Indices of frequency and temporal resolution as a function of level
RobynM. Cox
MemphisStateUniversity,
andDepartment
of Veterans
•4ffairsMedicalCenter,Memphis,
Tennessee 38105
Genevieve C. Alexander
Department
of Veterans
AffairsMedicalCenter,Memphis.Tennessee
38105
(Received15December1991;acceptedfor publication6 October1992)
Severalinvestigators
havesuggested
that between-subject
variationsin hearingaid benefitmay
be relatedto between-subject
variationsin auditoryresolution.Evaluationof thisrelationship
iscomplicated
by theneedto assess
resolution
at severalfrequencies
andlevelsin hearingimpairedlisteners
whomaynottoleratelengthytestprocedures.
Thispaperreportsdatafrom
normal-hearing
listenersfor abbreviated
indicesof frequencyandtemporalresolution.For a
particularfrequency-stimulus
levelcombination,
bothindicesmaybederivedfromthreepuretonethresholds
obtainedwith differentmaskingstimuli.Normsare reportedasa functionof
maskerlevelfor eachindexat fourtestfrequencies.
Changes
in theindicesasa functionof
maskerlevelwereconsistent
withexpectations
basedon relatedstudies
by otherinvestigators.
Normswereobtainedusinga supra-auralearphone.Comparison
of resultsfromsupra-aural
andinsertearphones
suggested
that low-frequency
soundleakagesubstantially
affectedthe
normsfor 500Hz. Reliabilityof individualindiceswasgoodto excellent.
PACS numbers:43.66.Yw, 43.66.Ts, 43.66.Mk [WDW]
INTRODUCTION
In viewof the wide variabilityin frequencyandtemporal resolutionobservedin individualswith the samehearing
Whenwe measurethe extentto whichamplificationimloss,and theprobablerelationshipbetweenauditoryresoluprovestheabilityof hearing-impaired
personsto understand tionandspeechintelligibility,it is reasonable
to hypothesize
speech,it is commonto observesubstantialbetween-subject that between-subject
differencesin hearingaid benefit (dedifferences,even for individualswith similar pure-tone finedasimprovedspeechunderstanding
ability) canbeparthresholds,especiallyif speechintelligibilityis measuredin
tially explainedby between-subject
differences
in auditory
the presenceof a competingnoise (Cox and Alexander,
resolutionvariables.Onefactorthat complicates
the evalua1991). Someinvestigatorshavehypothesizedthat thesebetion of anyrelationshipbetweenhearingaid benefitandresotween-subjectdifferencesin benefitmay be relatedto differlutionabilitiesis the effectof stimuluslevel.Bothfrequency
encesin the frequencyand/or temporal analysisabilitiesof
and temporal resolutionare known to vary with stimulus
the individuals' auditory systems (e.g., G!asberg and
levelin normal-hearinglistenersbut data describingvariaMoore, 1989).
tions with level in hearing-impairedlisteners are more
Many studies suggestthat frequency resolution is
scarce.If auditory resolutionvarieswith stimuluslevels,the
poorerthan normal,but widelyvariable,in individualswith
relationshipbetween resolution and hearing aid benefit
sensorineural
hearingimpairment(seeTyler, 1986,for a rewouldbeinfluenced
by thelisteninglevelsfor amplifiedand
view). Somestudiesof the relationshipbetweenfrequency unamplifiedspeech.The potentialimportanceof this factor
resolutionand speechintelligibility havefoundevidenceof a
is supportedby studiesshowingthat speechrecognitionis
significantlink betweenthesetwo variables(e.g., Glasberg stronglyrelatedto presentation
levelin hearing-impairedlisand Moore, 1989;ter Keurs et al., i 992; Thibodeau and Van
tenersand that there is often an optimal presentationlevel
Tasell, 1987) althoughothershavefailedto finda link (e.g.,
for which the highestscoresare obtained (e.g., Gutnick,
Dubno and Schaefer, 1992; Lutman and Clark, 1986).
Althoughtemporalresolutionper semaynot be affected
by cochlearhearing loss (e.g., Moore and Glasberg, 1988),
manystudiespresentevidencethat performanceon certain
temporalresolutiontasksis,on average,poorerthan normal
in listenerswith sensorineural
hearingimpairment,and that
1982; Yantis etak, 1966).
