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Received 29 October 1968
Perceptual Masking in Multiple Sound Backgrounds
RAYMONDCARHART,TOM W. T•LLnAN, AN• ELIZABETHS. GREETIS
Auditory ResearchLaboratory,NorthwesternUniversity,Evanston,Illinois 60201
Shifts in maskedspondeethresholdsduring severalconditionsof listening (monaural,homophasic,antiphasic,and with interaural time disparity) in the presenceof oneto four competingmaskerswere measured.
The maskersusedwerewhite noise,white noisemodulatedfour timesper secondby 10 dB with a 50% duty
cycle,the samenoisewith 75% duty cycle,connectedspeechby onemale talker, and connectedspeechby
a secondmale talker. Resultsfrom three experimentsthat employedvariouspermutationsof the aforementionedconditionsare reported. The findings,after equatingconditionsto equivalent maskerlevels,
were four. First, the modulatednoisewith 50% duty cycleproducedabout 3.5 dB lessmaskingthan that
producedby unmodulatedwhite noise.Second,the modulatednoisewith 75% duty cycleallowedonly about
1 dB lessshift than did the unmodulatednoise.Third, mixingonespeechtrain with noise(eithermodulated
or unmodulated)inducedabout 3.2 dB excessmasking.This excessis here termed perceptualmasking.
Fourth,perceptualmaskingroseto 6.6 dB when two speechtrainswere includedin the maskercomplex,
irrespectiveof whetheror not noisewas alsopart of the complex.The findingssupportthe hypothesisthat
successive
stagesof perceptualmaskingarise as the task of signalsorting becomesmore exactingfor the
listener. Related
studies are discussed in this connection.
INTRODUCTION
while
the aforementioned
two maskers
were concur-
rently present.Surprisingly,we foundthat maskingfor
HE experiments
reported
herewith
wereunder-spondeesincreasedabout 7.8 dB and intelligibility
taken to ascertainwhetherspeechintelligibility functions were shifted about 10.5 dB when the two
as gaugedby the thresholdfor spondeewordsis more
We commented,
adverselyaffectedby competingsoundsthat are mean- maskerswerepresentedsimultaneously.
No more than 3 dB of each increase can be attributed to the
ingfulspeechthan it is by comparablymodulatednoises
simpleaddition of the long-termaverageintensitiesof the
that lack meaning.Any such adverseeffect could be
two maskers (assumingthat they were sufficiently equiconsideredas excessmaskingdue to perceptualintervalent sothat their combinationapproximatelydoubledthe
ferences
amongthe severalspeechsignals.The question
over-all acousticpower). There thus remainedan excessin
at issuein the experimentswas whether perceptual
masking of at least 4.8 dB as gaugedby identification of
masking could be demonstrated,and, if so, what
spondeesand 7.5 dB as gaugedby preciserecognitionof
monosyllabicwords. This increasemust be attributed to
magnitude it exhibited.
other interactions between the maskers, interactions relatThe impetusfor theseinvestigations
camefrom obing either to the nature of the acoustic instability each
servationswe madein a studywherewe wereexploring
maskerpossessed
or to semanticinfluences(p. 1225).
variousfacetsof the maskingfor speechinducedby two
thesefindingsat somelength, making the
competing signals (Carhart, Tillman, and Johnson, We discussed
1968). The data from this previousstudy that are followingstatement,whichis pertinentto the present
pertinent to the presentdiscussion
are three setsof paper.
monauralresponses.
One set was obtainedwith only
The fact that combining the two maskers yielded sub-
modulated
noise as the masker.
The second set em-
ployed only connectedsentences
as the competition.
These signals,when each was operatingalone, were
almostidentical to oneanotherin their maskingeffects,
whether gaugedby the shift they causedin spondee
thresholdor by their effecton intelligibilityfor monosyllabicwords.The third array of data wasprocured
694
Volume45
Number3
stantial excessmasking beyond the 3 dB attributable to
simplepower summationis not inconsistentwith earlier
findings,but the mechanisminvolved remainssomewhat
obscure.Onepossibilityis that simplealgebraicinteraction
between burst and interburst levels of the two maskers was
highly detrimental,while a secondpossibilityis that per-
ceptualconfusions
wereinordinatelyenhanced
whenmultiple maskers
had the opportunityto interactandoneof the
1969
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PERCEPTUAL
MASKING
IN
maskerswas speech.Only further experimentationwill
revealwhichpossibilityis morelikely... (1226ff.).
We havesubsequently
gatheredexperimentaldata that
allow the foregoingpossibilitiesto be explored.The
presentpaper describesthese data, which comefrom
three studies,and discusses
their implications.As is
seen,one is carried to the conclusionthat the task of
abstractinga primary messagefrom multiple competition containingmeaningfulspeechis moredifficultthan
is accountedfor by the simplealgebraicspectrumof the
combined
maskers.
MULTIPLE
BACKGROUNDS
TABt.•. II. Differences in average masked thresholdscharacterizingfour monaurallisteningconditions.These differencesare
derived from the grand meansin Table I and expressedre performance in unmodulated white noise (Nm•Sm).Positive values
denote reduction in masking.
Threshold
Masker
Nm•Sm: Unmodulated white noise
Nm•'Sm:Modulated noise (4 MPS, 10 dB IBR,
50% dc)
CmSm:Sentences
CmNm•'Sm:
Sentencesand modulatednoi,se
difference
0.0
3.8
3.8
-- 1.1
I. PROCEDURES
The data to be consideredwere obtained as part of
competing
sentences
(CmSm);andcompeting
sentences
combined with modulated white noise (CmNm2Sm).
two major studiesdirectedtoward exploringthe r61eof Eachmaskerwaspresented
at nominallevelsof 60, 70,
interaural phaseand time differenceson perceptionof and 80 dB SPL (seeCarhart et al., 1968,for details).
