Effect of masker level on overshoot in running- and frozen-noise
maskers
Regine von Klitzing
DrittesPhysikalisches
Institut, Universitdt
G6ttingen,Biirgerstrasse
42-44,D-37073 GSttingen,
Germany
Armin Kohlrausch
a)
Institutefor Perception
Research
(IPO), P.O. Box 513,NL-5600MB Eindhoven,
TheNetherlands
(Received23 October1992;revised18 November1993;accepted13 December1993)
Maskedthresholdswere measuredwith running-and frozen-noisemaskers.The 5-kHz signal
was2 ms in duration.The maskerwaslow-passnoise(20 Hz-10 kHz); its total durationwas
300 ms. The overall level of the masker was 30, 50, or 70 dB SPL. The onsetof the signal was
delayedby 0, 3, 8, 18, 198, or 278 ms relativeto the onsetof the masker.In all frozen-noise
measurements,
thesignalwasaddedto the samefinestructureof the noise.Overshoot
in frozen
noisewasmeasured
for two startingphases
of the signalthat led to a 10-dBdifference
for large
signal-onset
delays.In all threeconfigurations
(runningnoiseandfrozennoisewithtwodifferent
signalphases)maskerlevelhad a similarinfluence
on overshoot.
At the intermediate
masker
level (50 dB SPL), a significant
amountof overshoot(up to 15 dB) wasobserved
in all three
conditions.
At the low and the highmaskerlevels,overshoot
wasverymuchreduced,and even
becamenegativein mostconditionsfor the 30-dB-SPLmasker.For the 50-dB frozen-noise
masker,the total variationof thresholdswith signalphasewas 8 to 11 dB for long signal-onset
delays,but only 3 to 6 dB for shortdelays.For the low- andhigh-levelmaskers,whereonly a
small overshootwas observed,the thresholdvariationwith phasefor a signalat maskeronset
wasthe sameas that for the long-delaycondition.An explanationfor the variationof signal
detectability
with maskerlevelis proposed
that refersexplicitlyto the compressive
input-output
characteristic of the basilar membrane at intermediate levels.
PACS numbers: 43.66.Dc,43.66.Mk, 43.66.Ba[HSC]
-- 15and45dB.Hefoundanincrease
ofovershoot
from0
INTRODUCTION
The term overshoot
refersto the fact that a brief signal
is sometimes
harderto detectif it is presentedat the onset
of a maskerthan if it is presentedwith a long onsetdelay.
About 30 yearsagothe first experiments
to investigatethis
phenomenon
were conducted(Samoilova,1959; Scholl,
1962;Zwicker, 1965a,b;Elliott, 1965). In the last decade
overshoothasreceivedrenewedattentionwhich can partly
be attributedto the fact that no satisfactoryexplanation
has beenofferedso far (e.g., Baconand Viemeister,1985;
Baconand Moore, 1986;Carlyon, 1987, 1989;Carlyon and
Sloan, 1987; Champlin and McFadden, 1989; McFadden,
1989;Bacon, 1990;McFaddenand Champlin, 1990;Bacon
and Smith, 1991;Schmidtand Zwicker, 1991;Carlyon and
White, 1992;Baconand Takahashi, 1992).
It is known that overshootis influencedby many parameterslike frequencyanddurationof the signal,spectral
composition
and level of the masker,prior stimulation,
hearing impairment, and even consumptionof aspirin by
subjects.Since theseeffectsand the possiblerelation to
neuralshort-termadaptationhavebeenreviewedin several
recentpublications(e.g.,Baconand Smith, 1991;Carlyon
and White, 1992; Bacon and Takahashi, 1992), we will
to about10 dB for an increasein the maskerspectrumlevel
up to q-5 dB. For all highermaskerlevels,up to a spectrum level of 45 dB, overshoot remained constant at this
value. Fastl (1976) useda uniform maskingnoiseof 16kHz bandwidthand an 8-kHz signalof 2-msduration.The
spectrumlevel in the 8-kHz regionrangedfrom -15 to
q-25 dB. Similar to Zwicker's results, overshoot increased
for spectrumlevelsup to 5 dB and remainedconstantfor
all higherlevels.The amountof overshoot,however,was
much larger than reportedby Zwicker, beingas much as
30 dB.
The results of more recent measurements revealed a
differentdependence
of overshooton maskerlevel.Bacon
(1990) measuredovershootfor a 10-ms4-kHz signal.The
noise had a bandwidth of 8 kHz and was presentedat
spectrumlevelsbetween-- 10 and 50 dB. Overshootgrew
nonmonotonically
with level,reachingan asymptoteof 10
to 15 dB at spectrumlevels of 20 to 30 riB, before decreas-
ing at higher levels.For a majority of the subjects,overshootat a 50-dB spectrumlevel was as small as it was at
--10
dB.
Carlyon and White (1992) measuredovershootwith
concentrate on the influence of masker level on overshoot.
broadbandmaskersfor signal frequenciesof 2.5 and 6.5
Zwieker (1965a) measuredovershootof a 5-kHz 2-ms
kHz. At 2.5 kHz, overshootvalueswereindependentof the
signalin a broadbandnoiseat spectrumlevelsbetween masker level. At 6.5 kHz, overshootfor most subjectswas
largerat a maskerspectrumlevelof 25 dB than at either5
a)Allcorrespondence
should
besenttoArminKohlrausch
at thisaddress. or 45 dB. However,two of the subjectsshowedeither no
2192 J. Acoust.Soc.Am.95 (4),April1994
0001-4966/94/95(4)/2192/10/$6.00
@ 1994Acoustical
Societyof America 2192
leveldependence
or an inversebehavior,with the smallest
overshootoccurringat the intermediatemaskerlevel.
Bacon and Takahashi (1992)
collected data for
maskerspectrumlevelsbetween20 and 40 riB. Their signal
was a 10-ms 4-kHz sinusold.For the majority of their
subjects,overshootdecreasedmonotonicallywith increasing maskerlevel.The rangeof individualovershootvalues
at the lowest masker level was 7 to 26 dB, while at the
highestlevel it was 2 to 14 dB.