Beforethe relationshipbetweenauditory resolutionand
hearing aid benefitcan be explored, it is necessaryto formu-
there are wide individual differences (e.g., Zwicker and
lateprocedures
formeasurement
of frequency
andtemporal
resolutionin hearingaid wearers.Most laboratorymethods
arearduousandmaybetoodemandingfor thetypicalelderly hearingaid wearer,especially
if we wishto generatereso-
Schorn,1982;Nelson and Freyman, 1987;Glasberget al.,
1987). Of the studiesexploring the relationship between
temporal resolutionand speechintelligibility, some have
found evidenceof a significantlink betweenthe two (e.g.,
Tyler etal., 1982;Dreschler, 1989), but somehavenot (e.g.,
Festenand Piomp, 1983).
lution data at severalfrequenciesand stimuluslevels.This
paper describesresultsfor two indices,one for frequency
resolutionand one for temporalresolution,that weredevelopedfor usewith hearingaid wearers.The presentdata encompassresultsobtainedfor normal-hearingsubjectsas a
functionof presentationlevel.The goalsof thiswork wereto
1068
J. Acoust.Soc.Am.93 (2), February1993
0001-4966/93/021068-08506.00
¸ 1993 AcousticalSocietyof America
1068
determine
whetherthesetemporalandfrequency
resolution 2. Procedure
indiceswouldproducepatternsfor normalhearersthatwere
The indicesweremeasured
usinga Bekesytrackingtest
consistent
with publisheddata basedon relatedbut more
method.Thresholdfor the pulsedtesttonewastrackedin
traditionalmeasurement
procedures;
to generatenormsfor
the presence
of eachof thethreecontinuousmaskingnoises.
each index at severaltest frequencies;and, to evaluatethe
reliability of the indices.
I. EQUIPMENT
I: DERIVATION
OF NORMS
A. Method
1. Resolution
indices
The frequencyresolutionindex (FRI) measurement
procedurewasbasedon a methodsuggested
by Pattersonet
al. (1982). The approachis an abbreviationof the notchednoisemaskingmethodusedby theseand manyotherinvestigatorsto determineauditoryfilter bandwidth.The FRI was
taken as the differencebetweentwo pure-tonethresholds.
The first thresholdwasobtainedin the presenceof a bandpasswhite noisemaskercenteredon the testfrequencyand
havinga bandwidthof 0.8foHz ( fo = testfrequency).The
secondthresholdwasobtainedin thepresence
of a notchednoisemaskerin whicha notch,_+0.3foHz wide,centered
on
the testfrequency,wasdefinedby two bandsof whitenoise
each0.4foHz in width. Notch slopeswere about200 dB/
octaveandminimumnotchdepthwas35 dB. This minimum
depthwaschosenbasedon a pilot studywhichdetermined
that the FRI wasessentiallyunchangedfor notchdepths
rangingfrom 50 to 30 dB when maskerspectrumlevel was
heldconstantat 40 dB. In practice,notchdepthsassmallas
35 dB were usedonly for low-levelmaskers.Typical notch
depthswere40-50 dB.
The temporal resolutionindex (TRI) measurement
procedurewasan adaptationof thetestfor temporalresolution factordescribedby Zwieker (1980). Like the FRI, the
TRI was taken as the differencebetweentwo pure tone
thresholds.
The firstthreshold
wasobtainedusingthesame
0.8f)-Hz bandwidthwhite noisemaskerasusedfor the FRI.
The secondthresholdwasobtainedwith the samemasking
noise(0.8fo-Hzbandwidth) 100% amplitudemodulatedby
a 14-Hz squarewave.
Thus,for onetestfrequency,
at onemaskinglevel,both
the FRI and theTRI werederivedfrompure-tonethresholds obtained with three different bandlimited white noise
The one-octaveband level of the bandpassand modulated
maskers
wasvariedfrom40 to 90dBSPLat all testfrequencies.For thenotchedmasker,thelowestsound-pressure
level usedwasthe lowestlevelat whichit waspossibleto maintain a 35-dB notchdepth (44 dB $PL at the three lower test
frequencies
and 52 dB SPL at 3000 Hz), 90 dB SPL wasthe
upperlimit.
A test run beganwith the maskerat the lowestlevel.
During a run, the levelof the maskerwasincremented
in ldB stepsat therateof 10dB/min. The testtonewaspulsedat
a rate of 2.5 pulses/s(rise/fall time = 15 ms) with a 50%
dutycycle.The subjectwasinstructedto respondby depressing a button when the tone was audibleand releasingthe
button when the tone was not audible. The level of the tone
wasvariedat a rate of 3 dB/s. If the buttonwasdepressed,
the tone level was decremented. If the button was released,
the tone level was incremented.
A reversal occurred when
the testtonelevelchangedfrom increasingto decreasing
or
vice versa. The levels of test tone and masker were recorded
at each reversal.
For all testfrequencies,
a whitenoisewith low-passcutoff below the frequencylimits of the primary maskerswas
addedat a spectrumlevel25 dB belowthat of the primary
maskersto ensurethat switchingtransientsor other distortion productswould be inaudible. In addition, to prevent
participationby the untestedear, white noiseat an effective
maskinglevel of 40-45 dB SPL was presentedto the contralateral earphone.
