Lecture program Psychoacoustics and instrumentation

1. Instrumentation
Used In The
Measurement of
Acoustic Signals
And Auditory
Function
Dr. Ghulam Saqulain
M.B.B.S., D.L.O., F.C.P.S
Head of Department of ENT
Capital Hospital, Islamabad

2. Objective Tests
 Acoustic Immittance
 Otoacoustic Emissions
 Auditory Evoked Potentials

3. Acoustic Immittance
 Instrumentation
 Ear Canal Measurements
 Static Immittance (compliance)
 Tympanometry
 Acoustic Reflexes

4. Instrumentation
3 tubes for immittance measures
 miniature loudspeaker which emits 220 or 226 Hz
(incident wave)
 miniature microphone which picks up sound in the
external ear canal (reflected and incident wave)
 air pump for positive and negative pressure in external
ear canal
Regular earphone on contralateral ear

5. Ear canal volume
 Ear canal volume predicted by intensity of reflected
sound

7. Ear Canal Volume:
 Adults
– Mean = 1.5 mL
– 90% range = 0.6 to 1.5 mL
 Children
– Mean = 0.7 mL
– 90% range = 0.4 to 1.0 mL

8. Static immittance
Static immittance: is the measurement of
tympanic membrane and middle ear mobility
 Normative Data for Adults
– Mean = 0.9 mL
– 90% range = 0.4 to 1.6 mL
 Normative Data for Children
– Mean = 0.5 mL
– 90% range = 0.3 to 1.0 mL

9. Tympanometry tracks changes in static immittance over
time

10. Acoustic Immittance
(Tympanometry)
A tympanogram is a graphic representation of how the
immittance of the middle ear is altered by changes in air
pressure in the ear canal.
It is a measurement of middle ear pressure determined by the
mobility of the membrane as a function of different amounts of
positive and negative air pressure in the external ear canal

11.  Tympanometry yields information about resting middle ear
pressure (tympanometric peak pressure) and is helpful in
identifying both:
 low impedance (eg, ossicular disarticulation, tympanic membrane scarring)
 and high-impedance (eg, otosclerosis, otitis media).
 Until relatively recently, most acoustic immittance devices allowed
measurements at only one or two probe frequencies (usually 226
and 678 Hz). The vast body of data relating various tympanometric
patterns to specific pathologic conditions is based on measures at
these two frequencies

12. Tympanometric (Jerger) types

14. Tympanometric Shape
(as recommended by ASHA)
Width of the tympanogram at the 50% amplitude point

16. Normative data for tympanomeric shape
 Adults
– Mean = 80 daPa
– 90% range = 50 to 110 daPa
 Children
– Mean = 100 daPa
– 90% range = 60 to 150 daPa

17. Acoustic Reflex
Although there are two middle ear muscles (tensor tympani
and stapedius), in humans, only the stapedius muscle
contracts in response to sound.
Contraction of the stapedius causes a change in the axis of
rotation of the stapes footplate, thus increasing the
immittance of the middle ear system.
This change in conductivity through the middle ear can be
measured indirectly as changes in acoustic immittance.

18. Acoustic Reflex Anatomy and Physiology

20. Acoustic Reflex Threshold
 ART for pure tones = 85 to 95 dB HL for normally hearing people
 ART for Speech = 10 to 20 dB less for pure tones
 Doesn’t change in people with cochlear loss (up to 50 dB HL)

21. Acoustic Reflex Decay
 In tests of acoustic reflex decay or adaptation a sustained stimulus is
presented at a suprathreshold level and the change in immittance is
measured for 10 seconds.
 If the magnitude of the response decays by more than 50% in this
interval, the test is considered positive for retrocochlear abnormality.
 Although this test theoretically can be performed for many different
stimuli, the observation of decay at 2000 or 4000 Hz is not considered
clinically significant (Givens and Seidemann, 1979).
 Thus the test typically is performed only at 500 and 1000 Hz.

22. Diagnostic Significance
Conductive Loss
Sensorineural Loss
 Cochlear Loss
 Retrocochlear Loss