8.11 TYMPANOGRAM CHARACTERISTICS
A tympanogram has a number of characteristics which are often used
for comparison and diagnosis by audiologists, and can be computed
automatically by the ZODIAC 901.
Volume
tympanogram
A volume tympanogram is a graph showing the measured volume as a
function of pressure. The measured volume will at no point be less than
the volume of the ear canal alone (ear canal volume, ECV). It is therefore
more instructive to generate a “compliance tympanogram”, in which the
origin has been moved up the
y
-axis to lie level with the flat part of the
curve. The (measured) volume is related to the compliance and the ECV
as follows:
volume = ECV + compliance
Compliance
tympanogram
The name “compliance” requires some elucidation. The audiologist is in-
terested in using the tympanogram to determine the compliance of the
eardrum. Compliance is conventionally expressed as a displacement di-
vided by a pressure. However, any transmission of sound by the ear-
drum is equivalent to an enlargement of the volume of the ear canal.
What is actually measured is this equivalent change in volume, which is
a useful representation of compliance for diagnostic purposes.
Middle Ear Pressure
The eardrum will be most compliant when the static pressures on oppo-
site surfaces of the eardrum are equal or only slightly different. The peak
(maximum) of the tympanogram occurs at this pressure, which may dif-
fer from atmospheric pressure, depending on conditions in the middle
ear. The difference between the pressure at which the peak occurs and
atmospheric pressure is therefore referred to as the Middle Ear Pressure
(MEP). This pressure is computed and recorded by the ZODIAC 901
when it performs a tympanogram sweep, and can be used automatically
for subsequent reflex tests on the same patient.
Static Compliance
The maximum increase in measured volume, i.e., the height of the peak
on a tympanogram, is the compliance at Middle Ear Pressure. This is
known as the Static Compliance (SC). The SC is computed and recorded
by the ZODIAC 901 when it performs a tympanogram sweep.
For comparing tympanometric data between patients or between
healthy ears and ears subject to disorder, it is helpful to have data which
express the shape of the tympanogram rather than its absolute dimen-
sions. After performing a tympanogram sweep, the ZODIAC 901 com-
putes and records two such parameters.
Gradient
The Gradient (Grad) is the value of compliance at which the width of the
“hump” of the tympanogram is 100 daPa, divided by the Static Compli-
ance.
Operation Manual
79
GN Otometrics
PRINCIPLES OF MIDDLE-EAR ANALYSIS
Chap. 8
ZODIAC 901 Middle-Ear Analyzer
TYMPANOGRAM CHARACTERISTICS
8.11
