Use Audiomoth with an external hydrophone

Problem solved! I now have separate power supplies to the AM and the preamp. The periodic noise is completely gone!! You're a true wizzard!!! Thanks a million! :-)

Oki, I'll try the separate power supply for the preamp!! Thanks!

Hi Alex, thanks for responding! I am using a Sandisk Extreme 64 GB U3 A2. I use two AA 3-packs (alkaline batteries) in parallel, each giving 4.5V.

Hi again, Alex. Now I have made progress with my external hydrophone case. I work with the Audiomoth Dev board. I have put a 2k7 resistor between signal input and GND, which seems to work fine. Between the hydrophone and the AM input there is a voltage preamp. However, I get a peculiar periodic noise, which screws up the triggering by weak porpoise click trains. It looks like this:

I have tried a wet ground, i.e. connecting the AM input GND to the water, but it does not help. I have wrapped the AM board in aluminum folie, also grounded in the water, but it does not help either. Do you have any suggestions on how to remove it? Thanks for any advice!! mats

Hi, now I am on it again! Please refer to my previous comments, where I describe how I try to adapt the AM 1.2.0 to a piezo ceramic hydrophone input. This time I have replaced the 12k resistor between audio in and GND with a 2k7, to increase the "swing range". I have a another hydrophone preamp than the Aquarian PA4, (now similar to the Etec A1001 (www.etec.dk), adding 50dB, between the hydrophone and the AM. I recorded a pinger in a small test tank, with the pinger resting on the bottom, ca 80 cm below the AM which was held close to the surface. The pinger main frequency is 70kHz, so I tested both 250kS/S and 384 kS/S. In both cases there were gaps in the recording, more with 384 (top image) than with 250. The AM settings were Low in the low gain range, and high pass filter 50kHz.

What can be the cause of this? And what to do about it?

Thanks in advance for any advice!

mats

Oki, that explains it! I am always using dB, so I just assumed it was dB. Thanks for the fast clarifications!!

OKi, got it! :-) The gain steps in the AM make me bewildered. You say the Low-Medium-High should give 4.3dB-15dB and 30dB, respectively. However, Hill et al. 2019 AudioMoth_A low-cost acoustic device for monitoring biodiversity and the environment, report much lower gain steps: 27.2dB-30.6dB and 32dB, respectively. And Lapp et al. 2023 Quantitative Evaluation of the Performance of the Low-Cost AudioMoth Acoustic Recording Unit, report -14.2dB for Low and -5.7dB for Medium, relative to the High setting. What are the correct values..? And yet anotherquestion: Jay Barlow told me that the bit depth at 384kS/s was only 12, compared to 16 with the other settings. Is this correct,and if so, what are the consequences for the recordings? Thanks!!

As I have described in an earlier post, I am working on an underwater setup for the Audiomoth, based on an earlier porpoise detector housing. It has a piezo-ceramic hydrophone with a frequency response suitable for recording dolphin and porpoise sonar clicks. I followed advice from Jay Barlow at NOAH and put a 12 kOhm resistor between signal and ground and connected the external jack detector to ground. I use an Aquarian Scientific PA-4 preamp, and it works fine in the high frequency range at lower gain levels but loses 11 dB from 30kHz to 150kHz. Of course this can be compensated for in the post-process, but it affects detection of weak clicks due to lower S/N ratio. I read in the Forum that you were working on a new external preamp for the Audiomoth – is there now a suitable one for my purpose? If so, what are the dimensions – the space in the housing is limited (diameter 67mm and length 80mm; most of this space is taken by the AM and an extra 3xAA battery casing).

Another possible issue is a suspicious “wrapping down” in the spectrogram. I recorded a pinger, with main frequency output in 50-85kHz range, but with some components between 100-120kHz, at a sampling rate of 250kS/s. The top spectrogram below is this AM recording, and the one below the same pinger (there is some variation in the pings) recorded with a Reason hydrophone using a NI USB-6351 A/D converter sampling at 333kS/s, and a custom-made software on my laptop.

As you can see, there are down-sweeping tones in the AM recording which are not there in the laptop recording. Is this an aliasing effect? As far as I have understood there is no anti-aliasing filter in the AM, right? I tried to remove this by applying a lowpass filter, with cutoff at 150kHz, but this did not work, possibly because it is not inserted before digitalization.

Any suggestions on how to deal with this?

Thanks for any advice!

mats

Hi again, the reason for my earlier question about the H2d hydrophone is that I want to put my AM in an underwater casingwith room for more batteries and connect it to an external hydrophone. This casing has a custom-made ceramic hydrophone which I connected to the input terminals on the AM PCB, via an Aquarian Audio PA-4 preamplifier (powered by a separate 9V battery). I have soldered a bridge between the AM input ground and the jack detect terminal, and inserted a 12kOhm resistor between ground and signal input, as suggested by Jay Barlow in an earlier note. This worked fine when I tested with the internal batteries. I recorded 120kHz “clicks”, similar to harbour porpoise sonar clicks, generated by a fishfinder. Then I went to the next step by replacing the internal batteries with an externa 9V battery cassette (6 AA batteries), connected to the + and – terminal on the AM PCB (the internal batteries were removed). I first tested this setup using the AM live app, with the AM setup as a USB Microphone. This worked fine, also capturing wav on the uSD card. Then I did the final test, trying to record on the uSD card. The AM Firmware Basic was installed without problem, and the AM config seemingly was OK as well. However, when I turned the switch to Default, there was no flashing by the red LED, and there was nothing recorded in the uSD card. I did the External SRAM check, and the green LED flashed both in Default and Custom. When I switched to Default with the uSD card not inserted properly, only the green LED flashed.

What may be wrong, and can it be fixed? Thanks in advance,

mats

Yes. It works very well with the H2d Hydrophone.