1. PSYCHOACOUSTIC AUDIOMETRY
AND
EVOKED PHYSIOLOGICAL
MEASUREMENT OF AUDITORY
SENSITIVITY
By
Dr. Syed Salman Hussaini

2. PURE-TONE AUDIOMETRY

The purpose of pure-tone audiometry is to determine
hearing threshold levels for pure tones.

The threshold of hearing is defined as the 'level of a sound
at which, under specified conditions, a person gives 50
percent of correct detection responses on repeated trials'.

'Specified conditions' means the type of sound and ways of
presenting the test sound.

The normal test sound is pure tone pulses at standardized
frequencies in the range of 125-8000 Hz and the normal
presentation mode is monaurally by means of a standardized
type of earphone.

3. 
Hearing thresholds are sensitive indicators of the functional
state of the peripheral part of the auditory sense organ.

Pure tone audiometry has become the standard method for
quantitative description of degree of hearing loss.

It also provides information regarding the localization of
the lesion that causes the hearing loss.

4. Equipment

A pure tone audiometer is designed in terms of a set of basic
functions.

Sinusoidal signals, pure tones, at standardized frequencies
in octave steps from 125 to 8000 Hz constitute the basic test
signals generated by the oscillator function.

In order to obtain improved resolution in the range where
most hearing losses occur, the intermediate frequencies
1500, 3000 and 6000 Hz are usually also included.

Equipment providing test signals at higher frequencies
(8000 to 16,000 Hz) than the conventional range is also
available.

6. 
Tone pulses are formed by the pulse former function which
controls rise time, duration and fall time.

The attenuator is a volume control that allows the change
of signal level in calibrated steps along a decibel scale.

For the normal manual procedures, as well as computer-
controlled versions, the optimum step size is 5 dB.

The attenuator scale is calibrated in decibel hearing level.

This scale has its zero at a sound level which for each tone
frequency corresponds approximately to the average normal
threshold of hearing for young, otologically normal subjects.

7. 
After suitable amplification, the signal is finally delivered to
one of the earphones.

The most commonly used earphone type is the supra-aural
Telephonics TDH-39 or its later versions Telephonics TDH-
49 and 50.

Larger circum-aural earphones, Sennheiser HDA-200, are
housed in relatively effective noise-excluding muffs.

Insert earphones of type Etymotic Research EAR-Tone 3A,
coupled to the ear canal by means of foam inserts, offer
another alternative with superior attenuation of ambient
noise compared to supra-aural earphones.

9. Bone conduction

Bone vibrator as an alternative output transducer, whereby
mechanical vibrations coupled to the skull bone at the
mastoid process behind the external ear are used to stimulate
the cochlea through bone conduction.

The sensitivity to detect these mechanical vibrations
depends on the sensorineural function alone with negligible
influence from the outer and middle ears.

Thus, a comparison of results obtained by air and by bone
conduction provides evidence for the localization of a lesion.

10. 
In addition to the direct mechanical pathway to the cochlea
there are three additional routes that may interfere.

1. The vibrations of the skull give rise to a relative motion
of the ossicular chain, the middle ear component.

2. They also give rise to vibrations of the walls of the
external auditory canal, in particular the outer cartilaginous
part - this is the external ear component.

When the external ear is open, this component is relatively
weak, but when the ear canal is occluded, e.g. by an
earphone, a hearing aid or a hearing protector, it may become
significantly larger.

11. 
3. Finally, a fluid component may also provide a pathway
for bone conduction stimuli to the inner ear via the
cerebrospinal fluid (CSF) and its connection to the
perilymph of the inner ear.

The actual stimulation of the cochlea by bone conduction
stimuli is determined by the vectorial sum of all these
components.

12. Masking noise

A test signal from a bone vibrator will reach both cochleae
at approximately equal levels.

In air conduction testing with earphones, a certain degree
of vibratory energy is transferred to the skull.

For the supra-aural earphones, the level of this vibratory
signal is on average 60 dB below that of the air conduction
signal generated, but may be as low as 40 dB below.

Thus, when testing the poorer ear in cases of asymmetrical
hearing loss, if the side difference is 40 dB or greater there is
a risk that the test signal heard is actually a bone conducted
signal picked up by the contralateral ear rather than an air-
conducted signal picked up by the poorer test ear.

13. Masking noise

In order to allow reliable measurements of hearing
thresholds for bone conduction in general, as well as for air
conduction in the poorer ear when there is a side difference
of 40 dB or more in air conduction hearing thresholds,
masking of the contralateral nontest ear is necessary.