The presentstudy is concernedwith level effectsin
overshootconditionsfor running-and frozen-noisemaskers.There are two reasonsto investigateovershooteffects
for frozen-noisemaskers.We know from recent experimentsthat thresholdsof short signalsin frozennoisecan
be much lower than thosein runningnoiseand that they
vary stronglywith the temporalpositionof the signalin
the noiseand with the signalstartingphase (Langhans,
1991; Langhansand Kohlrausch, 1992). These resultsfor
frozen-noise maskers indicate that with the lack of statis-
tical variability in the maskerwaveform,additionalcues
like slighttimbredifferences
causedby the additionof the
signal can be evaluatedby the subjects.It is unclear
whetherthe useof suchsubtlecuescouldhelp to detecta
signalplacedcloseto the onsetof a frozennoiseand thus
lead to smallerovershootvaluesthan for a running-noise
masker.
A
second motivation
for
the use of frozen
noise
we startedthe experiments
described
in thispaper,we were
interestedin checkingwhether this discrepancybetween
the model predictionand the majority of data is a consequenceof usingfrozennoisein the simulationand running
noise in the measurements,or whether the model is incor-
rectin thisrespect.•
I. METHOD
A. Stimuli and apparatus
All stimuli were generateddigitally,convertedto analog voltageswith a two-channel,16-bit D/A converterat
a samplingrate of 30 kHz, and low-passfilteredat 10 kHz.
To obtain the noisemasker,a complexspectrumof
frequencycomponentsbetween20 Hz and 10 kHz was
constructed.All frequencycomponentshad a constantamplitude, the phase values were randomly distributed between0 and 2•r. The inverseFFT of this spectrumresults
in a cyclicnoisesignalconsisting
of 2•5 samples
witha
periodof 1.1 s. In experiments
with frozennoise,a fixed
300-mspart of this time waveformwas used.This fixed
noisesamplewaspresentedin all intervalsof all runsfor a
given signaldelay (for more details,seeSec.I C below).
In contrast, in experiments with running noise,
different300-mssectionsof this 1.1-snoisewerepresented
to the listenerin every interval of a run. Thesenoisebursts
were copied from the long noisewaveform by randomly
stemmed from theoretical predictionsderived from a
modelfor temporalprocessing.
This modelhas beenappliedto nonsimultaneous-masking
dataandothertemporal
effectsand is describedin detail elsewhere(Pfischel,1988;
Kohlrauschet al., 1992; Dau etal., 1994). The relevant
choosing
oneof the2• values
asstarting
point.Forevery
elements of the model are a (linear) ba.,filar-membrane
levels of - 10, 10, and 30 dB. All noise bursts had a rect-
stagefollowedby a seriesof adaptive-gain-control
stages
angulartemporalenvelope,i.e., the durationof the onset
ramp was only influencedby the analoglow-passfilter at
with time constants between 5 and 500 ms that result in a
loglikecompression
for signalswith a stationaryenvelope.
Fast fluctuationsin the envelopeof an input signallike in
amplitudemodulationor the fluctuationsof a noiseenvelope are, however,transformedmore linearly and lead to
largervariationsin the internalrepresentatiion
of the stimulus.The detectioncriterionusedto predictthresholdsis
derivedfrom the stationarybehaviorin sucha way that,
for a stationarysignal,thejust-noticeable
changein levelis
1 dB at all levels.
For a signal placed at the onset of a frozen-noise
masker, the model predictsa somewhatlower threshold
than for a signalwith a long onsetdelay. The reasonfor
this lowerthresholdis that both the signaland the masker
are lesscompressedat the onset of the masker than at a
later point. The addition of the signal at t'.heonset of the
maskerthus leadsto a greaterchangein the internalrepresentationthan for onepositionedlater in the masker,and
thus the signalis detectedmore easily.
A negativeovershoot(or undershoot):fornoisemaskers has beenobservedonly in somelistenersunder a lim-
run, a new 1.1-snoisewaveformwas calculatedby usinga
new set of phasevalues.
The noise maskerswere presentedat three different
levels:30, 50, and 70 dB SPL, corresponding
to spectrum
10 kHz.
The signalwas a 5-kHz sinusoldwith a total duration
of 2 ms. It was gated with l-ms raised-cosine
on and off
ramps.The signalwasaddedto the maskerwith oneof the
following onset delays:0, 3, 8, 18, 198, or 278 ms. The
onsetdelay of the signalwas definedas the time between
maskeronsetand the onsetof the signal'sramp. In the
running-noise
measurements,
the startingphaseof the signal, definedwith respectto its envelope,was alwayszero.
In the frozen-noisemeasurements,
the startingphasewas
an additionalparameter.
At each masker level, the following three measurementswere performed:
( 1) Thresholdsof a signalplacedwith a 278-msdelay
in the noise bursts.This measurementwas performedfor
nine valuesof the startingphasebetween0 and 2•r in frozen noise(signalphaseincreasedin stepsof •r/4), and in
additionfor the signalin runningnoise.The valuesfor the
starting phases0 and 2•r, althoughphysicallyidentical,
were measuredseparatelyto testthe reliabilityof the data.
ited set of conditions (e.g., Osman and Raab, 1963; Wilson
(2) Measurement of the thresholds for six temporal
and Carhart, 1971;Bacon, 1990, Figs. 2 and 3; Baconand
Smith, 1991, Fig. 1; Baconand Takahashi, 1992, Fig. 2),
and little emphasishasbeengivento thesefindings.When
positionsof the signal,with onsetdelaysbetween0 and 278
ms. This measurementwas performedfor frozen noisefor
thosetwo signal-phase
valueswhich gavethe highestand
2193 J. Acoust.Soc. Am., Vol. 95, No. 4, April1994
R. von Kiltzingand A. Kohlrausch:
Effectof maskerlevel 2193
the lowest thresholds in the first measurement.