speechagainstmultiple maskersof three types' white This procedure
resultedin differences
in the average
noise, modulated white noise, and connectedspeech. powerof the severalmaskersundernominallyequiThesemaskerswere presentedaloneor were combined valent conditions. Corrections for these differences were
in variouswaysduring specifiedbinauralpresentations appliedto the data by adding2.6 dB to thresholds
for
(with monauralpresentationsas controlswhere appro- Nm•Smand for CmSmand by subtracting0.4 dB from
priate). In all instances,masking was measuredin thresholds for CmNmeSm.
terms of the thresholdfor spondeewords.The presentaTableI presents
the meancorrected
spondee
threshtion levels of maskers differed somewhat
from one
old for eachmaskerconditionat eachpresentationlevel
experiment to another, but otherwise the equipment alongwith the grandmeanfor eachmaskercondition.
and procedureswere essentiallythosewe have already Table II reportsthe differences
in the severalgrand
reported (Carhart et al., 1968). The one exceptionwas means.Hereperformance
in unmodulated
noiseistaken
that a new tape carrying 297 spondeeitems plus two as the reference.
trainsof competingsentences
(threeseparatechannels)
Two relationsare quicklyapparent.First, both the
was prepared accordingto our usual routinesso as to Nn?Smand the CmSmconditionsyielded a 3.8-dB
facilitate testingwith multiple speechcompetition.The reduction in masking beyond that attributable to
subjectswere young adults with normal hearing who reductionin averagemaskerpower.In otherwords,the
were selectedaccordingto the same criteria we have process
ofmodulating
whitenoisefourtimespersecond
used in the past.
to a depthof 10 dB with 50% duty cycleresultedin
II.
almost4 dB drop in the amountof maskingand con-
RESULTS
nectedspeech
yieldedthesamedrop.Oneshouldnotbe
surprised
at theparallelism
between
maskers,
sincethey
A. Experiment 1
were chosento be equivalenton the basis of data
Monaural spondeethresholdswereobtainedfrom six gatheredin our earlierstudieswith modulated
noise.
subjects(two men and four women:mean age= 17.6 Moreover,it isreasonable
to expectthat thepresence
of
years)in four maskerconditions:namely,unmodulated modulationin a masker, whether it be artificially
white noise(Nm•Sm);white noisemodulatedfour times induced or be the normal modulation of connected
per secondby 10 dB with 50% duty cycle (Nm2Sm); speech,
shouldfurnishacoustic
"windows,"whichthe
listener utilizes to advantage. The question is not
TABnE I. Means of masked spondee thresholds
a obtained
monaurally in the presenceof four maskers at three masker
presentationlevels.
Masker
Presentation level
60 dB 70 dB 80 dB
Nm•Sm: Unmodulated white noise 48.0
57.5
68.0
Nm•'Sm:Modulated noise(4MPS,
10dB IBR, 50% dc)
45.6 53.6 62.6
CrnSrn:Sentences
47.6
CmNm•'Sm: Sentences and modulated
noise
48.6
Grand
mean
57.7
53.6
60.6
53.9
53.9
58.6
68.6
58.6
a Expressed as equivalent SPL in dB re 0.0002 t•bar after correction re N •
for power-level differences of masker complexes.
whetherlessened
maskingshouldoccurhere,but rather
howgreatis its magnitude.
Second,
the composite
background
of sentences
plus
modulatednoiseyieldedan excess
shiftin thresholdof
1.1 dB. This overmasking
appearsat first glanceto be
so smallthat it might well be attributableto chance.
However, recall that the compositemasker we are
considering
still containedacoustic"windows,"althoughthesenow averagedcloserto 60 than to 125
msec(seeCarhartetal., 1968,p. 1227).Thesewindows
should have still allowed some reduction in masking re
performance
in continuous
white noiseof equivalent
power.Hence,sincethe reverseactuallyemerged
here,
TSe Journalof the AcousticalSocietyof America
695
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CARHART,
TILLMAN,
TABLE III. Means of masked spondeethresholds
a obtained
monaurally in the presenceof four maskers at three masker
presentationlevels.
Presentation
level
65 dB 75 dB 85 dB
Grand
NmlSm: Unmodulated white noise 52.0 63.0 73.0
•-NTm2Sm:
Modulated noise (4 MPS, 47.6 59.6 69.6
10 dB IB R, 50% dc)
•m3Sm: Modulated noise (4 MPS, 51.1 62.1 72.1
10 dB IBR, 75% dc)
Cm•-Nq'•m2Sm.
ß Sentencesand 50% dc
54.6 64.6 75.6
62.7
58.9
Masker
modulated
mean
61.8
64.9
AND
GREETIS
modulation characterizingsuch a composite(see
Carhartet al., 1968,p. 1227);yet, it is a maskerthat
roughlyduplicatesCN2physicallywhileremainingfree
of an5- linguisticor phoneticcomponent.
TableIII presents
the corrected
meansof themasked
spondeethresholdsfor eachmaskercondition,while
Table IV reportsthe differences
in the grandmeansfor
spondeethresholdsobtained in the several masker
conditions.
Again,performance
in unmodulatednoise
serves as the reference.
The three relations that characterize these data are
noise
immediatelyapparentin Table IV. First, as in the
a Expressed as equivalent SPL in decibels re 0.0002 •bar after correction
preceding
experiment,
a 3.8dB reductionin interference
re N • for power-level differences of masker complexes.
emergedwhenN s wasthe competingsound.Again,a
10 dB "window" with 50% duty cycleoccurringfour
one may reason that the task of sorting a primary times per secondwas lessdisruptiveby this amount
messagefrom both a competingmessageand a meaning- even after correctionfor the power reductionit inlesssignal causeda small amount of excessmasking. corporates.
Second,
alsoas in the precedingdata, the
The overmaskingwe observed,although not quantifiableon the basisof the data presentedsofar, is evidence
that the perceptualtask is more complicatedthan one
would expect if only the acousticinteractionsof the
maskingsignalswere critical.
B. Experiment 2
combinationof sentencesand N s produced modest
perceptual
interference
(2.2dB re performance
in N1).
Third,however,wenowhavea basisfor estimating
this
perceptualinterference,
or perceptualmasking,more
adequately.
