| The middle ear impedance expresses the relationship between input sound pressure at the tympanic membrane and the stapes velocity as a function of frequency. It is a quantitative description of how the output velocity is related to the input pressure. Systematic investigation of middle ear mechanical properties requires the presentation or measurement of very small displacements within the ossicular chain. The goal in the present research is to characterize the specific acoustic impedance (SAI) of the middle ear in the 14 day old post hatch chicken and in the chicken embryo at 19 days of incubation for each of 8 frequencies ranging from 100 to 4000 Hz. The procedure of this study was as follows. First, we calibrated a piezoelectric pusher for use as a direct mechanical driver of the stapes footplate (FP) using a laser interferometer, microphone, accelerometer, and LVDT. Second, we measured the particle velocity of the inner ear at the FP and sound pressure at the tympanic membrane (TM) as a function of frequency at both ages. From these measurements, we determined the SAI of the middle ear at two ages. In the piezoelectric driver calibration experiment, there was a relationship between the displacement, the frequency and the applied voltage for small displacements ranging from 0.2 to 300 nm p-p, and the displacement was linearly related to the applied control voltage. A mechanical vibration of known velocity was applied to the FP, and the relationship between the FP velocity and the CM amplitude was determined over a 60 dB intensity range. In direct driving measurements, the cochlear microphonic (CM) amplitude was robust for both hatchling and embryonic chickens, but embryonic CM was generally much smaller than that for the hatchling. FP velocity measurements during sound stimulation provide a basis for calculating the specific acoustic impedance (SAI) of the middle ear. We estimated FP velocity at two age groups using CM recordings calibrated to reflect FP velocity. In each anesthetized animal, acoustic stimuli were delivered to the acoustic meatus from an open field source. The SAI was calculated as the applied sound pressure (dyne/cm2) divided by particle velocity (cm/sec). At a given frequency, the SAI was comparable at all stimulus levels. The SAI for a hatchling ranged from 4244.2 ± 2487.31 dyne·sec/cm 3 (at 100 Hz) to 296.69 ± 221.20 dyne·sec/cm 3 (at 4000 Hz) and for embryo from 3296.64 ± 3022.59 dyne·sec/cm 3 (at 100 Hz) to 1209.40 ± 496.10 dyne·sec/cm 3 (at 4000 Hz). Instead of a progressive decrease in SAI, there was a substantial increase in the SAI with increasing frequency in embryos. We concluded from the data that a substantial restriction in the transfer of sound occurs at high frequencies, particularly those above 1000 Hz in embryos compared to post hatch animals. |
