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I have conducted passive acoustic monitoring at a nature reserve (a wetland, near a city and two medium-sized roads). At night only, I have a strange source of sound that goes on and off every now and then, ranging between ~9-18 kHz (see spectrogram from 1 AM).

Edit: At night I recorded only from 01:00 - 02:00 because the night is not the main focus of the acoustic monitoring. It is repeatedly occurring during this time window (aka not a singular event per night), every night (for weeks), in 2020 and 2021 (in both - autumn).

enter image description here

Edit: I am now adding another spectrogram where "three such sources" are seen "together": enter image description here

I do not know what the source is. Nature reserve managers are unaware of any water pump nearby, but I suspect this has to be the source of such a sound.

Can I rule out any biotic sources assuming no animal here (birds, bats, or insects) can produce such a broad range of frequencies? Any ideas about what could it be?

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  • $\begingroup$ Could you also post a spectrogram over the full night if you have (to see the periodicity of the events)? What time does it start for the first time in the night and what time does it definetly stop at the end of the night? Sunset/sunrise times could be helpful as well please. $\endgroup$
    – Noil
    Sep 18, 2022 at 7:55
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    $\begingroup$ I am afraid not, I don't have such recordings.. At night I recorded only from 01:00 - 02:00 because the night is not the main focus of the acoustic monitoring. It is repeatedly occurring during this time window (aka not a singular event per night), every night (for weeks), in 2020 and 2021 (in both - autumn). $\endgroup$ Sep 18, 2022 at 9:34
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    $\begingroup$ Some very short time series (1 to 10 ms) could also reveal the pulse type $\endgroup$
    – WMXZ
    Sep 18, 2022 at 10:25
  • $\begingroup$ since there are several sources, and a high frequency... this could point to an insect? or are there really no insect that make such noise in your area? $\endgroup$
    – lframond
    Oct 10, 2022 at 8:05
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    $\begingroup$ One possibility: It could be an "ultrasonic pest scarer" - even though that frequency range is not usually considered "ultrasonic", I've certainly heard pest-scarers that were audible by me. Any chance they could be in the area? Also, since the sound is audible or close-to-audible frequency, I recommend slowing it down by a factor of 2 and listening. You'll quickly get a clear feeling for whether it's biotic or artificial. $\endgroup$
    – Dan Stowell
    Oct 11, 2022 at 17:22

2 Answers 2

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The tonal sound below 1 kHz that varies in intensity could easily be traffic noise from nearby roads. (varying intensity an irregular occurrence), The broadband noise from 9 to 18 kHz on the contrary has steady spectral intensity and ends abruptly. This is typical for technical noise at a constant distance with a binary (on-off) characteristic, the reason for which can only speculated about.

Here is my attempt:

As noise occurs only at night, when the temperature goes down and therefore humidity goes up, it could easily be electric discharges of a close-by power line.

Maybe such discharges could also occur inside the recorder.

If you consider this a sound signal and carry out co-variate analysis (temperature, humidity, duration, etc), maybe you could pin-down the source

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  • $\begingroup$ Thank you! I rule out the recorder option for now, it is an SM4 by WildlifeAcoustics and I use many of those without seeing this pattern. I'll check where is the nearest powerline in relation to the recorder. Althout I wonder how come I see this turning on\off pattern (sometimes of several "sources" overlapping). I haven't seen such a pattern when I recorded under a powerline in the past. $\endgroup$ Sep 18, 2022 at 9:37
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Answering my question, following another season of acoustic monitoring:

These are sounds produced by an animal, and are not any sort of anthropogenic disruption (probably some Orthoptera of unknown sp.)

Using multiple recorders this autumn showed that these sounds are only within several tens of meters of a large pond. At one night an individual generated these sounds next to the ARU and allowed us to confirm it was an animal.

enter image description here

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  • $\begingroup$ It remains suspicious that amplitude and frequency is exctremely (better un-naturally) constant. $\endgroup$
    – WMXZ
    Dec 6, 2023 at 15:11

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