Transcript Immitance

Acoustic Immitance
(Impedance and
Admittance)
Lecture 9
Outline
 Concepts of Immitance

Admittance (compliance and Impedance)
 Tympanometry


Equipment
Procedures
 Interpreting Results
 Acoustic Reflexes
Immitance Concepts: Big Picture
 Wave strikes the TM


Some of signal is absorbed
by TM
Some of signal is reflected
back out the external canal.
Efficient transmission of sound
 Ideal condition: as much of the sound signal
“admitted” or go through as possible


Need a system that is highly compliant
Need a system that has low impedance
 OE & ME structures
Condition of TM
 Which TM would allow more sound to be
admitted (more compliant/low impedance)
 Which TM would provide less sound to be
admitted (less compliant/more impedance)
Components of Acoustic
Immitance
 Admittance – measures the flow of energy
through the middle ear system (compliance)
 Impedance- measures the opposition to the
flow of energy through the middle ear system
Components of Acoustic
Immitance
A high impedance system is heavy &/or stiff
and does not accept energy well.
A system that does not readily accept energy can
also be called a low admittance system.
IDEALLY for HEARING : want a low
impedance, high admittance system
Tympanometry
 Definition: Tympanometry is a dynamic measure of the
acoustic immitance in the external ear canal as a
function of ear canal air pressure change
 Used to test the condition of the middle ear and
mobility of the eardrum and the conduction bones by
creating variations of air pressure in the ear canal.
 Purpose
 Provides information regarding mobility
(compliance) of the tympanic membrane
 Guides us in determining the status of the ME
 Cross check assessment that aids in differential
diagnosis
Tympanometry Procedure
 A probe will be presented to the ear canal
and a seal will be formed
 A change in pressure will be presented that
the client will feel stopped up
 A sound will be presented through the probe
tip
 A microphone will record the amount of
sound that is reflected back
Tympanometry Equipment
Impedance Instrumentation
Introduces a pure tone into ear canal
through 3-function probe tip
Reflex Signal
Generator
(speaker)
Probe Tone
Generator
(speaker)
Probe
(Microphone)
Air Pressure
System
Tympanometry Procedure
 Ear canal is sealed with a probe tip – sizes vary
 Air pump creates a change in pressure in ear canal
 Pressure changes gradually:
 Positive +200 daPa (forces TM inward)
Negative – 200 daPa (forces TM outward)
 While pressure is changing, a sound transmitter
sends a sound wave to the TM
 Wave that is reflected from the TM is then picked up
by a mic in the probe
 The tympanometer measures energy of the reflected
sound.
Tympanometric Features obtained
1.
2.
3.
4.
5.
Shapes of tympanogram
Equivalent Ear Canal Volume
Static Acoustic Compliance
Tympanometric peak pressure
Tympanometric Width
Concepts
 If ME space is filled with air,


energy is absorbed by the TM, ossicles, and inner
ear structures
tracing will show a normal peak compliance
 If ME space is filled with fluid
 most of the sound is reflected back to the probe

from the stiff TM
Tracing will show a flat tympanogram
 If disruption of ossicles


large amount of energy will be absorbed into
the TM
tracing will display an abnormally high peak.
1. Tympanometric Shapes
 Type A: Normal
 Type As: Normal, but
shallow
 Type Ad: Possibly
disarticulated or flaccid
 Type B: Abnormal,
reduced eardrum
mobility
 Type C: Abnormal,
Negative Pressure
Or decaPascals
Shape
Type A – Normal Tympanogram
 The peak of the pressure curve falls between +50 and -150 daPa
of pressure
 Peak compliance falls between .2 and 1.8 mm
 Results indicate the absence of middle ear pathology
 Intact & mobile TM with normal eustachian tube
function
 If there is a hearing loss,
it is likely to be
SNHL!
Shape
Type As (shallow): Abnormal Tympanogram
 The peak of the pressure curve falls between +50 and -150 daPa of




pressure (WNL)
Peak compliance very low (well below .2 cm3)
Often associated with ossicular fixation or TM scarring (not middle ear
effusion)
May result in a fairly flat, non-fluctuating hearing loss
Eustachian tube function is normal
-300
-150
0 +50
+200
Shape
Type Ad (disarticulation): Abnormal Tympanogram
 The peak of the pressure curve falls between +50 and -150 daPa




