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The principles and technique of spectral phonocardiography have been described in detail in earlier publications from this laboratory (McKusick et al., 1954). The method provides detailed frequency-time-intensity plots of cardiovascular sound. It resembles in some respects the manner of operation of the auditory mechanism on these sounds. In essence the spectral phonocardiogram is made up of a large number of horizontal lines, each representing the output of an individual pass-band filter. In other words, in making the spectral phonocardiogram a selected segment of sound (in electrical form) is in effect put through a battery of a very large number of individual electrical filters each of which is tuned to a specific and different, although overlapping, pass-band.Spectral phonocardiography has the following advantages over conventional oscillographic phonocardiography (McKusick et al., 1955): (1) resolution in the time dimension is improved; (2) the wide intensity range of cardiovascular sound is better encompassed with simultaneous display of faint and loud components in their true proportions; (3) quality (timbre) is represented accurately; and (4) vibrations of non-cardiovascular origin are more easily identified. Spectral phonocardiography can do, or at least can be made to do, all the ear can. It can exceed the performance of the ear because (1) resolution in the time dimension is better, (2) it is not wed to one particular frequency response curve, (3) it is not handicapped by psycho-acoustic phenomena such as masking, and (4) permanent, quantifiable records are produced. METHODSSimultaneous recordings of heart sounds, electrocardiogram, and respiratory phase tracing were made on magnetic tape. The electrocardiogram and respiratory mark were recorded by means of frequency modulated carriers. A microphone (Altec 21-BR-150) of the condenser type was employed for pickup and transducing purposes. As a rule, quiet respiration was permitted during the recordings which were routinely made at the aortic area, at the pulmonary area, at the lower left sternal border (4th interspace), and at the apex. The tape recordings, raw data for the studies, were audited and selected sections analysed on the modified Bell sound spectrograph. In the spectral phonocardiogram the time dimension is on the abscissa as in most physiological recordings. The ordinate is frequency spectrum, not intensity. Intensity is represented by the degree of blackness of any given portion of the recording.

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