A new approach to the measurement of middle-ear impedance was described by Allen (1986). He demonstrated that middle-ear impedance measurements were obtainable in animals out to 33 kHz. Later, Keefe et al. (1992) and Voss and Allen (1994) presented methods for measuring middle ear impedance in humans based on this approach at frequencies as high as 10 kHz.
These impedance measurements were made possible by using a novel calibration method that calculates the impedance and sound-pressure characteristics of the probe itself. Once these (Thevenin) values of the probe system are known, the impedance of the ear can be estimated based on the measured sound pressure levels generated by a wideband signal (click or chirp) in the ear canal.
A comparison of the wideband impedance at the probe tip and the characteristic impedance of the ear canal can be used to estimate the pressure reflected from the middle ear. Energy reflectance may then be calculated as the square of the pressure reflectance of the ear.
Figure 3 [PDF] shows that energy reflectance is the ratio of sound power reflected from the middle ear to the incident power presented to the ear canal represented by the blue and red arrows, respectively. The green arrow represents the energy absorbed by the middle ear or the Energy Transmittance, which = 1- Energy Reflectance. If an ear has excessively high impedance and all the incident power is reflected from the middle ear, the ratio of reflected to incident power will be 1.0. If all of the incident energy is absorbed by the middle ear, the energy reflectance is 0.0. Thus in a system like the middle ear, which is essentially linear, energy reflectance can range from 0.0. to 1.0.
Adult Normative Data
Mean adult energy reflectance at ambient canal pressure is shown in Figure 4 [PDF] for 40 young adults at frequencies from 250 to 8000 Hz. These data were obtained in our lab using the Keefe reflectance system. The shaded area represents the 5th to the 95th percentile of the adult data. The mean energy reflectance is near 1.0 in the low frequencies, but decreases as a function of frequency, reaching a minimum of 0.2 near 4000 Hz and then increasing at higher frequencies. The minimum in the energy reflectance represents a frequency region of maximum energy flow to the middle ear.
One promising application of energy reflectance is in the evaluation of middle ear disorders. Energy reflectance data from two ears with middle ear disorders are shown in Figure 5 [PDF]. The shaded area represents the 5th to the 95th percentile of young adult data from Figure 4 [PDF]. One ear has otosclerosis, in which the stapes is fixed, resulting in a 40 dB conductive hearing loss (solid line), and the second ear has a disarticulation of the ossicular chain resulting in a 60 dB conductive hearing loss (dashed line, from Feeney, Grant, & Marryott, 2003). Note that in the case of otosclerosis, the energy reflectance is higher than the normal range in the low frequencies, but within the normal range at higher frequencies. However, the ear with the disarticulation has a very different pattern of energy reflectance with a deep notch around 700 Hz. Both ears had normal 226 Hz admittance tympanograms, suggesting that wideband energy reflectance at ambient pressure may be more sensitive to middle ear disorders than 226 Hz tympanometry. Energy reflectance may also prove helpful in sorting out middle-ear disorders in infants undergoing newborn hearing screening (Keefe, et al. 2000).
Another application of wideband reflectance techniques that holds promise is the measurement of the acoustic reflex. We have found that contralateral acoustic reflex thresholds are around 12 dB more sensitive when measured with the wideband reflectance method compared to a 226 Hz probe tone (Feeney, Keefe, & Marryott, 2003).
The development of commercial wideband reflectance systems is ongoing. We expect these methods to be integrated with clinical systems by the end of the decade.