The acoustic impedance locus for normal human ears.

The ear's complex acoustic impedance locus for frequencies from 200 Hz to 2 kHz was obtained for 20 subjects screened for normal hearing. The purpose of the experiment was to observe "kinks" in the total driving point impedance locus of the ear, as measured by acoustic impedance in the ear canal at a depth comparable to that of clinical audiometry. Such impendance bumps or loops are expected to contribute to irregularities seen in high-frequency clinical tympanograms. The transducer probe consisted of two 1/8 inch condenser microphones inserted in the ear canal. The instrumentation used a continuous sinusoidal probe tone sweeping the frequency range in 90 seconds. Analog circuitry performed synchronous detection and filtering and provided the resistance and reactance of the impedance derived from the sound pressure measured in the ear canal. Data were recorded in the form of an impedance locus in the complex plane. These loci demonstrated kinks suggesting middle ear resonances, at 604 +/- 99 Hz, 986 +/- 94 Hz and 1 361 +/- 87 Hz. Ninety-seven percent of the ears demonstrated a single kink while 78% displayed two. In addition, a group of 5 normal subjects were measured 5 times. The variation within subjects was 5 to 10% of the mean impedance. It is concluded that there are resonances within the middle ear structure that manifest themselves as easily observed bumps or loops in the ear's impedance locus.