To explain,the Na noiseconditionundermaskedby 0.9 dB. Accepting
thisundermasking
asthe
"window-effect"advantagein this caseand rememberMonaural spondeethresholdswere obtainedfrom six ing that the "window"magnitude
for N aand CN2were
subjects(four men and two women:mean age= 22.9 probablyrelativelycomparable,
onemayestimate
from
years) in the followingfour maskingconditions:un- thesedata that the perceptualmaskingresultingwhen
modulatednoise (NmlSm);white noisemodulatedfour both speechand modulatednoiseare combinedis
times per secondby 10 dB with 500/0duty cycle approximately
3 dB (2.2 dB plus0.9 dB). In other
(NmSSm);white noisemodulated in like manner, but words,maskingfrom competition
that is acoustically
with 750/0duty cycle(NmaSm);
andcompeting
sentences similarbut meaningless
(i.e., from N 3)producedabout
combined with the N s modulated noise (CmNmSSm). 3 dB lessdisruptionto perceptionof spondees
than
Each maskerwaspresentedat nominallevelsof 65, 75, occurredduring the CN • condition.•
and 85 dB SPL and again data are reported after being
To summarize,the foregoingdiscussions
presentdata
corrected as were the earlier data for average power illustratingthe followingfacts' (1) Reductionin maskdifferencesbetweenmaskers.The N aconditionrequired ingisallowed
by theacoustic
"windows"
in a modulated
that 1.1 dB be added to all thresholds in order to
achieve this correction.
The N a modulated noise was selected to simulate
noise. This reduction is between 3 and 4 dB when the
modulationpattern (N •) roughlysimulatesthat of a
singletalker. (2) The amountof reductionin masking
approximatelythe gap durationsin the pattern of is lessenedas the "window" is narrowed, being about
masker modulation resulting when connectedspeech 1 dB whenthe modulationpattern (N '•) roughlysimu(C) and the N smodulatednoiseare combined.True, N 3 latesa composite
of two talkers.(3) About3 dB of per-
doesnot reproduceexactly the pattern of amplitude ceptualmasking
occurs
wheninterference
withspondee
thresholdis producedby onetrain of connected
speech
TABLE IV. Differences in average masked threshold charac- coupled
with an N•-modulated
noise.Theseconclusions
terizingfour monaurallisteningconditions.Thesedifferencesare are confirmedand extendedby the relationsthat appear
derivedfrom the grand meansin Table III and expressed
re per-
formance in unmodulated white noise (NmlSm). Positive values
denote reduction in masking.
in the next set of data.
C. Experiment 3
Threshold
Masker
difference
_-•mlSm:Unmodulatedwhite noise
.NTm•Sm
' Modulated noise (4 MPS, 10 dB IBR,
50% dc)
.-NTmaSm
' Modulated noise (4 MPS, 10 dB IBR,
75% dc)
Cm.-NTm2Sm
- Sentences
and 50% dcmodulatednoise
696
Volume45
Number3
0.0
3.8
0.9
-- 2.2
Binauralthresholdsfor spondees
wereobtainedfrom
12 subjects(sevenmalesand five females:mean age
i Two othercomparisons
from our researchsupportthe view
that the N aconditionproduces
about 1 dB lessmaskingthanN •
and, hence,add credence
to the thoughtsjust expressed.
These
additionalcomparisons
involve data gatheredduringbinaural
listeningunderanalogous
maskercomplexes.
The corrected
reduction in homophasic
maskingby N a vs N 1 was 1.2 dB, whilefor
antiphasic
listeningthe analogous
reductionwas1.4 dB.
1969
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PERCEPTUAL
MASKING
IN
MULTIPLE
BACKGROUNDS
TABLEV. Meansof maskedspondee
thresholds
a obtainedbinaurallyin the presence
of threeinterauralrelations,duringninemasker
conditionsand at three presentationlevels.
Homophasic
Presentation
Masker condition
level
65 dB
75 dB
85 dB
Grand mean
N02N02uS0ß
N02S0:
Unmodulated white noiseb
One modulated noise
52.1
48.6
62.2
58.2
71.7
68.2
62.0
58.3
C0•S0:
C0•S0:
C0•N0•S0:
C01N0•N0•S0:
C01C02S0:
C0•C0•N0•S0:
C0•C0•N0•N02RS0:
One competingspeaker
Secondcompetingspeaker
One speakerand mod. noise
One speakerand unmod. noiseb
Both speakers
Both speakersand mod. noise
Both speakersand unmod.noiseb
48.7
48.0
53.1
55.1
57.7
58.2
59.4
57.3
58.2
63.7
65.1
69.5
68.7
69.2
66.9
66.8
73.0
75.2
78.7
79.3
79.8
57.6
57.7
63.3
65.1
68.6
68.7
69.5
Antiphasic
N?N?RS0:
N?S0:
Unmodulated white noiseb
One modulated noise
45.7
42.9
55.0
52.8
65.9
62.6
55.5
52.4
C•S0:
C?S0:
C•N?S0:
C•N?N?RS0:
C•C?S0:
C•C?N?S0:
C•C?N?N?uS0:
One competingspeaker
Secondcompetingspeaker
One speakerand mod. noise
One speakerand unmod. noiseb
Both speakers
Both speakersand mod. noise
Both speakersand unmod. noiseb
43.1
44.7
48.1
49.9
50.6
51.3
52.5
53.0
52.8
58.5
59.4
60.5
62.1
62.8
63.2
62.5
69.0
70.5
71.2
72.7
72.7
53.1
53.3
58.5
59.9
60.8
62.0
62.7
N0.8•N0 82uS0:
N0.8 So:
Unmodulated white noiseb
One modulated noise
Parallel time delay
47.3
44.0
C0.8•S0:
Onecompetingspeaker
C0.8•S0:
Secondcompetingspeaker
Co.8•No.8•So:
One speakerand mod. noise
Co.8•No.s•No.s•USo: Onespeakerandunmod.noisel'
Co.s•Co.82So:
Both speakers
Co.s•Co.8•No.8•So: Both speakersand mod. noise
Co.s•Co.8•No.8•No.82J•So:
Both speakersand unmod. noiseb
44.2
43.2
49.9
50.7
51.5
52.5
52.9
57.4
67.5
57.4
54.0
54.6
52.7
59.5
60.3
61.1
62.6
63.1
63.0
63.1
63.6
70.1
71.5
72.7
73.6
72.6
54.0
54.0
53.8
59.8
60.8
61.8
62.9
62.9
Expressedas equivalent SPL in decibelsre 0.0002 •bar after correctionre N=N=nor N t for power-leveldifferencesof maskercomplexes.
