June 29, 2002; RWB, MH. Two tests are shown here. One is to use only the signal from hydrophone #1, and to compare the resulting spectrogram with that obtained using the coherent sum of the four hydrophones, which has been the standard up till now. RWB justifies adding the hydrophone signals with the vague argument that while this does produce interference between hydrophones, that the randomness of the phase will result in the canceling out of the effects. MH argues that the interference may be coherent over long enough intervals to produce annoying variability in the response, and he backs this up with his observation that the signals from the humpback whales seem broken up in a way which would correspond to the interference going in and out of phase as the frequency varies. The effect for blue whales might be less obvious, since they are so dumb and just call at absolutely constant frequencies.

The result of this test (first try) is that the contrast seems better with only one hydrophone. The comparison is between (ch1 +ch2 + ch3 + ch4)/4 and ch1 alone, which would make the ch1 signal bigger in most cases, especially since it seems to have the largest gain of the four hydrophone channels. It seems that there is little to lose by using ch1 for the study of blue-whale calls. We will probably look at a full day's (day 255) data before drawing a final conclusion.

The two left-hand images show the result of passing a 8-kHz version of the signal through a wavelet filter which leaves only the d6 scale. We thought that this would select the 90-Hz component of the blue-whale call, but it seems instead to be selecting the lower-frequency component. Further work on this filtering scheme is needed and is underway.

beam formed signal	beam-formed signal, d6 scale (sampling at 8000 Hz), DB10 wavelet
hydrophone 1 only	hydrophone 1, d6 scale (sampling at 8000 Hz), DB10 wavelet