MEASUREMENTS OF THE ACOUSTIC INPUT IMPEDANCE OF CAT EARS - 10-HZ TO 20-KHZ

The acoustic input impedance of the ear is a useful measure of the beh avior of the middle ear and of its effect on the acoustics of the exte rnal ear. A high-impedance acoustic source with an integral microphone was designed for acoustic-impedance measurements. The source's Norton equivalent circuit was determined from measurements of the sound pres sure it generated in known acoustic loads. Tests on simple acoustic co nfigurations show errors in impedance measurements of less than 10% in magnitude and 7 degrees in angle over a frequency range from 10 Hz to 10 kHz with increasing errors at higher frequencies. Measurements at the tympanic membrane (TM) on five cat ears with widely opened middle- ear cavities show an impedance that is compliance-like below 0.3 kHz a nd approximately resistive above 2 kHz. With the cavities intact the i mpedance magnitude is somewhat larger for low frequencies, has a sharp maximum near 4 kHz, and at the highest frequencies is little affected by the state of the cavities. Impedance magnitude varies among ears b y a factor of 3. The pressure reflection-coefficient that is determine d from the impedance is frequency dependent with magnitude between 0.2 and 1. To characterize the motion transformation of the TM we calcula te the ratio of tympanic-membrane volume velocity to the velocity of t he mallear umbo, called here the kinematic area A(TK). This complex qu antity is constant with an angle of zero for frequencies below 0.6 kHz , but at higher frequencies both magnitude and angle of A(TK) vary wit h frequency.