lecture 2 b instrumentation used in the measurement of acoustic signals and auditory function
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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
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