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Phonocardiographic instrumentation tests using a square wave of sound pressure reveal differentiation spikes at the output of a dynamic microphone and at the amplifier-filter during initial onset of the test pulse. When the pulse reaches the plateau level, ringing oscillations can be observed in the amplifier-filter combination. Microphone differentiation and filter ringing are also present at removal of the test pulse. These artifacts can be observed as a frequency distortion at the beginning and end of the first and second heart sounds in the normal phonocardiogram. By observing the characteristics of these artifacts investigators and clinicians may be assisted in improving their understanding of heart sound recordings. It is suggested that clinicians use the square wave test to determine the distortion unique to their equipment.