(3) Thresholdsof a signalplacedcloseto the onsetof
the maskerburst at that temporalpositionfor which the
highestthresholdshad beenfoundin the secondmeasurement. The onsetdelay was chosenfor each subjectseparately and was either 0 or 3 ms. This measurement
again
wasperformedfor nine valuesof the startingphasein frozen noiseand in additionfor the signalin runningnoise.
All measurements
reported in this paper were performed diotically, i.e., masker and signal were presented
identically to both ears of the subjects.
0.05
0.00
-0.05
0.0
0.2
0.4
0.6
0.8
s
1.0
0.0
0.2
0.4
0.6
0.8
s
1.0
o.o
0.2
0.4
0.6
0.8
s
1.0
0.05
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ko.05
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B. Subjects and procedure
-0.05
The experimentswere performedby three subjects,
who had experiencein psychoacoustic
experimentsand
had severalhoursof trainingin overshootconditions.The
subjects
rangedin agefrom 24-28 years.The subjects
were
Time
seated in a sound-insulated booth and listened to the stimFIG. 1. Illustrationof the generationof the frozen-noisemaskerbursts.
uli via headphones(Beyer DT 880M with diffuse-field The top row showsthe 1-sfrozennoise.The signalonset(markedby the
equalizer).The computercontrolledthe experimentsand
arrow at the bottom) occurred at 0.6 s. The middle trace shows the
300-ms masker burst which was used in the measurement with a 278-ms
read the answersof the subjects.
delay between masker and signal onset. The bottom row shows the
The maskedthresholdswere measuredusing a stanmaskerfor the delayof 0 ms betweenmaskerand signal.
dard three-intervalforced-choice(3IFC) procedurewith
adaptivelevel adjustment.The three noiseintervalshad a
durationof 300 ms eachand were separatedby interstim- the desiredmasker-signalonset delay, a 300-ms section
surroundingthis time point was cut out of the long noise
ulus intervalsof 500 ms. In one randomlychoseninterval,
signalwith a rectangularwindow. This procedureis illusthe signalwas addedto the noisemasker.The task of the
subjectwas to specifythe interval containingthe signal. tratedin Fig. 1. The top row showsthe 1-snoisewherethe
The signal level was varied accordingto a two-down arrow (bottom) marks the positionof the signal.The situation for a 278-ms delay betweenmasker and signal is
one-upprocedure(Levitt, 1971). A run beganwith the
shown in the middle trace and the masker used for the
signallevelwell abovethe expectedthreshold.The initial
step size for the level changewas 10 dB and was halved O-msdelay is shownin the bottom trace.
(dB measure)after each secondturnaroundpoint of the
II. RESULTS
signallevel.When the stepsizebecamesmallerthan 1 dB,
it was set to the constant value of 1 dB and the run was
extended over 20 more trials. The threshold levels were
calculatedas mediansignallevelwithin theselast 20 trials
of eachrun. Typically, 4 to 6 reversalsof the signallevel
occurredduring the 20 trials. For eachconditionthis procedurewas repeatedfour times in differentsessions.
Data
in the figuresare calculatedas the median valuesfrom
these four runs and are shown together with the correspondinginterquartileranges.
A. Signal detectability as a function of masker level
In Fig. 2, the influenceof signal phaseon masked
thresholds in a frozen-noise masker of 50 dB $PL is shown
For differenttemporalpositionsof a short signal in
frozen noise, thresholdscan vary by more than 10 dB
(Langhans,1991). This valuecorresponds
to the order of
for a short-delay(a) and a long-delaycondition(b) by the
opensymbols.The closedsymbolsto the left in both panels
indicatethe thresholdsin runningnoisefor the samesignal
delay. The differentsymbolsindicateindividualdata from
the three subjects.
The resultsfor frozen noisealways show a maximum
at phase•r/2 and a minimum at phase3•r/2. In the longdelaycondition,the thresholddifferencebetweenthesetwo
phasesis 8 to 11 dB. The signalthresholdsfor running
noiseat thisdelayare higherthan the arithmeticaverageof
thosefor frozennoise,but they do not form an upperlimit
overshoot in running noise. If the signal was added to a
for the frozen-noise
fixed frozen-noisesample with a variable onset delay, a
possibleovershooteffectcouldbecomeinvisibledueto this
fine-structureeffect.Hence, it is very important that the
perimentsat 1 kHz (Langhansand Kohlrausch, 1992).
The measurementsin the short-delaycondition were
performedfor that signalpositioncloseto the maskeronset
that led to the highestthreshold(cf. Fig. 3). This time
point was 0 ms for subjectsAS and SM and 3 ms for
subjectRK. The phase dependenceof the thresholdsin
frozen noiseis much smallerthan in the long-delaycondition and amountsfor two of the subjectsto no more than
3 dB. The variation of thresholdwith phaseis largestfor
C. Construction
of the frozen-noise
measurements
signalis alwaysadded to the samefine structurein all
frozen-noise
measurements.
