Simultaneous masking.pptx

8/21/2019 Simultaneous masking.pptx

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Presenter:

Bebek Bhattarai

Faculty:

Dr. Vijay Kumar Narne


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Definition

Everyday situations

Masker--noise

Maskeesignal

Masked threshold

If one freq signal is masked by another frequency masker, then it indicates poofrequencybecause the system is not able to resolve the two freq separately.

Simultaneous masking

Non-simultaneous


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Frequency selectivity.

Filters in the cochlea.

What happen when there is damage to OHC???Reduced sharpness of Tuning Curves of BM and single neur


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How do we measure itDetection of a fixed freq signal in presence of masker

It is based on two conceptsAuditory filter shape

Excitation pattern


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Fletcher (1940)Cochlea as array of bandpass filters with overlappinpassband filter

Filters corresponds to particular place in BM


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Array of linear overlapping filters

In noisy situation, only on filter is used; whicmaximum SNR and freq close to signal

Only the noise in the filter will have effect,detection is dependent on the level of noise filter


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Non-linear filter which are level dependa

Information can be combined from diff f

Noise outside of band also affects the

detectionFluctuation (short) of noise can affectdetection


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1:

Signal at fixed intensity and freq

Masker of diff freq

Find detection level

Plot gives shape of filter

2:

Signal whose freq are systechanged

Input changed so as to keepconstant

Maxima at CF Minima at CF

If the filter is linear


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Auditory filters are not linear

It is level dependent; shape changes with level

Also filter shape depends on whether i/p or o/p is held constant.

Practically it is difficult, we can jus approximate

Measure i/p and o/p is difficult in human.

Hence filter shapes are obtained using masking methods following the assustated earlier.


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PTC Similar to tuning curve of BM or single neuron

Signal fixed at low level, usually 10dBSL

Masker can be NBN or sinusoidal signal (noise preferable)

Reduced formation of beats

With sinusoid beats can give cues about the presence of signal. Resulting PTC can be s

the exact curve Masker at diff freq presented

Level of masker required to just mask the signal is found out

Since signal at low level (10dBSL), it is assumed that:

Masker produces constant o/p at single filter

As low level only one filter is involved


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Off-frequency listeningAffects the PTC

If masker above the signal, maximum SMR at lower level thasignal frequency

If masker is below signal freq, maximum SMR is obtained for

above signal freq Because the PTC have rounded peaks


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Solution for off- freq listening would be notch noise

Here the procedure is similar, as finding threshold inpresence of noise; but the notch noise will be constanpresent

Notch noise reduced the off freq listening when freq soccurs.


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With notch noise techniqueEither the signal can be kept constant or the mcan be kept constant

Usually noise level is kept constant


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In notch noise techniquie: If the threshold drops rapidly with increasing the notwidth, it indicated sharply tuned freq curve

Whereas if the drop is slow,it indicates broadly tune

Also with the relation between the noise and the thresthe action of the filter can also be determined.


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Notch noise technique does not yield betterrepresentation of tuning curve if the filter is notsymmetric, as in:Hearing loss cases

High intensity level (only at moderate level the filter symmetric)

At high intensity level the notch should also be placeasymmetrically to get valid result.


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VARIATION WITH CENTER FREQUENCY

Bandwidth -3dB bandwidth or half power bandwidth

ERB (Equal Rectangular Bandwidth) {for auditory filters: ERB is 11% larger than -3dB bandwidth}


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F= CF

N= For Nor


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For normal listeners, the filter is relatively constexcept for very low and high frequencies.

Each ERBNcorresponds to about 0.89mm on B

Normal variation is not more than 10% from me

ERBN.


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VARIATION WITH LEVEL:Since filters are non-linear, there will be difference in thewith diff level.

If it was liner then the filters would have been symmetriclevels


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Studied with notch noise---symmetrical and assymetr

Findings:The slope of LF becomes less sharper at higher level

HF become more steeper at high level


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The concluded as:Filter at 1KHz is symmetrical at moderate intensity of no(30dB)

For other freq also symmetrical if the i/p level is 51dB/E1KHz

Low freq side become less steep with increasing intensiHigh freq side variable results:

@ 1-4KHz: slope increases with intensity

@ High CF slope decreases with increasing intensity


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Conductive hearing loss (Hg loss varies at diff f

Shift in threshold (with noise should be present

SPL or SL used to compare Normal and HI If SPL, TW at BM will be equal

If Sl, the SPL at BM will be diff.

Amount of masking or masked thresholdDiff level of hearing requires diff level to get mas


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Differential Diagnosis:Langenbeck (1950) studied on normal listeners, listewith noise induced hearing loss, with head injury anpresbyacusis.

Normal: comparative shift in threshold across freq

NIHL: similar to normal at LF; at HF elevated due to hg lo Presbyacusis: abnormal shift above 1.5KHz

Head injury: abnormal across freq

But no clear cut demarcation obtained

Noise was presented at 40dB SPL.


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Palva et al (1953) studied masking of pure tone usingbroadband noise in normal, SN hearing loss withrecruitment and SNHL without recruitment.

