APPENDIX A
–
1000 HZ TYPANOMETRY & MEATUS COMPENSATION
58
OTOWAVE 302+ INSTRUCTION FOR USE
10.
APPENDIX A
–
1000 HZ TYPANOMETRY & MEATUS
COMPENSATION
10.1.
TYMPANOMETRIC PROPERTIES
Tympanometric measurements of the ear are affected by a large number of physiological characteristics, but from a
clinical perspective the three most important, physical properties impacting the outcome of a tympanometrical
measurement are:
1.
Stiffness
2.
Mass
3.
Friction
Combined mathematical and electro-technical approaches have been developed to measure/calculate and predict the
stiffness of the ear drum and the middle ear. This led to the conversion of stiffness, mass and friction into equivalent
electrical impedances (
Z
):
1.
Negative reactance (stiffness)
2.
Positive reactance (mass)
3.
Resistance (friction), whereby friction can only be positive in passive systems.
For tympanometry however, it is more usual to consider the inverse of impedance, the so-called admittance (
Y = 1/Z
),
of stiffness, mass and friction:
1.
Susceptance (
B
, inverse of reactance)
2.
Conductance (
G
, inverse of resistance)
The units of all these admittance components are mhos (the inverse of ohms used for impedance).
The reason for using these inverse measures is because the admittances of the ear canal and middle ear components
can then be treated as being in series with each other, making their values easy to separate. If considered as impedances
these components are in parallel, which makes their separation much more difficult to calculate and to visualise.
For example, the ear canal admittance/impedance is often not of immediate interest and is removed from the
measurement as described later. For tympanometry, it is more of interest, to find the admittance/impedance of the
middle ear than the one of the ear canal.
When considering a simple stiffness like that of the ear canal air volume, its susceptance is positive and is related to the
commonly used term “compliance”. At low frequencies, such as 226Hz used in most tympanometers, the middle ear
and the ear canal air volume both behave quite like a simple stiffness and use of the term compliance is appropriate (to
an approximation). However, at higher frequencies such as 1000Hz, this simplification breaks down, as described in the
following section.
10.2.
TYMPANOMETRIC MEASUREMENTS
The main aim of tympanometry is to separate out the admittance contribution of the ear canal air volume (
Y
ec
) from
the total measured admittance (
Y
meas
), to find the admittance in the plane of the tympanic membrane (
Y
tm
). This
separation is variously called ‘baseline removal‘ or ‘meatus compensation‘. The value removed is often displaye
d
separately as the Ear Canal Volume. Note that when using a 226 Hz probe tone, one can substitute the word
compliance
for
admittance
in this description, with minor loss of accuracy, and the calculation is a simple scalar subtraction of the
magnitudes of the admittance values:
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