To meet this requirementa noise sampleof 1 s was
generated.Within this noisea fixedtemporalpositionfor
the signalwaschosenwherethe phasedependence
of the
threshold
wasmaximal(i.e.,about10dB).2According
to
2194
J. Acoust. Soc. Am., Vol. 95, No. 4, April 1994
thresholds
as we found
in recent
ex-
R. von Klitzingand A. Kohlrausch:Effect of masker level 2194
7O
70
dBSPI
dB SPL
65
o
QJ
•
6O
60
55
5O
50
I
]•/2
(a)
I
I
I
•
I
3]z/2
I
i
I
2]z
10
20
200
(a)
ms 280
Signal 13clay
SignalPhase
70
d@SPL
70
6•
dB SPL
65
.• •0
•
o... ......._
60
oJ
•
55
50
5O
(b)
0
•[/2
•
3•[/2
2•
Signal Phase
FIG. 2. Thresholds
of a 2-ms5-kHz signalin 300-msburstsof running
and frozennoise.The flat-spectrum
noisehad a bandwidthof 10 kHz and
a levelof 50 dB SPL. Opensymbols
indicatethe resultsin frozennoise,
filledsymbols
indicaterunning-noise
data.The abseissa
denotes
thestarting phaseof the signalfor the frozen-noise
results.Circlesare usedfor
subject
RK, diamonds
forsubject
AS andsquares
forsubject
SM. Median
andinterquartiles
of four runsfor eachparametervalueandsubjectare
shown.(a) Data for a short-delay
condition(onsetdelayof 0 msfor AS
and SM and of 3 ms for RK) and the (b} the data for a Iong-dday
dB SPL
6O
The amount of overshoot for the frozen noise was
55
terminedby placingthe signalat variousdelaysrelativeto
themaskeronset.Theresultsareshownfor eachsubject
in
a separatepanelof Fig. 3. The measurements
were performedfor thephasevaluesrr/2 (squares)and3rr/2 (tri-
5O
angles)whichcorresponded
to the maximumand the min-
The samemeasurements
were repeatedat two other
2195 J. Acoust.Soc.Am.,Vol.95, No.4, April1994
200 ms 280
70
that subject(SM) whois mostsensitive
in the short-delay
condition.In this condition,the running-noisethresholds
form an upperlimit for the frozen-noisevalues.
masker levels,namely 30 dB SPL and 70 dB SPL. The
20
Signal Detay
65
subjectsmeasureda smallerovershootof 3 to I0 riB. The
maximum in thresholddid not alwaysoccur for the O-ms
delay [seeFig. 3(a)].
I
10
(b)
conditions(onsetdelayof 278 ms}.
imumsignalthresholds,
respectively.
Experiments
at the
phase3rr/2 showan overshoot
of up to 15 riB. At rr/2 all
I
0
I
I
o
lO
(c)
20
200 ms 280
Signal Detay
FIG. 3. Maskedthresholds
of the2-mssignalin frozennoiseasa function
of the signaldelay.The ,-nasker
levelwas50 dB SPL. Squaresindicatethe
resultsfor thesignalphaserr/2 andtriangles
thedatafor thephase3rr/2.
Median and interquartilesof four runs for each parametervalueand
subjectare shown.(a) Resultsfor subjectRK. (b) Resultsfor subject
AS. (c} Resultsfor subjectSM.
R. yonKlitzing
andA. Kohlrausch:
Effectof rnaskerlevel 2195
80
30
dB
][
2O
]•..70dB
dB
o 25
z
60
•
50
•/SOdB
20
15
;0
30
--
_•_e..•
.....•
•
•/30dB
10
20
/.0
60 dBSPL
NaskeF
0
10
20
200
Level
ms 280
Signal Delay
FIG. 4. Maskedthresholds
of the 2-mssignalin frozennoiseasa function
of the masker-signal
delayfor the maskerlevels30, 50, and 70 dB SPL.
Squaresindicatethe resultsfor the signalphasest/2 and trianglesthe data
for the phase3sr/2. The data showthe medianand the corresponding
interquartilerangesof the 12 individualthresholdvalues.
FIG. 5. Masked thresholdsof the 2-ms signal in running noiseas a
functionof the maskerlevel.Signalthresholds
are expressed
as 10 log(E/
No). The parameteris the signaldelay. Squares:short delay condition
(medianof valuesfor 0, 3, and 8 ms delay); triangles:18-msdelay;
circles:48-msdelay;invertedtriangles:long-delayposition(medianof
valuesfor 198-and278-msdelay).Medianandinterquartiles
of fourruns
for eachparametervalueare shownfor one subject(RK).
resembleone periodof a sinusold.(2) The minimumof
thispatternoccursfor the70-dBmaskerat a phasevalueof
(RK) and includedmore maskerlevelsand signaldelays
than the main experiments.
The parameterin the figureis
the onsetdelay of the signal.At all signaldelays,thresholds vary nonmonotonicallywith maskerlevel. The variation is largestfor the short-delaycondition(squares),but
evenat delaysof more than 200 ms (invertedtriangles),
the relativedetectabilityvariesby 6 dB with maskerlevel.
All three curvesrepresenting
a signalpositionwithin the
samethree subjectsas beforeparticipatedin the measurementsdeterminingovershoot,while the influenceof signal
phaseon the thresholdwas studiedwith two of thesesubjects (AS and RK). The effect of signal phaseon the
thresholds
at these two levels can be summarized
as fol-
lows: (1) As at 50 dB SPL, the frozen-noise thresholds
3•r/2 (as for the 50-dB masker), but for the 30-dB masker
first 50 ms after masker onset reach their maximum at a
at a phasevalueof 5•r/4. (3) The total variationof thresholdswith phaseis about 8 dB at 30 dB and about 5 dB at
70 dB and is the samein the short- and the long-delay
positions.(4) Running-noisethresholdsare generallyin
the upper-half range of the correspondingfrozen-noise
maskerlevelof 50 dB. In the long-delaycondition,thresholdsincreaseup to a maskerlevel of 60 dB.
thresholds.
The experimentsin frozennoisehave shownthat both
the maskerleveland the startingphaseof the signalaffect
overshoot.In Fig. 6, the amount of overshootis representedasa functionof themaskerlevelfor bothphases•r/2
(squares) and 3•r/2 (triangles) in frozen noise and for
running noise (circles). Individual overshootvalueswere
calculatedin the followingway: The referencevalue was
the medianthresholdvalue of the eight singlemeasurements for the two longestdelays 198 and 278 ms. The
Overshootin frozen noise was again determinedfor
two differentphasevaluesof the signal.The resultsfor all
threemaskerlevelsare summarizedin Fig. 4. Data in this
figureare calculatedas medianvaluesfrom the 12 individual runsof the threesubjectsand are showntogetherwith
the corresponding
interquartileranges.Squaresindicateresultsfor a signalphaseof •r/2 and trianglesindicateresults
for a signalphaseof 3•r/2.
