In a previous post I mentioned that OpenAcoustic’s AudioMoth was going to be upgraded to allow users to attach an external microphone. This new version was released during winter 2020/21 and in this post I will share first impressions of using this and some comparisons with other recording gear.

Out of the box

The external 3.5mm jack does not come fitted and has to be soldered onto the six contacts on the top of the unit. It’s a bit fiddly, but even with my limited soldering experience I managed to do it OK. It is clearly stated on the OpenAcoustic’s website but worth reiterating that once you have attached the audio jack, the device will no longer fit in the standard AudioMoth case.

To use an external mic you simply plug it in – i.e. you don’t need to enable anything in the Configuration App. I opted for a Primo EM272 capsule mic as these are affordable and good quality. They come with a decent length of cable so can be routed into a parabolic reflector if required.

Comparisons

To field test the AudioMoth with external mic I ran the following equipment side by side over 2 nights in February 2021:

• Dodotronic Hi-Stereo parabolic mic recording to MixPre3 recorder, set to 28dB gain
• Wildlife Acoustics SM4mini, set to 24dB gain
• AudioMoth with EM272 external mic in a parabolic reflector; gain set to medium. The AudioMoth was put inside a small cliplock food storage box with a hole drilled in the end for the cable to exit. I crudely sealed the hole by wrapping a strip of an old bike innertube around the cable.
• AudioMoth in a standard AudioMoth case; gain set to medium

I haven’t done any formal measurements of signal strength. Instead, as for most casual nocmig purposes we visually check spectrograms to detect calls, I have just produced standard spectrograms as they would appear when processing a recording. I think this shows how easily a call recorded on the different gear would be detected by a human observer. In each of the following images the spectrograms are in the order Dodotronic (both channels, or just the strongest), SM4mini, AudioMoth with external mic, AudioMoth in box. Note that the recordings made with a parabolic reflector may not show a major improvement over those without a reflector if the bird was outside the main receptive zone in front of the reflector.

Redwing 27 Feb 2021

A single Redwing call. The note is identifiable on all equipment but weaker in the AudioMoths. There is some marginal improvement with using the external mic.

Little Grebe 27 Feb 2021

A pair of Little Grebe ‘scream’ calls. The upper harmonic was invisible on both AudioMoth recordings. The AudioMoth with external mic is definitely an improvement on the device in the case, but still inferior to the Dodotronic and SM4mini. I suspect I might have missed this call entirely on the recordings from the AudioMoth in the case.

Robin 27 Feb 2021

A single Robin flight call, visible on all recordings, but with detail slightly variable across the recordings.

Coot 27 Feb 2021

A sequence of high squeak calls. This bird was off to the left of the garden as evident from the Dodotronic channels where the call is much stronger on the left channel. The AudioMoth with external mic is almost on a par with the right channel of the Dodotronic, and noticeably better than the device in the case.

Water Rail 28 Feb 2021

A sequence of four calls. Again the AudioMoth with external mic shows a small but significant improvement on the device in the case.

Conclusions

Based on these recordings the AudioMoth with external mic showed some small improvements on the AudioMoth in the case, though they weren’t as significant as I was expecting. It is possible this is because the recordings were made using a parabolic reflector, and if none of the birds were in front of the reflector, this might not have provided the best test. The comparison below was made without the parabolic reflector and shows an improvement in detection of the Black-headed Gull call over the AudioMoth in the case. I’ll repeat this test when there are hopefully a few more birds moving and post an update. Personally, I think these small improvements, and the confidence of being able to have the device in a watertight box and microphone external mean this is still a better option than a normal AudioMoth.