N•N=n combinestwo modulated noisesinto one unmodulated white noise (ND.
= 21.1years)under27 listeningconditionspertinentto tionsper second,10dB modulationdepthand50% duty
thisdiscussion
aspart of a studyinvolving37 conditions cycle.One of thesemaskerswas designatedN =, and its
(seeCarhart,Tillman, andGreetis,1969).Eachsubject opposite number N =n. The other two maskers were
served for three 2-h sessions. Conditions were ranconnectedsentences,the first set spokenby one male
domizedin six blocks.These blockswere presentedin (C1) and the secondspokenby anothermale talker
different orders to different subjectsand conditions (C=). These sentencesand 30 spondees(by a third
within each block were randomized. Nine of the conditalker) were recordedon separatechannelsof a fourtionsof interestto us consisted
of homophasic
presenta- track tape so as to yield 297 time-lockeditems wherein
tion with all signalsin phaseat the two ears. Nine of the speechwas sufficientlycontinuousat the instant of
the remainingconditionsemployedan 0.8-msecinter- sportdeeutterance sothat the latter was fully maskable
aural time delay of the maskercomplexwith the spon- by the competition. These 297 items were presented
deesinteraurallyin phase.Hel-e•the maskingwas ad- sequentiallyto any singlesubject,but the startingpoint
ministeredso as to lead in the right ears of half the wasvaried from personto person?During presentation
subjectsand the left earsof the other half. The remain-
of the spondees,one or both of the sequencesof sentences could be removed from the stimulus complex
through attenuation. The samewas true of N • and N 2R,
two ears.
which were generated at the time of the experiment.
The nine conditions within each set were distinPhasereversaland time delay units wereincludedin the
guishedonthe basisof howthey combinedfourdifferent circuitry. This circuitry was similar to our earlier
ing nine conditionsinvolvedantiphasiclistening,with
the spondees(primary message)againin-phaseat the
maskers.
Two
of these were white
noise modulated
identically but with oppositetiming, i.e., the burst
interval of one coincided with the interburst interval of
the other so that when combinedthey produceda
continuouswhite noise.Both employedfour modula-
• These 297 items actually represented10 recurring randomizations of a singlelist of 30 spondeewords.However, three test
items were droppedbecausein thesethree instances,the spondees
occurredduring gaps in the competition,which precluded their
being fully masked.
The Journal of the AcousticalSocietyof America
697
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CARHART,
TILLMAN,
AND
GREETIS
TABLEVI. Shiftsin binauralmaskingin decibelsderivedfrom the grandmeansin Table V and expressed
for eachinterauralstate re
performance
in unmodulatedwhite noise(N•), hereproducedby combiningtwo modulatedwhite noises(N•'N•'e),for that sameinteraural state. Positive values indicate reduced masking.
Interaural
Masker condition
Nx"Nx"eS0:
Nx"S0:
Unmodulated white noises
One modulated noise
C•S0:
One competingspeaker
C•"S0:
Secondcompetingspeaker
C•N•'-S0:
One speakerand mod. noise
C•N•N•aeS0:
One speakerand unmod.noises
C•CxaS0:
Both speakers
CdC•"N•'S0:
Both speakers
and rood.noise
CdC•"N•'-N•"eS0:Both speakers
and unmod.noise
s
a N•N•
state
Time
Homophasic
Antiphasic
delay
0
3.7
0
3.1
0
3.4
4.4
4.3
-- 1.3
--3.1
-- 6.6
--6.7
-- 7.5
2.4
2.2
-- 3.0
--4.4
-- 5.3
-6.5
-- 7.2
3.4
3.6
-- 2.4
--3.4
--4.4
--5.5
-- 5.5
•I• combines two modulated noises into one unmodulated white noise (N•).
grounds.To explain' the original intent was that these
maskers should produce equal masking and, in that
sensehave equivalent effectivepower.• However, computationof the grandmeanof all SRT's obtainedin the
combinations listed below3:
presenceof eachsignalalonerevealedthat N •'wasmore
severeas a maskerthan were Csand Caby 2.9 and 1 dB,
(a) One modulatednoise(Na)
respectively.The latter two were thereforejudged to
(b) One competingspeaker(C1S)
have effectivepowersin the ratios of 0.513 and 0.794
(c) The other competingspeaker(CaS)
re N a, and the powersof all compositemaskerswere
(d) Both modulatednoises(NaNaRS)or (NIS)
computedusing thesevalues,i.e., 1.000, for N •' or for
(e) One modulatednoiseand one competingspeaker N •'•, 0.513 for C1, and 0.794 for Ca The result was a
(C1NaS)
series of correctionsthat equated all conditions to
(f) Both competingspeakers(C•CaS)
equivalentlevels.Specifically,sincewe wishedto refer
(g) Both noisesand one speaker (C•NaNaRS)or all the conditions to masking in unmodulated white
(C1N1S)
noise(NaN•'•S,or NIS, which,by the systemdescribed
(h) One noiseand both speakers(CICaNaS)
here, has a power value of 2.000 re N a) the resultant
(i) Both noisesand both speakers(C•CaNaNa•S)or correctionsapplied to the nominal masked thresholds
(C1C2N1S).
for the various conditionswere: N a add 3 dB; C•S add
5.9 dB; CaSadd 4 dB; NaNaRS(or NIS) no change;
Thus, the set of nine conditionsran the gamut from a
C•CaSadd 1.8 dB; C•NaS add 1.2 dB; C•NaNa•S (or
single masker (either meaningless
or meaningful)to
C•N•S) subtract 1 dB; CICaNaSsubtract 0.6 dB; and
quadruple maskers, two of which were competing
C•CaNaN•'•S(or C•CaN•S)subtract2.2 dB.
speech,and the other two weremodulatedwhite noise.
