reference purposes. Our methodology, however, makes it convenient to delay 

 further spectral analyses until particular sounds are encountered during 

 playback procedures. 



Spectral Analyses 



The sound spectrum analyzing system described in a previous section is capable 

 of producing visual displays (sonograms) of sounds fed into it from previously 

 recorded tapes. A choice of two types of sonograms is available. A standard 

 sonogram. Figure 3, provides a representation of the sound structure in terms 

 of frequency on the vertical axis and time on horizontal. Amplitude is 

 related to the degree of darkening of the marks produced on the paper but 

 cannot be determined with any certainty. 



A second type of sonogram. Figure 4A, provides a contour display with contour 

 outlines spaced at six decibel increments. The darkest of the seven levels is 

 the region of greatest amplitude and the overall dynamic range is 42 decibels. 

 Absolute acoustic magnitudes are not attainable without some point of refer- 

 ence and a calibration procedure which we have not undertaken. Relative ampli- 

 tudes, however, are readily detected and using the ever present background 

 noise as a rough base, some idea of the amplitude of the sounds relative to 

 background noise is attainable. Care must be exercised here, however, because 

 the distance of the subject from the hydrophone during recording has a profound 

 effect upon the relative intensity and apparent quality of a sound as detected 

 by the listener or recorder. This becomes quite apparent in the case of stac- 

 catos and cooing sounds of certain squirrelf ishes of the genus Holocentrus . 



Background Noise 



Snapping, crackling, and frying pan noises: 



The sonograms in Figure 5A and B are of special interest in a consideration of 

 the nature of the pervasive and ever present background snapping and crackling 

 which occurs in the vicinity of coral reefs. This sonogram represents a waxing 

 sequence of innumerable snapping sounds which, when perceived together, create 

 the impression of a piece of cold bacon being flung into a pan of hot grease. 

 This apparent flush of background noise lasts for a few seconds after which the 

 more stable but less intense level is regained. What the sound appears to be 

 is an increase in the frequency of occurrence and to some extent the amplitude 

 of the background snaps. Analysis of the frying pan noise emphasizes and clari- 

 fied to some extent the nature of the snapping. Two predominant modes in ampli- 

 tude are evident, one at around 3500 Hz, and the other at around 5500 Hz. When 

 sonograms made during the analyses of other sounds are examined, it is found in 

 almost all cases that the majority of the background noise is distributed around 

 these frequencies. A more dramatic representation which illustrates the two 

 amplifying modes of the frying pan noise quite clearly appears in Figure 5B. 

 This sonogram was produced by analyzing the noise at a very low gain setting, 

 thereby reproducing only those components of the sounds which were highest in 

 energy content. 



Two individual snaps appear in Figure 3A along with a pop from a reef squirrel- 

 fish. Consideration of the plots of the two snaps indicates that the bimodal 

 nature of the background noise is a result of bimodality in individual snaps 



VI-86 



