LOUDNESS.pdf / KUNNAMPALLIL GEJO

7/27/2019 LOUDNESS.pdf / KUNNAMPALLIL GEJO

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LOUDNESS

KUNNAMPALLIL GEJO JOHN

BASLP,MASLP



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The intensity of a sound refers to the physicalmagnitude which can be expressed as its power orpressure

The perception of the intensity is called loudness There is not a simple one to one correlation

between the intensity and loudness

Loudness changes when the bass and treble

changes



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Absolute Threshold

Threshold can be defined as the level at which the

sound is heard 50% of the time its prsented(0.5

probability)



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Absolute sensitivity

Minimum audible level: The absolute sensitivitydescribes how much sound intensity is necessaryfor a typical normal hearing individual to just detectthe presence of a stimulus

Two fundamental methods have been used MAP(minimum audible pressure): Testing subjects

threshold through ear phones and then actuallymonitoring the sound pressure in the ear canal thatcorresponds to these threshold



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MAF(minimum audible field)

Testing subjects threshold by use of a loud

speaker and the subjects leave the room and a

small mic is placed where the subjects head

had been.These measurements corresponds to

MAF



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Usually MAP curves fall below the MAF curves i.e. alower intensity is needed to reach the threshold inMAF than MAP

It was first demonstrated by Sivian and White and

the discrepancy of 6-10 dB is called the missing 6dB

According to them the discrepancy might be due to

Physiological noise picked up by the ear when its

covered by ear



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Binaural thresholds are better than monaural

ones



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MAP-MAF in water

Under water the performance will be impaired

because of heightened stress and demand on the

attention of divers

The immersion head under water result in a detectionthreshold at about 60dB SPL

Sivian(1943) speculated that water plugging the ear

would enhance hearing by bone conduction and he

estimated the hearing loss in water to be between 44-49 dB



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Hamilton reported that upward threshold shift of

35-45 dB in divers and no change in loudness for

occluded ear A study program was initiated to determine the

factors which limits mans ability to converse

under water as does in the air



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Procedure

2 subjects MAP,MAF measurements were done in

air. The maximum difference between the two was 4

dB at 4khz and below these frequency the deviation

was less than 2dB After the measurements were completed in air,

arrangements were made to make similar

measurements in water under same 2 subjects.



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All underwater measurements were made at theunderwater sound laboratory pond

The depth of the water was 50 feet and the temperatureat the time of test was 72-74F

Evidence indicated that hearing under water is primarilyaccomplished by bone conduction.

The increased velocity of sound in water caused areduced interaural time difference (ITD)and interauralintensity difference and also serve insertion of theirfingers into the ear caused no detectable difference in theunderwater thresholds



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Sivian 1943- the greatest loss in sensitivity

underwater is caused by impedance mismatch in air

and water also the bone structure of the body is

more closely match to sea water than is in the airand an increase in bone conduction reception might

be expected



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According to sivian 1943 reduction in sensitivity causedby impedance mismatch in water was around 40dB

The sensitivity of submerged ear may be further reduced

by factors such asunbalanced static pressure increasing with depth

The reduction of increased ambient sound field caused bythe acoustic softness or the swimmers head and the body



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Sivian also suggested that hearing through theear drum submerged in water and boneconduction may have approximate hearing

threshold at 1Khz and it is of the order of 45-50dB above the threshold of the air plus allow nessof the effect of unbalanced static pressure andthe pressure release effect of the swimmers body



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Theories of hearing under

water

Tympanic theory- Banner

under water hearing is accomplished, the same manner ashearing in air, however because the human ear is adapted(impedance match ) to function in air and the characteristicacoustic impedance of water is much greater than that of the air,

a substantial mismatch exist btw water and airBanner concluded that the human ear is not sensitive to

water born sounds as it is to air borne



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Acc to this model the sensitivity loss is frequency

dependent i.e. there will be no loss of sensitivity

at 100hz but a linear drop in sensitivity(12dB/octave) as frequency increases from 100-

5khz



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Sivians dual path approach

Under water hearing is mediated by both tympanicand bone conduction mechanism and they areapproximately equal by 1khz,at other frequency onepath way may predominate

One complication of dual path approach is that adeficiency in one route should not result in degradedunder water hearing

When under water hearing is compared to that in airthe two are not found to be equal



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Bone conduction model

Reysenback and Hann

Since the impedance of human skull is close to that of watersound is readily transmitted from water to cochlea through thesetissues and it bypasses the acoustically in efficient route of theexternal and the middle ear

