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PURE TONE AUDIOMETRY
Dr.A.GANESH BALA M.S ENT
ASSOCIATE PROFESSOR
VMMC - KARAIKAL
DEFINITIONS
SOUND- It is a form of energy produced
by vibrating object.
PURE TONE SOUND- sound produced by
a sine wave at a single frequency.
FREQUENCY- It is a number of cycles per
seconds & is measured by HERTZ.
PITCH- It is a subjective sensation
produced by frequency of sound.
INTENSITY- It is the strength of sound which
determines its loudness & is measured in DECIBELS.
LOUDNESS- It is subjective sensation produced by
intensity.
HEARING THRESHOLD- The lowest sound pressure
level ,at which under specified conditions, a person
gives a predetermined percentage of correct responses
on repeated trials.(minimum sound that a subject may
hear for a given tone or frequency)
DECIBEL – 1/ 10 of bel.
AIMS
Whether the subject has any definite auditory
disorders.
Whether the hearing loss is conductive,
sensorineural or mixed.
The degree of hearing dysfunction.
The results plotted graphically is called pure tone audiogram.
Instrument used to measure is called pure tone audiometer.
The range of normal human hearing is 20- 20,000 HZ.
The graph plotted in x –axis frequency in hertz are 250,500, 1000,2000,4000 & 8000 (cycles per second ).
In y –axis hearing loss in decibels from -10 to 110 db.
PURE TONE AUDIOGRAM
PROCEDURE - AIR CONDUCTION
TEST
Calibration of the instrument.
A reasonably noiseless test environment.
Position of headphones
Instructions to the patient.
Technique of air conduction test- First is Conventional
Method Hughson-Westlake technique , modified by
CALIBRATION OF INSTRUMENTS
ISO – 1964 Specifications for calibration
Electronic calibration – atleast once in 6 months
Biological calibration – should be done each day
before the audiometer is used.
Micro-processor based audiometers are used
nowadays.
A REASONABLY NOISELESS TEST
ENVIRONMENT
Level of Test Environment < Level of Masking
Sound
Cause A Threshold Shift In Normal Hearing
Subject
ISO / DIS 8253 – maximum permissible ambient
noise for the different frequencies required for air and
bone conduction tests.
Air Conduction - 25 - 30 db
Bone Conduction - 10 -15 db
POSITION OF HEADPHONES
Diaphragm of headphone – over the opening of
EAC ( >15 db)
No wax , discharge , cotton in EAC
Collapse of ear canal – supraaural earphones /
headphones are placed over ear – Cause small Air-
Bone gap.
INSTRUCTION TO PATIENT
Test needs should be thoroughly explained.
Little time spent in getting acquainted with the
patient and his problems prior to the test , helps in
establishing a rapport.
TECHNIQUES - CONVENTIONAL
METHOD
A detailed clinical history & examination should
precede the test.
Better ear is tested first, start with 1000 Hz & then
2k,4k,8k,10k,then 500 , 250 Hz.
If difference is more than 20 db then half octaves is
to be tested.
Tones are lowered in 10 db steps and increased in
5 db steps.
TECHNIQUES - CONVENTIONAL
METHOD
Second ear – may start with last frequency used to
test the first ear ( no need to start with 1000 Hz )
5-up-10-down ( tones are lowered in 10 db steps
and increased in 5 db steps )
STEP 3 - RAISE SOUND BY 10 db (i.e 60 db)
RESPONSE PRESENT –PROCEED TO STEP 5
RESPONSE ABSENT –PROCEED TO STEP 4
STEP 2 - RAISE SOUND BY 20 db (i.e 50 db)
RESPONSE PRESENT –PROCEED TO STEP 5
RESPONSE ABSENT –PROCEED TO STEP 3
STEP 1 - START WITH 30 db HL SOUND
RESPONSE PRESENT –PROCEED TO STEP 5
RESPONSE ABSENT –PROCEED TO STEP 2
METHOD OF ASHA
STEP 6 - RAISE SOUND BY 5 dbRESPONSE PRESENT – PROCEED TO STEP
7
RESP ABSENT - RETURN TO STEP 6 , I.E RAISE SOUND BY 5 DB TILL RESPONSE IS PRESENT THEN
PROCEED TO STEP 7
STEP 5 – LOWER SOUND BY 10 dbRESPONSE PRESENT – REPEAT STEP 5 , I.E AGAIN LOWER TILL
RESPONSE ABSENT THN PROCEED TO STEP 6
RESPONSE ABSENT –PROCEED TO STEP 6
STEP 4 – RAISE SOUND BY 10 db STEPS TILLRESPONSE PRESENT AT A PARTICULAR STEP –
PROCEED TO STEP 5
MAXIMUM OF AUDIOMETER BUT NO RESPONSE
OBTAINED – RECORD AS NO RESPONSE OBTAINED
METHOD OF ASHA
METHOD OF ASHA