Compared to the resultsat 50-dB masker level, the
B. Overshoot
as a function
of masker
level
value of overshoot resulted from the difference between
curves at 30- and 70-dB masker level are flatter. At 30 dB,
this referenceand the highestthresholdwithin the first 10
the signal is even easierto detect for a positionat the
ms after masker onset. If thresholds were lower in the
masker onsetthan at the end. For the signal phaseof 3•r/2
(triangles), such a negativeovershootoccursnot only in
the median values, but also in the individual data of each
onset region than the referencevalue, the lowest value was
subject.
The figurealsoindicatesa stronglevel nonlinearityin
signal detectability,in particular at short signal delays.
This nonlinear aspectof overshootmeasurementsis emphasizedin Fig. 5, wheresignalthresholds[10 log(E/No)]
in runningnoisehavebeenplottedas a functionof masker
level. These measurementswere performedby one subject
overshoot
2196
J. Acoust. $oc. Am., Vol. 95, No. 4, April 1994
chosen to indicate
the maximal
undershoot.
The
data
pointsin Fig. 6 representthe averagesof theseindividual
values.
In all three conditions, overshoot is maximal at the
intermediate masker level. In frozen noise, the maximum
overshootwasobtainedwith a signalphaseof 3•r/2 andthe
minimum overshootwas obtained with a signal phase of
•r/2. The overshootfor running noise fell between these
two extremes. At the lower masker level of 30 dB $PL, the
R. von Klitzingand A. Kohlrausch:Effect of masker level 2196
15
dB
10
oi
-5
30
50
Masker
dB
70
Level
FIG. 6. Overshoot as a function of the masker level. Circles indicate the
magnitudeof overshootin runningnoise.Squaresindicatethe frozennoisedata for the signalphaserr/2 and trianglesindicatethe resultsin
frozennoisefor the phase3zr/2. Averagevaluesof •:hethreelisteners.
averageovershootvalues are always negative.From the
nineindividualvaluesat this level,only thosein running
noiseand in frozennoisefor phase•r/2 of subjectRK are
FIG. 7. Vectordiagramsillustratingthe additionofa maskerand a signal
of the samefrequency.The solidvectorin both diagramsrepresents
IIhe
masker,andthethin vectorsrepresent
thesignalsfor relativephasevalues
of 0 (continuous)andof rr (dashed).The circlesaroundtheoriginof the
maskervectorindicatethejust noticeable
changein vectorlength.In the
left panelthiscriterionis a changeby a factortwo (the diameterhastwice
the lengthof the maskervector). The length of the two signalvectors
differsby a factorof 3 (9.5 dB). In the right panelthe thresholdcriterion
is a changeby a factor of 4. The length of the two signalvectorsnow
differs
bya factor
• (4.4dB).
An important additionalparameterin the vectordiagram is the relative length between the masker and tlhe
signalvectorfor a signalat threshold.If the criterioncor-
dB SPL, the amount of overshoot for the three conditions
responds
a doublingof the lengthof the maskervector(left
panel in Fig. 7), the amplituderelation betweenthe inphaseandthe out-of-phase
signalat thresholdis a factorof
threeor 9.5 dB. The righthalf of Fig. 7 showsthesituation
for a thresholdcriterionthat is equivalentto quadrupling
the lengthof the maskervector.In sucha situation,tlhe
amplituderelationbetweenthe in-phaseand the out-of-
is systematically
orderedfor all subjects,
thisis not thecase
phase
signal
isonly•, whichcorresponds
to a leveldiffer-
at the two extreme masker levels.
enceof 4.4 dB. Becausehigher thresholdsoccur, for instance,for a signalplacedat the onsetof the masker,tlhe
right half of Fig. 7 might representa short-delaycondition,
whereasthe left half might representa long-delaycondi-
positivewith a maximum of 3 dB. Small overshootvalues
are alsofoundat 70 dB SPL, althoughtlaevaluesare on
averagea few dB higherthan at 30 dB SPL and they are
negativein onlyoneindividualcase(subjectAS for signal
phase•r/2). While at the intermediatemaskerlevel of 50
III. DISCUSSION
tion.
A. Effects of signal phase
The resultspresentedin this paperdemonstrate
that
overshoot values of more than 10 dB can be obtained for a
frozen-noise masker at an intermediate
masker level. In
addition,the nonmonotonicdependenceof overshooton
maskerlevelis similarfor frozen-andrunning-noise
maskers. A third result is that the amount of overshoot in frozen
noisedependsuponthe startingphaseof the signal,althoughthe overallleveldependence
is similar.
Thresholdsof a shortsignalin frozen,noiseare influencedby thedeterministic
interaction
of maskerandsignal
and thusdependon the signal'sstartingphase.The phase
Thuswith increasing
signal-to-noise
ratioat threshold,
the variation of thresholdwith the starting phasedecreases.Another consequence
of this reasoningis a differencein overshootfor the in-phaseand the out-of-phase
signal.The threshold
amplitudefor thein-phase
signaldiffers by a factor of 3 betweenthe two vector diagrams
which corresponds
to a thresholdchangeof 9.5 dB. The
threshold
of theantiphasic
signalchanges
onlyby• which
correspondsto 4.4 dB.
Suchan idealizedpictureis in line with our resultsilar
the 50-dB-SPLfrozen-noise
masker(Figs. 2 and 3). For
the large onsetdelay, the thresholdsof the individualsub-
dependence
can be illustratedby usinga vector diagram
(Fig. 7). The maskerand the signalare bothrepresented
by a vectorand the anglebetweenthe two corresponds
to
the (relative) phaseof the two finestructures.The masked
jectsvary with the signalphaseby 8 to 11 dB [Fig. 2(b)].
For signals
placedat theonset,wherethresholds
in general
are muchhigher,the variationwith phaseis muchsmaller
[2 to 6 dB, cf. Fig. 2(a)]. The variationat the onsetposithreshold is reached if the sum of the two vectors matches
tion is largestfor subjectSM whosethresholdsare lowest.
a certain thresholdcriterion (circle in Fig. 7). If the
Finally, the amount of overshootis on average6 dB
maskerandthe signalare in phase,thiscriterionis reached smallerfor a startingphaseof •r/2 (highersignal-to-noise
witha minimal
length
ofthesignal
vector.
