Non-Ossicular Signal Transmission in Human Middle Ears: Experimental Assessment of the Acoustic Route with Perforated Tympanic Membranes

Direct acoustic stimulation of the cochlea by the sound-pressure difference between the oval and round windows (called the acoustic route ) has been thought to contribute to hearing in some pathological conditions, along with the normally dominant ossicular route. To determine the efficacy of this acoustic route and its constituent mechanisms in human ears, sound pressures were measured at three locations in cadaveric temporal bones [with intact and perforated tympanic membranes (TMs)]: (1) in the external ear canal lateral to the TM, PTM; (2) in the tympanic cavity lateral to the oval window, POW; and (3) near the round window, PRW. Sound transmission via the acoustic route is described by two concatenated processes: (1) coupling of sound pressure from ear canal to middle-ear cavity, H PCAV ≡ PCAV PTM, where PCAV represents the middle-ear cavity pressure, and (2) sound-pressure difference between the windows, HWPD ≡ (POW - PRW) PCAV. Results show that: H PCAV depends on perforation size but not perforation location; HWPD depends on neither perforation size nor location. The results (1) provide a description of the window pressures based on measurements, (2) refute the common otological view that TM perforation location affects the relative phase of the pressures at the oval and round windows, and (3) show with an intact ossicular chain that acoustic-route transmission is substantially below ossicular-route transmission except for low frequencies with large perforations. Thus, hearing loss from TM perforations results primarily from reduction in sound coupling via the ossicular route. Some features of the frequency dependence of H PCAV and HWPD can be interpreted in terms of a structure-based lumped-element acoustic model of the perforation and middle-ear cavities

Communications Biophysics

Contains a report on a research project.National Institutes of Health (Grant MH-04737-02)National Science Foundation (Grant G-16526

Acoustic Mechanisms that Determine the Ear-Canal Sound Pressures Generated by Earphones

In clinical measurements of hearing sensitivity, a given earphone is assumed to produce essentially the same sound-pressure level in all ears. However, recent measurements [Voss et al., Ear and Hearing (in press)] show that with some middle-ear pathologies, ear-canal sound pressures can deviate by as much as 35 dB from the normal-ear value; the deviations depend on the earphone, the middle-ear pathology, and frequency. These pressure variations cause errors in the results of hearing tests. Models developed here identify acoustic mechanisms that cause pressure variations in certain pathological conditions. The models combine measurement-based Thevenin equivalents for insert and supra-aural earphones with lumped-element models for both the normal ear and ears with pathologies that alter the ear\u27s impedance (mastoid bowl, tympanostomy tube, tympanic-membrane perforation, and a \u27high- impedance\u27 ear). Comparison of the earphones\u27 Thevenin impedances to the ear\u27s input impedance with these middle-ear conditions shows that neither class of earphone acts as an ideal pressure source; with some middle-ear pathologies, the ear\u27s input impedance deviates substantially from normal and thereby causes abnormal ear-canal pressure levels. In general, for the three conditions that make the ear\u27s impedance magnitude lower than normal, the model predicts a reduced ear-canal pressure (as much as 35 dB), with a greater pressure reduction with an insert earphone than with a supra-aural earphone. In contrast, the model predicts that ear-canal pressure levels increase only a few dB when the ear has an increased impedance magnitude; the compliance of the air-space between the tympanic membrane and the earphone determines an upper limit on the effect of the middle-ear\u27s impedance increase. Acoustic leaks at the earphone-to-ear connection can also cause uncontrolled pressure variations during hearing tests. From measurements at the supra-aural earphone-to-ear connection, we conclude that it is unusual for the connection between the earphone cushion and the pinna to seal effectively for frequencies below 250 Hz. The models developed here explain the measured pressure variations with several pathologic ears. Understanding these mechanisms should inform the design of more accurate audiometric systems which might include a microphone that monitors the ear-canal pressure and corrects deviations from normal

Communications Biophysics

Contains reports on two research projects.National Institutes of Health (Grant 5 P01 GM14940-05

Communications Biophysics

Contains reports on two research projects

Communications Biophysics

Contains reports on three research projects.United States Air Force (Contract AF19(602)-4112

Communications Biophysics

Contains a summary of research publications and reports on four research projects.National Science Foundation (Grant GP-2495)National Institutes of Health (Grant MH-04737-04)National Aeronautics and Space Administration (Grant NsG-496

Communications Biophysics

Contains reports on five research projects.National Institutes of Health (Grant 5 PO1 GM14940-03)Joint Services Electronics Programs (U. S. Army, U.S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E)National Aeronautics and Space Administration (Grant NGL 22-009-304

Communications Biophysics

Contains reports on three research projects

Communications Biophysics

Contains reports on four research projects.U.S. Air Force under Contract AF19(604)-411