Data werecollectedat four testfrequencies:
500, 1250,
2000, and 3000 Hz. For eachtestfrequency,two runswere
executedfor eachof the threemaskers.
This testprocedure
requiredabout40 min of testingtimeper frequency.
3. Subjects
Ten subjectsprovideddata for eachindex at each test
frequency.Exceptfor onesubjectat 3000 Hz, the sameindividualsservedat all frequencies.Subjects'pure-tonethresh-
oldsfor the testfrequencies
werelessthan25 dB SPL. Ages
rangedfrom 22 to 34 years,with a meanof 26.
maskers.
Theywill bereferredto asbandpass
(unmodulated-unnotched), notched (unmodulated-notched), and
modulated (modulated-unnotched). Wide-band modulated
B. Results
and discussion
and unmodulated
noiseswereproducedby a functionsyn-
For eachsubjectandtestfrequency,
maskingfunctions
weregeneratedfor eachof the threemaskers.For a given
werenotchedand/or bandlimitedusingtwo cascaded
twomasker,the functionwascomputedusingthe midpointsof
channelbrickwallfilters(Wavetek model852), and a bandtrackingexcursions
for bothtestrunscombined.
The analypassfilter (Frequencydevicesmodel9002), respectively. sis proceduresprovidedfor discardingany data where
thesizer (Hewlett Packard model 8904A). These noises
Signals
werepresented
viaa Telephonics
TDH49 earphone maskedthresholds
werewithin5 dB of threshold
in quiet
encased
in a Telephonics
MX41-AR supra-aural
cushion (however,it wasnotnecessary
to discardanydataon this
andspringheadband.Levelswerecalibratedin a standard6-
cm3coupler.The nominallevelof all maskers
wasthecorresponding
one-octave
bandlevelof thebandpass
masker.At
eachnominalmaskerlevel,the threemaskingnoiseswere
presentedat the samespectrumlevel.
1069
d.Acoust.
Sec.Am.,VoL93, No.2, February1993
basis).The numberof excursion
midpointsusedto produce
eachfunctionvariedacrosssubjects
because
trackingexcursionwidthsdiffered.The rangewas84 to 238,with a meanof
147data pointsper function.To generatea maskingfunction,thesemidpointdataweresubjected
to regression
analy-
R.M. CoxandG. C. Alexander:
Frequency
andtemporalresolution
1069
sisto establisha least-squares
best-fitline.
A preliminary analysiswas performed to determine
whether data for each masker were more accurately describedby a first-or second-order
polynomial.Bothfirstand
second-orderfunctionsweregeneratedfor eachsubject-frequency-masker
conditionand the standarderror of estimate
(S•) wasdeterminedfor eachfunction.The S,• data were
subjectedto a three-wayanalysisof variancewith test frequency,masker,and regressionorderasvariables.This analysisproduceda significantinteractionbetweenmaskerand
regressionorder (F[2,18] = 111.1, p<0.001). Post hoe
analysisrevealedthat, for the notchedmasker,the S• was
significantlysmaller ( p < 0.001) when a second-order
regression
wasusedwhereasregression
orderdid not significantly impact the Seefor the two other maskers.Basedon
thisoutcome,a first-orderpolynomialwasusedto produce
maskingfunctionsfor both the bandpassand modulated
maskers.A second-orderpolynomialwas usedto generate
functionsfor the notchedmasker.Usingtheseprocedures,
the meanSeeWasin the range2.0 to 2.2 dB, and standard
deviationsof Seeswere < 0.5 dB, for all maskers.
The resultingmaskingfunctionsareillustratedin Fig. 1.
This figuredepictsmaskingfunctionsdeterminedfor each
subjectfor the 3000-Hz testfrequency.The left paneldepicts
the two functions
that would be used to derive the FRI
and
the right panelshowscorresponding
data for the TRI. Resultsfor the othertestfrequencies
werequalitativelysimilar
to thesefor 3000 Hz. Functionsfor the bandpassnoise
showeda relativelysmallrangeof variationacrosssubjects.
For the modulatednoisemasker,between-subject
variability
wasconsiderably
larger.The notchednoisealsoproduceda
fairly widerangeof resultsacrosssubjects
andthe functions
generallywere substantiallycurved with a slope that increasedwith maskerlevel. Weber (1977) reportedsimilarly
curved growth of masking functions for notched-noise
maskerswith notchwidthsgreaterthan + 0.075.
