spectral phonocardiograph

spec·tral pho·no·car·di·o·graph

an instrument for recording the heart sounds in which the electrical changes created by the latter pass from a microphone through a series of filters, each of which is tuned to a particular frequency band; output from each filter activates a separate light source of brightness proportional to the intensity of the sound transmitted through that filter; the lights are arranged vertically in descending order of frequencies. A record is obtained by photographing the vertical row of lights.
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