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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
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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.
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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.
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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
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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
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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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