For eachtestfrequency,thesedata wereusedto generate normsfor eachindex.The procedurewasasfollows:( 1)
the maskingfunctionsfor the bandpassnoisewere normalized at a one-octaveband maskerlevel of 70 dB; (2) for each
subject,the two other maskingfunctionswere adjustedin
levelby the sameamountas the bandpass
function;(3)
means and unbiased standard deviations for the normalized
datawerecomputedfor eachfunctionacrossthe 10subjects
at 5-dB maskerincrementsencompassing
the testedrange;
(4) for eachmaskerincrement,the rangeof levelsestimated
to encompass
95% of the populationdataweredetermined.
The resultsareshownin Figs.2-5. Thesolidlinesdepict
the 95% rangeof the maskingfunctionfor the bandpass
masker after normalization
at a one-octave band level of 70
dB SPL. The dash-dotlinesdescribethe 95% rangeof the
maskingfunction for the correspondingnotchedmasker
(leftpanel) andthecorresponding
modulated
masker( right
panel). It is anticipatedthat only 5% of normalhearinglistenerswould producedata outsidethesebounds.Normalization of the maskingfunctionsfor the bandpassmasker
minimizedthe effectsof between-subject
differencesin detectorefficiency.Nevertheless,
evenafter normalization,between-subjectdifferenceswere substantialfor the other two
maskers.This is revealedby the relativelywide separation
betweenthe upperand lower boundsof the normsfor these
maskersin Figs.2-5 (the dash-dotlines). To allow accurate
reproductionof thesenormsby otherinvestigators,
thefour
curvescomprisingeachpanelwereempiricallyfitted with
second-order
polynomials.All of the derivedcurvesproduceda correlationin excess
of 0.999with theoriginalcurve.
Coefficients
for the resultingequationsare givenin Table I.
The slopeof the meanfunetonfor the bandpass
masker
wascloseto 1.0for all testfrequencies
(range0.98-1.01), as
500 Hz
go
/
70
80
FRI
60
50
7O
•
cn
•
r•
'T
30!
"r
I.--
,,, ..30
z
o
10•
I--
L,..I 10
r-
z
o
40
50
60
70
BO 90
40
50
60
70
80
90
ONE-OCTAVE
BANDMASKER
LEVEL(dB SPL)
d. Acoust.Soc. Am., Vol. 93, No. 2, February1993
60
70
BO 90
40
50
60
70
80
90
FIG.2.l•orms
formasking
functions
comprising
thefrequency
resolution
FIG. 1. Maskingfunctionsfor a 3000-Hz toneas a functionof the oneoctavebandsound-pressure
levelof thebandpass
musket.The right panel
depictsfunctionsfor thebandpass
(solidlines)andnotched(dashedlines)
maskers.
The left panelshowsfunctionsfor thebandpass
(solidlines)and
modulated(dashedlines}maskers.Funclionsaregivenfor all tensubjects.
1070
½0 50
ONE-OCTAVE
BANDMASKERLEVEL(dB SPL)
index(FRI, rightpanel)andthe temporalresolutionindex(TRI, left panel) at 500 Hz. The pair of solidlinesillustratesthe estimatedrangeof the
maskingfunctionfor the bandpass
maskerfor 95% of normalhearinglisteners,afternormalizationat a maskerlevelof 70 dB SPL. The pair of dashdot linesillustratesthe corresponding
rangeof maskingfunctionsfor the
notchedmasker(right) and modulatedmasker(left).
R.M. Cox and G. C. Alexander:Frequencyand temporalresolution
1070
•,
90
•9øi
1250Hz
5000
Hz
3000
Hz
TRI
ca 70
o
T
u'J
60
LO
-r
50
3:50
P-
p-
z
40
P-
30
o
/-
/
i,i
/
I--
ca 20
/'
N
'q
z
4-0
p-
.30
////
;
b.I
!
I'--
./
ca 20
/
b.I
N
10
•
10
;:2
o
0
40
z
50
60
70
80
90
40
50
60
70
80
O
Z
90
ONE-OCTAVE
8ANDMASKER
LEVEL(dB SPL)
40
50
60
70
80
90
4-0 50
60
70
BO 90
ONE-OCTAVE
BANDMASKERLEVEL(dR SPL)
FIG. 3. Sameas Fig. 2, for I250 Hz.
FIG. 5. Sameas Fig. 2, for 3000 Hz.
would be expectedfor simultaneousmaskingwith coincident testand maskerfrequency(e.g., Stelmachowicz
et aL,
1987).
