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10 October 2010 H.Dai 1
AUDITORYSENSITIVITY I
SPH 380 HEARING SCIENCES
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1830: Fechner published the first text on
psychophysics
About two centuries ago, on this day, Fechner
had the insight that sensation can be measured.
The day has been named in his honor as the
Fechner’s day——Happy Fechner’s day!
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10 October 2010 H.Dai 3
~Today’s Topics ~
Threshold of audibility
Methods for measuring thresholds
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10 October 2010 H.Dai 4
What is threshold?
CAN’T HEAR
S O
U N D
L E
V E L ( d B
S P L )
THRESHOLD
BELOW THRESHOLD
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10 October 2010 H.Dai 5
What is threshold?
CAN HEAR!
S O
U N D
L E
V E L ( d B
S P L )
THRESHOLD
ABOVE THRESHOLD
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10 October 2010 H.Dai 6
Thresholds of Audibility
• Thresholds of audibility (absolutethreshold): the lowest SPL needed todetect the sound.
• Scientists typically measure the SPL in
one of two ways (of placing themicrophone): – In the space where the listener’s head would
be, but in its absence; called the minimumaudible field (MAF) or
– near the eardrum, called the minimumaudible pressure (MAP).
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10 October 2010 H.Dai 7
MAP vs MAF
7 7
MINIMUM AUDIBLE PRESSURE
TYPICALLY OBTAINED WITH
EARPHONES.
ESTIMATES ACTUAL
PRESSURE AT TYMPANIC
MEMBRANE
MIMINUM AUDIBLE FIELD
IS MEASURED BY A
MICROPHONE PLACED IN
THE SOUND FIELD IN THE
POSITION WHERE THE
EAR USE TO BE
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10 October 2010 H.Dai 8
Calibration of Transducers
• Loudness Speaker: free field (MAF);
• Headphones (MAP)
--Supra-aural: 6-cc coupler --Circum-aural: artificial ear
• Insert phone (MAP): 2-cc coupler, or
a ear simulator
(ALL are used in hearing clinics)
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10 October 2010 H.Dai 9
100 500 1000 5000 10,000 Hz
125 dB
SPL
100 dB
75 dB
50 dB
25 dB
0 dB
MAP
MAF
Things to note about MAP and MAF
1. Best thresholds are in “middle” frequency regions (Why?)
2. Sound field threshold (MAF) is actually below 0 dB SPL for
some frequencies
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10 October 2010 H.Dai 10100 500 1000 5000 10,000 Hz
100 dB
SPL
75 dB
50 dB
25 dB
0 dB
MAP
MAF
3. MAF is lower than MAP, but their difference is a matter of
reference, rather than real sensitivity. MAF is lower, because
when the listener is present, the sound pressure at the ear drum
will be boosted from ear canal resonance, concha resonance,
head baffle and other features of the head and torso
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Factors affecting the shape of the threshold
curve (frequency dependence)
External and middle ear transfer functions play a role in shaping the
audibility curve.
From Pickles (1988)
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From Pickles (1988)
Behavioral vs Neural Threshold
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Hearing Level (dB HL) vs dB SPL
• Threshold curve standardized by ANSI. Sound
level expressed with reference to the standard
threshold curve is defined as dB HL (as in an
audiogram)
• A hearing loss of 25 dB at a particular frequency,
for example, will show up on an audiogram as a
threshold of 25 dB HL at that frequency
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10 October 2010 H.Dai 14
Standardized Absolute Thresholds as reference for
calculating the hearing level (HL)
40 dB SPL
or
15 dB HL
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HL (audiogram) vs SPL
0
25
50
75
100
dB
SPL
Threshold curve
(ANSI standard)
25 dB
250 1k 2k 4k
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10 October 2010 H.Dai 16
100 500 1000 5000 10,000 Hz
125 dB
100 dB
75 dB
50 dB
25 dB
0 dB
MAP
MAF
DISCOMFORT
PAIN
Pain & Discomfort Thresholds
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Level of Discomfort
and Threshold of Pain
• Level of discomfort: UCL: 100 to 120 dB
SPL
• Threshold of pain: above 120-130 dB SPL
• Dynamic Range (DR) of Hearing:DR = UCL - Threshold(Hearing) (90 dB at
100 Hz, 120 dB at mid freqs)
TEMPORAL
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Duration EffectsTEMPORAL
INTEGRATION:
LONGER DURATION,
LOWER THRESHOLD,UP TO 250 MSEC.