Table V summarizesthe corrected mean spondee
As in the past experiments,maskerswerecombined thresholdsfor the 27 experimentalconditionsat each
at preselectednominal levels, which in this instance nominal maskerlevel. The Table also givesthe grand
were65, 75, and 85 dB SPL. Again,therefore,the actual mean for each condition. The outstandingfeatures of
powerlevelsinvolvedvariedwith the numberand types the data aspresentedherearetwo. First, asanticipated,
of maskerscharacterizinga given condition. Correction the shift in mean threshold was about 10 dB from one
for thesepower-leveldifferences
had to be madebefore presentation level to another for each of the 27 condidata could be most meaningfully compared. The
tions involved. Actually, the grand mean for all such
procedureemployedwas similar to that used in the shiftswas exactly 10 dB, as it would be expectedto be,
past, exceptthat the effectivepowersof C• and Ca re sinceeach masker complexwas changed10 dB in the
that of N a (or N a•) were computed from the experitransitionto adjacentpresentations.Second,systematic
mental data, rather than estimated on theoretical differences between means for various masker condi-
assemblages,but was expandedto accommodatethe
new experimentalconditions.
Each set of nine conditions(i.e., the homophasic,
the
antiphasic,and the time delayedsets)consistedof the
tionsat the samepresentationlevel appearconsistently.
These several relations give one confidenceboth that
pliesbecausein thesethreeconditions,
N s and N ae interlockto the data have adequatereliability and that the trends
form a singleunmodulatedwhite noise (N•). Hence, thesecondi- describedrepresenttrue phenomenarather than artitions can also be designatedas N•S, as C•N•S and as C•CaN•S, facts due to instability of the data.
aNote that Conditions d (NaN"eS), q (C•NaNaeS), and i
(C•CaNaNaeS)can each be thought of as a complexcomposedof
one fewer masker than the foregoing method of designationim-
respectively.
The distinctionbetweenN s and Nae whenthe two
4 Masking by N "e was not measuredbecausethe methodwhereby it and N" were generated made these two signalsequivalent in
opposed
time delayssothat onemodulatednoisewaslateralized power, in spectrum and in modulation envelope. Thus, their
masking efficiencywas identical.
to the right ear and the other to the left ear.
were combinedis retained in our coding becausein some of the
other 11 conditionsof the total experimentN sand N aeweregiven
698
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Number 3
1969
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PERCEPTUAL
MASKING
IN
TABLEVII. Mean binaural maskingshifts derived by averaging
the changesfor the three interaural states (seeTable VI). Positive
values indicate reduced masking.
Mean shift
in threshold
re N•-N•-RS0
Masker
N•-N•aS0:
Unmodulated
N•-S0:
One modulated
white
noise
noise a
3.4
0
C•S0:
C•-S0:
C•N•2S0:
C•N•-N•aS0:
C•C•-S0:
C•C•-N•2S0:
C•C•-N•N•RS0:
One competing speaker
Second competing speaker
One speaker and modulated noise
One speaker and unmodulated noisea
Both speakers
Both speakers and modulated noise
Both speakers and unmodulated noise•
3.4
3.4
--2.2
--3.6
--5.4
--6.2
--6.7
MULTIPLE
BACKGROUNDS
The actual magnitudesof the relations just summarizedare better estimatedby averagingthe foregoing
changesin masking acrossinteraural states for each
conditionseparately.Table VII presentstheseaverages.
The three conditionswhere there was only a single
modulated masker (Nx•S0, CxlS0, and C•S0), each
exhibited
a mean binaural
reduction
in interference
of
3.4 dB. This completeequivalenceof meanswas to be
expected,sincethe relative power levelsof thesethree
maskers were estimated
on the basis of differences in
their uncorrectedspondeethresholdsand the data were
then correctedaccordinglyas describedearlier.
Table VII revealsthat all of the remainingconditions,
a N•N•
combines two modulated
noises into one unmodulated
white
i.e.,
thosein which more than one maskerwas present,
noise (N•D.
produced increasedinterferencewith perception of
spondees.The absolutevalues of these increases(re
Table VI reassembles the same information in terms
masking by N•Nx2•S0) range from 2.2 dB for the
of the mean shift in masking re that produced by CxlN•S0 conditionto 6.7 dB against a backgroundof
N2N2RS(or N1). Here, shiftsduringhomophasic
pres- all four maskers (CdCx•Nx•Nx•So). These values,
entation were computedre N02N0•RS0
(or NdS0), anti- however,do not give an uncontaminatedpicture of the
phasicre N•N•S0
(or NtiS0) and time-delayedre perceptual interferencearising from combinationsof
N0.8•N0.82•S0
(or N0.dS0).Descriptionof the findingsis maskers because these values are influenced by the
most easily summarized in terms of these various residual "window" effects,and these residual effects
may be expectedto vary with the number of signals
changesin maskingefficiency.
Note first in Table VI that the three setsof changes constitutingthe maskercomplex.