Since
a decreaseratioat [hreshold)thanfor a startingphaseof 3½r/2(lower
in the vector length cannot be detectedfor frozen-noise signal-to-noiseratio), a value in agreementwith the above
maskers
andshortsignals,
thresholds
arehighestfor a sig- example.
nal addedin antiphaseto the masker(ct: Langhansand
At the masker levels of 30 and 70 dB SPL, the influKohlrausch, 1992).
enceof the signalphaseis similar for the onsetand the
2197 J. Acoust.Soc.Am.,Vol.95, No.4, April1994
R. yonKlitzing
andA. Kohlrausch:
Effectof maskerlevel 2197
long-delayposition.Sucha resultis expectedfrom the reasoningwith the vectordiagram, if the thresholdcriterion is
natedby the high-threshold
fibers.Sincethesefibersexhibit
only a smallonsetresponse,overshootdecreases
for higher
not stronglyelevatedfor the signalat the maskeronset.
levels.
And indeed, overshoot in all conditions, for frozen noise as
well as for runningnoise,is smallerthan at 50 dB (with
one exceptionin the resultsof subjectSM for frozen noise
and phase•r/2). This resultagreeswith other recentstudies showinga relativemaximumof the overshootin runningnoisefor spectrumlevelsbetween20 to 30 dB (Bacon,
1990; Carlyon and White, 1992; Bacon and Takahashi,
1992), althoughthe maximum for our subjectsseemsto
occurcloserto a spectrumlevelof 10 dB (of. Fig. 6). The
different level dependencein the results reported by
Zwicker (1965a) and Fastl (1976) might be a consequence
of applyinga differentexperimentalprocedure(method of
adjustment),as was pointedout by Bacon (1990).
For the 30-dB-SPL masker, we observea negative
overshootfor the signalwith startingphase3•r/2 in frozen
noise.The effectis not very large, but it is the kind of
behaviorthat we expectedfrom our model predictions
mentioned
in the Introduction.
In contrast to our recent
simulations mentioned in footnote 1, however, the results
with runningnoisealso indicatea negativeovershootfor
two of the subjects.Thus it appearsthat a negativeovershootat low levelsis not a particularconsequence
of using
a deterministic masker, as our model predictions suggested.
B. Adaptation in auditory-nerve fibers and overshoot
Overshootand especiallythe nonmonotoniclevel effects have been related by severalauthorsto peripheral
adaptationin the two populationsof auditory nerve fibers
(e.g., Champlin and McFadden, 1989; Bacon, 1990; McFaddenand Champlin, 1990). In simplifiedterms,this argument is as follows. Fibers in the eighth nerve can be
subdividedin two groupsaccordingto their physiological
properties(e.g., Libermanand Simons,1985). About 75%
of the fibershavea low thresholdand a high spontaneous
activity, while the remainderhave high thresholdsand a
low spontaneous
activity. Thesetwo groupsof fibersalso
differ in their onsetresponse.
Low-thresholdfibersexhibit
a large onsetresponsethat decreases
over time due to adaptation. High-thresholdfibers,on the other hand, show
only a small onsetresponseand thus much lessadaptation
(Rhode and Smith, 1985, but see below). For low to mod-
eratemaskerlevels,the responseof the low-thresholdfibers
is dominant.These fibers respondto a short increment
(corresponding to the "signal" in an overshoot measure-
ments) in the level of a pulsedstimulus (the "masker")
with a constantincrementin firing rate, independentof the
temporal position of the increment relative to the
"masker" onset (e.g., Smith and Zwislocki, 1975). Since
the responseto the maskeradaptsover time, the neural
signal-to-maskerratio (Bacon, 1990) increaseswith increasing onset delay, corresponding to a decrease in
masked threshold. At high masker levels, the lowthresholdfibers are saturatedand the responseis domi2198
J. Acoust.Soc. Am., Vol. 95, No. 4, April 1994
Althoughit is generallybelievedthat peripheraladaptation is involved in overshoot, the above scheme is cer-
tainly not sufficientto explainall or evena major part of
the observedeffects(for a thorough discussion,see McFaddenand Champlin,1990). Probablymostpuzzlingare
the changes
in overshoot
dueto aspirinconsumption,
prior
noiseexposureor with sensory-hearing
impairedsubjects
(Bacon and Takahashi, 1992; Champlin and McFadden,
1989;McFaddenand Champlin,1990). In all theseconditions, overshootis reducedor disappears,becausesubjects
becomemore sensitivein the short-delaycondition. McFadden and Champlin (1990) discussedthe idea that in
aspirinas well as in TTS overshootexperiments,an unusual rebalancingeffect betweenlow- and high-threshold
auditory nervefiberscould take place.If, at intermediate
maskerlevels,subjectscoulduseinformationsuppliedby
high-threshold
fibers(insteadof informationsuppliedby
low-thresholdfibers as in "normal" overshootexperiments), overshootwould disappear.
However,this idea as well as the aboveargumentfor
the decreaseof overshootat high levelshaveto be questionedin the light of recentphysiologicalresultsby Relkin
and Doucet (1991). They showed that with a sufficient
interstimulustime of at least500 ms (a typicalintervalin
psychophysicalovershootmeasurements),high-threshold
fibersshowa similaronsetresponseaslow-thresholdfibers.
In the next sectionwe presentan alternative,probably
related,idea, which links signaldetectabilitydirectlyto the
nonlinearinput-output characteristicof the basilar membrane.Although we haveto admit that our consideration
is
not an explanationof overshootper se, it seemsto offer a
differentview on someof the above-mentioned
puzzling
effectsin the short-delayconditions.