Insert earphones have a much weaker mechanical coupling
to the listener's head, and therefore transfer less energy to the
skull.

When insert earphones are used, the level of the transferred
bone conduction component is at least 20 dB lower than that
generated by supra-aural earphones.

14. 
Masking is performed by the presentation of narrow-band
filtered random noise with a centre frequency equal to the
test tone frequency being used.

Thus, a clinical pure-tone audiometer needs a second
channel where random noise is generated, band-passfiltered,
attenuated and amplified in order to be presented as a
continuous sound through one of the earphones available.

The calibration of the attenuator when used for adjusting the
masking noise level is in decibel effective masking level.

15. Test environment

Psychoacoustic tests require a concentrated undisturbed
listener.

This concerns absence of visual and other sensory
distractions, acceptable ventilation and sufficiently low
ambient sound levels.

The requirement on ambient sound levels is of particular
importance in threshold audiometry in order to avoid test
stimuli of very low sound levels being masked by unwanted
sounds in the test room.

16. Test method

The test should always begin with the better ear if the test
subject is aware of a side difference.

A clear instruction to the subject is an essential part,
informing about the test procedure, the listening task, i.e.
trying to detect test tones that may be very faint, and the
response task, usually pressing a button in response to
detected tone.

Two alternative methods for the manual determination of
hearing thresholds are described.

17. Test method

The ascending method (modified Hughson-Westlake
method).

After familiarization by presenting a clearly audible test
tone, it is based on repeated ascents from inaudible to just
audible stimuli in steps of 5 dB.

As soon as the listener responds, the level is decreased by 10
dB and a new ascent is started.

The hearing threshold level is defined as the stimulus level at
which the listener first has given three correct responses after
three to five ascending series of stimuli.

The first test frequency is 1000 Hz followed by the higher
frequencies in rising order and finally the lower frequencies
in falling order.

18. Test method

The alternative manual method is the bracketing method,
which is based on alternating ascending and descending
series.

When a response has occurred in an ascending series, the
level is increased another 5 dB and a descending series is
started in 5 dB steps.

When no response occurrs the level is lowered another 5 dB
and a new ascending series is presented.

This is repeated until three ascending and three
descending series have been completed.

The hearing threshold level is the average of the three
lowest audible levels in the ascending series and the three
lowest audible levels in the descending series.

19. 
When masking is needed, narrowband masking noise should
be presented to the non-test ear.

This procedure is called the plateau-seeking method.

The masking noise is presented continuously to the
contralateral ear and is introduced first at the level where it is
just audible.

It is increased in steps of 5 dB as long as each increase in
masker level requires an increase in test tone level in order
for the test tone to remain audible.

When the masking level can be increased by three 5 dB steps
or more without affecting the audibility of the test tone, this
constitutes the masking plateau.

20. Screening audiometry

Screening audiometry is a simplified procedure which aims
at identifying those listeners whose hearing threshold levels
exceed a certain level, the screening level, without spending
time on those whose hearing threshold levels are better than
this level.

Thus, it allows a possibility of saving test time, which is
meaningful provided that no valuable information is lost.

The normal prerequisites for choosing a screening procedure
and a specific screening level are that the subjects identified
by the procedure can be offered a meaningful intervention
and/or that hearing threshold levels better than the screening
level represent variations within the normal limits.

21. Screening audiometry

Screening audiometry is often performed when testing
school children and a common screening level is then 20 dB
HL.

This level represents the statistical borderline for normal
hearing threshold levels, corresponding approximately to the
average plus two standard deviations.

22. Sources of error
PHYSIOLOGICAL SOURCES

The auditory pathways always have a certain spontaneous
activity, also in the complete absence of any audible sound.

This activity can be considered as a physiological Noise.

When a test signal is presented at a barely audible sound
level, it will introduce a certain regularity in the otherwise
irregular spontaneous activity in the auditory pathways.

Because of this irregularity there will be a certain degree of
randomness with regard to the probability of the test tone
being audible or not.

23. PSYCHOLOGICAL SOURCES

Each test is to start with the tester instructing the test subject
about the procedure and the subject's role in the process.

The test subject's concentration is probably the dominating
source of error.

Even in a relatively limited test session involving only air-
conduction testing, the goal of keeping a high concentration
level on the listening task without thinking of anything else is
virtually impossible to reach.

When the test involves not only air-conduction but also bone
conduction and masking, fatigue will of course affect the
listener's ability to concentrate and cooperate.

