Analyses of the Tympanic Membrane Impulse Response Measured with High-Speed Holography.

The Tympanic Membrane (TM) transforms acoustic energy to ossicular vibration. The shape and the displacement of the TM play an important role in this process. We developed a High-speed Digital Holography (HDH) system to measure the shape and transient displacements of the TM induced by acoustic clicks. The displacements were further normalized by the measured shape to derive surface normal displacements at over 100,000 points on the TM surface. Frequency and impulse response analyses were performed at each TM point, which enable us to describe 2D surface maps of four new TM mechanical parameters. From frequency domain analyses, we describe the (i) dominant frequencies of the displacement per sound pressure based on Frequency Response Function (FRF) at each surface point. From time domain analyses, we describe the (ii) rising time, (iii) exponential decay time, and the (iv) root-mean-square (rms) displacement of the TM based on Impulse Response Function (IRF) at each surface point. The resultant 2D maps show that a majority of the TM surface has a dominant frequency of around 1.5 kHz. The rising times suggest that much of the TM surface is set into motion within 50 µs of an impulsive stimulus. The maps of the exponential decay time of the IRF illustrate spatial variations in damping, the least known TM mechanical property. The damping ratios at locations with varied dominant frequencies are quantified and compared.

Effects of middle-ear static pressure on pars tensa and pars flaccida of gerbil ears.

It has long been known that static pressure affects middle-ear function and conventional tympanometry uses variations in static pressure for clinical assessment of the middle ear. However, conventional tympanometry treats the entire tympanic membrane as a uniform interface between the external and middle ear and does not differentiate the behavior of the two components of the tympanic membrane, pars tensa and pars flaccida. To analyze separately the different acoustic behavior of these two tympanic membrane components, laser Doppler velocimetry is used to determine the motion of each of these two structures. The velocities of points near the center of p. tensa and p. flaccida in response to the external-ear sound pressure at different middle-ear static pressures were measured in nine gerbil ears. The effect of middle-ear static pressure on the acoustic response of both structures is similar in that non-zero middle-ear static pressures generally reduce the velocity magnitude of the two membrane components in response to sound stimuli. Middle-ear under-pressures tend to reduce the velocity magnitude more than do middle-ear over-pressures. The acoustic stiffness and inertance of both p. tensa and p. flaccida are altered by static pressure, as shown in our results as changes of transfer-function phase angle. Compared to p. tensa, p. flaccida showed larger reductions in the velocity magnitude to small over- and under-pressures near the ambient middle-ear pressure. This higher pressure sensitivity of p. flaccida has been found in all ears and may link the previously proposed middle-ear pressure regulating and the acoustic shunting functions of p. flaccida. We also describe, in both p. tensa and p. flaccida, a frequency dependence of the velocity measurements, hysteresis of velocity magnitude between different directions of pressure sweep and asymmetrical effects of over- and under-pressure on the point velocity.

Acoustic responses of the human middle ear.

Measurements on human cadaver ears are reported that describe sound transmission through the middle ear. Four response variables were measured with acoustic stimulation at the tympanic membrane: stapes velocity, middle-ear cavity sound pressure, acoustic impedance at the tympanic membrane and acoustic impedance of the middle-ear cavity. Measurements of stapes velocity at different locations on the stapes suggest that stapes motion is predominantly 'piston-like', for frequencies up to at least 2000 Hz. The measurements are generally consistent with constraints of existing models. The measurements are used (1) to show how the cavity pressure and the impedance at the tympanic membrane are related, (2) to develop a measurement-based middle-ear cavity model, which shows that the middle-ear cavity has only small effects on the motion of the tympanic membrane and stapes in the normal ear, although it may play a more prominent role in pathological ears, and (3) to show that inter-ear variations in the impedance at the tympanic membrane and the stapes velocity are not well correlated.

Middle ear pathology can affect the ear-canal sound pressure generated by audiologic earphones.

