Bat calls at low frequencies

Thanks Davidlee for your response, that sounds more plausible!

The effect is simply due to aliasing. According to the Nyquist-Shannon sampling theorem a digitally sampled recording can only reproduce properly frequencies up to half of the sampling rate (the Nyquist frequency). Frequencies greater than this cannot be reproduced unambiguously and will be aliased - ie "folded down" into the the Nyquist range - when converted back to an analog signal. The effect is that frequencies between one half and one times the sampling rate will appear inverted, as though they have been reflected by a line represented by the Nyquist frequency. As the frequency increases the aliased "image" will decrease in frequency until it reaches the zero-frequency line, whereupon it will be "reflected" back upwards. So frequencies between one and one and a half times the sample rate will appear as normal but shifted downwards by the sampling frequency.

The pulses in recording rat1.wav are clearly upside-down FM/CF bat calls with a terminal frequency of about 11-12 kHz. The sampling frequency is 48kHz so the original terminal frequency would have been around 36-37kHz.

The pulses in rat2.wav & rat6.wav are un-inverted pipistrelle pulses with a terminal frequency of about 3-6 kHz. These have suffered second order aliasing and so the original terminal frequencies would have been 51 - 54 kHz - compatible with soprano pipistrelle.

There is no way of correcting for aliasing once the waveform has been sampled. The best solution would be to add an analog low-pass anti-aliasing filter to remove the higher frequencies before sampling. Since this isn't possible without redesigning the AudioMoth the only remedy is to increase the sampling frequency to greater that double the highest frequency that may be present and take the hit on the file size.

Alternatively, if you can identify the aliased signals then the most economical solution is just to live with the problem and ignore them.

Thanks Matthew for your response. The sound in the sonogram should not be real but an artifact, as we don't have bats calling at such low frequencies. Or perhaps that sampling you mention work similar to a frequency division system and that's why it looks and sounds so similar to calls recorded with this system?

Pancho,

You are most likely correct. The problem you are experiencing is probably much simpler than the idea I was trying to explore.

The microphone will still be receiving the ultrasound (which we know it is sensitive to and the silicon wafer will thus be vibrating which causes a signal to be sent to the A/D converter which it then has to interpret. I'm no expert in the way a MEMS mic works but I imagine that it is sending a similar kind of analogue signal to the A/D converter as any other microphone. If the A/D converter is just taking samples from the analogue wave form then it is going to come up with a result of some sort and it probably looks a lot like your first sonogram.

At that point the AudioMoth could introduce a filter based on what frequency it is sampling at, but it does not appear from your results to be doing this. (Maybe a filter could be programmed or maybe there is already filtering hardware on the board or A/D converter that is not being engaged? Commercial recorders like the SongMeter have hardware filters that are engaged by software or manually with dip-switches or jumpers)

If there is no built-in hardware solution then you will have to try a pre-mic filter to solve your issue. Ultrasound is lousy at going around corners so any solid, non-resonant barrier that you can position between the mic and the bats should moderate the ultra-sound.

Lower frequencies are much better at travelling through and around barriers so you should be able to detect lower frequency bird calls preferentially to the bats but it will take a bit of experimenting to find your frequency window.

Another glimmer of hope for your study is the plans to build AudioMoth with a microphone port so you can use any old mic you like. Then you can select a mic that has poor high frequency response or even better, one that has a built in low-pass filter.

Your problems are of particular interest to me because I'm trying to build an enclosure for a µMoth to be worn by a Koala. I'm really only interested in sounds below 3 kHz that are coming from inside the Koala so I'm making the enclosure sealed and experimenting with the best shapes and wall thicknesses to transfer the low frequency sounds while excluding all the high frequency sounds (like the sound of things rubbing on the case). I hope I don't have to sample at a rate higher than 8 kHz in order to post process the higher frequencies out of my recordings. AudioMoth mic does have an excellent low frequency response (certainly into infrasound) which is why I'm interested in it for my application but the high frequency response could become problematic.

Hi Matthew, it is not possible what you say, because, as you see in my first post, I had 3 recorders at the same place, at the same moment, and the results are completely different. I had 2 audiomoths, both with medium gain, and the records are different deppending only just on the sample rate you set. Even I tried in an open field, with no possibilities of echo effects, and there is still the same problem.

Hi Matthew,

Thanks for your reply. The audiomoth was housed in just a plastic zip bag.

The pipistrelles I usually get around home are Soprano Pipistrelles (Pipistrellus pygmaeus), which usually emit calls from 50kHz to 60kHz, sometimes even lower. Also Common Pipistrelles (Pipistrellus pipistrellus) are around, which emit calls from c42 to c50kHz.

Is there any physical event/process (e.g. echo?) that can create lower harmonics of an ultrasound? I rarely think so, but there is no other option I can think of! I don't really remember the gain settings I set, but I'd say it happens regardless this.

Thank you,

Rob

I'm wondering how you have the audioMoth housed. Could it be secondary harmonics generated in the housing? (Like the rattle you get in a badly installed stereo when certain frequencies are hit).

The frequency range of some of those recorded pulses is quite a high percentage for a bat (rat6.wav shows pulses ranging from 3 kHz to 5 kHz) which if it was a frequency division issue might translate to a 40% variation (say 28 to 40 kHz) which would not be normal for search phase calls (but I am not familiar with your pipistrelles). So maybe they are some other kind of bat call or not a bat and are a "real" noise.

What gain settings are you applying to the Audio Moth?

Dear all,

I am as well recording NFC (with an Audiomoth 1.1.0) and also routinely catching bad calls at very low frequencies (below 10 kHz). In this case, they should probably be Pipistrelles, which call at 50 kHz or more.

I guess it could be related with the signal processing of the device.

Could you give some insight into this matter?

Kind regards,

Ferran Pujol

I have exactly the same problem. I want to record bird NFC, but I don’t know why, bat calls appear in low freqüencies, and the place where I'm recording, it's full of them, so I cannot hear nothing else (it is really annoying).

Here you have 3 spectrograms of the very same moment: Audiomoth (1) with sample rate configuration at 32kHz:

Audiomoth (2) with sample rate configuration at 192kHz:

Ordinary Olympus recorder:

It is like if the Audiomoth transforms bat calls to low frequencies just to show them. That's a big problem, because if you want to avoid that bat calls match with bird calls, you need to set the highest sample rate, and this way, files are so big that just in 1 or 2 nights the SDcard is full. Also, since you get very heavy files, it is very slow to process them.