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Cardiographs designed for recording heart sounds at the thoracic body surface. These instruments consist of a main unit that includes a recorder, electroacoustic transducers, signal processors, and detachable probes that include audio sensors (e.g., microphones) at the distal tip that are fixed to anatomic areas on the thorax. The action of heart muscle causes low-frequency sounds, typically in the range of the audio spectrum that are detected at the body surface and transmitted to the recorder which is capable of reproducing their characteristics in an amplitude versus time graph (i.e., a phonocardiogram). Phonocardiographs are used to assess low-frequency sounds (associated with atrial and ventricular gallops) and also relatively higher frequency sounds (associated with mitral regurgitation and ventricular septal defect). The devices can assess both normal sounds (known as S1 and S2 sounds) and abnormal sounds (known as S3 and S4) whose presence may indicate heart dysfunctions*. Some phonocardiographs are capable of recording signals at frequencies below the threshold of the audio spectrum (i.e., subsonic, nonacoustic vibrations) that are mainly produced by the apex of the heart (apexcardiography); the phonocardiographs may also include as an integral unit a single-channel electrocardiograph with appropriate electrodes to work as a reference for the phonocardiogram.
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