OBJECTIVE: To determine how the ear-canal sound pressures generated by earphones differ between normal and pathologic middle ears. DESIGN: Measurements of ear-canal sound pressures generated by the Etymtic Research ER-3A insert earphone in normal ears (N = 12) were compared with the pressures generated in abnormal ears with mastoidectomy bowls (N = 15), tympanostomy tubes (N = 5), and tympanic-membrane perforations (N = 5). Similar measurements were made with the Telephonics TDH-49 supra-aural earphone in normal ears (N = 10) and abnormal ears with mastoidectomy bowls (N = 10), tympanostomy tubes (N = 4), and tympanic-membrane perforations (N = 5). RESULTS: With the insert earphone, the sound pressures generated in the mastoid-bowl ears were all smaller than the pressures generated in normal ears; from 250 to 1000 Hz the difference in pressure level was nearly frequency independent and ranged from -3 to -15 dB; from 1000 to 4000 Hz the reduction in level increased with frequency and ranged from -5 dB to -35 dB. In the ears with tympanostomy tubes and perforations the sound pressures were always smaller than in normal ears at frequencies below 1000 Hz; the largest differences occurred below 500 Hz and ranged from -5 to -25 dB. With the supra-aural earphone, the sound pressures in ears with the three pathologic conditions were more variable than those with the insert earphone. Generally, sound pressures in the ears with mastoid bowls were lower than those in normal ears for frequencies below about 500 Hz; above about 500 Hz the pressures showed sharp minima and maxima that were not seen in the normal ears. The ears with tympanostomy tubes and tympanic-membrane perforations also showed reduced ear-canal pressures at the lower frequencies, but at higher frequencies these ear-canal pressures were generally similar to the pressures measured in the normal ears. CONCLUSIONS: When the middle ear is not normal, ear-canal sound pressures can differ by up to 35 dB from the normal-ear value. Because the pressure level generally is decreased in the pathologic conditions that were studied, the measured hearing loss would exaggerate substantially the actual loss in ear sensitivity. The variations depend on the earphone, the middle ear pathology, and frequency. Uncontrolled variations in ear-canal pressure, whether caused by a poor earphone-to-ear connection or by abnormal middle ear impedance, could be corrected with audiometers that measure sound pressures during hearing tests.

Acoustic input impedance of the stapes and cochlea in human temporal bones.

The acoustic input impedance of the stapes and cochlea ZSC represents the mechanical load driven by the tympanic membrane, malleus and incus. ZSC was calculated from broad-band measurements (20 Hz to 11 kHz) of stapes displacement made with an optical motion sensor and of sound pressure at the stapes head in a human temporal-bone preparation. Measurements were made in 12 fresh temporal bones with the round window insulated from the sound stimulus. Below 1 kHz, the magnitude of ZSC was approximately inversely proportional to frequency, and ZSC angle was between 0.10 and -0.20 periods. This behavior is consistent with a mixed stiffness and resistance. Between 1 and 4 kHz, ZSC was resistance-dominated with a magnitude between 40 and 100 mks acoustic G omega that was roughly independent of frequency, and its angle was between -0.12 and 0 periods. Between 4 and 7 kHz, the magnitude of ZSC was either constant or increased with frequency while ZSC angle was near 0. Between 7 and 8 kHz, both ZSC magnitude and angle decreased sharply with frequency, and both increased somewhat at higher frequencies. The input impedance of the cochlea ZC was estimated in one ear from ZSC measurements made before and after draining the inner ear fluids. ZC was stiffness-dominated below 100 HZ, and resistance-dominated from 100 Hz to 5 kHz. The frequency-dependent magnitude of ZSC in our bones is similar to those reported by other investigators in cadaver temporal bones (Nakamura et al., 1992; Kurokawa and Goode, 1995). Our ZSC measurements are qualitatively similar to theoretical predictions (Zwislocki, 1962; Kringlebotn, 1988), but are a factor of 3 greater in magnitude, implying that ZSC may be more resistive and stiffer than previously thought. We found inter-ear variations of a factor of 4 (12 dB), which may explain some of the clinically observed variations in size of the air bone gap in individuals with middle ear lesions or after middle-ear reconstructive surgery.

Cochlear nonlinearities inferred from two-tone distortion products in the ear canal of the alligator lizard.

Distortion products ( DPs ) evoked by two-tone stimuli at frequencies F1 and F2 were measured in the ear-canal sound pressure of the alligator lizard. The largest sound pressures measured, other than those at F1 and F2, where at the cubic difference frequencies 2F1-F2 and 2F2-F1. All cubic DPs were greatly reduced by destruction of the basilar membrane, which suggests that its nonlinear properties are the source of the DPs . Measurements following acoustic overstimulation show a complex relationship between the magnitude of DPs and cochlear state, as assessed by measurements of cochlear potential, and indicate the existence of multiple nonlinear sources within the inner ear. Relative magnitudes of the DPs and their dependence on stimulus level suggest that the inner-ear DP sources are cubic nonlinearities. The DPs are not highly sensitive to either average stimulus frequency or stimulus frequency separation, suggesting that the nonlinear processes are within the macromechanical processes of the inner ear. Contrary to some interpretations of ear-canal DP measurements in mammals, we conclude that DPs need not be associated with hair-cell processes and are not particularly useful indicators of cochlear health.