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Edited: Aug 30, 2023

AudioMoth tiny files

Good afternoon

We have a number of AudioMoth 1.2.0 with firmware version 1.8.0 or 1.8.1. and we use them to record bats.

Until recently they worked fine, but gradually they started producing thousands of nonsense tiny files (39-61 kb) and only a handful of good ones scattered here and there, even in areas with many bats. In some cases they produce only the tiny ones, and no big/good files at all. The settings are the same as before (see end of message for an example).

Yesterday I tried one device with settings in the audible range and it worked well (on my desk). Then I used the usual settings for ultrasounds and put it outside overnight - today I found out that the problem was still there.

We are trying with new alkaline batteries like Energizer and sd cards suggested by Open Acoustics

Any ideas how to solve it?

Thanks in advance

Sample rate (Hz) : 384000

Gain : Medium

Sleep duration (s) : 1

Recording duration (s) : 5

Active recording periods : 2

Recording period 1 : 00:00 - 03:30 (UTC)

Recording period 2 : 14:30 - 24:00 (UTC)

Earliest recording time : ---------- --:--:--

Latest recording time : ---------- --:--:--

Filter : Band-pass (11.0kHz - 144.0kHz)

Trigger type : Amplitude

Threshold setting : -32 dB

Minimum trigger duration (s) : 0

Enable LED : Yes

Enable low-voltage cut-off : Yes

Always require acoustic chime : No

Use daily folder for WAV files : No

Disable 48Hz DC blocking filter : No

Enable energy saver mode : No

Enable low gain range : No

6 comments

6 Comments

Alex Rogers

Aug 30, 2023

Hi, It seems to have written a correct T.WAV file. If I expand the file using the tool in the Config App I get the full 5 second file that is expected.

The sleep period and the recording period are both quite short so the AudioMoth will typically struggle to keep to the schedule once there are many files on the SD card. We'd normally use a 55 second (595 second) record period and a 5 second sleep period to minimise the amount of time that is lost opening and closing files. I don't think the above factor is the issue here however. The example file contains no trigger events within the 5 second recording period. This may be because there were no audible bats within the interval, or it may be a microphone problem. With the setting you have, you should be able to rub your hands together in front of the microphone to trigger longer recordings. You will see the red LED flashing as it is writing the file to the SD card. If you record at the default 48 kHz, without a case, and count in a normal voice about 40 cm from the AudioMoth, do you get a good quality recording?

Alex

Alex Rogers

Aug 30, 2023

Replying to

Try turning off the amplitude trigger, remove the filter, and set the sample rate back to 48 kHz. Then try a recordings in DEFAULT mode of your voice. If you can't hear your voice clearly then it looks like the microphone of this AudioMoth has succumbed to exposure / water damage.

pangeos

Aug 31, 2023

Replying to

Hi, thanks again. I tried as you wrote but I cannot hear my voice, just a constand, uniform hiss.

The same happened with these custom settings:

Sample rate (Hz) : 48000

Gain : Medium

Sleep duration (s) : 5

Recording duration (s) : 5

Filter : -

Trigger type : Amplitude

Threshold setting : -68 dB, -100 dB

However, at -32 dB I get only tiny empty files (0 seconds length before expansion, like the first I uploaded). There is no difference in the "amount" of hiss or file size between -68 dB and -100 dB (as in the default setting) and file lenght is 5 seconds.

Do you think it was caused by the prolonged exposure to high temperatures (abient temperature during the day may reach 40 Celsius, appartently higher for device and its case). This summer was rather hard for some of my devices.

I cannot see any signs of defromation or corossion.

This specific device was set ca 7m above the sea and had some salt on the case and inside, but other devices have similar problems with no such treatment.

Could it be detachement of the microphone. Is there a way to fix this?

Pangeos

Alex Rogers

Aug 31, 2023

Replying to

Yep, when you change the amplitude threshold below -68 dB you are detecting the hiss and getting the full 5-second recording. Temperate may be an issue but humidity and condensation is possibly a greater danger to the microphone. Andy - andy@openacousticdevices.info - will be able to advise on changing the microphone. It's pretty straightforward for someone with some experience of electronics and access to a hot air

pangeos

Aug 30, 2023

This a typical example of the files produced