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

Chapter 7: CorrelationLogarithmic vs.quadraticamplitude axisSpectrogram correlations can be performed using spectrograms with eitherlogarithmic or quadratic amplitude axes (henceforth referred to aslogarithmic or quadratic spectrograms), as long as both spectrograms are ofthe same type. Although logarithmic spectrograms are often more visuallyinformative because of their wide dynamic range, quadratic spectrogramsmay sometimes be more appropriate for purposes of correlation. If quadraticspectrograms are used, the values being correlated at each point in time arethe spectrum intensities (in watts/m 2 /Hz) at each frequency; if logarithmicspectrograms are used, the values being correlated are the spectrum intensitylevels (in dB). 1 For spectrograms whose peak normalized correlations areclose to 1, the two types of spectrograms yield very similar results. (Foridentical signals, of course, either type of spectrogram yields a correlation of1.) For any given pair of signals, the correlation values from quadraticspectrograms are often lower than those from logarithmic spectrograms(Figure 7.9).10.80.6Peak correlationQuadraticspectrograms0.40.200 0.2 0.4 0.6 0.8 1Peak correlationLogarithmic spectrogramsFigure 7.9. Peak normalized correlation coefficients for spectrograms of 78pairs of signals. One point is plotted for each pair of signals, showing the peakvalues when quadratic versus logarithmic spectrograms were correlated. Thediagonal is the line of equal peak correlations.In a given set of signal pairs being correlated, the rank orderings of peakcorrelation coefficients obtained using quadratic and logarithmicspectrograms are not necessarily exactly the same, although they are generally1 If the electric calibration paradigm is selected, the values that are correlated arespectrum powers (in watts/Hz) or spectrum power levels (in dB).118 Canary 1.2 User’s Manual