On the influence of the middle ear muscles upon changes in sound transmission.

The influence of artificially induced impedance changes on the frequency response of the middle ear has been investigated in three experienced listeners. The tensor-reflex has been simulated by application of positive and negative air-pressure to the outer ear canal. In this test-situation, the thresholdshift ("l) obtained is greatest in the surrounding of the first resonance of the middle ear ("l = 8 dB at 500 Hz). For bone-conduction, the effect is weaker by about 2 dB than for air-conduction. The change in sound transmission is constant for all SPL's and is mainly due to the impedance change of the middle ear. In contrast the influence of the stapedial reflex is strongly dependent on SPL. In the range between hearing-threshold and 70 dB (HL) the attenuation is only 2 dB below l kHz. At higher levels the attenuation may amount to 30 dB. Thus excitation of the cochlea remains nearly constant. We suppose that the intensity-dependent influence of the stapedius reflex on sound transmission is due to a change of the stapes motion. The regulating device works without considerable distortion but with an attack-time of about 100 ms. The phenomenon of conductive recruitment in otosclerosis is probably due to the lack of this regulating effect by the fixed stapes. One function of the stapedial reflex could be the protection of the ear from high amplitudes during phonation and shouting. Furthermore, because frequencies above l kHz are also attenuated, a protective function of the stapedial reflex in industrial noise exposure can be assumed.