auditory localization lynn e. cook, aud occupational audiologist nnmc, bethesda, md
TRANSCRIPT
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AUDITORY LOCALIZATIONAUDITORY LOCALIZATION
Lynn E. Cook, AuD
Occupational Audiologist
NNMC, Bethesda, MD
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How do we tell where a sound How do we tell where a sound is coming from?is coming from?
LOCALIZATION The ability to identify
the direction and distance of a sound source outside the head
LATERALIZATION Occurs when
headphones are used, and the sound appears to come from within the head.
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LOCALIZATIONLOCALIZATION
Complex perceptual processSensory integration of a variety of cuesStill no consensus on how these cues are
weighted, the frequency range over which each is viable, the regions of auditory space where each is important, and relative accuracy of each.
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Horizontal Localization Horizontal Localization (L vs R)(L vs R)
Perceived by comparing the signal input between two ears
Interaural time difference (ITD)Interaural phase difference Interaural level difference (ILD)
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ITDITD
Sounds arrive earlier at the ear closest to the source. The difference in arrival time=ITD– Dependant on speed of sound and size of head
ITD = 0 for frontally incident sound
ITD ~ 0.7 msec for 90° azimuth (maximum)
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Interaural Phase DifferenceInteraural Phase Difference
Coincident with the time delay (ITD) Varies systematically with source azimuth and
wavelength due to distance from source and refraction around the head
Useful for frequencies up to about 700 Hz. Sound envelope provides similar information for
higher frequencies, but to a lesser degree Dominant cue for horizontal localization for
frequencies up to 1500 Hz.
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Interaural Level Difference Interaural Level Difference (ILD)(ILD)
Due to head shadow effectsHead and pinna defraction attenuates sound
at far ear, while boosting the sound at near ear.
Greatest for high frequency soundsMost pronounced for frequencies>1500Hz.About 20 dB at 6K, almost 0 at 200 Hz.
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Horizontal localization poorest Horizontal localization poorest at 1500 Hz.at 1500 Hz.
Most precise at 800 Hz, esp. Most precise at 800 Hz, esp. when source is directly in front when source is directly in front
of listener.of listener.
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Horizontal LocalizationHorizontal Localization
Low Frequencies / Timing Cues DominateHigh Frequencies / Intensity Cues
Dominate
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Accurate horizontal Accurate horizontal localization is possible ONLY localization is possible ONLY when the relevant acoustic when the relevant acoustic cues are clearly audible in cues are clearly audible in
BOTH EARSBOTH EARS
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Vertical localization Vertical localization (Up/Down)(Up/Down)
Determined from pinna cuesListener’s intimate knowledge of complex
geometry of pinna helps pinpoint elevationFor freq’s above 5KShoulder reflection causes changes in signal
in 2-3 K range
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Front/Back LocalizationFront/Back Localization
Less understoodSpectral balance = primary cue
– Hi freq sounds boosted by pinna when they arrive from the front; attenuated when from behind
MOST COMMON LOCALIZATION ERROR!
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Reducing ambiguityReducing ambiguity
Head movement– Feasible for sources up to 18’– Listener must be able to turn head, and source
must be repeated or be continuous for sufficient time to allow multiple head orientations
– Provides info re: front vs. back & distance– Cues are found in variance in ITD’s and ILD’s
as listener moves head
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Reducing ambiguity (con’t)Reducing ambiguity (con’t)
Non acoustic cues may also contribute– Visual cues– Source familiarity
Comparison with stored patterns– Once head reaches final size and distance between ears,
nothing will change these stored patterns except ear disease, trauma, or hearing changes
– Can adapt to stable unilateral hearing loss, assuming sound remains audible on both sides.
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Why is auditory localization Why is auditory localization important?important?
