I concur with Gianni and Steve that the level threshold and dynamic range (=
gray level scaling for the various sound levels) will greatly influence th=
e visual clarity of a spectrogram. Due to the limitations of the analog pri=
nting technology and the electronics of the ancient Kay Sonagraph machines,=
the dynamic range of those sonagrams was pretty small. I guess it was less=
than 35 dB (which means that sound components below -35dBFS were hidden). =
Nowadays, digital spectrograms can display dynamic ranges well beyond 100 d=
B. However, I believe that such large dynamic ranges are not useful if you =
just want to display the essential properties of an animal sound. On the ot=
her hand, a large dynamic range could be useful for detecting very soft sou=
nd components or for evaluating the background noise floor.
Another parameter that influences the clarity of a spectrogram is the tempo=
ral resolution. The old Kay Sonagraph machines supported two analysis bandw=
idth settings: narrow (45 Hz) or wide (300 Hz). Bird song researchers usual=
ly used the wide setting. This relatively high bandwidth provides a good te=
mporal resolution (1/300Hz =3D 3.3 ms), which preserves the rapid frequency=
modulations that are present in many bird songs. The users of digital spec=
trogram software tend to use inappropriately small analysis bandwidth setti=
ngs (they often believe that large FFT size numbers were always an advantag=
e). However, a FFT size of 512 points with a Hamming window and a sample ra=
te of 22.05 kHz provides a bandwidth of 56 Hz only, which will smear the te=
mporal details of the sound (temporal resolution =3D 18 ms). So, one should=
increase the bandwidth by selecting a smaller FFT size, a better window sh=
ape (FlatTop instead of Hamming) and/or by applying zero padding (reducing =
the frame size).
You might try this out with the free Avisoft-SASLab Light software:
http://www.avisoft.com/downloa_.htm or http://www.avisoft.com/avisoftdemo.e=
xe
Use the command =E2=80=9CAnalyze=E2=80=9D/=E2=80=9DSpectrogram Parameters..=
.=E2=80=9D and try different FFT and Frame sizes. The resulting Bandwidth i=
s displayed at the top of this dialog box.
Regards,
Raimund
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