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Chapter 5

HEARING

The mind is for seeing, the heart is for hearing.g g

Ch5--1

Two aspects of hearing will be considered:» Anatomy and physiology of the hearing

organs from the visible external organs of theorgans, from the visible external organs of the ear, to the point where sound signals are transformed into neural activity

Outer ear, middle ear, inner ear

» Perception of sound -- the sensations we experience in response to the input of a varietyexperience in response to the input of a variety of sound signals

Loudness level and loudness

Pitch (simple tone, complex tone)

Differential threshold

Masking

Ch5--2

g

Localization and lateralization

Hearing: The Hearing OrgansHearing: The Hearing Organs

Ch5--3

THE OUTER EAR» Includes the AURICLE or PINNA (the visible portion of» Includes the AURICLE, or PINNA (the visible portion of

the ear), and the EXTERNAL AUDITORY MEATUS (ear canal)

» The ear canal is an air-filled tube that is slightly more» The ear canal is an air-filled tube that is slightly more than 25 mm (2.5 cm; about 1 in.) long

» The canal is an acoustic tube that is open at outer end and closed at inner end by the TYMPANIC MEMBRANEand closed at inner end by the TYMPANIC MEMBRANE(ear drum)

» Sound waves impinging on the external ear are transmitted through the ear canal to the tympanictransmitted through the ear canal to the tympanic membrane

» The tympanic membrane is set into forced vibration with a frequency equal to the frequency of the applied

Ch5--4

with a frequency equal to the frequency of the applied force

Outer Ear (cont’d)» The external ear and the ear canal form an

ACOUSTICAL RESONATOR, an air-filled tube that is open at one end (outer) and closed at the other (inner)(inner)

» Sound waves with frequencies corresponding to the resonant frequency will have greater amplitudes of vibrationvibration

» The resonant frequency is approximately 3500 Hz, and pressure on the ear drum for frequencies of about 3500 Hz will be as much as 10 times greaterabout 3500 Hz will be as much as 10 times greater than the pressure observed at entrance to the canal

Ch5--5

THE MIDDLE EAR» Contains the AUDITORY OSSICLES, the three smallest

bones of the body that compose the OSSICULAR CHAIN

MALLEUS (hammer)INCUS (anvil)STAPES (stirrup)STAPES (stirrup)

Middle ear chamber is an air-filled cavity» Malleus attached to tympanic membrane (ear drum)» Malleus attached to tympanic membrane (ear drum)» Vibratory motion of the ear drum is transmitted to the

malleus, then to the incus (which acts as a fulcrum between the other two bones) and then to the stapes

Ch5--6

between the other two bones), and then to the stapes

Why do we need a middle ear?» 99.9% of air-borne sound, upon encountering the oval

window, would be reflected back. We would lose about 30 dB of sound energygy

» The middle ear acts as a transformer, and increases sound pressure by slightly more than 30 dB (not 38 dB

t t d i th t t) ti ll i l llas stated in the text), essentially recovering nearly all that would have been lost with the air-fluid barrier

How does the transformer work?How does the transformer work?

» AREAL mechanism

» OSSICULAR LEVER

» MEMBRANE BUCKLING

» Combined benefit from all three mechanisms

Ch5--7

17.7 x 1.3 x 2 = 46:1

20 log 46 = 33 dB

How does the transformer work?

AREAL h i» AREAL mechanism

Area of footplate of stapes is much smaller than area of tympanic membrane --- thus, pressurearea of tympanic membrane thus, pressure on footplate is greater by a factor of 17.7:1, which increases the sound pressure

OSSICULAR LEVER» OSSICULAR LEVER

Lever action provides an additional advantage of 1.3:1 over a single straight boneof 1.3:1 over a single straight bone

» MEMBRANE BUCKLING

The curved membrane of the ear drum provides a lever-like advantage of 2:1

» Combined benefit from all three mechanisms

17 7 1 3 2 46 1

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17.7 x 1.3 x 2 = 46:1

Finally, 20 log 46 = 33 dB

THE INNER EARTHE INNER EAR» System of fluid-filled cavities

in temporal bones of skullInner Ear comprises:» semicircular canals (maintaining

eq ilibri m and balance)equilibrium and balance)» COCHLEA (mechanical vibrations

transformed to electrical signals to be transmitted to the central nervous system (CNS) for processing