STEP 7 – 6 STIMULI PRESENTED AT SAME LEVEL
3 RESPONSE CORRECT –RECORD AS HEARING
THRESHOLD FOR THAT PARTICULAR FREQUENCY
LESS THAN 3 CORRECT – RETURN TO STEP 6
STEP 6 – SOUND RAISE BY 5 DB
RESPONSE PRESENT –PROCEED TO STEP 7
RETURN TO STEP 6 , I.E RAISE SOUND BY 5 DB TILL
RESPONSE IS PRESENT THEN PROCEED TO STEP 7
BONE CONDUCTION
COMPRESSIONAL / DISTORTIONAL BONE CONDUCTION
Vibratory energy ( Sound ) reaches the cochlea
Alternate expansion and compression of cochlear shell(due to flexiblity of round window memb and cochlear
equeduct)
Movement of cochlear fluid
Displacement of basilar membrance
Leads to changes that result ultimately in sound being heard
BONE CONDUCTION INERTIAL BONE CONDUCTION
Vibratory energy ( Sound ) strikes the skull
Sets the skull into vibration
Ossicles in middle ear lag behind & do not move due to
inertia of ossicles
Sets up relative motion b/w footplate of stapes & cochlear
fluid deep to oval window
Vibration of cochlear fluid
Movement of basement memb Sound to be heard
BONE CONDUCTION
OSSEO-TYMPANIC BONE CONDUCTION
Vibratory energy ( Sound ) reaches the skull
Skull starts vibrating
Sets into vibration the column of air in EAC
Partially transmitted to TM
Thro’ the ossicles in the middle ear
To Cochlear fluid of inner ear ( like air - conducted sound)
PROCEDURE - BONE
CONDUCTION
Calibration of instrument
Reasonably noiseless test environment
Placement of bone conduction vibrator
Instructions to the patient
Technique – same as air conduction.
PROCEDURE - BONE
CONDUCTION
MASTOID PLACEMENT OF BONE VIBRATOR
Tension of spring metal headband over the mastoid -
500 gm / sq.cm
Should be free of hair as possible , should not touch
the pinna
A constant sound is given and B.C vibrator is moved
over the mastoid – till a point of maximum
sound is heard.
When ears without any conductive pathology are
covered by earphones or occluded by any
apparatus , there is a false increase in the bone
conduction threshold for the lower frequencies -
PROCEDURE - BONE
CONDUCTION
FRONTAL PLACEMENT OF BONE VIBRATOR
Adv : less variation of the amount of tissue b/w bone
conduction vibrator and skull bone & lesser
artifacts
Correction factor has to be subtracted
15 db for 250 - 500 Hz
10 db for 1000 - 4000 Hz
LEFT RIGHT
AIR CONDUCTION SOUNDS ARE HEARD
UNMASKED
MASKED
BONE CONDUCTION
UNMASKED
(MASTOID)
MASKED
MASKED (
FOREHEAD)
AIR CONDUCTION SOUNDS NOT
HEARD
UNMASKED
MASKED
BONE CONDUCTION
UNMASKED
(MASTOID)
MASKED
MASKINGNoise presented to the non-test ear to prevent it from
responding to a signal presented to the test ear
WHEN TO MASK ?
- All bone conduction
- When interaural attenuation is more than 45 db .
- When air conduction more than 45 db HL
- Cross – hearing in air conduction if
AC ( test ear ) – BC ( non test ear ) > IA
HOW TO MASK ?
For bone conduction
- minimum masking = B t +( A m – B m)
For air conduction
- minimum masking = At – 40 + (Am – B m)
Maximum masking = B t + 45
MASKING
STEP 2
Obtain unmasked threshold by AC in both ears
STEP 1
For BC tests always mask the contralateral ear . For AC tests always mask contralateral ear if test ear is
comparatively poorer ear & sounds of 45 db or more are being used for the AC tests.
MASKING
Subject Hear Test Tone Subject does not hear test tone
Actual Threshold Proceed To Step 4
STEP 3
Present masking sound in non-test ear at 15 db above AC threshold of non-test ear & present the
test- tone at unmasked threshold in test ear
MASKING
Subject Hear Test Tone Subject does not hear test tone
STEP 4
Raise test tone by 5 db
Raise masking noise by 5
db
Raise masking noise by 2-3 steps
of 5 db till subject hears tone &
then proceed step 5
Raise masking sound by 2-3 more
steps of 5 db. If subject still hears –
Actual Threshold. If doesn’t hear -
Go back to step 4
Subject hears tone --- Subject
does not hear tone
Go to step 5
MASKING
STEP 5
Alternately raise test tone & masking sound in 5 db steps till subject continues to hear the test tone in spite of 2-3 consequetive increments of 5 db of
masking noise. This is actual threshold.
