Middle ear transmission in cats with experimentally induced tympanic membrane perforations

A small perforation (2 mm2) was placed in the posterosuperior quadrant of the feline tympanic membrane (TM). Sound pressures (amplitudes and phases) required for a 10-μV round window cochlear microphonic (CM) output were measured at third-octave intervals from 200 to 4000 Hz (a) in front and behind the TM, (b) in open and closed sound systems, and (c) before and after perforation, as were the corresponding voltages across the transducer. Sound pressure (SP) changes in front of the TM after perforation revealed low-frequency losses, identical in shape and magnitude for both open and closed sound systems, that vary inversely with frequency at a rate of 12 dB/octave below 1600 Hz. The transducer voltage changes, concomitant with SP’s in front of the TM necessary to produce the criterion CM, paralleled the SP changes only in the open system. However, considerably larger voltage changes were found in the closed system, especially between 630 and 2000 Hz. Furthermore, from the SP’s measured on both sides of the TM, the mean effective sound pressure (SPeff) acting on the TM was calculated. The changes in SPeff, taken in reference to a constant CM output, demonstrate that a small TM perforation produces a 10-dB frequency-independent loss in transmission.