1. Acoustic Immitance (Impedance and Admittance)
Lecture 9

2. Outline Concepts of Immitance Tympanometry Interpreting Results
Admittance (compliance and Impedance)
Tympanometry
Equipment
Procedures
Interpreting Results
Acoustic Reflexes

3. 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.

4. 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

5. 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)

6. 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

7. 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  

8. 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

9. 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

11. Tympanometry Equipment

12. 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

13. 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.

14. Tympanometric Features obtained
Shapes of tympanogram
Equivalent Ear Canal Volume
Static Acoustic Compliance
Tympanometric peak pressure
Tympanometric Width

15. If ME space is filled with air,
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.

16. 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

17. 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!

18. 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

19. 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

20. 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

21. 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

22. Air, Fluid or Disruption of ossicles?

23. 2) Equivalent Ear Canal Volume
Measured in cm3
Made at the beginning with +200 daPa of pressure
Normal values
Children cm3
Adults cm3

24. 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

25. 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: ml or cm3
Adults: ml or cm3

26. 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: daPa
Adults: daPa
Generally > 150 daPa associate with fluid

27. Discriminating Fluid from a Perforation
Both will be Type B!!!
Equivalent Ear canal volume is the key

28. Summary Tymp Interpretation
Peak
Ear Canal Volume
Type A
Present and occurs around 0
Normal Volume
Type B (fluid)
No Peak
Type B (perf)
Large
Volume
Type C
Negative Peak

29. 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

30. 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
Anatomy
Physiology (loud sounds)
Levels elicitted (85 dB SL)
Softest level = Acoustic reflex threshold
Get this even for many people with hearing loss up to severe types
Ipsilateral and Contralateral

31. 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

32. 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

33. Reflex Arc: Right Ipsi and Contra pathway

34. Reflex Arc: Left Ipsi and Contra pathways

35. RE cochlear pathology

36. VIIIth nerve pathology

37. R facial nerve pathology

38. Four Possible AR outcomes
Present at normal levels usually 85 dB SL
dB HL
Absent at limits of testing
> 120 dB HL
Present at lower SL
< 80 dB HL
Present at high SL (elevated) –
> dB HL
1- example 1000 Hz Th = 10, present at 95 dB HL, SL = 85
2- obvious
3- example 1000 Hz th = 60, present at 95 dB HL, SL = Basically normal performance because IHC intact to some degree
4- example 1000 Hz th =10, present at 125, SL = 105

39. Outcome …