(WNL)
Peak compliance very high or off chart
Associated with ossicular disarticulation
May result in a fairly flat, non-fluctuating hearing loss
Eustachian tube function is normal
-300
-150
0 +50
+200
Shape
Type B: TM Retracted, Poorly Mobile
 Peak is absent/poorly defined and at markedly
negative middle ear pressure (>-200 daPa)
 Max compliance = below normal range
Tympanometric width is outside normal limits
-300
-150 0 +50
+200
Shape
Type C:
 There is a clearly defined peak, but it falls
on the negative side of the chart, indicating
negative middle ear pressure
 Peak pressure is seen at greater than 150 mm (moved to left)
 Peak compliance may be normal
 Diagnosis: Eustachian tube dysfunction,
may cause a very mild conductive loss, or
hearing can be WNL
Air, Fluid or Disruption of
ossicles?
2) Equivalent Ear Canal Volume
 Measured in cm3
 Made at the beginning with +200 daPa of
pressure
 Normal values


Children .3-1.5 cm
Adults .6-2.0 cm3
3
3) Tymanometric Peak Pressure
 Machine will automatically record the peak
pressure point
 Observe the peak pressure value in daPa
 Normal Values:
>-150 to –200 daPa considered abnormal
4) Static Acoustic Compliance
(admittance)
 Most machines make this measurement
automatically at the peak compliant point of
the tracing
 Low values suggest reduced compliance
High values suggests excessive compliance
 Normal Values:
Children: .25- 1.5 ml or cm3
Adults: .3-1.7 ml or cm3
5) Ear canal pressure: Tympanometric
Width
 Defined: interval in daPa between sides of
tympanogram at one half of the peak
admittance value
 Normal range:
Children: 80-159 daPa
Adults:
51-114 daPa
Generally > 150 daPa associate with fluid
Discriminating
Fluid from a Perforation
 Both will be Type B!!!
 Equivalent Ear canal volume is the key
Summary
Tymp Interpretation
Peak
Ear Canal
Volume
Normal Volume
Type B (fluid)
Present and
occurs around
0
No Peak
Type B (perf)
No Peak
Large
Volume
Negative Peak
Normal Volume
Type A
Type C
Normal Volume
Acoustic Reflex
 Contraction of the stapedial muscle to loud
sounds
 Results in reduction of sound transmission to
cochlea

5-10 dB of attenuation
 Reflex occurs both contralaterally &
ipsilaterally
Acoustic Reflex
 If NH, at sound levels 85 dB SL, stapedial
muscle contracts, pulls stapes away from OW
 Contraction results in TM movement that can
be measured
Acoustic Reflex Testing
 Reflexes typically tested
at:

500, 1000, 2000 and
4000 Hz
 Begin at 85 dB HL and
increase level up to
120dB HL
Set-up for acoustic reflex testing
 Immitance set up the same as for
tympanogram testing
 Usually conducted immediately after
tympanogram is completed
 Set–up to measure admittance of the
eardrum as well as present a loud signal
Reflex Arc: Right Ipsi and Contra
pathway
Reflex Arc: Left Ipsi and Contra
pathways
RE cochlear pathology
VIIIth nerve pathology
R facial nerve pathology
Four Possible AR outcomes
 Present at normal levels usually 85 dB SL

80-100 dB HL
 Absent at limits of testing

> 120 dB HL
 Present at lower SL

< 80 dB HL
 Present at high SL (elevated) –

>100-120 dB HL
Outcome …