No significant difference among group data in terms omasked thresholds.

The levels of noise presentation were 40, 50& 60 dB


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Lightfoot et al (1956): BBN at 60 & 80 dB SPL in 30 hearing impaired subjects and

95 dB SPL in another 30.

They found that the elevation in masked thresholds in SN was at least 10 dB above than normal at frequencies 2000above whereas it was less than 10 dB in conductive heari

Significant variation within group.


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Webster et al (1950) masked threshold increasedfor every 10 dB absolute threshold elevation above(0 dB HL).

Higher masked thresholds in females, above 40 and those without musical training.

Suggested elevated masked thresholds could be anindicator of wide critical bandwidth and poor freqresolution.


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Tyler et al (1982) reported more masking than norlow level of noisebut had normal masked thresholhigh noise levels in two hearing impaired.

This could be

Frequency resolution is better at high intensity in

impaired, which contradicts to normal (Scharf et alChange in efficiency of the detector mechanism.


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Filter shape or detector mechanismElevated threshold in noise due to widened auditoryless effective detector mechanism.

Tyler et al (1982) suggested that the nonlinear growth o

masking might also be better understood in terms of chathe detector mechanism.


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Watson and Tolan (1949) used tone on tone masking with a combination of500/2000, 1000/3000 as masker/signal respectively in individuals with hearimpaired.

The presentation level was increased gradually in 10 dB steps and reportfour subjects did show a less change in signal threshold with increase ilevel.

At the same time de Bruine-Altes (1949) reported higher than normal mas

thresholds at signal frequencies of 2000 and 4000 Hz for a 400 Hz mas


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Tyler et al (1980) reported that pronounced upward of masking is generally confined to the regions ofabsolute threshold loss


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Leshowitz et al (1975) and Carney and Nelson (1976)Normal sharply tuned PTCs in regions of normal absothresholds

Abnormal broadly tuned PTCs in regions of elevatedabsolute thresholds

Correlation between threshold and the sharpness of tvaries markedly across studies

No systematic differences in PTCs have been repobetween cochlear losses of different origin, such as noinduced, Menieres disease, ageing, and hereditary lo


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Carney and Nelson (1983):Compared simultaneouscurves from normal-hearing and hearing-impaired listusing probe tones that were either at similar SL's orsimilar sound pressure levelsfor both types of liste


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Tuning curves from the hearing-impaired listeners areerratic, broad, and/or inverted, depending upon thefrequency region of the probe tone and the frequencycharacteristics of the hearing loss.


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Similar manner to analogue filters

Q10dividing the signal frequency by the bandwmeasured 10 dB up from the tip.

Bonding (1979) has chosen a slightly different aHis measurement of tuning is defined as the disdB between the tip of the PTC and the level wPTC is 1 octave wide..


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Tyler et al (1979) quantified the high and lowfrequency slopes of the PTCs to find asymmetr

Slope calculated between two masker freq.

Which might not give better representation.

It might be misleading if we are evaluating W-shPTC

Low freq more curvilinear.


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Tyler et al (1983) used a tip-to-tail difference(the difference between masker level in the

the PTC and the masker level at the tipof thPTC).

Used to quantify the efficiency of the detectmechanism (cochlear amplifier).

Stelmachowitz, et al. (1985): Compared Q10concluded abnormal upwards spread of main HI.


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Davidson & Melnick (1988):Compared PTCs in simultaneous masking usinmethods:2 IAFC (transformed up down procedure)

Bekesy tracking

More masking seen in the Bekesy method whiccould be because of subjects criteria for thresh

May be due to lack of choice (no force choice)


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Problem for both quantification and interpretation of P

Possibility is that some form of temporal interaction bthe masker and the signal is being detected

Viemeister suggested that these listeners may be heamodulation or beating over an extended frequency ra

Off frequency listening


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Leshowitz and Lindstrom (1977) noted that in onelistener with a NIHL, 68 dB SPL 4800 Hz masker assufficient to mask a 75 dBSPL 5400 Hz signal.

This was called over masking.

Hoeksta (1979) noted that in some cases masker lmay be 20 dB less than the signal level.


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Beats

Off frequency listening

Comparison between normal and HI


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A brief signal is more easily masked when presenthe onset of maskerrather than temporal center folong duration masker.

In this case, more masking takes place and hence

tip gets broader, if at onset.


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PTCs were sharpest when the signal was at the tecenter of the masker, and broadest when it was atbeginning of the masker


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PTC was virtually independent of the temporal posof the signal

Reason may be:

Mechanisms responsible for sharpening the PTC wtime in normal-hearinglisteners are ineffectiveinlisteners with moderate-to-severe sensorineural hloss


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Normal Hearing Hearing Impaired


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Auditory filter becomes more highly tuned with timonset of stimulation.

The spectral splatter occurring at the onsetof a mcauses a transient increase in masking.

The neural adaptation accounts for the phenomen


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Reduced cochlear gain leads to elevated thres

Steeper input-output functions lead to steepergrowth of loudness level

Broader cochlear tuning leads to poorer frequeselectivity.


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Simultaneous masking.pptx

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