The test environment should be designed for optimum
concentration.

24. METHODOLOGICAL SOURCES

The instruction of the test subject is an important part of any
psychophysical test.

The various parameters of the actual test method, i.e. the
presentation of the test tones, their duration, intervals
between successive test tones and order of varying the test
tone level, are other factors that may affect the test result.

An important part of the standardized method is also the
definition of hearing threshold level and how it is determined
according to the subject's response pattern relative to the
presented stimuli.

25. PHYSICAL/ACOUSTICAL SOURCES

Supra-aural earphones are supposed to provide a close fit
to the outer ear without leakage, but exceptions often occur
in practice.

A leakage usually affects the lowest test frequencies, making
the actual sound levels generated at the ear drum lower than
they should be without leakage.

At the highest test frequencies the wavelength of the sound is
of the same order of magnitude as the dimensions of the
enclosed cavity underneath the earphone.

Even relatively small changes in position of the earphone in
relation to the ear canal opening may affect the sound
pressure at the ear drum.

26. 
The placement of the bone vibrator on the mastoid process
behind the pinna has to take place without the aid of any
clear anatomical landmark.

The use of insert earphones makes the placement somewhat
easier. The only important factor to control here is to insert
the foam ear tip by its full length into the ear canal.

Ambient sound levels constitute another potential source of
error.

Correct calibration of the test equipment is obviously
important.

The equipment may change slowly over time due to ageing
in electronic components or it may change suddenly.

27. Clinical applications

An air-conduction pure-tone audiogram is the basic test
measure which is used to express the degree of hearing loss.

It is an important basis for assessing the needs for intervention,
including the fitting of hearing aids.

A complete audiogram, i.e. using both air- and bone-
conduction, provides important information for the topical
diagnosis in terms of differentiation between conductive and
sensorineural disorders.

Equal loss for air and bone-conduction indicates a
sensorineural lesion whereas a larger loss by air-conduction
than by bone-conduction, an air-bone gap, indicates the
presence of a conductive lesion.

The air-bone gap at a single frequency needs to be at least 15
dB in order to be considered statistically significant.

28. Special tests with pure tones

In clinical applications related to hearing aid fitting, use is
being made of test methods describing the loudness
function of the ear to be fitted as a basis for compression of
input sound levels to be provided by the hearing aid for the
ear with reduced dynamic range.

A simple way to provide additional information is to
measure the uncomfortable loudness level (UCL), as a
measure of the upper limit of the dynamic range.

Stimuli may be either pure tones or narrow bands of noise,
presented as pure tone or noise pulses at increasing levels
until the subject indicates the highest acceptable loudness.

29. SPEECH AUDIOMETRY

The measurement of speech recognition ability is an
important functional complement to pure tone audiometry.

Speech audiometry is a more complex procedure.

In pure tone audiometry, the listening task is simply
detection of the stimuli.

However, in speech audiometry the usual task is not
detection but recognition which requires both detection and
identification of the phonemes and recognition of sets of
phonemes as words.

Thus, depending on the choice of speech test material and
the listener's task, the test result reflects not only the auditory
function, but is also affected by cognitive and linguistic
functions.

30. Equipment

Equipment for speech audiometry is usually integrated with
a clinical pure-tone audiometer.

Recordings should preferably be digital. The digital
technology provides the significant advantage of constant
quality of the recording, independent of how many times it
has been used.

Live voice should be avoided where possible.

In conventional clinical speech audiometry, the test signal is
presented monaurally by means of earphones.

Contralateral masking may be needed to prevent the test
signal from being heard by the better ear when testing the
poorer ear in cases with large asymmetries.

31. 
Speech audiometry is often performed in sound field where
the speech signal is presented by means of a loudspeaker.

This test situation is much more difficult to standardize than
when earphones are used.

The characteristics of the test room are of great importance.

A free sound field can be achieved only in an anechoic
room, where no reflective surfaces are present and only the
direct sound from the loudspeaker will reach the listener.

32. 
In sound field audiometry, the loudspeaker should be
placed in front of the listener at a distance of at least 1 meter
and at the same height as the head of the listener.

If the speech signal is to be presented with a background of
competing noise, the noise may be presented either through
the same loudspeaker as the speech or through a pair of
loudspeakers located on either side of the frontal
loudspeaker, at a recommended angle of ± 45°.

When the competing noise is presented through the two
noise loudspeakers, the result of the speech recognition test
is affected by the listener's ability to take advantage of
binaural hearing.

