Welch Allyn TM 262 Auto Tymp - User Manual - Manual (Page 13)

Revised 4/11/08 1-5
Introduction
Tympanometry and Gradient
Tympanometry is an objective technique used since the late 1960’s to measure the mobility
compliance) and the pressure within the middle-ear system. During tympanometry, a low pitch
tone (i.e., 226 Hz probe tone) is presented to the ear canal via the light-weight probe. The probe
tone is used to measure the compliance changes within the middle-ear system while air pressure
within the hermetically sealed ear canal is varied from a positive to a negative value. A positive
pressure within the ear canal space, in the absence of middle-ear pathology, causes the middle-
ear system to stiffen up or become less mobile. This is caused by the pressure difference
between the sealed ear canal space and the middle-ear space which forces the tympanic
membrane to stretch inward. A stiffened middle-ear system displays little or no compliance. As
the pressure within the ear canal is brought back toward atmospheric (ambient or 0 daPa)
pressure, the pressure difference between the ear canal space and the middle-ear space is
reduced in normal ears.
At or near atmospheric pressure (0 daPa), the greatest amount of sound (probe tone) enters the
middle-ear system. In other words, this is the air pressure value where the middle-ear system
displays the maximum amount of compliance (admittance).
When the air pressure within the ear canal is then reduced to a negative value with respect to
the middle-ear space, a pressure difference is once again established and the middle-ear system
becomes less compliant. Therefore, by varying the pressure within the ear canal, it is possible
to make a series of compliance measurements by means of the probe tone. The tracing which
depicts these compliance changes is referred to as a tympanogram. The point of the
tympanogram which represents the point of maximum compliance (admittance) is the
compliance peak of the tympanogram. The air pressure (pressure at the peak) where this
compliance peak occurs approximates the pressure within the middle-ear system, since
maximum mobility is only possible when there is little or no pressure difference between the
ear canal and the middle-ear space. Compliance is measured with respect to the ability of an
equivalent volume of air to conduct sound and the scientific quantity used is cm
3 . Air pressure
is measured in decaPascals (daPa).
NOTE: 1.02 mm H20 = 1.0 daPa