Table VIII presents estimates of the amount of
are highlyparallel.The samerelationsappearedduring
homophasicas during antiphasic or time-delayed perceptual interference after correction for residual
presentations.Second,observethat the shiftsclusterin "window" effects.The magnitudesof our estimatesof
a clear-cutpattern re the N•N • (or N 1) referent. The these residual "window" effects are shown in the third
three conditionsinvolvingonly one competingmasker columnof the Table. The followingis the rationalefor
(i.e., Nx•S0,CxtS0,and Cx•S0)exhibitedpositiveshifts the values shown there. Experiment 2, it should be
(reductionsin masking)rangingfrom 2.2 to 4.4 dB.5 recalled,revealedan advantageover receptionin an
By contrast,the two conditionsinvolvingmeaningless unmodulated backgroundof about 1 dB when the
noiseplus onetrain of competingspeech(CdNx2S0and NmaSmcondition was operating. C•iC?S0 constitutes
CdN•Nx•S0) exhibited negative changes (excess a roughlyanalogouscombinationof modulations,sowe
masking)rangingfrom --1.3 to --4.4 dB. Finally, the presumeit too allowed about a 1-dB advantage. By
three conditionsinvolvingthe presenceof two compet- contrast, we may assume that this advantage coming speechtrains (CxlCx2S0,
CdCx•Nx•So,
and CxlCx2- pletelydisappearedwhenN • and N • combinedbecause
N•2N•2RS0)were characterized within each interaural these two modulated noiseshad their envelopesdovestate by transition to still more negativeshifts. These tailed so that the combination eliminated modulation.
latter rangefrom --4.4 dB to -- 7.5 dB. Thus, considering the over-all spanin the modificationsin amountsof
TABLE VIII. Perceptual masking induced by adding other
effectivemasking,the efficiency
in perceivingspondees competing signals to a single backgroundsignal. Perceptual
againstcompetitionwas alignedin four distinct levels. interference estimated by adding to the observed increase in
masking (re spondeethresholdin N•'N•'•S0), • the reduction in
Level 1 was epitomizedby a singlemodulatedsignal masking attributable to the residual "window" effect characterthat producedless masking than unmodulatedwhite izing the maskercomplex.
noise, which constitutes the second level. Moderate
Observed
Estimated
excessmaskingcharacterizedLevel 3. Here, masking
Composite
increasein
residual
Estimated
was producedby two competingbackgroundsounds,
masker
maskingre "window" perceptual
one of which was speech.Finally, at Level 4, the most
conditions
singlemasker
effect
interference
severeexcessmaskingoccurred.This level appeared
C•IN•"S0
2.2
q1.0
=
3.2
whentwo unrelatedspeechtrains werepresentin the C•IN•"N•"•S0
3.6
q0
=
3.6
maskercomplex.
C•IC•'So
* Throughoutthe restof thispaper,"x"s areusedassubscripts C•IC•'N•'So
to identify maskercomplexeswheneverspecificinteraural rela- C•C•'N•N•'•S0
tionsare not beingdiscussed.
For example,N•"S0refersto N0"S0,
N•'S0, and N0.a"S0
in the composite,i.e., to all conditionswhere
only a singlemodulatednoisewas the interferingsignal.
• N•2N• •
5.4
6.2
6.7
combines two modulated
qqq-
1.0
0.4
0
=
=
=
6.4
6.6
6.7
noises into one unmodulated
white
The Journalof the AcousticalSocietyof America
699
noise.
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03:08:53
CARHART,
TILLMAN,
AND
GREETIS
Hence, we estimate the residual "window" effect to be
shift than physicalconsiderations
alonewould lead one
to predict. It was reasonedthat the compositemasker
tions. An intermediatesituationmay be postulatedfor was introducinga perceptualcomplicationcausingthis
the Cxi Cx2Nx •.So complex. Here, three modulated excess,which was thereforecalledperceptualmasking.
maskers occurred in random relation to one another.
That is, when changesin maskingwere correctedto
Acoustically,burst intervals of one masker often filled allow for both differencesin effectivepower and varithe gapsin another.Hence,on the average,the intervals ations in residual "window" effects (see Table VIII,
where gapsin all three signalscoincidedwere shortened Expt. 3), the outcomeled to the conclusion
that a backto about a fourth of the mean duration of the gaps ground consistingof one talker in combination with
("window")in any singlesignal.Othersegments
of the modulated noise (N •') invoked about 3.2 dB of percomposite masker of course took on complicated ceptual masking. Results of Expt. 3 also indicated
summationalcharacteristics.However, the substantial that essentiallythe sameorder of perceptualmasking
shorteningof "window" duration wasprobably the cri- (3.6 dB) occurredin the presence
of unmodulatednoise
tical feature here, sincethe advantageof the "window" plus the singletalker. Moreover, a new magnitude of
effectwas largely lost by the drastic shorteningin the perceptualmasking (6.6 dB) appearedas soonas a
gapsin the maskercomplex.We have not measuredthe secondcompetingspeechsignalwas introducedin the
magnitude of this small residual advantage that per- maskercomplex,and now it madealmostno difference
sistedhere, but it seemsreasonableto assumethat it whether meaninglessnoisewas alsopresentor not.
probably approximated0.4 dB.
III.
DISCUSSION
When the observed increasesin masking due to
combining maskers (Column 2 of Table VIII) are
The foregoingrelationshipslead us to postulate a
correctedby the residual"window" effectswe have just
very simplemodelto describethe perceptualsequence
discussed,the estimatesof perceptualinterferencethat
involved in sorting a primary signal from a masker
emerge are those shown in the last column of Table
complex.This model is one whereinthe presenceof
VIII. Theseestimatesalign themselvesin a very simple
competingspeechcomplicatesthe task of signalsepapattern. In the two instanceswhere only a single ration the auditor must perform.We here presentthis
train of competingspeechwas present (CxlN•2S0and model in terms of the effectswe observedin measuring
CxlN•2N•'RS0),the deteriorationin spondeethreshold
maskedthresholdsfor spondees.
