[The effect of occlusion. Part I. The mechanism].

To clarify the mechanism of the occlusion effect the following experiments were performed on 18 healthy and 4 patients with conductive impairment: 1) measurement of the improvement in bone conduction threshold after the ears were covered with occluding devices of different volumes (earphones and calibrated spherical glass enclosures), 2) measurements of sound pressure level (SPL) changes in the meatus under the same conditions. The increase of SPL under covers was found, which was parallel to the improvement of bone conduction threshold in covered ears. Both phenomena were closely related to the volume of the enclosures: the larger the volume the smaller the change in SPL and in the threshold shift was found. In ears with a conductive loss--only the SPL was increased without any change in the bone conduction threshold. It was concluded that the occlusion effect is caused by the resonance phenomena in close meatus: the increased SPL induces the bone conducted threshold shift via the normal conductive apparatus (bone-air conduction). That explains the lack of threshold changes in conductive lesions. In the open meatus acoustic energy radiated by the bony walls becomes dissipated.