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App. 1 MADSEN Zodiac immittance methodology and features
App. 1 MADSEN Zodiac immittance methodology and
features
App. 1.1
An introduction to immittance
Immittance measurements are used to determine the middle ear's ability to transfer sound waves to the inner ear, as well
as assess the impact of middle ear mechanisms that alter this ability.
The purpose of the middle ear is to enhance the incoming sound wave in order to overcome the higher impedance of the
inner ear fluids, thus allowing the air-borne sound wave to be transferred into a liquid wave without being deflected.
The main mechanism used to achieve the sound wave transformation is the difference in the surface area of the tympanic
membrane and the stapes footplate.
Also, the middle ear includes the three middle ear bones or ossicles, malleus, incus and stapes, whose lever action adds to
the sound pressure at the oval window. Any alteration to the middle ear system, such as fluid or air pressure build-up in
the middle ear cavity or inhibition of the ossicular chain movements, will result in an inefficient transfer of acoustic energy
through the middle ear. This results in sound waves being reflected back through the outer ear.
App. 1.1.1
Immittance testing
Immittance testing as used in MADSEN Zodiac is characterized by the following:
A probe tone is delivered through the transducers of the probe into the ear. The microphone of the probe then measures
the acoustic energy that remains in the ear canal. As the ear canal air pressure or middle ear muscle activity alters the
mobility of the middle ear system, different amounts of remaining acoustic energy can be measured at the probe, depend-
ing on the amount of air pressure or muscle activity applied.
The maximum acoustic energy admittance (the point at which the most acoustic energy enters the middle ear) is obtained
when the middle ear mobility is maximized. This occurs when the ear canal air pressure is equal on both sides of the
eardrum membrane, and the middle ear muscles are in a neutral state.
When using a 226 Hz probe tone, the acoustic admittance of air enclosed in a 1 cc cavity is 1 mmho. Therefore, in 226 Hz
immittance measurements, the admittance unit mmho is interchangeable with the volume units cubic centimeters (cc or
3
cm
) or milliliters (ml) as a measure of the acoustic
applicable for the 226 Hz probe tone. Further, because the middle ear admittance is primarily determined by stiffness
(compliant susceptance) in this frequency region, the middle ear compliance can only be considered isolated using low fre-
quency probe tones.
In MADSEN Zodiac, you can choose between any of the compliance or admittance units
when you use a 226 Hz probe tone. When you use a higher frequency probe tone, all admittance components are meas-
ured and the unit is accordingly always
For reflex testing with MADSEN Zodiac, the volume unit
the corresponding numerical values are multiplied by factor 1000.
1 For a comprehensive understanding of immittance vector principles, see e.g. Margolis & Hunter (2000), Acoustic Immit-
tance Measurements. In: R.J. Roeser, M. Valente & H. Hosford-Dunn(Eds.)
Stoppenbach (2002), Basic Principles of Acoustic Immittance Measures. In: J. Katz (Ed.)
Fifth Edition
140
1
mmho
.
compliance
. This 1:1 admittance versus volume equivalence is only
.
µ
l can also be chosen for 226 Hz measurements. When
cc
,
cm3
, or
mmho
Audiology Diagnosis
, or T.L. Wiley & D.T.
Handbook of Clinical Audiology,
Otometrics - MADSEN Zodiac
respectively,
µ
l is used,
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