We wishit understood
attributable to perceptualfactorswas a little more than at the outsetthat we wouldnot expectthe magnitudeof
3 dB. By contrast,in thosethree instanceswhere both
perceptualmaskingto remainconstantif the type of
trains of competing speechwere present, perceptual
speechin the primary signalwere changed,but we
maskingroseto estimatedvaluesof about 6.5 dB. Here,
think it safe to assumethat the same sequencesof
it did not make any substantial differencewhether
perceptualsortingwouldneedto occur.
meaningless
backgroundnoisewaspresent(C.•1C•2N/2S0
Accordingto our model,the simplesttask whenone
or C•IC•2Nx2Nx2•S0)or was absent (C•C•2S0).
islisteningagainstcompetition
occurswhentheprimary
The three setsof data reviewedaboveform a sequence
message(target signal)is mixed with a singlebackthat gives insight into the phenomenonof perceptual
groundsignal.Here, the originalacousticconglomerate
masking, as this phenomenonis revealed in changes
needonly be dividedonce,i.e., into target signaland
that occur in the masked thresholdfor spondeeswhen
backgroundsignal.Our data indicatethat under such
measuredfor subjectswith normal hearing.
a circumstance,
the maskermay be eithera meaningless
Experiment i revealedthat the singlemaskersused
signal(noise)or a meaningful
one(speech).
in the present study possessedmodulation characWe suggestthat a secondlevel in complexityof
teristics allowing a little lessthan 4 dB reduction in
listeningappearsin the presence
of two maskerswith
maskingre thresholdsobtainedin unmodulatedwhite
highlydissimilarcharacteristics
provided
oneof theseis
noiseat equivalentlevel. Thesedata alsorevealedthat
meaningful
speech.
øNow the taskismoredifficult.The
combiningthe two maskerscausedthe spondeethreshauditor'snervoussystemmustnowperforma two-stage
old to becomesomewhatpoorerthan wouldbepredicted
sortingprocess.
One stageconsists
of segregating
the
on acousticgroundsalone.
nonspeech
component
fromthe total stimuluscomplex.
Experiment2 confirmedthe presenceand magnitude
The otherstageisthe separatingof the primarymessage
of the "window" effect for white noise modulated four
0 dB in the CxlNx•'Nx•'RS0and CxlCx2Nx2Nx2RSo
condi-
times per secondby 10 dB with 50% duty cycle. It
showedthat the reduction in masking drops to about
fromthe otherspeechsignal.Onemay reasonthat this
secondlevel of complexitycan be copedwith successfully only if the primary message
containsmore in-
1 dB whenthe modulationpattern is changedto 75% formation than when the masker consistsof only one
duty cycle, which is approximatelythe modulation
6 It is conceivable that two nonmeaningful signals that are
envelopethat characterizesthe combinationof consufficientlydistinctivefrom one anotherwouldproducesimilar
nectedspeechwith the 50% duty-cyclenoiseto form increase
in taskcomplexity
particularly
if at leastoneconsisted
of
the compositemasker (C.•N•2). This latter type of unpredictably
modulatednoise.However,this possibilitystill
backgroundwas found to produceabout 3.2 dB more awaitsexperimentalexploration.
700
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Number 3
1969
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PERCEPTUAL
MASKING
IN
signal. Suchadditional informationcan be acquiredin
only one way, since we are dealingwith situationsin
whichcompetitionsbetweensoundsare occurring.This
way is to give the primary messagehigher intensity
MUI•TIPLE
BACKGROUNDS
relatively unstable from subject to subject, when he
masked a gated sinusoidby simultaneouslypresenting
a noise and
a sine wave.
Green
concludes
that
two
mechanismsfor sensoryprocessing
wereoperatinghere,
relative to the masker, i.e., to make the signal-to- one involving one type of masker and the secondthe
competitionratio more favorable. In our experiment, other type. However, the aberrant resultshe obtained
an increaseof 3.2 dB in spondeelevel was required to when the sinusoidal masker was not identical in frequencyto the gated signalled him to state, "The rather
supplythis extra information.
A further degreeof complexitymay be presumedto startling results with off-frequencymasking indicate
enter the picture as soonas the backgroundcontains our ignoranceas to the exact mechanismsresponsible
two competingspeechtrains, whether or not meaning- for maskingin that situation" (p. 1525). The thought
lessnoiseis alsopresent.Here we may envisiona multi- we are suggestingnow is that in our study equally
interactto shift spondee
ple sorting task that is lesseasily performedbecause poorlyunderstoodmechanisms
threespeechsignalshave to be untangled.7 Still more thresholdsmore than simpleadditivity would produce,
information must be preservedin the primary signal but that in this case, the mechanismsdepend on linbefore it can be adequately isolated from its competi- guistic functions, rather than relatively simple sensory
tion. This further increase in available information can
processingof the type Green describes.
Our findings parallel rather well the increase in
only be brought about by making the signal-to-competition ratio still more favorable. In our experiment, masking that both Miller (1950) and Pollack and
the spondeelevelhad to be increased3.4 dB re the level Pickett (1958) reported as accruingwhen a second
required in the sorting task where one masker is non- backgroundtalker was addedto a first suchtalker. The
meaningful, while the other is speech. The over-all test materials employed in their studies were not
increasere the still simplertask of copingwith only one identical with ours,and exact comparisonsare therefore
masker is 6.6 dB.
impossible. However, sufficient parallelism existed
The foregoingmodelmay be helpful to our thinking, between their work and ours to allow the following
interpretation.The total thresholdshiftsthat resulted
but it is not essential. The basic consideration is that a
unique additivity of masking emergeswhen speech in these earlier studies when the second talker was
competeswith speechin a complexbackgroundof the added were in the range of 8-10 dB. These shifts
type employedin our experiments.This unique addi- probably includedabout 2.5 dB due to partial elimintivity, which we find it convenientto term perceptual ation of the "window" effect and a substantial further
masking, cannot be consideredto be a manifestationof increasedue to perceptualinterferenceamongthe three
the sametypesof excessadditivity in maskingthat have speechsignals ("target" signal plus two trains of
been describedby Bilger (1959) or by Green (1967). competition).
To generalize,the experimentaldata we havegathered
Our own studies have neither used maskers nor manipulated signalrelationshipsin mannersanalogousto the bring us to the conclusionwe have already expressedin
proceduresusedby either of theseexperimenters.How- various ways: namely, when spondeethresholdsare
ever, both their data and ours suggest that excess measuredunderthe conditionswe employed,perceptual
additivity in masking, defined as unusual change in masking (or semantic interference,if you prefer)
threshold for detection or for perception of a probe appears, provided the masker complex consists of
stimulus,can be the product of cumulativeinterference either a speechtrain pitis meaninglessnoiseor of two
involvingtwo or more independentmechanisms.We are speechtrains. Moreover, this perceptualmasking is
not attempting here to suggestwhat thesemechanisms greaterin the latter instance.In our study, the magniare, and we have alreadypresumedthat the mechanisms tudesof thesetwo manifestationsof perceptualmasking
invokedvary with the listeningtask. The point to be were about 3.2 and 6.6 dB of excessmasking, respecstressedis that excessadditivity is demonstrablein tively.
