking is caused by the AM, I get this and it varies as to exactly what it looks like/how bad it is depending on the batteries used. Non-rechargeable good quality batteries should minimise this - I think it may be to do with not supplying a high enough voltage (I'm not aware that it affects the operation of the AM in any other way). Regarding the 1600 files - the only tool to reduce this currently is the amplitude and filtering (fingers crossed the triggering software isn't too far away from being made available!). I use a high pass filter set to 15khz and amplitude threshold of 384 (the next step up starts to eliminate the quieter bat passes but should keep the example you have provided), though the latest software just released now uses a percentage for the amplitude threshold and I have yet to discover what the equivalent is! There is software from other companies that can scan the recordings and remove those without bat calls in (as well as suggest the species recorded), however they are relatively expensive (in the £100s). It may also be worth testing the AM in dry weather out of the box to check how the vent affects the recordings - these can reduce amount of higher frequency sounds reaching the microphone (albeit around 100khz+ which isn't too bad, but can could cause issues when you really get serious with bat call analysis!). I'd strongly suggest buying the book "Bat Calls of Britain and Europe A Guide to Species Identification by Jon Russ" when it is released in August. It'll tell you all you need to know about what bats you're recording as well as having info the species and how to record bats and analyse the calls. There is an earlier version "British Bat Calls" also by Jon Russ, which is still great for starting out, but I'm expecting the new book to supersede it (quite a lot has been learned about bats calls since). I expect you'll be able to pick up some cheap 2nd hand copies when the latest version is released if you don't fancy spending so much. Also check out the "Bat Call Sound Analysis Workshop" on facebook - a great group with both beginners and experts where you can upload recordings and ask for help etc. Hope that all helps!