Audiology—Terminology and Tests

14 Audiology—Terminology and Tests

14.1 Decibels and Decibel Scales

• Subjective loudness is proportional to equal multiples of sound intensity

• Bel = log10 (sound intensity/intensity reference (10–12 W/m2); 10 × ratio = dB

• Sound intensity ∝ sound pressure2; sound intensity (dB) = 20 log10 sound pressure level

• Doubling sound intensity = 3 dB increase; each 10 dB increase = 3.3 increase in sound pressure

• Sound pressure level scale (dB SPL)—produces curved audiogram in normal circumstances because the auditory system is less efficient at detecting sounds at certain frequencies

• Hearing level scale (dB HL/ISO)—designed so that 0 dB = expected threshold for every frequency—reference zero = group of otologically normal young adults; 0 dB HL is the intensity needed for a normal ear to perceive sound 50% of the time

• A-weighted scale (dB A)—reduced contribution of the lowest and highest frequencies; used in industrial and noise-exposure settings

14.2 Clinical Hearing Testing

• Free-field thresholds equate to the voice and distance level the subject gets >50% correct

• Use number and letter combinations

• Mask nontest ear with tragal rub (attenuate by ~50 dB) for whispered and conversational speech, Barany box (attenuates by ~90 dB when held at right-angle to ear) for loud voice

• Test order: whisper 2 ft (60 cm) then 6″ (15 cm), conversation 2 ft then 6″, loud voice 2 ft then 6″

• If cannot hear whisper at 2 ft, subject is hearing impaired (normally can hear this at 12 ft [3.5 m]); if can hear this pure tone thresholds will be better than 30 dB

• If hear whisper at 6″ or conversation at 2 ft 6″, threshold in range 30 to 70 dB HL

• If only hear loud voice, thresholds >70 dB HL

14.3 Tuning Fork Tests

• Usually use 512-Hz tuning fork as tone does not decay too quickly or produce too much vibration

• Activation by finger compression produces sound level ~70 dB; on knee (without pain) ~90 dB

• Weber test (Fig. 14.1):

figure Tuning fork placed in midline

figure With unilateral or asymmetric HL if conductive localizes to affected ear, if sensorineural localizes to nonaffected ear

figure Can detect as little as a 5 dB loss, but error rate up to 25%

• Rinne test (Fig. 14.2):

figure Hold tuning fork next to ear, then onto mastoid process—which sound is heard loudest? (Originally described as fork held next to ear until no longer heard, then see if heard via bone)

figure Positive when air conduction (AC) > bone conduction (BC); i.e., normal ears or SNHL in test ear

figure Negative when BC > AC (conductive loss in test ear)

figure In dead ear get false negative due to transcranial stimulation of nontest ear; hence need to mask (e.g., with Barany box)

figure Test less sensitive than Weber; for ABG 20 dB, sensitivity 50 to 70%, specificity ~95%; for ABG 40 dB, sensitivity and specificity ~95%

• Stenger test:

figure Can help detect patient with a nonorganic unilateral HL

figure Relies on fact that if pure tones of same frequency but different intensities are presented simultaneously to each ear, patient will only be aware of the louder stimulus

figure Present two tuning forks, but one to “deaf” ear closer; if genuine will hear fork in good ear, if “fake” will only be aware of sound in “deaf” ear so will deny hearing anything

figure Equivalent test can be performed with audiometer

14.4 Pure-Tone Audiometry (Fig. 14.3)

• A subjective test of a patient’s hearing compared with an established “normal”

• Test performed according to standard protocols (British Society of Audiology)

• Soundproofed facilities needed for definitive/diagnostic testing

• Test order of frequencies 1, 2, 4, 8 kHz then 500 and 250 Hz; repeat 1 kHz again to check within 5 dB of first result

• Subjectively better-hearing ear tested first

• Test to within 5 dB (to account for test–retest variability)

• Testing involves ascending in 5-dB increments until patient gets 50% responses correct, then descending 10 dB and ascending in 5 dB again

• BC at 250 Hz causes more vibration than sound, so test 0.5 to 4 kHz range (test–retest variability >10 dB, so differences only considered significant if at least this)

14.5 Loudness Recruitment

• Subjective loudness sensation greater than expected with an increase in sound intensity

< div class='tao-gold-member'>

Jul 4, 2016 | Posted by in OTOLARYNGOLOGY | Comments Off on Audiology—Terminology and Tests
Premium Wordpress Themes by UFO Themes