33. Speech level

Hearing level for speech, expressed as dB HL, is mesaured
as the speech level relative to the reference level for speech.

Reference level for speech is defined as the 'median value
of the speech recognition threshold levels of a sufficiently
large number of otologically normal persons, of both sexes,
aged between 18 and 25 years inclusive and for whom the
test material is appropriate'.

34. Speech material

The speech test material may differ widely from nonsense
combinations of consonants and vowels, so-called logatoms,
to natural connected speech.

The simplest test items such as logatoms or monosyllabic
words primarily reflect the function of the peripheral part
of the auditory system.

Bisyllabic words provide considerable redundancy
(repetition) - only parts of the complete word may be
sufficient for the listener to recognize in order to be able to
guess correctly by using his linguistic knowledge.

35. Speech material

Sentences may be designed to offer low or high redundancy.

When high-redundancy sentences and connected speech is
used as test material, the listener's cognitive and linguistic
functions will have significant effects on the test results in
addition to the auditory function.

Logatoms and single words provide the best test reliability
at the expense of validity.

Sentences and connected speech provide increased validity
but at the expense of reliability.

36. Speech in background noise

In order to simulate real life situations, speech audiometry
is often performed with the speech signal presented against a
background noise.

The degree of masking of the speech signal by the noise
depends on the sound pressure level of the noise in relation
to the sound pressure level of the speech signal.

This relation will affect to what extent the speech signal will
be audible above the noise.

The difference between speech level and the sound pressure
level of the noise is called the signal-to-noise ratio or the
speech-to-noise (SIN) ratio, expressed in dB.

37. Test method

The 'speech recognition threshold level' is defined as the
lowest speech level at which the speech recognition score is
equal to 50 percent for a given test subject, a specified
speech signal and a specified manner of signal presentation.

The instruction of the test subject before commencing the
actual measurement is essential.

The tester is to inform about which ear to be tested when
using earphones, what type of test items will be presented,
and the listener's response task, which usually is to repeat
orally what he thought he heard.

The recommended procedure that is most commonly used
starts with familiarizing the listener by presenting the first
test item at a clearly audible level.

38. Test method

A suitable starting level is a hearing level for speech that is 20-30
dB above the average pure tone hearing thresholds at 500, 1000
and 2000 Hz.

Then the speech level is reduced in steps of 5 dB, presenting two
test items on each level, until the listener no longer responds
correctly to all test items.

At this level, a set of test items, consisting of at least ten items, is
now presented.

If more than 50 percent of the items are correctly recognized, the
level is reduced by 5 dB and another set of test items is presented.

This descending procedure is repeated until the score at a certain
level is below 50 percent.

The speech recognition threshold level is the integer value of
the level corresponding to 50 percent correct.

39. When speech recognition in a background of competing noise
is to be determined, this can be carried out according to two
alternative methods.
1. The speech signal and the noise are presented at fixed sound
levels and the speech recognition score under these
conditions is determined.
2. The speech signal is presented at a fixed level and the noise
level is varied in order to determine the SIN at which the
listener's speech recognition score reaches a certain value,
usually 50 percent correctly recognized test items.

40. Sources of error

An important factor in speech audiometry is the effect of the
listener's linguistic skills.

The test lists should present words that are included in the
listener's vocabulary.

This is important when testing children with a constantly
expanding vocabulary.

The language used for testing should be the listener's native
language.

Especially in more difficult listening situations, the
difference in performance between native and non-native
listeners becomes a significant factor.

41. 
Cognitive functions constitute another area that influences
speech recognition ability, in particular speech recognition in
noise.

It is well known that cognitive functions on average
deteriorate progressively above approximately 70-75 years
of age.

Thus, not only reduced auditory acuity, but also cognitive
decline, may affect speech recognition performance among
elderly listeners.

42. CLINICAL APPLICATIONS OF
SPEECH AUDIOMETRY

In general, the speech recognition threshold (SRT) is
expected to agree with the average pure tone hearing
threshold levels at 500, 1000 and 2000 Hz pure tone average
(PTA) within 10 dB.

If the SRT is significantly poorer than the PTA, this may
indicate a retrocochlear or central lesion.

If, on the other hand, SRT is significantly better than the
PTA, this might indicate nonorganic hearing loss.

43. 
For listeners with normal or only mild cochlear hearing
loss, the maximum speech recognition score should reach
100 percent or close.

This also holds for pure conductive hearing loss, although
obviously the speech level has to be raised above normal in
order to reach this maximum.

