I'm trying to record owls calls with the AM in the IPX7 case but I'm not getting anything, while I know that they are there calling. Looking at the spectrum it seems that the low frequencies are very weak in my recordings. The case+AM works fine when recording bats. My current assumption is that the case does not play nice with below 1kHz sounds. How do people deal with this problem? Does turning up the gain while filtering high frequency signals do any good, or should I use a different kind of enclosure?
Thanks
Wim van Dam
Solvang, CA
If you are only interested in the owl calls then low pass filtering the 48kHz recording, or recording at 8kHz, would significantly reduce the noise floor as it increases the oversample rate by a factor of 6.
The slight amplification of the high pitched calls was what we observed last year when trialling the case for bird song. It might be interesting to try the case without a vent also. We were hoping to be able to use a test lab at the University of Southampton to do a real comparison last summer but the lockdown prevented us from doing that.
I repeated the test after allowing the case to settle for a good 13 hours (though the membrane already went flabby a few minutes after closing). Done with playback of Ural owl (Strix uralensis) and common chaffinch (Fringilla coelebs) calls, the loudspeaker was directly facing the recorder ~2.3 m away, with Ural owl call at approx. 85 dB RMS at 1m. Both sounds were first played at an enclosed recorder, then at the same recorder free-standing on a desk. It was set at maximum gain and 48 kbps sampling rate. Again, the case attenuated the owl's call by 20 dB, whereas the finch's call was amplified by 4 dB this time. This forum won't let me attach a picture, so I uploaded it to https://www.nib.si/images/stories/EKOS/comparison.jpg It shows oscillograms and spectrograms of all mentioned calls, pasted together (same scale throughout). We'll try putting out recorders in pairs - one enclosed and one in a ziplock bag - at our monitoring locations next week, to see what we get outside. But from what I've seen, it looks like the official cases aren't fit for our purpose, unfortunately. We'll also try one of the 3-d printed designs later, but it might take a while to get all the pieces.
Did you manage to measure some time after the case closure? The over pressure of closing the case definitely seems to increase the attenuation at low frequency and then this reduces as the pressure equalises over a few hours.
We just stumbled on this problem ourselves. Setting up monitoring of Ural owls, a quick overview of a week's worth of recordings near a nesting box yielded no calls whatsoever. Then we tested with playback at close range and the call was attenuated almost 20 dB by a freshly closed case! (375 Hz peak frequency, RMS amplitude measurement) In comparison, finch's song (3500 Hz) was only attenuated 5 dB. It would be nice to publish a real frequency response curve so people can make a more informed decision when purchasing. For us, this result is quite disappointing.
Hi Wim, The case and the acoustic vent does reduce the sensitivity to lower frequency calls somewhat, particularly in the first few hours after closing the case when it is typically slightly over pressured (and the vent bulges slightly). You can improve the signal to noise ratio at lower frequencies by trying a lower sampling rate. This increases the oversampling that is performed so background noise is reduced and the signal should be stronger in comparison. Ziplock bags seem to be almost completely acoustically transparent, but are obviously less robust for deployment in harsher environments or for longer periods. Alex