Pure Tone Audiometry SPA 4302 Summer A, 2004
The Pure-Tone Audiometer • Electronic device that generates tones for determining hearing thresholds • Manufactured to specifications of the American National Standards Institute (ANSI) • Air/Bone Conduction • Testable frequencies (A/C): 125, 250, 500, 750, 1000, 1500, 2000, 3000, 4000, 6000, 8000 Hz • Testable frequencies (B/C): 250 through 4000 Hz • Masking control available
Test Environment • Background noise may affect audiometric results by elevating thresholds • Three ways room noise may be attenuated • Earphone enclosure device • Insert earphones – foam tipped receivers that are inserted directly into the ears • Sound isolated chambers
The Patient’s Role • Patients must be aware that they are to indicate when they hear a tone • Patient response: hand raise, finger raise, signal button, vocal response, play • False responses • False negatives: patient fails to indicate when they’ve heard a tone - misunderstood or forgotten instructions, feigning or exaggerating loss • False positives: patients responds when no tone is presented – usually occurs when there are long silent periods in the test
The Clinician’s Role • Convey task instructions to patient • Ensure understanding • Patient position • Position patient so they cannot observe the clinician’s movements
Air-Conduction Audiometry • Specifies hearing sensitivity at various frequencies • Can’t tell whether deficit is conductive or sensorineural, or mixed • Earphone placed with diaphragm aimed directly over canal opening • Be careful of canals that collapse due to the pressure of the earphones – use insert earphones if this is a potential problem
Air-Conduction Audiometry • Test the known or suspected better ear first • Test at the octave points and the mid-octaves (750, 1500, 3000, 6000 Hz) if there is a difference of 20 dB or more between adjacent octaves • Begin at 1000 Hz – easily heard by most and high test-retest reliability
Measuring a Threshold • Start presenting pure tones at 30 dB HL • No response? Raise the level to 50 dB HL • Still no response? Raise the level in 10 dB increments • Whenever person responds, lower the level 10 dB • Whenever no response, raise the level in 5 dB • Threshold=the lowest level at which the patient can correctly identify the tone presentation at least 50% of the time, with a minimum of 3 responses at a given level.
Air-Conduction Audiometry • Pure-tone average (PTA)=average of air conduction thresholds obtained at 500, 1000, and 2000 Hz in one ear • Useful for predicting threshold for speech • Percentage of Hearing Impairment • Ignores audiometric configuration and looks only at average hearing loss • Often confusing and misleading to patients
Air-Conduction Audiometry • The Audiogram • Frequency (in hertz) on the x-axis, Intensity (in dB HL) on the y-axis • Moving left to right, frequency increases; moving top to bottom, intensity increases • Symbols are placed to correspond to threshold at a given frequency:
The Audiogram • Thresholds by frequency • Hearing by air and bone transmission
Bone-Conduction Audiometry • 3 Mechanisms of Bone Conduction • Distortional Bone Conduction • Inertial Bone Conduction • Osseotympanic Bone Conduction • Bone Oscillator Placement Mastoid process, or, Forehead
Bone-Conduction Audiometry • Occlusion Effect • When the ears of patients with normal hearing or SNHL are covered or occluded, there is an increase in intensity of sound delivered via a bone oscillator • Affects 1000 Hz and below • Result of increase in SPL in the ear canal when the outer ear is covered • Markedly decreased when insert phones are used (as opposed to supra-aural headphones)
Bone-Conduction Audiometry • No matter where the oscillator is placed, you can never be sure which cochlea is being stimulated! (more on this to come) • Frequencies usually tested: • 250, 500, 1000, 2000, and 4000 Hz • Symbols for bone conduction are only connected on the audiogram (with dashed lines) when there is a conductive or mixed loss.
Audiogram Interpretation Look at: • hearing sensitivity by AC • hearing sensitivity by BC • AC/BC difference (a.k.a. the air-bone gap) No air-bone gap = normal or SNHL AC worse than BC = conductive hearing loss • Watchout: low frequencies at high levels via BC can be perceived as a tactile signal!
Another Thing to Watch Out For: • Cross Hearing: sound delivered to one ear but perceived in the other ear. • Interaural Attenuation (IA)—How much sound it takes to reach the other side: • Air conduction IA = 40 dB • Bone conduction IA = 0 dB • Danger for cross-hearing • For AC—If AC threshold in the test ear, minus IA, is greater than or equal to the BC threshold of the opposite ear • For BC—If Air-bone gap of test ear exceeds 10 dB Insert Phones >60 dB
Masking • Masking—keeping the non-test ear “busy” in order to ensure that it is actually the test ear which is responding • Noises used to mask: • White noise—has approximately equal energy per cycle & covers a broad range of frequencies • Narrowband noise—made up of frequencies that immediately surround the pure tone being tested • Insert earphones recommended because: • They lessen the occlusion effect • They provide much more interaural attenuation
Effective Masking: Calibration of the noise • dB EM (Effective Masking) describes the level to which a threshold will shift in the presence of a given level of noise • So, 45 dB EM should raise the threshold for a tone to 45 dB HL in the ear in which both are presented.
Masking • Masking for air conduction • “Shotgun” Approach • Minimum-noise method • Maximum-noise method • Plateau method • Masking for bone conduction • Similar to air conduction • Beware of occlusion effect, and overmasking
Computerized Audiometry • Using a device remotely operated by a computer and data is stored • Computer can control all aspects of testing and masking and analyze patient responses • Used more often for military, industrial, and educational applications (large number of people to test)