Compatible external mics for ultrasound--current status?

Hi Alex,

Thanks for quick reply.  This is good to know.  The reason I was considering this is because I’ve been a little disappointed so far in the AudioMoth’s (AM) performance, so I was thinking ahead a little about whether using an external mic would improve things.  I’ve been running an AM alongside my Song Meter Mini Bat for comparison for some tests.  Not because I think it should perform as well, since that would be unreasonable considering the price difference.  But to just have a baseline of bat presence I could be confident in to see how well the AM performed.

After a few short tests, the AM is averaging only about 1/10th of the total identifiable calls of the SMMB, and only about 2/3 of the species.  But before drawing any firmer conclusions, I probably need to do some more experimenting/testing.  I’ve been using the guidelines shared by the good folks at SonoBat, whose software I use to analyze bat calls from all my devices.  They recommended to not use the filtering, but instead record in individual large time blocks and then use the large file parser (one of the SonoBat software tools) to parse those large files into individual call sequences.  And then finally to run those individual call sequences through the regular Sonobat analysis software like the shorter files ordinarily recorded by triggered devices like the SMMB.  These are the settings I’ve been using as recommended by SonoBat:

384 kHz Sample rate

Gain set at one step above Low

Sleep duration 5min

Recording duration 58:00

No filtering

All Advanced options unchecked expect Enable energy saver mode

My tests have used an AM in the IPX7 Waterproof Case, mounted right next to my SMMB on an elevated pole ~ 8’ high.

I’m thinking I should at least try increasing that Gain.  I believe SonoBat’s thinking on this (they recommend fairly low Gain levels on devices like the SMMB too) is to minimize the number of “dirty” (excessive noise) files.

I will say that I was pleased to see that the AM recorded some decent Myotis sequences (screenshots from SonoBat software below), since I think I remember reading somewhere that a weakness of the device was recording higher-frequency species like Myotis.

If you have any ideas/suggestions for improvement, I’d love to hear them.

Thanks, Colin

Hi Colin,

I'd definitely try increasing the gain. We typically use medium gain, which is a factor of x2 compared to your setting. The detectors will likely work okay with slightly saturating signals as they are predominantly dependent on the fundamental frequency. Also, if you can, using a sample rate of 192kHz will increase the signal-to-noise ratio a bit. You can use the downsample tools in the Config App to change a 384kHz recording into something very similar to what AudioMoth would produce at 192kHz (not exactly the same as the SD card writes will occur at a different rate which generates a bit of background noise in the recordings).

You can also see the effect of downsampling in the AudioMoth Play web app https://play.openacousticdevices.info.

Alex