Auditory Sensitivity I

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10 October 2010 H.Dai 1

AUDITORYSENSITIVITY I

SPH 380 HEARING SCIENCES




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!




~Today’s Topics ~

Threshold of audibility

Methods for measuring thresholds




What is threshold?

CAN’T HEAR

S O

U N D

L E

V E L ( d B

S P L )

THRESHOLD

BELOW THRESHOLD




What is threshold?

CAN HEAR!

S O

U N D

L E

V E L ( d B

S P L )

THRESHOLD

ABOVE THRESHOLD




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).




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




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)




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



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



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)



From Pickles (1988)

Behavioral vs Neural Threshold




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




Standardized Absolute Thresholds as reference for

calculating the hearing level (HL)

40 dB SPL

or

15 dB HL



HL (audiogram) vs SPL

0

25

50

75

100

dB

SPL

Threshold curve

(ANSI standard)

25 dB

250 1k 2k 4k




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




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




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




Temporal Integration: You can trade signal

power for duration (time-intensity trade)

Temporal window (250-ms)




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




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




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)




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




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




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




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 ~




Other factors affecting

threshold• Instructions to listener

• Monaural versus binaural listening

• Location of sound source in a sound

field




Instruction Effects




One ear or two ears?

2 ears ~ 3 dB

improvement

Location of Sound Same side




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.




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.




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




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



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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Auditory Sensitivity I

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