On the basisof the reports by Zwicker and Schorn
(1982, 1990), we wouldpredictthat the slopesof the mean
maskingfunctions
for the modulated
noisewouldapproximate 0.5. The obtained mean functions were close to this
prediction,
rangingfrom0.53to0.42 (thesefunctions
would
bisectthe TRI dash-dotlinesin Figs.2-5). However,there
wasa systematic
decrease
in slopewith increasein testfrequency.The fourmeanslopes
were0.53,0.50,0.43,and0.42
at testfrequencies
500, 1250,2000,and3000Hz, respectively. Studiesof gapdetectionin bandlimitednoisessimilarto
thoseusedin this studyhaveshownthat normal-hearing
listenerscan detectshortergapsat higher test frequencies
(e.g.,Shailerand Moore, 1985). Basedon thisoutcome,it
wasnotsurprising
to observe
apparentlybetterperformance
TABLE i. Coefficients
of second-order
polynomials
describingthe upper
and lower boundsof normsshownin Figs. 2-5. For each polynomial,
Y =/•o +/•x + .82x
•, wherey = testtonethreshold,
afternormalization
(dB), and x = one-octaveband masker level {dB SPL). BP = bandpass
masker,NCH = notchedmasker,MOD = modulatedmasker.
Hz
500
Masker
BP
Bound
Upper
Lower
500
500
NCH
MOD
BP
•,
0.0022
-- 0.0022
46.872
-- 0.8527
0.0127
16.159
-- 0.5066
0.0111
Upper
Upper
26.472
0. i 149
-- 12.423
0.9542
8.9214
0.5092
-- 25.442
1.4697
0.003
-- 0.003
0.0035
-- 0.0035
Upper
58.624 -- 1.6244
0.0209
Lower
27.911
0.0102
Upper
i 1.57
-
i.2783
90
2000 Hz
m 80
ul
NCH
1.2973
Lower
Lower
1250
0.6788
18.228
Upper
Lower
1250
3.8995
--
,
1250
MOD
Lower
TRI
70
2000
BP
Upper
Lower
0.3667
5.8259
0.6408
1.4003
0.6972
-- 21.179
1.3283
0.0017
-- 0.0017
0.0023
- 0.12)023
•o 6o
2000
hi
ac
'T
50
NCH
Upper
Lower
105.89
31.339
-- 3.4657
0.0368
-- 1.2504
0.0179
p-
La
z
o
1-
40
2000
Upper
Lower
bd
P-
MOD
30
3000
20
BP
Upper
Lower
14.466
-- 3.2339
3.873
23.209
0.2691
0.5923
0.6026
1.3595
0.0019
-- 0.0019
0.0027
-- 0.0027
b.I
N-- 10
3000
NCH
Upper
Lower
O
Z
4-0 50
60
70
80
90
40
,50 60
70
60
90.425
i 19.88
-- 2.5124
0.0267
-- 4.0686
0.0383
90
ONE-OCTAVE
BANDMASKER
LEVEL(dB SPL)
3000
MOD
Upper
Lower
21.301
-- 5.4767
0.2453
0.6021
0.0013
-- 0.0013
FIG. 4. Sameas Fig. 2, for 2000 Hz.
1071
J. Acoust.Sec. Am., Vol. 93, No. 2, February 1993
R.M. Cox and G. C. Alexander:Frequencyand temporal resolution
1071
at higherfrequencies
in thepresentstudy,sincethetemporal
resolutiontaskrequireddetectionof a toneduringbriefgaps
(36 ms) in the maskingnoises.
Figure 6 depictsthe meanTRI functiondeterminedfor
eachtestfrequency.Thesefunctionswereobtainedby subtractingthemeannormalizedmaskingfunctionfor themodulatedmaskerfrom that for the bandpassmasker.They describethe extent to which the typical listenerwas able to
exploitthe brief intervalsin the maskingnoiseto improve
detection of the test tone. Each TRI function rose monotoni-
callywith maskerlevel,indicatingthat thismeasureof temporalresolutionproducedimprovedperformanceasmasker
level increased. A similar outcome can be seen in the data
25
,>5.,2o
z
z
o
•-
15
o
n,
10
>=
z
b.l
La
5
[] _2?00
HZ
O 3000 Hz
\'
[3
generatedby Zwicker (1980) and Zwicker and Schorn
I
I
I
I
I
I
I
I
I
I
4-5 50 55
60 65 70
75 80
B5 gO
( 1982, 1990), usingsimilarprocedures.Studiesof temporal
gap detectionthresholdshavealsotypicallyproducedimONE-OCTAVE
BANDMASKER
LEVEL(dR SPL)
provedperformance
with increasing
level(e.g.,Fitzgibbons,
FIG. 7. MeanFRI functionfor eachtestfrequency.
Eachfunctionwasde1983;ShailerandMoore, 1983), althoughotherapproaches rivedbysubtracting
themeannormalized
masking
functionforthenotched
to the quantificationof temporalresolutionhavenot always maskerfrom that for the bandpassmasker.
shownthis trend (e.g., Nelson and Freyman, 1987).