» Total volume of inner ear: a small grape

Ch5--9

The CochleaThe Cochlea» Snail-like shape with length of 3.5 cm completes 2 3/4

turns

Divided into three regions along most of lengthby the COCHLEAR PARTITION» SCALA MEDIA (filled with endolymph)

» SCALA VESTIBULI (filled with perilymph)

» SCALA TYMPANI (filled with perilymph)

Ch5--10

Ch5--11

Organ of Cortig

Ch5--12

ORGAN OF CORTI comprises a large number of cells lying on the basilar membrane» Contains HAIR CELLS, which are sensory receptors

that accomplish the mechanical to electricalthat accomplish the mechanical to electrical conversion

» Auditory nerve enters through modiolusTh i di id d i b h PILLARSThe organ is divided into two parts by the PILLARS, or RODS OF CORTI» On inner side is single row of INNER HAIR CELLS g

(about 3,500)» On outer side are three or four rows of OUTER HAIR

CELLS (about 12 500)

Ch5--13

CELLS (about 12,500)

Organ of Corti (cont’d)» Organ of Corti is covered by the TECTORIAL

MEMBRANE th ti f th t ll t f ili fMEMBRANE -- the tips of the tallest row of cilia from each of the outer hair cells are in contact with tectorial membrane

» Basilar membrane is displaced by vibration of fluids

» Cilia from hair cells are stimulated to move by shearing fforces

» The nerve at the base of the hair cell initiates a neural potential, and that electrical signal is transmitted by thepotential, and that electrical signal is transmitted by the auditory nerve

Ch5--14

Th h i f ili ti l t fib tThe shearing forces on cilia stimulate nerve fibers at base of outer hair cells, and mechanical energy is converted (transduced) into electrochemical activity by hair cellshair cellsComprises on the order of 30,000 nerve fibers» Each fiber arises from a few hair cells

f» Each hair cell excites several nerve fibersNerve fibers excited by stimulation of hair cellsSignals arrive at auditory cortex located in the temporal g y plobe -- a PLACE-FREQUENCY arrangement of the basilar membrane is preserved» What is meant by place-frequency arrangement?y p q y g

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Travelling wave on basilar gmembrane sorts sounds by

frequencyfrequency

high frequencies low frequencies

base apex0

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distance along basilar membrane

The Perception of SoundThe Perception of Sound

The discipline of psychoacoustics is a branch of psychophysicsp y p y» Quantification of sensations of listeners

to physical, acoustic events (signals) --p y ( g )measurement of psychological correlates of physical signals

Ch5--17

Ch5--18

Symptoms of SNHLy p• Raised thresholds:

helped by amplification

• Wider bandwidths:Wider bandwidths: no help possible

• Recruitment (restricted dynamic range): partly helped by automatic gain controls in modern digital aids

• Often accompanied by tinnitus

Ch5--19

• Often accompanied by tinnitus

h U f t bl

Normal hearingImpaired hearingH

igh Uncomfortable

elou

nd le

veSo

Low

Below threshold of hearingBelow threshold of hearing

Let’s amplify the sounds!

Ch5--20Low Frequency High

L Below threshold of hearingBelow threshold of hearing

h U f t bl

Compression amplificationH

igh Uncomfortable

elou

nd le

veSo

Low

Below threshold of hearing

Ch5--21Low Frequency High

L Below threshold of hearing

Physical Versus SubjectivePhysical Versus Subjective

When we refer to the intensity or frequency of a sound wave, we focus on PHYSICAL ATTRIBUTES of sound

Loudness and pitch, on the other hand, SUBJECTIVE ATTRIBUTESare SUBJECTIVE ATTRIBUTES

The subjective attributes are the h l i l l t f th h i lpsychological correlates of the physical

characteristics, but they are distinctly different from one another

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different from one another

LOUDNESS LEVELLOUDNESS LEVEL

» LOUDNESS LEVEL of comparison signal “is defined as the intensity (measured in decibels) of a 1000 Hz tone that sounds equal inof a 1000 Hz tone that sounds equal in loudness (Denes and Pinson, 1993) to the reference signal” (1000 Hz).

