User's Manual - Cornell Lab of Ornithology - Cornell University

Chapter 7: CorrelationSpectrogram correlationsInterpretingspectrogramcorrelationsThe peak value of a spectrogram correlation function can provide anobjective, well-defined, repeatable, and comparable measure of the similarityof two spectrograms. (The correlation measurement panel can be configuredto display the peak value in a selected part of a correlation function; seeChapter 6.) However, care should be exercised in the interpretation ofspectrogram correlations because the type of “similarity” that theymeasure is simple and narrowly defined, and may or may not beappropriate to the research question being asked.Spectrogram correlation is not a tool for generalized “pattern recognition”.The ranking of similarities among a group of spectrograms based on peakcorrelations may conflict with a human’s subjective ranking based either onlistening to the sound or on examining the spectrograms. For example,spectrogram correlations are insensitive to similarities in “shape” of twospectrograms if they differ in frequency, but such similarities are striking to ahuman observer. Figure 7.8 shows correlations between two pairs ofsyllables from within the song of a painted bunting. To a human, syllables 2and 6 look and sound more “similar” than syllables 2 and 5 because theyhave a more similar pattern of frequency modulation. But because thefrequency ranges of syllables 2 and 6 barely overlap, their peak correlation ismuch lower than that between syllables 2 and 5, which overlap moreextensively in frequency (even though they differ more in pattern offrequency modulation).(a)1 2 3 4 5 6 7(b)syllable 2 * syllable 6(peak = .294)(c)syllable 2 * syllable 5(peak = .549)116 Canary 1.2 User’s Manual