The curvedmaskingfunctionsfor the notchednoise
wereindicativeof a regionof maskerlevelswherefrequency
resolution
abilitieswereat a maximum:bothhigherandlower maskerlevelsproducedsmallerindices.Theseresultscan
beseenin Figs.2-5 but are especially
clearin Fig. 7 which
depictsthe meanFRI functionfor eachtestfrequency.
To
producethesefunctions,the mean normalizedmasking
function for the notched masker was subtracted from that
for thebandpass
maskerat eachtestfrequency.Thesemean
functionsdescribethe extentto which the typicallistener
wasable to benefitfrom the spectralnotchin the masking
noiseto improvedetectionof thetesttone.As Fig. 7 shows,
theinfluence
ofmaskerlevelonFRI wasquitemarkedforall
fourfrequencies.
The maskerlevelproducing
themaximum
FRI wassomewhatdifferentacrosstestfrequencies,
with
octavebandlevelsrangingfrom 60 to 70 dB SPL. The magnitudeof themaximumFRI increased
slightlywith testfre-
0 500
Hz
35
quency,rangingfrom 20 dB at 500 Hz to 26 dB at 3000Hz.
Spreadof maskingstudiesreportedby ZwickerandJaroszewski (1982) and Lutfi and Patterson (1984) indicate
that low levelmaskersproducedownwardspreadof masking effects,whereashigh level maskersproduceupward
spreadof maskingeffects.This resultis consistentwith the
changingshapeof auditoryfiltersas a functionof level (see
Moore and Glasberg,1987;Glasbergand Moore, 1990,for
reviews).Theseobservations
providea basisforinterpreting
the FRI data in the presentstudy.It seemslikely that the
effectsof the notched-noise
maskerwere dominatedby
downward spreadof maskingfrom the high-frequency
maskingbandat lowmaskerlevelsandby upwardspreadof
maskingfrom the low-frequencymaskingband at high
maskerlevels.At each testfrequency,therewas a narrow
regionofmaskerlevelsfor whichneitherupwardnordownward spreadof maskingwas substantialand thesemasker
levelsproducedthe maximumFRI values.
Lutfi and Patterson(1984) reported that the masker
spectrumlevelproducinga symmetricmaskingpattern(not
dominatedby upwardor downwardspreadof masking)was
about 35 dB at 1000 Hz whereas Zwicker and Jaroszewski
z
25
o
10
(1982) estimateda levelof 40 dB SPL to producesymmetric
effectsat this frequency.Glasbergand Moore (1990) note
that the maskerlevelfor whichthe auditoryfilter is roughly
symmetriccorresponds
to approximately51 dB SPL/ERB.
At the testfrequencies
usedin this study,the corresponding
spectrumlevelswould be approximately32, 29, 27, and 26
dB for 500, 1250, 2000, and 3000 Hz, respectively.
In the presentstudy, the masker spectrumlevels yield-
0
I
40
I
I
4,5 50
I
I
I
I
I
55
60
65
70
75
I
I
BO B5
I
<:JO
ONE-OCTAVE
BANDMASKERLEVEL(dB SPL)
FIG. 6. Mean TRI functionfor eachtestfrequency.Eachfunctionwasderivedbysublractingthemeannormalizedmaskingfunctionfor themodulated maskerfrom lhat for the bandpassmasker.
1072
J. Acoust.Sec. Am., Vol. 93, No. 2, February1993
ing maximumFRI data werehypothesised
to correspondto
regionsof symmetricmaskingeffects.Usingthedataof Fig.
7, the spectrumlevelswere found to be 44.5, 30.5, 28.5, and
31.7 dB at 500, 1250,2000, and 3000 Hz, respectively.Exceptfor the somewhathigh valueat 500 Hz (seeexperiment
II), theseresultscorrespondwell to the levelscitedby other
investigators
asproducingsymmetricmasking,lendingsup-
R M. Cox and (3. C. Alexander:Frequencyand temporalresolution
1072
portto ourinterpretation
of theFRI datain termsof a progression
fromdownward
spreadof masking
to symmetric
masking
andthento upwardspreadof masking
effects
as
masker level is increased.
Figure 7 revealsthat the mean FRI functionobtained
for 500 Hz wasconsiderably
lesscurvedthan thosefor the
threeothertestfrequencies,
suggesting
lessupwardspreadof
n
90
......................
,•
•m80 --• TDH49
ER
q 70
•-
50
masking at high notched-maskerlevels. Furthermore, the
octavebandmaskerlevelproducingthe maximumFRI was
higher (70 dB) at this frequencythan at the other testfrequencies.We postulatedthat this differencemight be the
result of low-frequencyleakageunder the TDH-49 ear-
• 4o
phones.