It also postulated that 2 cochlea are not independently stimulatedunder water as they are in air due to cross conduction of soundthrough the skull and it impairs the sound localization in humans



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Its suggested that the so called underwater hearing loss isnot a loss at all but rather that the thresholds of sensitivity i.e.on the mechanical relationship between sound transmissionin water and the anatomy of human head

The observed threshold are consequence of the mechanicalforce/amplitude arrangements i.e. those which under watersound travel through air in a high amplitude low forcemodel(Af) yet through a fluid such as water as high force lowamplitude (aF).the external and middle section of the earfunction to increase the force from its air born level to one

which will interfere with viscous fluid of the inner ear



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Hearing in air is Af aF where as hearing

under water involves a third step

(aF Af aF) with all the reduction in

efficiency that this multiple change implies.

Ie the external and middle ear mechanism are

not needed, sound waves enters the cochlea

directly through the skull



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Model of underwater hearing



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This model specifies that the human dynamic range

of hearing is reduced to 55-60dB from one in air

which can exceed 130 dB

Under water sound does not decay as rapidly as itdoes in the air

Because due to elevated threshold of detect ability

the divers may not be aware of the actual intensity

of some of the high energy sound that theyexperience



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Loudness level of a sound is the sound

pressure level of a 1 khz tone that is as loud

as the sound

Its unit is Phon



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Loudness level

The magnitude of intensity needed in order for tones of differentfrequencies to sound equally loud---equal loudness level

Procedure One tone is presented at a fixed intensity level and serve as the

reference tone. The other tone is then varied in level until theloudness is judged equal to that of the reference tone.

The traditional reference tone was 1khz

Steven (1972) suggested the use of 3150hz where thresholdsensitivity is almost acute



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If the experiment is repeated for different

reference tone intensity the result is a

series of contours like the figure



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Equal loudness contour curve are of similar

shape to the threshold curve but tend to become

flatter at high loudness level

i.e. the rate of growth of loudness differ for tones

of different frequencies

The rate of growth of lower frequency and to

same extend for higher frequency is higher thanfor middle frequencies



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Loudness matching

The listener is required to vary the intensity of one

stimuli so that it sound as loud as a standard

stimulus with a fixed intensity

The procedure reveals how the physical parametersof sound (frequency and bandwidth) affect the

loudness and also how loudness is affected by

intrinsic factors of the listeners ear (eg; presence of

SN component)



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Method for measurement of equal

loudness contours

Fletcher and Munson(1933)

They took up 11 subjects, hearing thresholds

were determined and rule out for any pathology

Method used:Loudness balanced method

First the subject heard the sound being tested

and immediately afterwards the reference tone

each for a period of one second after a pause ofone sound. The subjects were required to estimate

whether the reference tone was louder or softer



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Other methods

Magnitude estimation method -

(frequently used)

Magnitude production method

Cross modality method



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Robinson and dadson

120 subject of normals-ruled out for any pathology

Method:Method of constant stimuli

Observer task is to simply judge the inequality of

loudnesss for pair of pure tone,one at fixed intensityand the other variable.



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Innitial comparison to the standard

reference tone of 1khz and corresponding

equal loudness relation to variousfrequencies.



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Fletcher-munson Robinson-Dadson

Linear increaser in loudness

from 500hz-20khz.

Obtained phon curve up to 120

phones

Relatively high steeper

at low

frequncies.(20Hz-

100Hz)

Relatively flat response

at 100hz and 1000hz.

Obtained phon curve

up to 100 phones.



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Similarities:

Both ELCs have notch at 4 khz.

Both ELCs are in 10dB steps

Flat response of loudness seen at highloudness levels.



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Loudness scaling-SS Stevens

2 methods

Magnitude estimation

Magnitude production In ME, sounds with various levels are

presented and the subject is asked to assign

number to each one according to its

perceived loudness



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Magnitude production

The subject is asked to adjust the level of a

test sound until it has a specified loudness

either in absolute terms or relative terms that

of a standard. for eg: twice as loud,4times as loud and so on.



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The real function lies somewhat in between the

2.so the unbiased fn may be obtained by using

the method of psychological magnitude balance

suggested by Hellman and Zwislocki.

This method involves the tracking the geometric

means of ME and MP along the intensity axis

and loudness matching.



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Steven suggested that the perceived loudness L

is a power function of physical intensity I.

.03

L=KI

I.e. loudness of a given sound is proportional to

its intensity raised to the power .03



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This low states that sensation grows as a

power of stimulus level

The exponent shows the rate at which thesensation grows with stimulus magnitude.