For more pronounced cochlear lesions, maximum speech
recognition score may be significantly below 100 percent.

This is likely to be due to both various kinds of distortion
caused by the cochlear lesion, as well as the typical large
difference in sensitivity between low and high frequency
ranges.

44. 
Retrocochlear lesions often show significantly poorer
performance than expected according to their pure tone
audiogram.

Patients with central auditory lesions are likely to perform
relatively normally in conventional speech audiometry in
quiet but below normal on tests using dichotic speech or
distorted speech.

45. Otoacoustic emissions

Otoacoustic emissions (OAE) are acoustic signals emitted
from the cochlea to the middle ear and into the external ear
canal where they are recorded.

They are generated by active mechanical contraction of the
outer hair cells, spontaneously or in reponse to sound.

There are four types of OAEs:

spontaneous OAEs (SOAE),

transient evoked OAEs (TOAE),

distortion product OAEs (DPOAE), and

stimulus frequency OAEs (SFOAE).

46. Otoacoustic emissions

All four types of OAEs are recorded with a sensitive, low
noise microphone that is placed in the sealed external ear
canal.

When OAEs are evoked, the sealed probe includes a tube
for sound delivery to the ear canal, in addition to the
recording microphone.

The microphone records all sounds in the ear canal, and
these include, in addition to OAEs, the sound evoking the
OAEs when TEOAEs or DPOAEs are recorded, as well as
other patient-generated and ambient sounds.

47. TRANSIENT-EVOKED EMISSIONS

The delay between stimulus offset and onset of the evoked
emissions varies between 4 ms, for high frequencies, and 20
ms for low frequencies.

This temporal separation helps in visual identification and
separation of the transient-evoked emissions from the
stimulus that evoked them, that is also recorded.

Thus, TEOAEs are typically presented as an amplitude/time
plot of the acoustic waveform recorded from the ear canal.

TEOAEs greater than 20 dB sound pressure level (SPL) can
be recorded from newborns, while responses from children
and adults range between 10 and 15 dB SPL.

The most effective stimulus to evoke TEOAEs is a tone
burst in the mid-frequencies.

49. DISTORTION PRODUCT EMISSIONS

DPOAEs are generated in the cochlea in response to two
simultaneous pure-tone stimuli (primary tones).

This tonal response is not present in the eliciting stimuli, and
is therefore referred to as a distortion.

Because DPOAEs are separated in frequency from the
eliciting stimuli, they can be recorded in the presence of the
stimulating tones and separated from them by spectral
analysis.

Their magnitude is very small (5-15 dB SPL), approximately
60-70 dB below the level of the stimuli used to evoke them.

DPOAEs are attributed to nonlinearity of motion of the
outer hair cells, particularly at low stimlus levels.

51. Neurogenic evoked potentials

Electric signals generated by the auditory pathway from the
cochlea to the cerebral cortex are the most reliable and
widely used physiological estimators of auditory sensitivity.

Such recordings belong to a class of electrophysiology called
'evoked potentials', or 'event-related potentials', defined as
changes in voltage that occur at a particular time before,
during or after a change in the physical world and/or some
psychological process that gave rise to these voltage
changes.

When evoked by a stimulus in the physical world outside the
brain, they are called 'exogenous', whereas if evoked by a
psychological, cognitive process within the brain they are
termed 'endogenous'.

52. COCHLEAR POTENTIALS

Cochlear potentials that can be recorded include the cochlear
microphonic potentials, the summating potential and the
compound action potential of the auditory nerve.

These potentials are most readily recorded in the
electrocochleogram (ECog), which can be recorded from an
electrode that is inserted as close as possible to the cochlea.

In electrocochleography, a needle electrode is inserted through
the tympanic membrane to rest on the promontory.

In case of a perforated eardrum, a ball-tipped electrode is
inserted through the perforation to rest on the round window.

Transtympanically recorded cochlear potentials are 20 times
larger than those recorded noninvasively from the ear canal,
and 10 times larger than those recorded noninvasively from an
electrode resting on the tympanic membrane.

54. 
Cochlear potentials can be recorded noninvasively from the
ear canal using an electrode resting on the tympanic
membrane.

The cochlear compound is recorded as a major negative peak
of a few uV, called N1 or action potential (AP), at
approximately 1.5 ms after stimulus onset at the eardrum,
followed by a minor negativity called N2, at approximately
2.5 ms.

The summating potential (SP), preceding N1 as a negative
step-like deflection from baseline.

