otometrics Madsen ZODIAC 901, Руководство пользователя

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Frequency for
tympanometry It is important to perform this measurement at a frequency which is not
too high, with the undesirable result that the changing pressure in the
cavity would cease to be the same everywhere in the cavity. This is an
expression of the requirement that the wavelength of the chosen fre-
quency should be much longer than the largest dimension of the cavity.
For mechanical or acoustic impedance measurements on the ear, the fre-
quency to be used has been standardized at 226 Hz.
The expression P V log V is related to the mechanical impedance of the
outer ear cavity, but is not equal to it. This is because it relates the acous-
tic pressure to the change in volume, not to the acoustic velocity. The ve-
locity is a function not only of the change in volume, but also of the
frequency. However, at a fixed frequency there will be a one-to-one cor-
respondence between the mechanical impedance and the volume of the
cavity. Tympanometry is concerned with measuring, not the absolute
value of the mechanical impedance, but changes in its value.
8.8 TENSIONING OF THE EARDRUM
As explained above, the impedance of the healthy ear under normal con-
ditions will be determined not only by the volume of the outer ear, but
also by the impedance of the inner ear as transformed by the auditory
ossicular chain and presented at the eardrum.
The deflection of the eardrum in response to acoustic vibrations is very
small, and its behaviour is normally linear. That is to say, its mechanical
impedance is independent of the sound pressure level. Its deflection is
proportional to the sound pressure.
Nonlinearity of the
eardrum However, for changes in pressure which are significantly greater than
those produced by the normal range of sounds, the deflection of the ear-
drum is non-linear. Its impedance is no longer independent of the ap-
plied pressure. The greater the pressure (or depression), the stiffer it
becomes.
If the ear-drum becomes tensioned for any reason, most of the acoustic
energy arriving at it will be reflected, and the mechanical impedance
measured will be determined predominantly by the volume of the outer
ear alone.
The nonlinearity of the eardrum is what makes tympanometry possible.
Tensioning the
eardrum Tensioning of the ear-drum may be achieved by applying a positive or
negative standing air pressure to the outer ear. The inner ear normally
remains at (or close to) atmospheric pressure provided the ear is healthy,
i.e., the Eustachian Tube is functioning correctly. The term “tympanome-
try” is commonly understood to mean the measurement of mechanical
impedance while varying the static (standing) pressure in the outer ear.
The resulting mechanical impedance characteristic (the tymapanogram)
is a very effective indicator of conditions of disorder in the middle ear.
76 Operation Manual
PRINCIPLES OF MIDDLE-EAR ANALYSIS
Chap. 8 GN Otometrics
TENSIONING OF THE EARDRUM
8.8 ZODIAC 901 Middle-Ear Analyzer