C. Basilar-membrane nonlinearities and signal
detectability
In the input-outputcharacteristic
of the basilarmembrane (BM), we can roughlydiscriminatethree level regions (e.g., Johnstoneet al., 1986; Yates, 1990; Ruggero
and Rich, 1991). At low levels up to about 40 dB above
absolutethreshold,the responseis rather linear. At intermediate levels between about 40 and 80 dB, the function
becomesstronglycompressive.This compressivebehavior
is assumedto be a consequence
of the saturationof the
activeprocesses
and to work instantaneously.
At the highest levelsabove about 80 riB, the function again tends to be
more linear. This compressivecharacteristicis only ob-
served for signal frequenciesclose to the resonantfrequencyof a particularplaceon the BM. For frequenciesof
about half an octave (or more) below the resonant fre-
quency, the input-output characteristicbecomeslinear
(e.g., Sellick et al., 1982; Johnstoneet al., 1986). This
frequency-dependent
function of the mechanicsis also reflected in the rate-intensityfunctionsof auditory nerve fibers (Yates et al., 1990).
R. von Klitzingand A. Kohlrausch:Effect of maskerlevel 2198
Such a compressivecharacteristicshouldbe reflected
in a reduceddifferentialsensitivityfor levelsin the midlevel
range (midlevel"hump"). This changein sensitivitywill,
however,only be substantial(in terms of changesin the
measuredthresholds),if the detectiontaskrequiresa large
changein amplitude,as for shortstimulusdurations.Only
the case of sensorineuralhearing loss, the input-output
characteristic
of the BM
will be more linear than in a
healthyear (Yates, 1990) and (ideally) no midleveldeterioration
should occur. Overshoot measurements for sub-
jects with permanentsensorineural
hearinglossby Bacon
and Takahashi (1992) revealed maximum overshoot val-
been observedin intensity discrimination.experimentsfor
clicks (Raab and Taub, 1969). Here, a maximal value for
uesof only 5 dB, while a controlgroupof normal-hearing
subjectsreachedovershootvaluesbetween7 and 26 dB.
The differencewasprimarily due to lowerthresholdsin tke
short-delayconditionfor the hearingimpairedsubjects.In
addition, the short-delaythresholdsvaried much more linearly with maskerlevel for thesesubjects(of. Fig. 1 in
AI/I
Bacon and Takahashi, 1992).
then the differentialeffectof compression
will be large
comparedto the limitedresolutionof psychoacoustic
measurements.
Such an increased threshold at intermediate
levels has
of 1.2 was observed for a click level of 40 dB SL. This
valuewaslargerby a factor3 than the resultsat clicklevels
of either 20 or 80 dB SL. Carlyon and Moore (1984) reported a much larger midlevel deteriorationin intensity
discrimination
for 30-mssignalsthanfor 225-mssignals.A
similar resultis mentionedby Plack and Viemeister(1992)
who observedin preliminaryexperiments
that the "midlevelelevationin the Weberfractionis alsolargerfor brief
signals"(p. 1909).
This reasoningis alsoof relevancefor maskingexperimentsusingshortsignals.The peakamplitudeof a 5-kHz
signalof 2-msduration,placedwith a largeonsetdelayin
runningnoisewith a spectrumlevelof - 10 dB, is about38
dB comparedto the typical peak amplitudeof the noise
within the 5-kHz auditory filter of 30 dB (of. the very
instructivewaveformpicturein Zwicker, 1965a,Fig. 12).
At a 20-dB highernoiselevel the BM becomescompressive. Assuming,for simplicity,a linear characteristicfor
input amplitudesup to 50 dB and a compression
ratio of
1:3 for peak amplitudesabove 50 dB, the differencebetweennoisepeaksand signalpeakat the BM outputis no
longer 8 dB (as at the lower level), but only 2.7 dB. This
differenceoccursbecausethe noiseamplitudesare still in
the rangeof linear transformation,while tlhehighersignal
amplitude undergoescompression.In order to reach an
8-dB differenceafter compression,
the signallevel would
haveto be increasedby another 16 dB.
As we have shownin Fig. 6, the thresholdsignal-tonoiseratio of a 2-ms signalvariesnonlinearlywith masker
levelfor all onsetdelaysandis highestin thelevelregionof
strongBM compression.However, the an•tountof nonlinearity decreases
with increasingonsetdelay. If one wants
to explainthis resultpurelywith the input-outputcharacteristicof the BM onehasto assumethat tlheamplification
causedby the activeprocesses
is reducedin the courseof a
longer noisemaskerand that the input-output characteristic becomes more linear. Such an idea has been discussed
by Schmidt and Zwicker (1991) who mention the time
constants of the outer hair cells of several milliseconds and
the delayedactivationof the efferentsystemas possible
sourcefor such a change.In our opinion, however,the
current experimental data seem insuffici[entto state a
changeof BM mechanicswithin a few ten milliseconds.
Nevertheless,the influenceof the various manipulations on signal detectability in the short-delay condition
showsmany parallelswith what would be predictedfrom
BM properties.If the cochlearamplifieris disruptedas in
2199 J. Acoust.Soc. Am., Vol. 95, No. 4, April 1994
McFaddenand Champlin (1990) observedthat after
aspirinadministration,
the absolutethresholdfor tonalsignals was increasedand the sensitivity in the short-delay
overshootconditionwas improved,i.e., thresholdswere
lowered. As the authors discussthoroughly,aspirin is
known to affect the active micromechanicalprocesses
on
the BM in a similar way as doesTTS and permanenthearing loss,i.e., it makesthe input-outputcharacteristic
of the
BM more linear.
BM mechanicsalso offers an explanationfor the
smallerlevelnonlinearitiesobservedwith longersignals.If
the durationof a signalis increased,its thresholdlewfl
decreases.
As soonas the peakamplitudeof the signaland
the typical noisepeaksare of similar magnitude,the compressionwill affectboth the noiseand the signalsimilarly
and the nonlineargrowth of the maskedthreshold--and
thus overshoot--at intermediatelevelswill disappear.
