otometrics Madsen ZODIAC 901, Руководство пользователя

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of this impedance is the impedance presented by the oval window, as
transformed by the hammer, anvil and stirrup. It is because of this that
impedance measurements taken at the entrance to the outer ear can sup-
ply detailed information on conditions in the middle and inner ear.
The arrival of very loud sounds in the middle ear would result in over-
loading of the mechanisms of the ear. This would cause distortion and
reduce the effectiveness of the sense of hearing. If the loudness were ex-
cessive, permanent damage could occur.
Stapedius reflex When the central nervous system perceives a rise in sound level in either
ear, it issues an instruction to the
tensor tympani and stapedius muscles in
both ears to contract. Contraction of these muscles results in a tensioning
of the ear-drum. The tense ear-drum presents a higher impedance to the
outer ear, and consequently most of the energy in the loud sound is re-
flected back the way it came.
This mechanism is part of the body’s reflex system. It is known as the
auditory, or
stapedius , reflex. It serves as a form of automatic volume
control. Its speed and effectiveness may be determined by applying a
loud sound to the ear and measuring the timing of the change in the me-
chanical impedance of the outer ear.
8.7 MEASURING ACOUSTIC IMPEDANCE
Acoustic impedance If air is injected into a cavity, the pressure inside the cavity will increase.
Provided the temperature does not change, and the cavity walls are
rigid, the relationship between the volume
V and the pressure P will
conform to Boyle’s Law:
P = k / V
where k is constant. Differentiating the right-hand expression gives:
dP / dV = k log V
i.e.,
dP = dV P V log V
This relationship tells us that the change in pressure resulting from the
injection of a small, known volume of air
dV will depend not only on dV ,
but also on the initial pressure P and the volume V of the cavity. Since
dV and P are known, measuring dP would enable V log V , and hence V ,
to be calculated.
The practical and convenient way to make this measurement is to use a
calibrated electroacoustic transducer to inject a sound wave into the cav-
ity. The resulting value of
dP can be measured using a calibrated micro-
phone connected to the cavity.
Operation Manual 75
GN Otometrics
PRINCIPLES OF MIDDLE-EAR ANALYSIS
Chap. 8
ZODIAC 901 Middle-Ear Analyzer
MEASURING ACOUSTIC IMPEDANCE
8.7