Previous
studies
haveshownthat acoustic
leakage
Q lo
under the TelephonicsMX41-AR cushion/headbandassemblymaybecomesubstantial
asfrequencies
decrease
be-
o
z
N
o
40 50
low about 400 Hz (Cox, ! 986). This would havethe effectof
60
70 80
90
40
50
60
70
80
90
ONE-OCTAVE
BANDM•KER L•EL (dB SPL)
reducingthelevelin thelow-frequency
bandof thenotched
maskerat 500 Hz by 5-10 dB. This would, in turn, decrease
FIG. 8. Norms for maskin[ functionscomprisin[the frequencyresolution
upwardspreadof maskingeffectsat highmaskerlevelsdue
to thisband.Additionaldatawereobtainedto explorethis
index(FRI, rightpanel)andthe(emporalresolution
index(TRI, leftpanel) at 5• Hz. Dashedlinesdepictresultsfor TDH49 earphoneand solid
linesshowcorresponding
datafor ER-3A earphone.
Eachpairof linesillus-
hypothesis.
II. EXPERIMENT
tratesthe estimatedran[e of the maskingfunctionfor 95% of normalhearin[ listeners,after normalizationat a one-octavemaskerlevelor70 dB SPE.
I1: NORMS FOR INSERT EARPHONE
To evaluatethe likely effectsof low-frequencyleakage
underthe Telephonicsearphonecushion,additionalnorms
(both FRI and TRI) were generatedfor the 500-Hz test
frequencyusingan insertearphone(EtymoticER-3A) cou-
pledto the earcanalwith a compressible
foamearplug.Becausethe foamearplugmakesa tight sealwith theearcanal,
low-frequency
acousticleakagewasessentially
eliminated.
A. Method
I. Subjects
Ten normal-hearing
subjects
participated,eightof them
alsoservedin experimentI to generatenormsat 500 Hz for
thesupra-auralearphone.The twonewsubjects
weresimilar
greaterfor the insertearphoneand the width of the normal
rangeof resultswas reducedat highermaskerlevels.This
resultisin agreement
withourhypothesis
thatleakageunder
the supra-auralearphonecushionreducedthe level of the
low-frequency
bandof thenotchedmasker,resultingin reducedupwardspreadof maskingat highermaskerlevels.
The outcomesuggests
that presentation
of stimuliusingan
insertearphonereducedlow-frequency
soundleakageand
also generatedmore consistentearcanallevelsacrosssubjects,thusdecreasing
variability.
Thedifferentoutcomeforthetwoearphones
isseenvery
clearlyin Fig. 9, whichillustratesthe meanFRI functionfor
to thosein theoriginalgroup.
2. Procedure
Instrumentation,measurementprocedures,and methodsfor generatingmaskingfunctionsand index functions
were identicalto thoseusedin experimentI. Levels were
30
,oiFRii1
TDiH4
.....
25
calibrated
in a standard
HA-2 2-cm3couplerusingtheprocedurerecommended
by the earphonemanufacturer.
B. Results
20
and discussion
Figure8 depictsthe FRI andTRI normsobtainedusing
the insertearphone(solid lines) and comparesthem with
thecorresponding
normsobtainedwith the supra-auralearphonereproducedfrom Fig. 2 (dashedlines). Examination
of thisfigurerevealsthat thenormsfor thebandpass
masker
forbothearphones
(thetwoupperpairsoflinesinbothpanels) were almost identical.
In contrast, the norms for the notched masker (lower
pairsoflines,leftpanel)hada substantially
different
pattern
fortheinsertearphone
(solidlines)andthesupra-aural
earphone(dashedlines): The maskingfunctionslopewas
1073
J.Acoust.
SOC.
Am.,Vol.93,No.2, February
1993
10
I
I
I
I
I
I
40
45
50
55
60
65
I
I
I
I
I
70
75
80
85
90
ONE-OCTAVE
BANDk{•KER •EL
(dB SPL)
FIG. 9. MeanFRI andTRI functions
at 500Hz obtained
usingTDH49 and
ER-3A earphones.
R.M.CoxandG.C.Alexander:
Frequency
andtemporal
resolution
1073
each earphone.The function for the insert (ER-3A) ear-
phoneis similarto that for the supra-aural(TDH49) earphoneat low maskerlevelsbut divergessystematically
as
masker level increases. The octave band masker level that
producedthe maximum meanFRI wasreducedfrom 70 dB
for the supra-auralearphoneto 60 dB for the insert earphone.This valueis moreconsistent
with the supra-aural
earphoneresultsfor higherfrequencies
whereacousticleakagewas not expectedto occur (seeFig. 7). In addition, the
45 50 55 60 6,5 70 75 80 8.5 90
corresponding
spectrumlevelof 34.5 dB is in closeraccord
ONE-OCTAVE
BANDblASKER
LEVF_J_
(dB SPL)
with levelsdetermined
by previousinvestigators
to produce
symmetrical
maskingandto correspond
to a symmetrically FIG. 10. Mean lest-retestdifferencesfor frequencyresolutionindices
(FRI) and letuporalresolutionindices(TRI) asa functionof maskerlevel.
shapedauditory filter.