MASKING
Over masking
If the masking sound used is so loud thai it crosses over from the non-test ear and obliterates or mask the test signal in test ear , the subject will not hear the test signal in the test ear until it is much above the actual threshold.
Under masking
Undermasking leads to false A-B gaps and is usually the result of operator inexperience or a failure to follow masking rules.
Low-frequency A-B gap occurred and it disappeared when proper masking was applied.
MASKING
MASKING DILEMMA
In pts with b/l moderate to severe CHL ( large AB Gap ) ,
the plateau in Hood’s plateau method of masking may be
unidentifiable.
Max and Min masking may be equal (or) min masking may
become more than max masking
SOUNDS USED FOR MASKING
WHITE NOISE
Broadband or wideband noise ( equal amt of sound
of all frequencies , starting from low to very high
frequencies )
NARROW BAND NOISE
Narrow band of noise centered on test tone freq.
with 100 – 200 Hz above and below that freq.
COMPLEX NOISE
Low freq. fundamentals plus the multiples of that
freq. up to 4000 Hz
INTERPRETATION OF AUDIOGRAM
0-25 db - Normal
26-40 db - Mild deafness
41-55 db - Moderate deafness
56-70 db - Severe deafness
71-90 db - Very severe deafness
Above 90 db - Profound deafness
% of Handicap
Formula for calculating % of handicap for unilateral
deafness – { [( a+b+c+d)÷ 4 ]- 25} x 1.5
%
Bilateral – [ (5x +y) ÷6 ]%
Pitfalls
- Vibrotactile stimulation
- Faulty response in bone conduction tests at 3 k
& 4 k Hz.
PITFALLS
DISADVANTAGES
Improper technique
Improper test condition
Improper test instrument
Improper examiner
Subjective & time consuming test
Does not assess main features of hearing- frequency
discrimination & temporal resolution of sound.
Does not identify nature of the pathology
Bone conduction test does not assess true sensoneural
reserve.
CLINICAL TESTS WITH PTA
WEBER’S TEST USING BONE VIBRATOR
TO identify the better ear in SNHL and the ear
with larger AB Gap in suspected CHL
BC vibrator is placed in centre of pt’s forehead
and a 1000 Hz sound of 0 db is presented
Above 1000 Hz is avoided – resonsce in skull
may blemish the results
Above 10 db above threshold is not used – may
heard by air conduction
CLINICAL TESTS WITH PTA
BING TEST OR OCCLUSION TEST
BC Vibrator is placed over mastoid process
BC threshold at certain freq. b/w 250 – 1000 Hz
ascertained twice , once with EAC open & EAC
occluded.
No change in BC thershold – Conductive
deafness
Hearing threshold found better with EAC
Occluded – No Conductive deafness
CLINICAL TESTS WITH PTAGELLE’S TEST
BC Vibrator is placed over mastoid process of test ear
Audiometer ear phone is placed over the other ear
Masked BC hearing threshold of test ear is determined at air pressure of -400 , 0 , +400 mm of water pressure.
In normal middle ear - the BC hearing threshold is found to become poorer at -400 & +400 as compared to 0.
In stapes fixation - no change in BC threshold
STENGER TEST
May be used to identify unilateral or asymmetrical
functional hearing loss.
When both ears are stimulated simultaneously by a
tone equal in frequency and phase, the auditory percept
is lateralized to the ear with better hearing.
When speech stimuli are used, the test is called a
Speech Stenger test or a Modified Stenger test.
NORMAL
MILD CHL
MODERATE SNHL
VERY SEVERE MIXED DEAFNESS
CHL SHOWING LARGER AB GAP -
OME
CHL WITH VERY LARGE AB GAP –
OSSICULAR DISCONTINUITY
CARHART’S NOTCH - OTOSCLEROSIS
FLAT AUDIOGRAM – SNHL
STRIAL PRESBYACUSIS
DESCENDING AUDIOGRAM – SNHL -
OTOTOXICITY
SNHL – DIP IN 4OOO HZ –
ACOUSTIC NEUROMA
SNHL – MOD DEGREE IN LOW FREQ –
EARLY ENDOLYMPHATIC HYDROPS
TROUGH SHAPED – CONGENITAL
SNHL