Allows us to pinpoint a sound of interestLocate the position of another personLocate direction and distance of a moving
sound sourceAllows us to quickly locate and attend to a
speaker, esp. in multi-talker situations
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Visual localizationVisual localization
Just as accurate, but not nearly as efficientNot possible in low or reduced light
situations, or when the source of the sound cannot be visualized
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Effects of hearing loss on Effects of hearing loss on localization ablilitylocalization ablility
Horizontal localization ability decreases with increasing low freq. hearing loss (below 1500 Hz)
Sounds must be audible (at least 10 dB above threshold)
Vertical localization ability decreases with increasing high freq. hearing loss
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Unilateral hearing lossUnilateral hearing loss
Severely disrupts horizontal localization ability
Front to back localization remains intact (other studies dispute this)
Vertical localization only slightly affected provided the other ear is adequate
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Monaural localizationMonaural localization
May be possible, but not as accuarate as binaural localization
Time delay between direct and pinna-reflected sound is the dominant cue for monaural localization
Skill disrupted when pinna is taped flat, filled with putty, or bypassed with glass tubes
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RepetitionsRepetitionsplus head movementplus head movement
First occurrence of the sound random in terms of spatial orientation
Listener makes effort to turn towards source for second repetition
Third repetition with head at third (random) angle provides refined information
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Conductive hearing lossConductive hearing loss
Results in marked decrease in localization ability– As conductive component increases, the
amount of B/C information becomes dominant where there is no interaural attenuation
– Conductive hearing loss also causes disruption in phase information critical to localization
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How do we measure How do we measure localization ability?localization ability?
No standardized way to directly measure this ability
Must be done through your own pinnae, therefore headphones tests (lateralization tasks) are not the same thing, even when head transfer functions are considered.
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Effects of noise on localizationEffects of noise on localization
Greatest decrease in accuracy found in judgment of front/back differences
Up/down errors occur with less frequencyLeast influence on left/right judgments
Accuracy decreases as S/N decreases
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Source Azimuth in Noise Test Source Azimuth in Noise Test (SAINT) (SAINT) Vermiglio 1999Vermiglio 1999
Listener sits in clock-like array of 12 speakers
Task is to detect a signal (pistol shot, female vocalization) in quiet and in noise (helicopter noise, crowd noise) for a variety of presentation azimuths
May be tested under headphones (no pinna cues for horizontal localization)
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Hearing in Noise Test (HINT)Hearing in Noise Test (HINT)Soli and Nillson, 1994Soli and Nillson, 1994
NOT a localization testMay, however, provide indirect proof of
binaural superiority as many subjects with unilateral loss will fail the portion of the HINT where noise is directed towards the good ear.
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Establishing an audiometric Establishing an audiometric standardstandard
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Suggested guidelinesSuggested guidelines
Applicants must have adequate and usable hearing in both ears, particularly for the all-important speech frequencies
SRT MUST BE 25 dB OR BETTER IN EACH EAR WHEN TESTED UNDER HEADPHONES
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Suggested guidelines, con’tSuggested guidelines, con’t
Low frequency hearing loss in one or both ears averaging 50 dB at the frequencies of 500 and 1000 Hz. should be disqualifying in and of itself , regardless of performance on any other applicable audiometric tests
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Suggested guidelines (con’t)Suggested guidelines (con’t)
Conditions involving fluctuating hearing loss such as Meniere’s disease should be disqualifying until such a point occurs that the hearing loss remains stable for at least 30 days. If the thresholds of 500, 1000, and 2000 Hz. differ by 25 dB or more in either ear, for two audiograms separated by at least 48 hours, hearing levels may be considered unstable.
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Suggested guidelines (con’t)Suggested guidelines (con’t)
Unresolved or chronic conductive hearing loss in one or both ears, where air/bone gap exceeds an average of 25 dB at the frequencies 500 and 1000 Hz, should be disqualifying until or unless the condition can be successfully resolved through medical and/or surgical means
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Use of hearing aidsUse of hearing aids
Hearing aids alter both time and intensity cues Digital processing can delay the sound by several
msec., signal is further delayed as it travels through tubing, transducers, etc.
Vented hearing aids allow listener to receive two different signals, which can cause ambiguity in time, phase, and intensity cues
Coupling of device to ear eliminates critical pinna cues needed for vertical and front/back localization