Error barsgiveone standarddeviation.
The right panelof Fig. 8 compares
the TR! normsobtainedwith thesupra-auralandinsertearphones.
The results
for the modultednoiseare similarbut at slightlydifferent
levelsfor the two earphones:
For a givennominalmasker the data suggested
that therewereno systematic
differences
level, tone thresholds in the modulated masker were a few
in testreliabilityat differenttestfrequencies.
Thusdatawere
decibelslower in the insertearphonecondition.This outcombinedacrossfrequenciesfor analysis.Meansand unbicomemay be attributableto the slightlyhigherlevelproasedstandarddeviationswere computedfor the difference
ducedin the earcanalby the insertearphone.The resultwas
dataat 5-dBmaskerincrements.
The resultsaregivenin Fig.
a slightlylargermeanTRI for the insertearphoneat all
10. For each index, the mean differenceat each maskerlevel
nominalmaskerlevels,asillustratedin Fig. 9. Comparison wasbetween0.0 and 0.6 dB, suggesting
that there was no
of thesedatawith thosein Fig. 6 revealsthat the inserteartrend for a systematicdifferencebetweenthe two tests.In
phonedataat 500Hz arein goodagreement
withthesupra- addition,only oneof the standarddeviations(FRI: 90 dB)
aural earphonedataat higherfrequencies.
waslargerthan 2.5 dB and mostwereconsiderably
smaller
than this. This outcome indicates that both indices demonIII. EXPERIMENT
II1: RELIABILITY
stratedgoodrepeatabilitywithinthemaskerlevelrangetest-
OF INDIVIDUAL
ed.
DATA
A. Method
IV. GENERAL
I. Subjects
Overall, thesesimplifiedmeasuresof frequencyand
temporal resolutionproducedresultsfor normal hearers
that wereconsistentwith data reportedin severalprevious
investigations
that usedsimilar or relatedproceduresand
stimuli.At eachtestfrequency,the rangeof resultsfor normal-hearinglistenerswasfairly wide, but the individualindiceswerequitereliable.Further researchis neededto determinethe extentto whichperformanceof hearing-impaired
listenerstypicallyfallsoutsideof thesebounds.
It shouldbe kept in mind that the norms presentedin
Table I applyto measurement
of FRI andTRI obtainedwith
TelephonicsTDH49 earphones,MX41-AR supra-aural
cushions,and springheadband.This transducerassembly
waschosenbecauseit is in widespreadusein both research
and clinical endeavors.However, the resultsof experiment
II suggestthat FRI and TRI data obtainedat 500 Hz with
this earphoneare stronglyinfluencedby leakageunder the
Nine normalhearingsubjects
servedin thispart of the
study.Six of themalsoservedin the groupusedin experiment I. The three new subjectsweresimilar to thosein the
originalgroup.
2. Procedure
Reliabilityof the indiceswasevaluatedby comparing
two independentmeasurementsof both FRI and TRI functions. A total of 16 test-retest measurements of both indices
were made.Five subjectsprovideddata at two frequencies.
Oneprovideddataat threefrequencies.
Testfrequencies
for
the 16 measurements
weredistributedas follows:eight at
3000 Hz, six at 500 Hz, and two at 2000 Hz. Instrumenta-
tion, measurement
procedures,
andmethodsfor generating
masking functions and index functions were identical to
those usedin experimentI. TDH49 earphoneswere used.
For 15 measurements, the two sets of data were obtained
consecutivelywithout removing the earphones.For one
measurement, the two sets of data were collected several
daysapart.
B. Results
and discussion
Two FRI functionsweregeneratedfor eachdatasetand
differencesbetweenthe two functionswere determinedby
subtractingthesecondfunctionfromthefirst.The sameprocedureswere followedfor the TRI functions.Inspectionof
1074
J. Acoust. Soc. Am., Vol. 93, No. 2, February 1993
DISCUSSION
cushion-headbandassembly.Although this doesnot invalidate the norms at 500 Hz, thesedata might be of limited
usefulness.For measurementsat 500 Hz, it would be reason-
ableto usetheER-3A earphoneinstead.However,thebandwidth andoutputlimitationsof thisearphonemakeit unsuitablefor testsat high frequenciesor high levels.
ACKNOWLEDGMENTS
This work was supportedby the Department of VeteransAffairs RehabilitationResearchand DevelopmentSer-
R.M. Cox and G. C. Alexander: Frequency and temporal resolution
1074
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