AudioMoth is a general-purpose device that has been used in a wide variety of applications worldwide from monitoring poachers gunfire, to tracking crickets. The basic branch of the firmware is highly configurable and reflects this general approach (at the time of writing in summer 2020 – version 1.4.4).
Hence some degree of experimentation is needed to find the best settings that work for UK bats. The testing process we followed is summarised on this page, with the full results in this spreadsheet. Please review this Audiomoth whitepaper for an explanation of the features being tested.
If you have any feedback on this testing or your own Audiomoth findings, then please do get in touch.
SUMMARY of Testing results & Outcomes
Sample Rate
The number of bats detected by AutoID in recordings using (near) continual recording was tested with sample rates of 192, 256 and 384 kHz. These recordings were also manually IDed for verification. Summary below:
- Result – The number of bats detected using sample rates of 192 and 256 kHz is broadly similar, while 394kHz records 20 to 35% less bats.
- Outcome – Use 256kHz. As we are in a Lesser Horseshoe (LHS) area we standardised on 256kHz. In areas without LHS, it would be sensible to use 192kHz to reduce filesize and benefit from the internal oversampling to get cleaner recordings.
- Some people have suggested using 394kHz to allow identification of myotis spp. However rather than detect the start frequency of Myotis call, this is just likely to detect the frequency at which the AM mic becomes sensitive enough to detect the call. An external ultrasound mic. may be a solution here.
Full details of the Sample rate testing on this page.
Gain
Using all 5 gain settings the number of bats Auto ID detected in recordings using (near) continual recording was tested. Summary below:
- Result – High gain was the worst, with Low gain second worst. The three medium settings (Low/Med, Med & Med/High) were similar with Medium usually slightly the best of the three.
- Outcome – Use Medium Gain
High Pass
For UK bats the Audiomoth team recommends a high pass of 50kHz, this is as hardcoded into the versions of the triggering firmware produced by the AM team (versions 1.0.9 and 1.0.10).
We tested this by using a range of frequencies for high pass filter, the numbers of bats Auto ID detected in recordings using (near) continual recording was tested using Med and High gain.
The aim was to reduce the Audiomoth internal noise recorded, and more importantly to compensate for the non-linear frequency response of the AM Mic, by finding the highest ideal setting for high pass that still allows reliable detection of Noctules at 20kHz. Summary below:
- Result – This “ideal” setting of High pass is dependent on Gain
- Outcomes – For Medium gain use 25kHz, for High Gain use up to 45kHz.
AMPLITUDE THRESHOLD
For each setting of gain, a range of Amplitude thresholds was used. The total size of the files recorded was measured, and the number of bats detected in these recordings was noted.
Effect of gain and amplitude threshold on file size
Our testing has found that the biggest variable on filesize is, not surprisingly, background noise.
When deployed in a quiet location we have found that both the configured Amplitude Threshold AND Gain setting control the resulting filesizes. Put simply, the higher the gain, the higher the Amplitude Threshold needed to reduce filesize.
We found selecting an optimal setting of Amplitude Threshold for a set gain to be very difficult as there is not a linear response, with the setting more of a “cliff face”.
Click here for full testing results.
Effect of file size on the number of bats detected
The number of bats detected in every recording set of gain vs. amplitude threshold was analysed. Summary below:
- Results – The number of bats detected remains broadly constant until the total file size is approx 10% that of continual recording (CR). As the total file size falls below 10%CR the number of bats detected gradually reduces.
The truncated files were also expended using the Audiomoth config. app to their full 100% CR size, and bat AutoId reapplied to the expanded files.
- Results – In all cases, the expanded files resulted in fewer bats autoIDed than original truncated files. While this may be due to elimination of some false positives, manual ID verification of a small sample showed this was not always the case.
Summary of Amplitude Threshold testing
The aim of this testing had been to produce recommendations as to the ideal settings of amplitude threshold for detecting UK bats. We have not been able to do this. The main problems encountered were:
- Variability of the night time background noise, which can change rapidly between nights or even within a night.
- The smaller variability between the internal noise of individual Audiomoths.
- The sensitivity of both the gain and amplitude threshold on the resulting file sizes mean there is a small “Goldilocks” band in which “small and good” bat recording can be made.
- We have found no way to predict this “Goldilocks” setting. The AM team are clearly aware of this, as in their whitepaper they recommend sampling in the field using the red flashing LED to try and establish the right setting. But this is very difficult as it would need to be done for every device in every location, and more critically it would not allow for noise variability within the night time recording period.
Outcome of Amplitude Threshold testing
For UK bat recordings the only way we can see to ensure reliable recordings is not to use amplitude threshold, or set it at very conservative setting that may frequently result in near continual recordings.
Duration testing
For the Somerset landscape scale surveys we plan to use rechargeable batteries. We are currently testing how many nights Audiomoth can record for in various configurations.
Future work
Experimental Triggering Firmware
There is also an experimental branch of Audiomoth firmware developed in 2019 by the team at Southampton working with Adrian Bicker (versions 1.0.9 and 1.0.10). This is hard-coded for southern UK bats and uses ultrasound triggering rather than a Amplitude Threshold.
We are doing some comparison testing of this triggering firmware vs. amplitude threshold.
There are also discussions ongoing about progressing this triggering firmware, to make it configurable.
FUTURE AUDIOMOTH TESTING PLANNED
We still have open questions and plan the testing below. However if anyone has already tested these, or knows of others test results then please do get in contact.
- Triggering 1.0.9 vs Amp.Threshold 1.4.4 Firmware vs Wildlife Accoustics SM4+Bat – Using the “best settings” from testing to see which detects the most bat passes.
- Case – Is a wire mesh vent, or the porelle vent of the official case better at letting bat sound pass to the AudioMoth?