Middle-ear function in the guinea pig

Abstract

The velocity and displacement of parts of the guinea pig middle ear were measured over a broad frequency range using the Mössbauer technique. Measurements under free-field/closed-bulla conditions gave a displacement curve with a slope of 6 dB/octave at frequencies above 3 kHz, breaking to a much steeper slope (30–40 dB/octave) above 25 kHz. Under closed sound-system/open-bulla conditions, the slope beyond 0.5 kHz was 7–8 dB/octave. These data indicate that a high-frequency displacement slope of 12 dB/octave (as used in mass-limited middle-ear models) does not exist in the guinea pig. Up to 25 kHz, the incus displacement is about 5 dB below the malleus, but larger differences exist at higher frequencies. The relative phase of the incus is near 0° to 6 kHz, then shows an increasing phase lag. The fusion of malleus and incus thus preserves an efficient ossicular linkage to high frequencies. At low frequencies, highest eardrum displacements are in the inferior portion, but above 15 kHz are on or near the malleus. At low frequencies, isoamplitude contours indicate that the drum does not vibrate like a stiff plate.