D2 Tympanometry: Function of the Middle Ear

Topic

The tympanometer creates an airtight seal in the external ear canal using a probe and, through that same probe, emits a fixed-frequency test tone toward the tympanic membrane—around 226 Hz for adults, or a higher frequency near 1,000 Hz for newborns. This is because a newborn's external ear canal and middle ear are softer and more compliant than an adult's; consequently, a low-frequency test tone cannot distinguish the specific contribution of the tympanic membrane and ossicular chain from that of the surrounding structures in such a soft ear.

While the probe continuously emits the test tone, the tympanometer also gradually alters the pressure within the sealed canal, sweeping through a range that goes from a pressure higher than that of the middle ear to one lower than that of the middle ear, and back again. At each pressure level within the sealed canal, the tympanometer measures how much of the test tone's energy passes through the tympanic membrane and ossicular chain rather than reflecting back toward the probe; this amount of energy passing through the tympanic membrane is known as acoustic admittance.

The acoustic admittance measured at each pressure level depends on the difference between the pressure in the sealed canal and the pressure in the middle ear—that is, the air pressure behind the tympanic membrane, on the side facing away from the probe. When the pressure in the sealed canal equals the pressure in the middle ear, the tympanic membrane is not being pushed in any particular direction and vibrates with maximum freedom; thus, acoustic admittance reaches its peak value at that exact point. The greater the difference between the sealed-canal pressure and the middle-ear pressure—in either direction—the more stretched and rigid the tympanic membrane becomes, causing acoustic admittance to drop below its peak value.

By sweeping through the entire range of sealed-canal pressures and recording acoustic admittance at each point, the tympanometer plots a single-peaked curve; this peak encapsulates, in two data points, everything the model has described so far. The peak's position along the sealed-canal pressure axis indicates the middle-ear pressure, as the peak occurs exactly where the sealed-canal pressure matches the middle-ear pressure, thereby eliminating the pressure differential. Conversely, the peak's height indicates maximum compliance: the highest level of acoustic admittance achieved by the tympanic membrane and ossicular chain when vibrating with maximum freedom, free from any stretching pressure differential.

Since both the peak's position and height depend exclusively on the behavior of the tympanic membrane and ossicular chain under varying pressures, the tympanometer provides an objective measure of middle-ear function based on this single peak: a middle-ear pressure far outside the normal range, or abnormally high or low maximum compliance, indicates a deviation from normal behavior in the tympanic membrane, the ossicular chain, or the middle-ear pressure—all without the tympanometer needing to determine, from that peak alone, exactly which of the three is malfunctioning.

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