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The first heart sound was studied in 20 normal subjects. The phonocardiogram (PCG) was recorded from apical and mid-precordial areas using microphone with a flat response curve from 0.2 to 8,000 Hz. It was stored, together with a simultaneous electrocardiogram, on an FM analog tape recorder (linear frequency response from 0 to 4,000 Hz), fitted with a filter with weighting curve B according to the American National Standard Institute. A linear (SPL) recording was also made. The signal was fed through a digital converter into a minicomputer and the frequency distribution of the first heart sound was analyzed using Fast Fourier Transform. These data were stored and the average spectra were calculated for both B and SPL. The SPL spectra from both apex and mid-precordium showed a maximum intensity of about 80 dB between 12-20 Hz, decreasing progressively to a constant level of 35 dB between 110-120 Hz. The spectra obtained from both areas using filter B showed a maximum intensity of 40-50 dB between 20-60 thereafter. It is important to emphasize that the dB values in B and SPL are absolute, since they refer to a standard reference weighting. It appears that the SPL recording is more valuable in that it allows the study of all components of the PCG signal. The spectra obtained in this study will be used as a standard for future research in various pathological conditions.

Phonometric study of the human first heart sound.