» The unit of measurement of LOUDNESS LEVEL is the PHON

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Ch5--24

LoudnessLoudnessS S Stevens (Harvard Psychoacoustic Laboratory) inS.S. Stevens (Harvard Psychoacoustic Laboratory) in 1930’s attempted to define relation between LOUDNESS LEVEL (in phons) and LOUDNESS (expressed in sones)

An experimental method (MAGNITUDE ESTIMATION)» First loudness of a 1000 Hz sinusoid at 40 dB SPL is defined as 1» First, loudness of a 1000 Hz sinusoid at 40 dB SPL is defined as 1

SONE: 1 SONE = 40 PHONS

Ch5--25

Magnitude Estimation cont’d

P t L ith t» Present L with two sinusoids of same frequency

» L adjusts intensity of one until it is just, for example, j ptwice as loud as the other. That defines 2 sones.

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Pitch and FrequencyPitch and Frequency

PITCH is the subjective attribute most closely related to frequencyJust as loudness is not governed exclusively by intensity, pitch is not governed exclusively by frequency» For sinusoids, pitch can be altered by

h i i t itchanging intensity» For complex waves, the piano for example,

pitch is affected little by changes in intensity

Ch5--27

pitch is affected little by changes in intensity

Frequency: 100-Hz Sine WaveFrequency: 100 Hz Sine Wave1.0

0 Spectrum

0 0 05-1.0

Amplitude against frequency

Time (s)0 0.05

Waveform

1

amp

Amplitude against time

100 Hz frequency

Ch5--28

q y

Frequency: 500-Hz Sine WaveFrequency: 500 Hz Sine Wave1.0

Spectrum0

Amplitude against frequency

Time (s)0 0.05

-1.0

Waveform

1

amp

1

amp

Amplitude against time

frequency100 frequency500

Ch5--29

q y

Excitation pattern of complex tone on bmbm

25.02004001600

resolvedunresolved

600800

15.0

20.0

200400600800

0.0

5.0

10.0

base-5.0

base apexlog (ish) frequency

Output o f 1600 Hz filter Output o f 200 Hz filter

0

0.5

1

1.5

21/200s = 5ms

0

0.2

0.4

0.6

0.8

11/200s = 5ms

Ch5--30-2

-1.5

-1

-0.5 0 0.2 0.4 0.6 0.8 1

-1

-0.8

-0.6

-0.4

-0.2 0 0.2 0.4 0.6 0.8 1

The unit of pitch is the MEL -- 1000 mels are d fi d th it h f 1000 H t ( t 40defined as the pitch of a 1000 Hz tone (at 40 phons) -- Therefore,» 500 mels is pitch of some tone that sounds» 500 mels is pitch of some tone that sounds

half as high» 2000 mels is pitch of some tone that p

sounds twice as high

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Ch5--32

The Pitch of Complex TonesThe Pitch of Complex TonesComplex tones such as music, spoken words, p , p ,etc. also evoke the sensation of pitchFor many complex, periodic sounds, the pitch is

i t d i il ith th f d t lassociated primarily with the fundamental frequency, fo

Earl Stanley Gardner and “the case of theEarl Stanley Gardner and the case of the missing fundamental frequency”» Combine 700, 800, 900, and 1000 Hz» Pitch corresponds to a frequency of about 100 Hz --

the common difference frequency -- the missing fundamental frequency

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fundamental frequency

Th i i f d t l f ( t’d)The missing fundamental frequency (cont’d)

» Combine 400, 600, 800, and 1000 Hz -- to what , , ,frequency does the pitch correspond?

200 Hz (the missing fundamental, the 00 (t e ss g u da e ta , t eperiodicity pitch, or the residue pitch)

» Add 500 700 and 900 Hz to that series above --» Add 500, 700, and 900 Hz to that series above to what frequency does the pitch correspond?