The Ecog is affected by auditory sensitivity in the range of
1000 to 4000 Hz and is independent of the subject's state of
arousal or the effects of drugs.

56. AUDITORY NERVE AND
BRAINSTEM POTENTIALS

Auditory nerve and brainstem evoked potentials (ABEP) are
optimally recorded from the scalp by disc or cup electrodes
in response to high intensity clicks presented at a rate of
approximately ten per second.

The normal waveform includes a series of five to seven
voltage oscillations, approximately 1 ms apart during the
first 6-10 ms after stimulus onset.

The first peak in the sequence, peak I, is the only one to
survive section of the auditory nerve central to the internal
auditory canal, placing its generator in the cochlea.

It is synchronous with N1 of the ECog

57. 
The second peak, II, is synchronous with proximal
auditory nerve activity.

Overlapping activity from the auditory nerve and from the
cochlear nucleus.

Peak II is generated in the viscinity of the auditory nerve's
entry into the brainstem.

Peak III generators span the lower brainstem between the
cochlear nucleus, through the trapezoid body to the superior
olivary complex.

For practical clinical purpose the generators may be
attributed to the lower pons.

58. 
The fourth component is not always identified.

Peak IV is usually partially merged with V, creating a bifid
IV-V complex.

Generators of this complex are the upper pons, between
the superior olivary complex, through the lateral lemniscus,
with possible contribution from the inferior colliculus.

For clinical purposes, the IV -V complex can be attributed
to the upper pons and its junction with the midbrain.

60. 
Audiometric ABEPs measures include the lowest stimulus
intensity at which a response is detected (detection
threshold).

Peak latencies measure the time lapse between stimulus
onset and the time of highest synchronous activity.

Multiple factors that contribute to peak latency have led to
the definition of interpeak latency difference measures.

The most widely used interpeak latency differences are
V-I, between cochlea and ponto-midbrain junction
III-I, between cochlea and ponto-medullary junction, and
V-III, along the pons

61. 
A significant interaural difference in respective measures,
in response to left and right ear stimulation, may be
indicative of a unilateral functional abnormality.

ABEPs are affected by a variety of nonpathologic factors
which include the subject's age, body temperature and
gender, as well as stimulus factors such as frequency
composition, intensity, presentation rate and envelope.

ABEP peak latencies shorten with increasing stimulus
intensity.

During childhood, the peak amplitudes are typically larger
than in adults, particularly component I which is often larger
than V.

63. TYMPANOMETRIC ACOUSTIC
REFLEX

Tympanometry is the continuous measurement of middle ear
impedance as air pressure in the sealed external ear canal is
varied.

The measurement device delivers a tone toward the tympanic
membrane, and the impedance or admittance of the middle
ear are quantified based on the intensity and other properties
of the tone in the ear canal.

The graph describing the mechanical properties of the
middle ear as a function of pressure in the external ear canal
is the tympanogram.

64. TYMPANOMETRIC ACOUSTIC REFLEX

Tympanograms typically show compliance (the inverse of
stiffness) as the measured aspect of impedance, and there are
three main types:

Type A, which shows a clear peak of compliance between 0
and -100 mm of water, associated with normal function;

Type B, where no peak in the compliance is noted, typically
associated with fluid in the middle ear cavity; and

Type C, resembling type A but peaking at a pressure more
negative than -100 mm of water, most commonly found in
patients with inadequate ventilation of the middle ear, such
as with Eustachian tube dysfunction.

66. TYMPANOMETRIC ACOUSTIC REFLEX
 A variant of type A (As) with abnormally low compliances
(unusually high impedance) is found in patients with fixation
of the ossicular chain, such as in otosclerosis.

Conversely, an abnormally high compliance (unusually low
impedance) type A tympanogram (Ad) is typical in patients
with disruption of the ossicular chain, rendering the ear
drum abnormally mobile and compliant.

When the tympanic membrane is ruptured or perforated,
tympanometry cannot be conducted and the ear canal volume
indicated by the tympanometer, when the sealing of the canal
is verified and measured, is larger than 2.5 cc.

67. 
The acoustic impedance of the ear changes when the
muscles of the middle ear contract.

The acoustic reflex is the contraction of the middle ear
stapedius muscle, attached to the posterior part of the stapes,
in response to medium to high intensity sounds.

The reflex arc includes the auditory nerve, brainstem
neurons connecting the cochlear nucleus ipsilateral to the
stimulated ear with bilateral neurons in the motor nuclei of
the facial nerve.