Proposed method of evaluating the pressure correction made necessary by diffraction.—The diffraction of sound around the diaphragm of the microphone ordinarily used in the measurement of the instantaneous pressure in a sound wave causes the indicated pressure to vary from equality with the actual pressure in the undisturbed wave at low frequencies, to twice this pressure at high frequencies. Because of the mathematically irregular shape of the conventional microphone and its mounting the effect cannot be calculated. It is proposed to evaluate the correction for diffraction by employing a standard spherical mounting of which the diaphragm occupies a small area about the pole; the increase in pressure for this mounting can be calculated theoretically, and the correction for other mountings can then be obtained by experimental comparison.