100 Hz100 Hz

Ch5--34

Hearing: Differential ThresholdsHearing: Differential Thresholds

ABSOLUTE THRESHOLD minimumABSOLUTE THRESHOLD -- minimum intensity required to detect a signalDIFFERENTIAL THRESHOLD -- how smallDIFFERENTIAL THRESHOLD how small a difference can L detect?» Called a DIFFERENCE LIMEN (DL), or a

JUST NOTICEABLE DIFFERENCE (jnd) -- “limen” is the German word for thresholdthreshold

» DL is not a constant -- varies with both intensity and frequency

Ch5--35

y q y

How many different pure tones can an ordinary listener detect?» Hold loudness level constant at 40

phonsAbout 1 400 distinguishable frequenciesAbout 1,400 distinguishable frequencies

» Hold frequency constant at 1000 HzAbout 280 distinguishable intensitiesg

» Co-vary frequency and intensity --nearly 400,000

Ch5--36

Hearing: Masking EffectsHearing: Masking Effects

The interest is in how the presence of aThe interest is in how the presence of a certain sound “drowns out” -- makes it difficult to hear -- other soundsdifficult to hear other sounds» Listening to car radio with windows open in

traffic

» Talking to a partner in a crowded, noisy room

The noise that makes it difficult to hear the intended signal is called a MASKER, or MASKING NOISE

Ch5--37

How are masking experiments conducted?» Measure two thresholds:

Threshold for signal (dB S)g ( )

Threshold for signal + masker (dB S + N)

Measure of masking?

dB masking = (dB S + N) – (dB S)

If measure of masking is a ratio, why can we simply subtract: (dB S + N) (dB S) ?simply subtract: (dB S + N) – (dB S) ?

Ch5--38

I th i t h ld S i t it» In the experiment, we can hold S intensity constant and vary N, or we can hold N constant and vary Sconstant and vary S

» Importantly, the amount of masking depends not only on the intensities of the masker andnot only on the intensities of the masker and the signal, but on the spectrum and temporal characteristics of the masker and, in the case ,of speech as a masker, on the linguistic complexity of the signal

Ch5--39

Pure tones as maskers: two important findings:

» At moderate intensities, tones provide greater masking for sinusoids of similar frequency rather than sinusoids at remote frequenciesthan sinusoids at remote frequencies

» Upward spread of masking:

Low-frequency tones can mask high-frequency q y g q ytones

High-frequency tones produce less masking of l f tlow-frequency tones

Ch5--40

Noise as a masker» Noise is the summed result of many y

sinusoids in combination

» With noise as the masker and a sinusoid as the signal, the most important sinusoidal components of the masker are

fthose frequencies that lie closest in frequency to the signal

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Hearing: Binaural EffectsHearing: Binaural Effects

The experiments discussed to this pointThe experiments discussed to this point have involved MONAURAL, or MONOTIC, presentation of a signal -- the signal is p g gdelivered to only one earIn ordinary listening, however, a single

d h b th b t tsound wave reaches both ears, but not necessarily in identical ways (BINAURAL EFFECTS))» Signal intensity may not be the same for

both ears

Ch5--42» Time-of-arrival at the two ears may not

be the same

Interaural time-difference -ITDITD

tt R

t L

ITD = t R - t L

Maximum c 0.6 ms

Ch5--43

Localization: Interaural Time Difference

Ch5--44Processed in Medial Superior Olive

Localization:Localization:Interaural Level Difference

Ch5--45

Processed in Lateral Superior Olive

A second binaural effect concerns INTRACRANIAL LATERALIZATION, which the textbook incorrectly calls yLOCALIZATION

An experiment:An experiment:» Two identical sine waves are presented

binaurally to L through earphonesbinaurally to L through earphones

» L hears a single, fused image arising from within the cranium close to median plane (inwithin the cranium close to median plane (in line with L’s nose)

Ch5--46

Experiment (cont’d)» Now, introduce some differences for the two ears

» Time-of-arrival is under control of the experimenterRE signal for example can be made to lead LE signal orRE signal, for example, can be made to lead LE signal, or

RE signal can be made to lag LE signal

» Listener attempts to identify the direction from which the sound iarrives

If LE signal lags, the origin moves toward the RE: origin moves toward leading earg

If time-of-arrival is equal but intensity is decreased for RE, the origin moves toward LE:

origin moves toward ear with greater intensity

Ch5--47