The Acoustical Impedance of the Guinea‐Pig Middle Ear and the Effects of the Middle‐Ear Muscles

The acoustical input impedance of the guinea‐pig middle ear was measured in the frequency range 100–10 000 Hz, using a highimpedance volume‐velocity source and a probe‐tube microphone. The impedance was measured both in the normal ear and with the tympanic membrane removed; the latter measurement permits accurate characterization of the middle‐ear cavities themselves. The data was compared to a slightly modified version of the middle‐ear model of Zwislocki [J. Acoust. Soc. Amer. 35, 1034–1040 (1963)], and new parameter values were calculated to match the present data. We also measured the time couraes of transient impedance changes caused by spontaneous contractions of the middle‐ear‐muscles of the anesthetized animals. These transient changes were compared to the middle‐ear‐muscle effects predicted by the model. [Supported by the Medical Research Council of Canada.]