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Possible influence of experiment site and method on results

Posted by wild_soundscape on 07 Jan 2012 at 13:30 GMT

A very interesting contribution to the subject. However, I have a few queries concerning the method.

[1] Specific noise spectrum
The compressor noise in audio S1 appears to contain potentially significant components in the middle frequencies (4 kHz-9 kHz) that are effectively absent from other types of anthropogenic background noise. For example, my own measurements of semi-distant continuous vehicular traffic noise show that, although loud (often approaching 50-60 dB above auditory threshold), it has few if any significant components above 2 kHz. As the middle frequencies present in the compressor noise used in these experiments are also apparently present in the vocalisation spectra of all the subject species, how likely is it that the results obtained may be more or less specific to the experimental setup?

[2] Relative loudness
Given the findings of Mathevon et al on bird song propagation (Singing in the Rain Forest: How a Tropical Bird Song Transfers Information, PloSONE February 2008, Volume 3, Issue 2, e1580), it seems to me that the relative loudness and spectra of the vocalisation and the noise should ideally be determined at the position from which the vocalisation is produced, as this is both a more direct measure of the subjective influence of noise on the subject and potentially more representative of the actual vocalisation. This would require rigorously controlled positioning of the microphone to ensure that the vocalisation plus noise and the noise alone are recorded under as near as possible identical conditions. It is not clear from the paper that this has been taken into account.

[3] Allowance for microphone response curve
Assuming the microphone used in these experiments is the AT815b, it is an interference tube "rifle" microphone exhibiting, as an intrinsic property of its technology, a very frequency-dependent pickup pattern and a very uneven on-axis response. According to the manufacturer's data sheet, the -10 dB pickup pattern is about twice as wide at 1 kHz as at 5 kHz and almost 3 times as wide at 200 Hz. The microphone also has a very uneven on-axis frequency response with a sharp 3 dB dip at 4 kHz and significant treble emphasis with an 8 dB peak around 9 kHz. I would ideally expect this kind of experiment to be conducted using strategically positioned omni-directional flat response measurement microphones, although I accept that this would make the procedure more difficult to carry out. If a "shotgun" microphone must be used, it would be good practice to calibrate it and apply corrective equalisation to the raw recorded tracks.

No competing interests declared.