Since hearing aid receiver acoustical output is commonly measured by an artificial ear wherein a microphone replaces the eardrum, the measurement yields data only on nominal pressure at the eardrum. A correction factor must be applied to the artificial ear output to relate it to the free field performance of the ear (which is the norm). In other words, substituting pressure operation for free field operation of the human ear changes its frequency response, and this change must be allowed for in determining the corrective characteristics of a hearing aid. The variation is computed and is then compared with subjective test results. The sum of this correction and one for the obstacle effect of the human body wearing the hearing aid transmitter are then determined and it is concluded that such an over‐all corrections so great that it cannot be neglected in determining or comparing hearing aid characteriistics. The over‐all correction is then applied to representative instrument characteristics.