Acoustic immittance measurements objectively assess middle ear function using tympanometry, acoustic reflex thresholds, and acoustic reflex decay. Tympanometry involves placing a probe in the ear canal to measure how acoustic admittance changes as pressure is varied. Normal tympanograms are Type A, while abnormal types include flat (Type B), negative pressure (Type C), stiff (Type As), and flaccid (Type AD). Acoustic reflex thresholds measure the lowest level needed to elicit the stapedius muscle reflex, providing information about the middle ear, cochlea, auditory nerve and brainstem. Acoustic reflex decay tests the sustainability of the reflex over 10 seconds of continuous stimulation.

1. Acoustic Immittance
Measurements
Ozarks Technical Community College
HIS 125

2. 
Objective measurements of acoustic
impedance and acoustic admittance
◦ Impedance = opposition to flow of sound
through auditory system
◦ Admittance = ease with which sound flows
through the auditory system

Consist of the following:
◦ Tympanometry
◦ Acoustic reflex thresholds
◦ Acoustic reflex decay
Acoustic Immittance Measurements

3. 
Objective measure of middle ear function

Procedure
◦ determines the amount of energy transmitted by the middle
ear system
◦ A probe is placed in the ear canal consisting of three ports:
 a loudspeaker, manometer pressure pump, and microphone
◦ A 226 Hz tone is introduced by the loudspeaker while the
manometer pressure pump automatically and slowly varies
the pressure in the ear canal from +200 to -400 daPa
(decapascals).
◦ In the meantime, the microphone measures the change in
intensity (in dB SPL) in the ear canal as the pressure is
varied.
 As admittance decreases (i.e., the eardrum is stiffer and more
sound is reflected off the tympanic membrane), the measured
SPL increases. As admittance increases (i.e., the eardrum is
more compliant and less sound is reflected off the tympanic
membrane), the measured SPL decreases.
Tympanometry

4. Admittance decreases,
Reflected SPL increases
Admittance increases,
Reflected SPL
decreases
Normal Tympanogram

5. 
Three outcome measures:
static admittance (SA) or maximum
compliance= point along the tympanogram where
the amount of reflected SPL is the least
 ear canal volume (ECV)=physical space between
the end of the probe assembly and the tympanic
membrane
 middle ear pressure (MEP)= pressure along the
x-axis of the tympanogram where the eardrum is
the most compliant or where pressure is equal on
both sides of the tympanic membrane

Tympanometry

6. Parameter
Normal Range
Ear Canal Volume
0.65-1.75 mL or cm3
Middle Ear Pressure
- 100 to + 50 daPa
Static Admittance
0.3-1.4 mL
Normal Tympanometry Values for
Adults

7. 

Any middle ear abnormality will result in some
abnormality on the tympanogram
Tympanograms are classified by type:
◦ Type A = all 3 outcome measures are normal
◦ Type B = no static admittance, the TM is immobile;
normal ECV suggestive of middle ear fluid, while an
enlarged ECV suggests a TM perforation or patent tube
◦ Type C = negative MEP (i.e. eustachian tube dysfunction)
◦ Type AS= stiff/hypocompliant; SA is reduced (i.e.
otosclerosis)
◦ Type AD = flaccid/hypercompliant; SA is greater than
normal (i.e. ossicular discontinuity)
Abnormal Tympanograms

8. Type B=flat
*Note: based on the
enlarged ECV, this
patient has a patent
tube or a hole in the
TM
Type C=negative
pressure
Abnormal Tympanograms

9. Type As=stiff,
hypocompliant
Type AD=flaccid,
hypercompliant
Abnormal Tympanograms

10. 
If otoscopy reveals significant cerumen
accumulation, a tympanogram will allow
you to determine if an accurate
audiogram can be obtained.
◦ If tympanometry is normal in the presence of
excess cerumen, then there is enough of an
opening in the ear canal for accurate hearing
testing to occur.
Other use of tympanometry

11. 
The introduction of a loud sound to the
ear canal of either ear results in the
acoustic reflex, or the contraction of the
stapedius muscle, which causes the
tympanic membrane to stiffen.
◦ This results in a change in middle ear
immittance, which is measured as an increase
in dB SPL by the microphone in the probe
assembly.
The Acoustic Reflex

12. 
Acoustic reflex thresholds (ARTs) = the
lowest level in dB HL at which the acoustic
reflex can be elicited (deflection ≥0.02 ml)
◦ Uses the same bracketing technique that is used in
obtaining puretone thresholds
◦ Measured at 500, 1000, 2000, and 4000 Hz
◦ Useful in determining site-of-lesion
 Directly evaluates middle ear status and indirectly
evaluates cochlear and retrocochlear status
◦ ARTs can be measured ipsilaterally (stimulus is in
the probe ear) and contralaterally (stimulus is in
the non-probe ear) via a supra-aural or insert
earphone
Acoustic Reflex Thresholds

13. 
Here is a graph of acoustic reflex testing
at 500, with threshold being established
at 85 dBHL

14. Hearing Status
Expected Acoustic Reflex Threshold (dB HL)
Normal
70-100 dB HL
Conductive Loss
Elevated or Absent
Cochlear Loss
Normal sensation level (SL) or reduced SL
Neural Loss
Normal SL, elevated, or absent
Interpretation of Acoustic
Reflexes

15. Acoustic Reflex Pathway
From: Martin & Clark, Introduction to Audiology
•As you can see from the
reflex pathway, the
measurement of ARTs not
only provides information
regarding the status of
the middle ear
system, but also of the
inner ear, auditory
nerve, regions of the
lower auditory
brainstem, and the facial
nerve.

16. 
Acoustic reflex decay is a measure of the sustainability
of the acoustic reflex, or how long the stapedius muscle
can remain contracted during continuous stimulation.

Procedure:

Outcomes:
◦ Reflex decay is measured with contralateral stimulation at 500 and
1000 Hz at 10 dB SL (re: contralateral acoustic reflex threshold).
◦ The tone is presented continuously for 10 seconds and the patient
is instructed to remain as still as possible.
◦ Negative reflex decay indicates that the magnitude of the
stapedial reflex contraction did not decrease by ≥50% in the first
five seconds of testing
◦ Positive reflex decay indicates that the magnitude of the
stapedial reflex contraction decreased by ≥50% in the first five
seconds of testing
 Positive acoustic reflex decay is highly suggestive of a retrocochlear
pathology and further evaluation by an otologist is strongly
recommended.
Acoustic Reflex Decay

17. 
Negative (upper)
and positive
(lower) acoustic
reflex decay at
1000 Hz.

In positive decay,
the stapedius
contracts initially,
but then “lets go”
Acoustic Reflex Decay