Instrumentation Used In The Measurement of Acoustic Signals And Auditory Function

Dr. Ghulam SaqulainM.B.B.S., D.L.O., F.C.P.S

Head of Department of ENTCapital Hospital, Islamabad

Objective Tests

Acoustic Immittance Otoacoustic Emissions Auditory Evoked Potentials

Acoustic Immittance

Instrumentation Ear Canal Measurements

Static Immittance (compliance)

Tympanometry

Acoustic Reflexes

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

Ear canal volume

Ear canal volume predicted by intensity of reflected sound

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

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

Tympanometry tracks changes in static immittance over time

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

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

Tympanometric (Jerger) types

Tympanometric Shape

(as recommended by ASHA)Width of the tympanogram at the 50% amplitude point

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

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.

Acoustic Reflex Anatomy and Physiology

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)

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.

Diagnostic Significance

Conductive LossSensorineural Loss

Cochlear Loss

Retrocochlear Loss