otometrics Madsen ZODIAC 901, Руководство пользователя

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The static pressure or depression typically required to tension the ear-
drum sufficiently to reflect half of the sound power impinging on it is
300 Pa, i.e., 0.3% of atmospheric pressure. This may be compared with
the RMS (root means square) sound pressure of the threshold of pain,
which is around 100 Pa (at the entrance to the ear canal).
Notice that a static pressure of this magnitude will not cause any pain or
discomfort in the healthy ear, unlike an acoustic pressure of the same
magnitude.
Audiological reflex The ear-drum may also be tensioned by the application of a loud sound
to either ear, resulting in the contraction of the middle-ear muscles of
both ears. The term “audiological reflex”, or simply “reflex”, is com-
monly taken to refer to the measurement of mechanical impedance dur-
ing the application and removal of a loud sound, either to the same ear
(“ipsi”), or to the other ear (“contra”). This phenomenon may be ob-
served and measured at a sound pressure level considerably below the
threshold of pain.
8.9 THE PRINCIPLE OF TYMPANOMETRY
Acoustic-flow
tympanometry The method used by the ZODIAC 901 for measuring the impedance of
the ear canal is sometimes referred to as acoustic-flow tympanometry.
The entry to the ear canal is sealed using a suitably dimensioned eartip,
and a probe inserted through the eartip. The probe contains three fine
air channels. One is connected to a pump which can raise or depress the
static pressure in the ear canal. A second carries a source of sound of
known amplitude, referred to as the probe tone. The third gives access to
a calibrated measuring microphone, which is used to measure the result-
ing sound level in the ear canal.
Reflection of sound at
the eardrum The amplitude of the sound needed to produce a given measured sound
level in the ear canal will be proportional to the volume of the canal. The
ZODIAC 901 uses this principle to calculate the volume. If the eardrum
has been tensioned, giving it a large impedance mismatch, so that it re-
flects most of the sound energy arriving at it, the calculated volume will
be that of the outer ear alone. However, if the eardrum is functioning
normally, so that most of the sound energy arriving at it is transmitted
onwards to the inner ear, the calculated volume will be considerably
greater. Intermediate states of the eardrum will result in intermediate
calculated volumes.
Operation Manual 77
GN Otometrics
PRINCIPLES OF MIDDLE-EAR ANALYSIS
Chap. 8
ZODIAC 901 Middle-Ear Analyzer
THE PRINCIPLE OF TYMPANOMETRY
8.9