App. 1 MADSEN Zodiac immittance methodology and features
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MADSEN Zodiac
High frequency probe tones
Since the B/G viewing is of interest only for high frequency probe tones, this method is clinically valuable in conjunction
with multi-frequency tympanometry where the normative data is based on resonance frequency. However, even when
measuring with a single high frequency probe tone, the B/G components provide a clearer picture of the state of the
middle ear, especially when testing the infant population, where the ear is still undergoing maturation and resonance
frequency alterations.
App. 1.5.4
Component compensation
Interpretation of tympanograms relating to high frequency probe tones may be difficult because no straightforward
approach is available.
Baseline compensation
To assess the middle ear admittance alone, the ear canal contribution to the total ear admittance must be eliminated.
This is called baseline compensation.
•
Using low frequency probe tones
Baseline compensation is achieved by subtracting simple ear canal admittance from the total admittance, but this is
only valid when using low frequency probe tones such as 226 Hz. The reason is that the middle ear is typically
stiffness controlled at 226 Hz, and accordingly the middle ear
Mass
elements will not influence the admittance curve.
Admittance is therefore made up of
Compliance
and
Conductance
properties alone.
Compliance
and
Conductance
properties vary similarly throughout the pressure sweep, making it possible simply to
subtract the ear canal
Compliance
and
Conductance
measured at one extreme point in the pressure sweep. In other
words, the stiffness and friction alone contribute equally to each point of the admittance curve throughout the
pressure sweep, including the extreme point where the ear canal admittance is determined.
•
Using higher frequency probe tones
The covariation between
Susceptance
and
Conductance
is no longer true when the probe tone frequency is
increased, approaching a
Mass
controlled frequency region. As the influence on the
Susceptance
curve of
Mass
elements increases when approaching the peak pressure, the relationship between the
Susceptance
and the
Conductance
will no longer remain the same throughout the pressure sweep. When this happens, you can no
longer subtract the ear canal admittance (measured at one extreme point of the sweep) from each point of the total
admittance curve.
•
Baseline compensation for Susceptance and Conductance curves
However, using baseline compensation is appropriate for the
Susceptance
and the
Conductance
curves individually.
I.e., you can subtract the isolated ear canal
Susceptance
measured at e.g., +200 daPa from every
Susceptance
point throughout the sweep, taking variations in
Mass
and
Compliance
contribution into account. Likewise, you
can subtract the ear canal
Conductance
from each point of the
Conductance
curve. After these separate
component compensations, the middle ear admittance can be correctly calculated from the two compensated
Susceptance
and
Conductance
curves.
The resulting admittance tympanogram
Each point of the resulting admittance tympanogram describes the difference from the admittance of the ear canal alone.
Since an absolute difference cannot be negative, the admittance values in the component compensated admittance curve
will always have a positive value throughout the curve, and even a notched non-compensated admittance tympanogram
will typically become single peaked when compensated through this procedure. The reason for this is that regardless of
whether the admittance change is due to a change in
Mass
or
Compliance
contribution, it is still an absolute change
relative to the ear canal admittance reference point.
