APPENDIX A – 1000 HZ TYPANOMETRY & MEATUS COMPENSATION
53
D-0115696-C (OM038)
– Otowave 302 Operating Manual
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 lead 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 visualize.
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 chapter.
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 displayed 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:
𝑌_𝑡𝑚 =
|
𝑌_𝑚𝑒𝑎𝑠
|
−
|
𝑌_𝑒𝑐
|
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