Sound Dynamics & Tests of Hearing

Decible Levels

Whisper 30 dB

Normal Conversation 60 dB

Shout 90 dB

Discomfort in ear 120dB

Pain in ear 130dB

Pitch is sensation of a particular frequency, dB is that of intensity

stapedial reflex is elicted with a sound of 70-100 dB

normal person can hear 20-20000 Hz

audiometric testing done 125-8000Hz

white noise-all frequencies if sound-comparable to white light

Narrow band noise-used in masking-it's a white noise without certain frequencies (above & below some patricular frequency having been filtered out)-used in pure tone audiometry

speech noise-noise having frequencies in speech range(300-3000 Hz)-all other frequencies filtered out

Masking-essential for all bone conduction tests-for air conduction test is required only when the difference in hearing between the two ears exceeds 40dB

Clinical Tests of Hearing

1.Finger Friction Test

2.Watch Test-practically obsolete

3.Speech (Voice Test)-conversational/forced whisper (use of spondee words by examiner)

4.Tuning Fork Tests- tests AC/BC

a)-Rinne Test (AC>BC is normal-positive Rinne)

BC>AC negative Rinne-minimum air-bone gap of 15-20 dB

• Rinne test equal or negative for 256 Hz but positive for 512 Hz=> air bone gap 20-30 dB
• Rinne negative for 256 & 512 Hz but positive for 1024 Hz => air bone gap 30-45 dB
• Rinne negative for 256, 512 & 1024 Hz => air bone gap of 45-60 dB

Negative Rinne for 256, 512 & 1024 Hz indicates minimum AB gap of 15,30&45 dB respectively

False negative Rinne-in severe unilateral SNHL-correct diagnosis made by masking non test ear Barany's noise Box while testing for bone conduction-Weber lateralzation also helps

b)-Weber Test-done with 512 Hz (detects 15-20 dB)

• lateralized to worse ear in conductive deafness
• to better ear in sensorineural hearing loss

c)-ABC- cochlear function- comparison with examiner

d)-Schwabach's-ABC with an occluded meatus (reduced in SNHL, lengthened in CHL)

e)-Bing Test- effect of occlusion of meatus

• Bing Positive-normal person/SNHL hear louder with occlusion of meatus
• Bing Negative-CHL appreciates no change

f)- Gelle's Test- test of bone conduction, examined is the effect of increased air pressure in ear canal on hearing-has been replaced by tympanometry to find out stapes fixation

• Positive in normal persons & SNHL-decreased hearing on increased pressure
• Negative in fixed/disconnected ossicular chain

Audiometric Tests

1. Pure Tone Audiometry:

-Measure of cochlear function

-Air Conduction Threshold measured for tones 125,250,500,1000,2000,4000 & 8000 Hz

-Bone conduction threshold measured for 250,500,1000,2000 & 4000 Hz

-AB gap measured (normal individual has no AB gap in audiometry)

Shadow Curve- obtained from non-test better ear when difference between two ears is 40dB or more above air conduction thresholds-Masking is required if difference is>40dB(done by employing narrow band noise to the non-test ear)

2.Speech Audiometry:

-patient's ability to hear & understand speech is measured.parameters studied are

• Speech Reception Threshold-normally SRT is within 10 dB of the average pure tone threshold of 3 speech frequencies(500,1000 & 2000 dB).SRT better than pure tone average by more than 10 dB suggests a functional hearing loss

• Discrimination Score(DS) or Speech Recognition Score (SRS)

Normal 90-100%

Slight Difficulty 76-88%

Moderate difficulty 60-74%

Poor 40-58%

Very Poor <40%

Optimum Discrimination Score (ODS)-expressed in %

Half Peak Level (HPL) expressed as dB-a derived figure from audiogram

• Normal- ODS 100% @30 dB,HPL-15 dB
• Conductive Deafness of 40 dB-ODS100% @70 dB,HPL-55 dB
• SNHL of 40 dB-ODS never reaches 100%, constant beyond a level
• Retrocochlear loss- Roll over curve obtained, score falls below ODS beyond a frequency

3.Bekesy Audiometry-no longer used

4.Tympanometry-220 Hz tone delivered

• Type A-normal tympanogram
• As-fixation of sosicles-otosclerosis/malleus fixation-lower compliance@ambient air pressure
• Ad-ossicular discontinuity/thin & lax TM -high compliance at or near ambient pressure
• Type B-flat or dome shaped graph- no change in compliance with pressure changes-seen in middle ear fluid or thick TM
• Type C-maximum compliance occurs with negative pressures in excess of 100 mm H2O-seen in retracted TM & may show some fluid in middle ear

Acoustic Reflex: Presence of stapedial reflex @ lower intensities (40-60 dB) than usual 70-100dB indicates recruitment & thus a cochlear type of hearing loss

Stapedial Reflex Decay- VIII th nerve lesion - if a sustained tone of 500-1000 Hz delivered 10dB above the acoustic reflex threshold,for a period of 10 seconds, brings the reflex amplitude to 50%, shows abnormal adaptation

Absence of stapedial reflex when hearing is normal indicates lesion of facial nerve, proximal to the nerve to stapedius.

Special Tests of Hearing

1. Recruitment:

• phenomenon of abnormal growth of loudness
• the ear which doesn't hear low intensity sounds begins to hear greater intensity sounds as loud or even louder than normal hearing ear
• poor candidates of hearing aid
• cochlear lesions (Menier's Disease,presbycusis)
• Alternate binaural loudness balance test- used to detect recruitment in unilateral cases

2. Short Increment Sensitivity Index

• Patients with cochlear lesions distinguish smaller changes in intensity of pure tone better than normal persons & those with CHL or retrocochlear lesions
• CHL- SISI score<15%
• Cochlear Lesion-SISI score 70-100%
• Nerve Deafness- SISI score 0-20%

3. Threshold Tone Decay Test:

• measure of nerve fatigue
• normal person can hear a tone continuously for 60 seconds, nerve fatigue-it's less
• result expressed as dB decay
• decay> 25dB=> retrocochlear lesion

4. Evoked Response Audiometry:

• Electrocochleography (EcoG)-measures electrical potentials arising in the cochlea & CN VIII in response to auditory stimuli within 5 mili seconds, the response is in the form of cochlear microphonics,summating potentials & action potentials of VIII th nerve-finds threshold of hearing in young infants & children to within 5-10 dB
• Auditory Brain Stem Responses-non-invasive- ECOLI,MA-7 wave forms I to VII-wates are studied for absolute latency, inter wave latency, amplitude

5.Otoacoustic Emissions: produced by outer hair cells of cochlea

6.Central Auditory Tests:designed to find defects in central auditory pathways & temporal cortex

7. Hearing Assessment ininfants & Children:

• Screening procedures- arousal test & auditory response cradle
• Behaviour Observation Audiometry- Moro's Reflex, Cochleo Palpebral Reflex & Cessation Reflex
• Distraction techniques
• Coonditioning Techniques
• Objective Tests