App. 1 MADSEN Zodiac immittance methodology and features
147
MADSEN Zodiac
•
Mass Susceptance
is greater with high frequencies and gradually decreases with lower frequencies.
The total
Susceptance
is the algebraic sum of the positive
Compliance
and the negative
Mass
contributions, jB + (-jB).
Positive Susceptance
If a major portion of the probe tone energy flows through the
Compliance
elements of the ear,
Susceptance
is positive.
The ear is in this case stiffness controlled.
This is true when the probe tone frequency is less than the resonance frequency of the ear.
Negative Susceptance
If a major portion of the probe tone energy flows through the
Mass
elements, then the
Susceptance
is negative. The ear
is in this case
Mass
controlled.
This is true when the probe tone frequency is greater than the resonance frequency of the ear.
Zero Susceptance
If equal amounts of probe tone energy flow through the
Compliance
elements and
Mass
elements, then the
Susceptance
is zero. The ear is in this case neither stiffness nor
Mass
controlled. This means that the probe tone frequency is the same
as the resonance frequency of the ear.
Baseline compensation
To quantify the middle ear
Susceptance
, it is helpful to baseline compensate the
Susceptance
curve. This is achieved
through subtraction of the ear canal
Susceptance
.
The tympanic membrane is tensioned with high air pressure so that the probe tone reflects off the tensed tympanic mem-
brane, and accordingly a negligible portion of the probe tone is admitted into the middle ear.
At high pressures, we only measure the
Susceptance
of the air that is enclosed between the probe tip and the tympanic
membrane, resulting in the ear canal
Susceptance
. This
Susceptance
is mainly
Compliance Susceptance
since the
Mass
element in this enclosed air is negligible. The value of the notch in the
Susceptance
tympanogram can then be
determined from the corresponding value on the vertical axis, and the relation of the probe tone frequency to the ear
resonance frequency can be estimated according to the above description.
App. 1.5.2
Conductance, G
Conductance
is the amount of energy that dissipates as heat due to the friction in the ear system. Friction occurs because
of contact between moving molecules in the system.
Conductance
typically increases when more energy reaches the
structures of the middle ear when the sweep pressure approaches the tympanometric peak pressure.
Being a frictional element,
Conductance
can never take a negative value.
App. 1.5.3
B/G viewing of tympanograms
Viewing dual component (B/G) tympanograms is preferable to admittance magnitude (Y) tympanograms in the higher probe
tone frequency region.
This is because the
Mass
elements have an increasingly pronounced impact on the tympanogram with higher frequencies.
When the probe tone frequency approaches the
Mass
controlled frequency region, the increasing
Mass
and the decreasing
Compliance
contribution to
Susceptance
will pull the
Susceptance
curve down, and cause notching of the curve.
Notching, if sufficiently pronounced, will also show in the total Admittance curve, and may make the curve difficult to
interpret, especially about normative data.