Exponent1:sensation grows at a fasterrate

than physical magnitude



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Steven propose the Sone as the unit ofloudness.

Sone:loudness of 1 khz tone at 40dBSPL

1 khs tone with a level of 50dBSPL is usuallyperceived as twice as loud as a 40dB tone and hasa loudness of 2 sones.

This relation dont hold good for loudness level

below 40dB.ie at low levels loudness change morerapidly with sound levels.



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Sone=2(phone-40)/10

Critisism for loudness scaling-Paulton1979

Technique used seem very susceptible tobias effect and result are affected byfactors such as

Range of stimuli presented

Firs stimulus presented

Instruction to subject



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Range of permissible response

Symmetry of response change

Various other factors related to experience,motivation, training and attention



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Theoretical objection

Triesman(1964)-pointed out that there are 2

stages involved in obtaining a loudness

judgments

First satge the stimulus evoked a loudness

sensation in the listener



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In the second stage the listener gives a

number which is related to some way to the

magnitude scale, such as a logarithmic scale

There is no easy way to determine what

number scale the listener is giving.



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Psychophysical power law

Describes the way in which sensation grow as

stimulus intensity is increased.

Webbers law-EH Webber(1834) The generalization that a just noticeable

difference /change in stimulus magnitude is a

constant proportion of initial magnitude



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OR

In other words for an increment in a

stimulus to cause it to be just noticeably

different from the one preceding it, would

have always to be a constant fraction of

capacity of a late state in the auditory

system

A high level of internal noise mightcharacterize the later stage.



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The limitation of peripheral processing would

then only appear when a relatively small

population of fibers may convey the

peripheral information.



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Parameters of loudness/Factors

affecting loudness

Spectral parameters

Equal loudness contours

Band width

Intensity parameters

Loudness function

Duration parameters



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Back ground variable

Masking

Loudness enhancement

Listener variable Binaural summation

Recruitment

Auditory fatigue



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Band width

Band width of signal refers to the range of

frequencies occupied by its different

elements. The loudness of a signal held at a given

overall SPL doesnt increased when its

bandwidth is increased from that of a

single frequency tone to what is calledcritical band width



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An increase of band width beyond critical

band width still keeping the total SPL does

increase the loudness because the signal

occupies more than one critical band.



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Intensity parameters

Loudness is a monotonic fn of stimulus

intensity, the change of loudness with a

change of sound pressure level is moreaccurately represented by decibel scale

doesnt reflect the relation between

loudness and sound pressure accurately

either a graph that plot the change ofloudness as the sound pressure is varied

is called loudness functionKUNNAMPALLIL GEJO JOHN


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The loudness function can be plotted in 2

ways

Geometric function

Linear function.



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Geometric function

It represent the ratios of loudness and make use

of multiplications.

The unit of loudness on the vertical scale is

sone

The loudness in sones of any other sound is

numerically equal to the ratios of its loudness to

the loudness level at 40 phons.

A sound as twice as the 40 phon reference has a

loudness level of 2 sones.



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LINEAR FUNCTION

The loudness scale is laid out in equal interval

Each point in the scale is an addition to rather

than a multiple of loudness values of the previous

point.

With increase in the SPL for equal loudness

interval number of sones increased by an equal

amount irrespective of the starting point.



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Duration parameters

The loudness of a short sound, e.g., a burst of white noise,depends on its duration (Scharf 1978).

Successive noise bursts equated in acoustic power andincreasing in duration from, say, 5 ms to about 200 ms areperceived not only as longer and longer but also as louder andlouder.

Loudness is thus determined by a temporal integration ofacoustic power.

This temporal integration implies that a percept of loudness is infact the content of an auditory memory.



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Measurements of the variation of absolute threshold withduration indicate that, over a certain range of durations, thethreshold corresponds to a constant energy rather than aconstant power (Garner and Miller, 1947).

In other words, over this range of durations, the ear behaves as ifit were a perfect energy integrator, although there is somedebate as to whether the actual neural mechanisms involvedinclude a "true" long time-constant integration device.



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loudness is also related to stimulus duration,

although, as one might expect, there is

considerable variability in the results.



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Using a procedure in which listeners wererequired to match the loudness of tone burstsof variable duration to that of a continuous

reference tone, Boone (1973)showed thatloudness is also proportional to the totalenergy of the tone, so that as the duration ofa tone of constant power is increased, its

loudness also increases.



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Stephens (1974) replicated these results but

showed in addition that this relationship is highly

susceptible to the experimental procedure and the

instructions given to the listener. In particular, at long durations it is hard to make a

judgment of the total loudness of the sound, rather

than the loudness at a particular instant or over a

short time period.