Time-intensity trade:
10 fold change in
duration trade for 10 dB
change in threshold
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10 October 2010 H.Dai 19
Temporal Integration: You can trade signal
power for duration (time-intensity trade)
Temporal window (250-ms)
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The “Classic” Methods
• Method of Adjustment: Listener controls
stimulus and “adjusts” it until a responsecriterion is met
• Method of Limits: Experimenter controls
stimulus and reverses direction of variable(e.g., intensity) when listener changes judgment
• Method of Constant Stimuli: Experimenter
preselects several stimuli and presents themin random order for a listener response.Proportion of correct response obtained for
each stimulus—Psychometric Function
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Method of Adjustment
Pro: Can be useful for estimating the
range to which the listener will respond
Con: Response may be strongly
influenced by listener biases
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Method of Limits
• Typically used in the clinic to measure
threshold of hearing
• Examiner starts with a tone that is audible and
then reduces the intensity in 10 dB steps until
the patient stops responding
• Examiner reverses direction of intensity
change and increases in 5 dB steps until thepatient responds
• Sequence is repeated 2 more times (descend,
ascend)
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Example of Method of Limits
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
50
4030
20
10
0
y
n
yy
yn
nn
y
y
nn
ny
nn n
y
nn
y
Threshold ~ 25 dB
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Method of Constant Stimuli: although time
consuming, it generates a psychometric function,
which has more complete info than just threshold
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
50
4030
20
10
0
yn
n
n
Threshold ~ 25 dB SPL
yy
n
yy
y
n
n n
y
n
n n
n
yy
y
Trial S i g n a
l L e v e l ( d B
S P L ) Example
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Psychometric Function: Threshold
(defined as SPL @ 50%)
0 10 20 30 40 50 dB SPL
P e r c e n t “ y e s ”
100
80
60
40
20
0
S
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Thresholds
• MAF vs MAP• Shape of threshold curve
• Behavioral vs neural threshold
• dB HL
• Upper limit of hearing and dynamic range
• Duration effect (temporal integration)
Pros and Cons of the Measurement Methods• Method of adjustment
• Method of limit (clinic application)
• Method of constant stimuli (psychometric function)
~Summary ~
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Other factors affecting
threshold• Instructions to listener
• Monaural versus binaural listening
• Location of sound source in a sound
field
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Instruction Effects
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One ear or two ears?
2 ears ~ 3 dB
improvement
Location of Sound Same side
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10 October 2010 H.Dai 30
Location of Sound – Same side
(ipsilateral) or opposite side
(contralateral).
Head shadow occurs for sounds that have short
wavelengths re: head size. Difference ~ 15 dB.
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How well are the ears isolated
when you use regular
earphones?SOUND ON THE
LEFT AT 50 DB
WILL “LEAK”OVER TO THE
RIGHT AFTER A
LOSS OF
ABOUT 40 DB.
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BONE CONDUCTION
7
A TUNING FORK SET
INTO VIBRATION AND
PLACED IN CONTACT
WITH THE SKULL WILLCREATE THE
SENSATION OF SOUND
THROUGH BONE
CONDUCTION.
Bone Conduction Thresholds in dB
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100 500 1000 5000 10,000 Hz
125 dB
100 dB
75 dB
50 dB
25 dB
0 dB
Bone Conduction Thresholds in dB
re: 1 dyne of force
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10 October 2010 H Dai 34
How is the ear stimulated by
bone conduction sound energy?
• Intertia…the skull moves to and fro but theinertia of the ossicles causes them to lagbehind the motion of the skull and produces
relative motion of the ossicles
• Compression…the vibrations moving throughthe skull create compressions andrarefactions squeezing on the inner ear capsule
• Osseo-tympanic…the skull moves and the jaw doesn’t move…creating a net relativemotion on the floor of the ear canal andproducing “sound” in the ear canal
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