otometrics Madsen ZODIAC 901, Руководство пользователя

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8. PRINCIPLES OF MIDDLE-EAR ANALYSIS
The ZODIAC 901 Middle-Ear Analyzer is designed for screening and
clinical diagnosis of audiological patients using objective measurement
methods.
The ZODIAC 901 measures the mechanical impedance of the ear, a pro-
cess known as tympanometry. In contrast to audiometry, mechanical im-
pedance measurements are not influenced by the patient’s conscious
response to the measurement stimulus.
The real-time measurement capability of the ZODIAC 901 enables it to
perform dynamic tests of the patient’s reflex, threshold and Eustachian
Tube Function.
The following explanation of tympanometry is included in order to pro-
vide the user of the ZODIAC 901 with sufficient understanding of the
physics and physiology of middle-ear measurement to enable him or her
to make the most effective use of this advanced and versatile instrument.
8.1 MECHANICAL IMPEDANCE
Sound and vibration Sound is a particular case of mechanical vibration at frequencies within
the audible range, namely more than about 20 Hz and less than about 20
000 Hz. Mechanical vibration comprises a backwards-and-forwards mo-
tion about a mean position. This motion takes place in the substance
transmitting the vibration—no vibration can be transmitted in a vac-
uum.
The backwards-and-forwards motion is characterized by changes in vi-
bration pressure
p which are proportional to the vibration velocity c of
the motion (except at very large amplitudes where the substance may
become nonlinear).
p = c Z
Mechanical
impedance The constant of proportionality, Z , is known as the mechanical imped-
ance. It is a characteristic of the substance or structure which is subject to
the vibration. Stiff, dense substances such as bone have relatively high
mechanical impedances, since a large pressure is required to produce a
small deflection (or motion). Air has a relatively low mechanical imped-
ance, since it is neither stiff nor dense, and a small pressure results in a
relatively large deflection.
Acoustic impedance In the special case of audible vibrations, the term “acoustic impedance”
is used instead of “mechanical impedance”.
Operation Manual 71
GN Otometrics
PRINCIPLES OF MIDDLE-EAR ANALYSIS
Chap. 8
ZODIAC 901 Middle-Ear Analyzer
MECHANICAL IMPEDANCE
8.1