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Back ground variable

Masking; Reduction of loudness by a back

ground noise is called masking.

The masking sound raises the thresholdfor the signal and reduce its loudness

The partial masking not only depends on

intensity but also bandwidth and frequency



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Frequency and band width

The slope of the loudness function under masking

increases when the band width of the masking noise

is narrowed while its overall level is kept

constant.(hellman1970 and Zwicker,1963) NBN is the better masker than WBN for pur tone.

Low freq are better masker of high freq but high freq

req more energy to mask low freq.



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Loudness enhancement

In contrast to the reduction in loudness produced by soundspresented simultaneously with the target, sounds presentedbefore the target can sometimes produce an increase inloudness.

Max enhancement occurs when the enhancement and the target

sound have same freq (Zwislocki & Sokolich, 1974),

the effect is attenuated (but not eliminated) when the two tonesare presented to different ears (Galambos et al., 1972; Elmasian& Galambos, 1975).



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Listeners variable

Loudness of a signal is more on binaural

hearing than in mono aural hearing due to

binaural summation.



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Binaural summation

If perfect binaural summation occurred between two

ear then the judged loudness in monaural sound

would be half of the loudness in the binaural

hearing. Hellman and zwislocki(1963) and Mark(1978) leads

to hypothesize that total loudness sound is the

listener sum of the loudness resulting from each ear.



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Marks (1987) reported a phenomenon

called super summation by virtue of whichthe binaural summation is greater thantwice of the mono aural loudness when abinaural tonal stimuli is heard over a partial

masking by noise. Fletcher and Munson concluded that

stimulus of a given SPL would sound twiceas loud as mono aurally



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Application

The increase in loudness due to binaural

hearing has a particular advantage for

hearing impaired person, as it reduces the

power requirement of amplification.



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Loudness Adaptation

It refers to the apparent decrease in the loudness of

a signal thats continuously presented over a period

of time.

The signal appears to be softer even though theintensity is same

Its the general property of the sensory system that

neural response to long duration stimulation decays

rapidly after stimulus onset to reach a steadyequilibrium state.


Schraff (1983) based on the experimentation


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Schraff (1983) based on the experimentation

using magnitude estimation provide a cohesive

report as follows.

There is noticeable amount of variability amongsubject in terms how much adaptation

experience.

Loudness of a pure tone adapts when its

presented to the subject at level up to 30dB SL

(appx).

Later Miskiewics et al(1973) found that loudness

adaptation also occurs above 30dB SL for highfreq tone (12,14,16 Khz)



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There is more adaptation for high freq tone

than low freq tone or noises.

Adaptation appears to be same regardless of

presentation signal to one or both the ears.



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Effects of Sensory Hearing Loss on

Loudness: Recruitment

Sensorineural hearing impairment ischaracterized by elevated thresholds for thedetection of sounds in quiet.

Despite this loss in sensitivity, a sound at a high

intensity might sound equally loud to a hearingimpaired listener as it does to a normally hearinglistener.

In other words, there is an abnormally steepgrowth of loudness with intensity in the impairedear. This phenomenon is called recruitment,



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Now its termed as sof tness impercept ion

Due to in adequate functioning ofOHCs,the low intensity sounds are not

perceived and the high intensity sounds

are perceived as the way normal personperceive it.

This leads to rapid growth at supra

threshold levels.



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Evans (1975) has suggested that recruitment may be a result ofthe reduced frequency selectivity usually associated with hearingloss (Tyler,1986).

As the intensity of a pure tone is increased, excitation spreadsacross the basilar membrane so that the number of nerve fibers

excited also increases. The broad auditory filters in impaired ears will give rise to a

greater spread of excitation with increasing intensity than occursin normal ears



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Models of loudness

Loudness is often regarded as a globalattribute of a sound, so that we usually talkabout the overall loudness of a sound rather

than describe separately the loudness inindividual frequency regions.

An exception to this arises from the models of

loudness that calculate the "specific

loudness" of a sound in each frequencychannel it excites to obtain an overallloudness measure by summation.


Zwicker (1958; Zwicker & Scharf, 1965)


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developed a model of loudness based on the

excitation pattern.

The model consists of a number of stages. First, the input stimulus is passed through a fixed

filter representing the transfer characteristics of

the outer and middle ear.

Above 2 kHz the form of the filter is given by theinverted absolute threshold curve.

Below 2 kHz, Zwicker assumed that the transfer

function is flat.