The last applicationof this argumentis the specific
influenceof maskerbandwidth on short-delaythresholds
in overshootconditions.Usually, no overshootis observed
for maskers with a bandwidth
of two critical bands or less
(e.g., Zwicker, 1965b;Fastl, 1976/77; Baconand Smith,
1991). From experimentsthat combinedon-frequency
narrow-band maskerswith flanking bands of higher and
lower frequencyit was concludedthat the presenceof
maskercomponents
abovethe on-frequency
bandwascrucial for a largeovershoot(McFadden, 1989;Schmidtand
Zwicker, 1989). On the other hand, Carlyon and White
(1992) pointedout that at a frequencyof 6.5 kHz, which
is higher than thoseusedin the other cited studies,a substantial overshoot was obtained for a relative masker band-
width of only 0.3 times the centerfrequency.
A narrow-bandsignalproducesmain excitationon the
BM at the place tuned to its center frequencyas well -'rs
excitation at more basal placeson the membranethat are
tuned to higher frequencies(sometimesreferred to as upward spreadof excitationor as accessory
excitation).Since
the characteristicof the BM is linear for the accessory
excitation, a short increment in a narrow-band noise of
intermediatelevel can be detectedmore easilyat a place
tuned to a frequencyabovethe noise,sincethere, no compressionoccurs.As long as the subjectscan rely on information from off-frequencychannelstuned abovethe signal
frequency, signal thresholdsshould grow linearly with
masker level.
In overshoot measurements,two conditions have been
R. von Klitzingand A. Kohlrausch:Effect of maskerlevel 21õ0
usedthat reducethe availabilityof accessory
excitationand
that lead to largeovershootvalues:(a) Increasingthe relative masker bandwidth beyond about 20% to 30%
(Zwicker, 1965b; McFadden, 1989; Bacon and Smith,
1991;Carlyonand White, 1992); (b) choosinga veryhigh
frequencyfor the on-frequency
band (Carlyon and White,
1992).
A third condition has been applied in intensitydiscriminationexperiments,where similar nonlinearlevel
lyon, M. v. d. Heijden, D. Hermes, N. Versfeld and, in
particular,the two reviewersSid Baconand Craig Champlin provided helpful commentson earlier drafts of this
paper. Technical assistanceby M. v. d. Heijden, L. Martensand G. Kirschmann-Schr6der
in preparingthe figures
is gratefullyacknowledged.
This studywassupportedby a
grant from the Deutsche Forschungsgemeinschaft
(Ko
1033/3-1).
extension
ofthismodelto running-noise
maskers
revealed
that
effectsas for overshootare observedfor high-frequency IA recent
for sucha maskerno undershoot
is predicted(Dau, 1992;Dau et al.,
signals.Surroundingthe signalwith a notchednoiseand
1994). Thresholdsremainconstantfor all (simultaneous)positionsof
thus masking spread of excitation, reducesthe perforthe signal.We explain this behavioras follows. For a running-noise
mance at intermediatelevelsmarkedly comparedto the
masker,the detectionis limitedby the statisticalpropertiesof the noise.
condition without a notched noise (Carlyon and Moore,
The statisticalvariation in the internal representationis larger for the
1984).
onsetpart and smallerfor the later part of the noise.In a parallelmanner, the increasein internalexcitationcausedby the additionof the
signalvarieswith its temporalposition.In summary,the relativedetectability of the signalis not increasedcloseto the maskeronset.
We would like to finishwith the qualificationthat, in
order to understandthe overshootphenomenon,it is certainly not sufficientto consideronly BM properties.For
example,overshootwith broadbandon-frequencymaskers
as well as the probablyrelatedmidlevelhump in intensity
discriminationare largestat frequenciesabove 1 to 2 kHz.
2Thechoice
ofsucha temporal
position
forthesignal
wasinspired
byour
previousexperiencewith frozen-noisemaskerswhere the most interesting effects in frozen noise--like the monaural-diotic threshold
difference-•occurredat such positions (Langhans and Kohlrausch,
1992).
No BM measurements are known to us that show a differ-
ent behavior of active processesbelow and above 1 kHz.
Neural adaptation processesor even more central
partsof the hearingpathwaymustbe takeninto accountin
understanding
overshoot.However, we think that on the
basisof our aboveargumentsthe contributionof BM mechanicsis of greaterrelevancefor this phenomenonthan
arguedin other recentarticles(e.g., McFadden, 1989;Bacon and Smith, 1991;Carlyon and White, 1992).
Bacon, S. P. (1990). "Effect of masker level on overshoot," J. Acoust.
Soc. Am. 88, 698-702.
Bacon,$. P., and Moore, B.C. J. (1986). "Temporaleffectsin simultaneouspure-tonemasking:effectsof signalfrequency,masker/signalfrequencyratio and maskerlevel," Hear. Res. 23, 257-266.
Bacon,S. P., and Smith, M. A. (1991). "Spectral,intensiveand temporal
factorsinfluencingovershoot,"Q. J. Exp. Psychol.43, 373-399.
Bacon, S. P., and Takahashi, G. A. (1992). "Overshoot in normal-
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Noteaddedin proof.It wasbroughtto our attentionby
Brian C. J. Moore that somerecentpublicationsconclude
that active processesmight be less involved in cochlear
tuningat low frequencies(Rosenand Stock,1992;Wilson,
1992). Rosen and Stock measured auditory filter bandwidths between 125 and 1000 Hz at various levels. Gener-
ally, filter bandwidthsincreasedwith level, with the strongesteffectat 1000Hz and little or no effectat 125 Hz. In
the conclusion,they refer to a remark in a reviewarticleby
Wilson that at low frequencies"there appearsto be a much
smallerinvolvementof the active process"(Wilson, 1992,
p. 81). Unfortunately,thisstatementby Wilsonis not sup-
ported by a reference.Nevertheless,such a difference
would
be consistent
with
our discussion of the role of
basilar-membrane nonlinearities in overshoot situations,
sinceovershootseemsto be absentat low frequencies.
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