2. DEFINITIONS
AUDIOLOGY AND ACOUSTICS:
Audiology refers to the study of hearing disorders through the
hearing evaluation as well as the rehabilitation of the patients.
Acoustic pertains to hearing and perception of sounds.
Acoustics is the science that deals with the sounds and their
perception.
3. SOUND:
Sound is a form of energy, produced by any vibrating object.
Produced by compression and rarefaction of molecules of
medium in which it travels.
FREQUENCY AND PITCH:
Frequency refers to the number of cycles per second. Hertz (Hz).
(Hz).
Pitch is a subjective sensation produced by frequency of sound.
4. PURE TONE:
A single frequency sound is called a pure tone such as 250, 500 or
1,000 Hz.
Threshold is the lowest sound pressure level at which under
specified conditions the person gives predetermined percentage
of correct responses on repeated trials.
In PTA, thresholds of hearing in decibels for various pure tones
from 250 to 8,000 Hz are measured.
COMPLEX TONE:
The sound, which has more than one frequency, is called a
complex sound such as voice and speech.
5. INTENSITY AND LOUDNESS:
The intensity is the strength of
sound and determines loudness.
Decibel (dB)
Loudness is a subjective
sensation produced by intensity.
6. TYPES OF HEARING LOSS
1.Conductive hearing loss:
Diseases, which interfere with conduction of sound from
external ear to the stapedio-vestibular joint, result in
conductive hearing loss (CHL).
1. External auditory canal obstructions: wax and inflammatory
swelling.
8. 2.Sensorineural hearing loss:
The diseases of cochlea (sensory type) and CN VIII and its
central connections (neural type) result in sensorineural
hearing loss (SNHL).
Retrocochlear HL refers to lesions of 8th nerve and central
auditory connections.
Sensory (Cochlear): Presbycusis, mumps, noise-induced.
Neural: Neural presbycusis and acoustic neuroma.
9. 3.Mixed hearing loss:
Has elements of both conductive and SNHL.
The air-bone gap in PTA indicates conductive element while
while impairment of bone conduction indicates SNHL.
The most common causes of mixed hearing loss (MHL) are
otosclerosis and COM.
10. Need for evaluating the hearing loss:
Hearing assessment facilitates in finding out following
features of hearing loss:-
„
Type of hearing loss: Conductive, sensorineural or mixed.
„Severity of hearing loss: Mild, moderate, moderately severe,
severe, profound or total.
11. „Site of lesion: The diseases of external ear, tympanic
membrane and middle ear result in CHL. Clinical examination
and audiometry are helpful in finding site of lesion.
Special hearing tests can differentiate between different types
of SNHL such as cochlear, retrocochlear and central.
12. „Cause of hearing loss: may be congenital, traumatic, infective,
neoplastic, degenerative, metabolic, ototoxic, vascular or
autoimmune process.
13. Methods of hearing evaluation are classified into two
groups subjective and objective:
Subjective methods which need patient’s co-operation and
reliability Tuning fork test, PTA and speech audiometry.
Objective methods which can be done even in children
include impedance, electrocochleography (EcoG), BERA and
oto-acoustic emission.
14. PTA
Non-invasive subjective test is a
graphic recording of hearing level
both quantitatively and
qualitatively.
Audiometer is an electronic
device that generates pure
tones.
The intensity of these tones are
either increased or decreased in 5
dB steps.
15. Audiometer is so calibrated that the
hearing of a normal person is at zero
dB level.
AC thresholds are measured usually
for tones of 250, 500, 1,000, 2,000,
4,000 and 8,000 Hz.
BC thresholds are measured usually
for 250, 500, 1,000, 2,000 and 4,000
Hz.
Speech frequencies range from 500–
2,000 Hz
16. PURE TONE AVERAGE: The pure
tone average is an average of the
AC thresholds at 500, 1,000, and
2,000 Hz frequencies.
„AIR-BONE GAP:
The BC thresholds are measure of
cochlear function.
17. The difference between the
thresholds of AC and BC called
air-bone gap (A-B gap) is a
measure of CHL.
„Decibel: A decibel is an
arbitrary logarithmic unit of
intensity of sound. Hearing is
measured on a biologic scale.
18. Method:
Audiometry is done in a
soundproof room (ideal) or a
quite room.
First AC and then BC is
recorded separately for each
ear.
The pure tones are presented
to the ears by headphone (for
AC) and vibrator (for BC).
19.
20. The graph on which these
thresholds are charted is
called audiogram.
For right ear red color and
for left ear blue color pencils
are used.
AC continuous line, BC
interrupted (broken) line is
used for recording.
21.
22. MASKING:
Air conduction sounds “crossover” occurs when a 45-50 dB
difference exists between AC thresholds of two ears.
BC sounds “crossover” may occur even at 0 dB difference
between the BC thresholds of the two ears.
The narrow-band noise in the non-test ear (better hearing) is
employed for the masking.
23. Presenting a noise into the non test ear so that its blocked
acoustically and does not participate in hearing test.
In AC masking is required if the AC of testing ear is >45dB, as
the lower limit of inter-aural attenuation(IA) is 40-45dB.
Cross hearing is suspected if AC(TEST)-BC(NON TEST) > IA
24. If masking sound presented to the non test ear is not loud
enough to eliminate the non test ear from participating in the
test, it is undermasking.
if masking presented to non test ear is loud and crosses and
blocks the cochlea of test ear, it is overmasking.
25.
26. When difference between the AC of
two ears is 40 dB or more, the better
ear is masked.
Masking avoids getting a shadow
curve from the better ear.
Masking of the non-test ear is
necessary during the BC studies.
27. SOUNDS USED FOR MASKING:
► White noise(wideband/broadband noise)
► Narrow band noise(more effective)
► Complex noise
28. INTERPRETATION OF PTA:
► AC threshold
► BC threshold
► A-B gap
Conductive hearing loss:
AC threshold > 30dB
BC threshold < 20dB
A-B gap > 25dB
29. Sensorineural hearing loss:
AC threshold > 30dB
BC threshold > 20dB
A-B gap < 20dB
Mixed hearing loss:
AC threshold > 45dB
BC threshold > 20dB
A-B gap > 20dB
30.
31.
32.
33.
34.
35.
36.
37.
38. USES OF PTA:
Measure of thresholds of AC and BC tells the degree and type
of hearing loss (mild-profound, CHL/SNHL).
The progress of the disease and response to the treatment can
be documented (improving/progressive).
The type and necessary setting of hearing aids can be
determined.
39. The degree of handicap for medico-legal purposes can be
determined.
Speech reception threshold (SRT) can be predicted.
40. Limitations of PTA
Inaccuracy of audiogram due to improper technique/examiner/test
condition/audiometer.
Subjective and time consuming test.
Does not assess all features of hearing.
Does not identify the nature of pathology.
Multiple factors dependency.