The rise in absolute threshold with decreasing

frequency is assumed to be caused by an

internal low-frequency noise.KUNNAMPALLIL GEJO JOHN


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In the second stage, an excitation pattern

for the stimulus is calculated ..

Excitation is plotted as a function of

frequency on a Bark scale

Finally, excitation is converted into specific

loudness (or loudness per critical band),N'



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Following Stevens, N' is assumed to be related to

excitation intensity,E, by a power law.

N ' = CE where C and are constants and < 1.

Zwicker and Fastl (1990) estimated to be 0.23.

This relationship works for excitation levels well

above absolute threshold.



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To account for the steep growth of loudness nearabsolute threshold the equation was modified as

follows:

where ESIG is the excitation produced by thestimulus and ETHRQis the excitation at absolutethreshold.

The overall loudness of the sound is defined as

the area under the specific loudness pattern. Inother words, the total loudness is the sum of theloudness across each critical band.



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This model has been modified by Moore andGlasberg (1986, 1994).

They assumed that, below I kHz, the form of theinitial filter is given by the inverted equal loudness

contour at 100 phon Above I kHz the filter shape is given by the inverted

absolute threshold curve.

Excitation patterns are calculated from auditory filtershapes they derived in earlier work (Moore &

Glasberg, 1983; Excitation is converted into specific loudness

according to the following relationship:



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Notice that when ESIG

= ETHRQ

the specific loudnessis0.

Hence, near absolute threshold, a small change in

excitation produces a largeproportionalchange in

specific loudness. Equation (4) can account here fore, for the steep

(proportional) growth of loudness with level near

absolute threshold.

------------(4)



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In the model of Moore and Glasberg, the

overall loudness of the sound is calculated by

integratingpositive specific loudness values

across the specific loudness pattern, asbefore.

In this case, however, the specific loudness

pattern is plotted on an "equivalentrectangular bandwidth" (ERB) frequency

scale rather than on the Bark scale



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The modified model is quite successful at

predicting the variation in loudness with

intensity, frequency, and bandwidth (Moore &

Glasberg,1994), supporting the view that loudness is

intimately related to the frequency selectivity

of the peripheral auditory system, and not justto the physical intensity of the sound per se.



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DIFFERENTIAL SENSITIVITY

The DL is the smallest perceivable difference

in dB between 2 intensities ( l) or the

smallest perceivable change in Hz between 2

frequencies( f) We may think of the JND in 2 ways

One is the absolute difference between the 2

and the other is as the relative differencebetween them.



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The relative DL is obtained by dividing the

absolute DL by the value of the starting level.

If the starting level I is 1000 unit and the DL

(delta I) is 50 units then the relative DL

I/I =50/1000=0.05.

This ratio is called the weber fraction.



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An important concept in psychophysics is

webers law, which states that the the value of

I/I(weber fraction) is a constant (K)

regardless of the stimulus level orI/I=K


Loudness perception in


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p p

pathological ears

Recrui tment

Sensorineural hearing impairment is characterizedby elevated thresholds for the detection of sounds inquiet.

Despite this loss in sensitivity, a sound at a highintensity might sound equally loud to a hearingimpaired listener as it does to a normally hearinglistener.

In other words, there is an abnormally steep growthof loudness with intensity in the impaired ear. Thisphenomenon is called recruitment,



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Due to in adequate functioning ofOHCs, the

low intensity sounds are not perceived and

the high intensity sounds are perceived as

the way normal person perceive it. This leads to rapid growth at supra threshold

levels.



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Evans (1975) has suggested that recruitment may be

a result of the reduced frequency selectivity usually

associated with hearing loss (Tyler,1986).

As the intensity of a pure tone is increased, excitation

spreads across the basilar membrane so that the

number of nerve fibers excited also increases.

The broad auditory filters in impaired ears will give rise

to a greater spread of excitation with increasing

intensity than occurs in normal ears



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Loudness Adaptation

It refers to the apparent decrease in the

loudness of a signal thats continuously

presented over a period of time.

The signal appears to be softer even thoughthe intensity is same.



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Schraff (1983) based on the experimentation using

magnitude estimation provide a cohesive report as

follows.

There is noticeable amount of variability amongsubject in terms how much adaptation experience.

Loudness of a pure tone adapts when its presented

to the subject at level up to 30dB SL (appx).

Later Miskiewics et al(1973) found that loudnessadaptation also occurs above 30dB SL for high freq

tone (12,14,16 Khz)



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There is more adaptation for high freq tone

than low freq tone or noises.

Adaptation appears to be same regardless of

presentation signal to one or both the ears.



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LOUDNESS.pdf / KUNNAMPALLIL GEJO