The existenceof perceptual masking as we have
various ways. B ilger, it can be recalled, studied how
maskingfor frequenciesoutsidea band of noisecombine observedit is consistentwith the general trend of
with critical-band-typemasking. When masker levels experimentsthat have beenperformedon multichannel
were high enough he obtained 6 dB threshold shifts listening, although techniquesdiffered sufficientlyso
insteadof the $ dB to be expectedfrom simpleaddi- that specific findings cannot be cross checked. For
tivity. Greenobtainedsomewhatsimilarresults,albeit example, numerous studies aimed at maximizing
efficiencyof suchtasksasreceptionof aircraft massages
7The implication of this statementmust not be misunderstood. by controltower operatorshave beenperformed.These
We are notpresumingthat the listenermust achieveunderstanding
of all threesignals.In fact, he mustprobablyinhibit the penetra- studies are epitomizedby the following references:
tion into his consciousness of the semantic nuances of the comBroadbent, 1954 and 1958; Webster and Solomon,
peting signalswhile achievingsuchpenetrationfor the primary 1955;Websterand Sharpe,1955.The wholepurposeof
signal. Our findings suggestthat such inhibition is more difficult
to achievewhen it must be imposedon two competingmessages, these studieshas been to discoverways of reducing
rather than on only one.
semanticinterferenceamongverbalmessages
by manipThe Journalof the AcousticalSocietyof America
701
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03:08:53
CARHART,
TILLMAN,
AND
GREETIS
ulating the whole-messagetest items, the physical successful
oneare suppressed.
Accordingto this schema,
arrangement of signal sources,and the arrays of com- comprehension
is a gameof 'hot and cold' . . ." (p.
petition that characterize control-tower operation. 594). And here we need merely add that there are
Manifestationsof perceptualmaskingare alsofound in circumstances,such as those encompassedby our
a variety of other investigations.Illustrative of such experiment,when the game becomesmuch harder to
resultsare Schubertand Schultz's (1962) observation play becausemultiple incomingpatterns must be unthat masking by a talker's own voice or by multiple tangledbeforethe correlationwith storedpatternsthat
voicesis greater than when another voice or backward results in understanding can occur. The conflict, and
speechconstitutesthe competition;Hogan andHanley's hence the degreeof perceptualmasking, is enhanced
(1963) finding that understandingof short-answer when the incomingsignalsare highly similar, i.e., are
sentencesagainstcompressed
and expandedcontinuous all speech.
speechdroppedwhen the numberof competingtalkers
IV. SUMMARY
increased;Treisman's (1964) obse•wationthat there is
Three sets of spondeethresholdsgathered in our
a greater decreasein efficiencyof repeatingconnected
prose when two irrelevant channels,each carrying a researchstudiesdemonstratethat multiple background
speechmessage,have to be rejectedthan when only one signalsproducemaskingin excessof that attributable
irrelevant channelmust be discarded;and Mowbray's to the power summationof these signals.This excess
(1964) demonstrationthat repetitionof wordspresented interferencewith receptionof spondeesmay be termed
to one ear is momentarily disruptedby an occasional perceptualmasking.The major relationshipsapparent
in our data (after adjustmentto equatebackground
and randomly timed word to the other ear.
Our findings,demonstratingasthey do two successive powerlevel) are four. First, a reductionin maskingof
degreesof perceptualmasking,interdigitatewith other about 3.5 dB, re maskingby white noiseof comparable
forms of data, such as thosejust mentionedand those level, appearedwhen white noisewas modulatedfour
that led Broadbent to state "...
there is in the listener
timesa secondby 10 dB with 50% duty cycle.Second,
somefactor which discardspart of the presentingin- this reduction decreased to about 1 dB when the same
formation,but which is not sensorymasking" (1956, noise was given 75% duty cycle, a condition that
p. 533) and to say "...
until attentional selectionof roughlysimulatesacousticallythe randomcombination
one voice takes place none of the componentsof a of either two modulatednoises,two speechtrains or a
babble of voice is appreciablyeffectivein producing noiseand a speechtrain. Third, mixingonespeechtrain
response"(1952,p. 54). He stresses
that the processes with noise(either modulatedor unmodulated)induced
involvedin sucha selectionare not completelyrandom about3.2 dB of overmasking(or perceptualmasking)
and the probabilityof adequateselectionis changedby as revealedby the shift in spondeethreshold.Fourth,
propertiesof the contributing competingsoundsand combiningtwo speechtrains (whetherwith or without
meaningless
noise)increasedperceptual
by the state of the listener(1958). Here, it is pertinent accompanying
to recall two statementsby Licklider (1952). First, he maskingabout 3.4 dB to a total excessof 6.6 dB. These
says,"The processof speechperceptionis analyzedinto relationshipsappear to be independent of whether
three main operations' (1) translation of the speech presentationis monaural or binaural (homophasic,
signalinto a form suitablefor the nervoussystem; (2) antiphasic,or with interaural time disparity). We
stagesof perceptualmaskidentification of discrete speech elements, and (3) hypothesizethat successive
comprehensionof meaning" (p. 590). Second, he ing ariseas the signalsortingtask becomesmore exactattributes comprehension
of meaningto a neurological ing and discussseveralstudieshaving a bearingon our
processof matching incomingpatterns to stored pat- findingsand interpretation.
terns, remarking,"When a storedpattern turns out to
ACKNOWLEDGMENT
have a strong correlationwith the incomingpattern,
that confirmationcausesactivation of other patterns
We gratefully acknowledgethe financialsupportof
closelybonded to the successfulone. When the cor- the National Institute of Neurological Diseasesand
relation turns out to be low, patterns closeto the un- Blindnessthrought grants.
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1969
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PERCEPTUAL
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