otometrics Madsen ZODIAC 901, Руководство пользователя

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The reciprocal of Z is the mechanical, or acoustical, admittance. The term
“acoustic immitance” is used collectively to refer to either of these pa-
rameters.
Measurement of changes in the acoustic impedance of the ear forms the
basis for tympanometry.
8.2 MISMATCHING OF MECHANICAL IMPEDANCES
During the transmission of a mechanical vibration through a homogene-
ous medium or along a homogeneous transmission line, the ratio of vi-
bration pressure to vibration velocity is equal to the characteristic
mechanical impedance of the transmission medium.
Mismatching of
impedances If the vibration encounters an inhomogeneity in its passage, i.e., a region
where the mechanical impedance changes from one value to another,
then, in general, a part of the energy in the vibration will be reflected
back towards the source, and only a proportion of the energy will con-
tinue in the direction of the original wave, past the inhomogeneity. The
impedances are said to be mismatched.
In a system whose purpose is to transmit vibrational energy, such as the
ear, this is a functional disadvantage, since some of the energy fails to ar-
rive at its intended destination. An inhomogeneity in this kind of system
is most likely to arise at a boundary between adjoining components of
the system.
8.3 MATCHING OF MECHANICAL IMPEDANCES
Matching of
impedances The ear collects as much as possible of the acoustic energy arriving at the
ear-lobe by means of two natural mechanisms which minimize reflec-
tions.
·
The structure of the outer ear is such as to effect a continuous transition
from the relatively low mechanical impedance of the open air to the in-
termediate impedance of the eardrum.
·
The middle ear incorporates a mechanical broad-band impedance
transformer. A broad-band transformer is a device whose structure is
such that it presents differing impedances at its input and output.
8.4 RESISTIVE LOSS OF VIBRATION ENERGY
Resistive loss of
vibration energy Resistive loss occurs when a mechanical vibration or sound wave passes
through a material which is subject to hysteresis, and part of the energy
is absorbed into the material, and thereby converted into heat.
72 Operation Manual
PRINCIPLES OF MIDDLE-EAR ANALYSIS
Chap. 8 GN Otometrics
MISMATCHING OF MECHANICAL IMPEDANCES
8.2 ZODIAC 901 Middle-Ear Analyzer