1987,volume16,pages175-186
Perceotron,
for
Relativeeffectivenessof threestimulusvariables
l o c a t i n ga m o v i n gs o u n dsource
LawrenceD Rosenblum11,ClaudiaCarello,RichardE Pastore
NY 13901,
USA
Binghamton'
of NewYorkat Binghamton,
StateUnrversrty
form1 April1987
in revised
Recerveo
20 May1986,
canuseat leastthreetypesof
Abstract.A studvis reportedin whichit is shownthat observers
acousticvanablesrhat indicatereliablywhen a moving sound sourceis passing:interaural
the Dopplereffect.and amplitudechange.Each of thesevanableswas
remporaldifferences,
movingsound
in indicatingwhena (simuiated)
preientedin isoiationand eachwas successful
Thesethree variableswere put into competitioniwith eachindisourcepassedan observer.
catingthat closestpassageoccurredat a different trme)in an effort to determinetheir relative
impoitance.It was found that amplitudechangedominatedinterauraltemporaldifferences
in
which.rn turn, dominatedthe Doppiereffect stimulusvariable.The resultsare discussed
on the
First, it is possiblethat subjectsbasedtheir judgements
rermsof two inrerprerations.
the stimuliusedinvolved
of eachstimulusvariable.However.because
porenrialdiscriminabilitv
easrlydiscriminablechanges.subjectsmay instead have based their judgementson the
situationconditions.The
of a stimulusvariablefrom differentenvironmental
independence
thesecondinterpretation.
supports
orderingobtained
domrnance
I Introduction
The psychoacousticliterature on locaiization has. for the most part, been concerned
'.vith situations in rvhich sound sources are stationary. Under such circumstances.
rely on interaurai temporal (inciuding onset) and intensity differlisteners generall-rencesor. rvhen the source sound is familiar. on the overall intensity of the sound (von
B e k e s v1 9 4 9 1R o s e n z w e i g1 9 6 1 ) .O u t s i d eo f e x p e r i m e n t asl e t t i n g sh, o w e v e r ,i t i s m o r e
commonpiacefor souncisourcesand listenersto move about. In attemptingto understand how a blind person knows when it is safe to cross a city street,for example,one
must understandwhat sorts of acoustic variables reliably indicate when cars, trucks.
and so on. that is movinq sound sources.have passedby. The stimulusvariablesthat are
relevant to the localizationof moving sound sourcesinciude interaurai temporal and
amplitude differences. the Doppler effect. (monauralt amplitude change, and the
reverberanrproperties of a sound. In the relativeiv few investigationsthat have been
concerned u'ith moving sound sources, however, the reiative importance of these
yariableshas not been evaluated.For example.Perrott and his colleagues(Perrott et al
1 9 7 9 :P e r r o t ta n d M u s i c a n t 1 9 8 1 ) d i d n o t a t t e m p ta f i n e - g r a i na n a l y s i so f t h e d i f f e r e n t
\ t i m u l u sv a l a b l e s t h a t u n d e r l i et h e p r o f i c i e n c yo f o b s e r v e r sa t i u d g i n gt h e v e l o c i t yo f a
s o u n d s o u r c em o v r n ga r o u n d t h e i r h e a d sl s e e a l s o G r a n t h a m 1 9 8 6 ) .I n t h e s t u d i e st h a t
h a v e c n t a i l e dh e a d p h o n ep r e s e n t a t i o lnA l t m a n a n d V i s k o v 1 9 7 7 ; G r a n t h a m1 9 8 3 1 '
l o c a l i z a t i o nr a c t u a l l vl.a t e r a l i z a t i o nat n d v e l o c i t vj u d g e m e n t so f o n l v o n e t y p e o f s o u n d s o u r c e v a n a b l e h a v e b e e n t e s t e d : t e m p o r a l d i s p a r i n ' b e t w e e nl a n d t h u s . i n t e r a u r a l
p h a s eo i d i c h o t i c a l l yp r e s e n t e ds t i m u l i .
W e h a v ec h o s en t o p u r s u et h i s l i n e o f r e s e a r c hb v e x a m i n i n st h r e e t y p e so f a c o u s t i c
r a r i a b l e sr h a r m r c h t a i d , r h s e r r e r si n l t r c a t i n sm o v i n g s o u r c e s :( ) n g o i n g il n t e r a u r a l
t c m p o r a lc l i f f e r c n c e st h. e D o p p l e r e t f e c t .a n d a m p l i t u d ec h a n g e .T h e f i r s t s t e p w a s t o
I -{uthor to whom requesrsior reprints should be sent. Presentaddresses:Haskins Laboratortes.
, n i v e r s i t yo t
1 7 0 C r o g ' n S t r e e t .N e w H a v e n . C T 0 6 5 1 1 . U S A : D e p a r t m e n to f P s y c h o l o g yU
. T 0 6 2 6 8 .U S A .
C o n n e c t i c u tS. t o r r s C
L D Rosenblum,C Carello,R E Pastore
l/o
verifythateachtypeofstimulusvariableis.infact,usedbyhumanobservers.Aswe
w i l l s h o w , e a c h o f t i r e t h r e e t y p e s o f v a r i a b l e s - i s g e n e r a t e d l a w f u lmaking
l y b y a p ajudgements
rticularevent.
to be useful to listeners
Therefore. each type in oorutio" ought
Becausethe location of a moving object changes
about particular aspectsof that event.
c o n t r n u a l i y ' w e a s s e s s e d . l o c a l i z a t i o n ' b y a s k i n g l i s t e n e r s t o j uare
d g eused
w h ein
n alocalisimulated
that the"different variables
sound source passed them. Assuming
ln
whether or not they are differentially effective'
zation. we then sought to determine
by
we
assessed
This
others?
the
type than on
other words do listeners rely more on one
(that is, each indicating that closest
competition
into
variables
stimulus
puttine the three
p a s s a g e o c c u r r e d a t a d i f f e r e n t t i m e ) a s w e l l a S n o t i n g t h e i r r e l a t i v evariables
u s e f u l nin
essin
descriptions of the three stimulus
isolation. We rviil beein with detailed
their reiativeimportance'
oJ.t to setthe stagefor predictionsabout
between the onsets of repeated dichotr
A systematic .rrung" in-t"rnpo.ur disparitv
the movement of a sound source'
reliably
indicate
cally presented stimuii is sufficient to
anywhere on either side of the
locatld
Thus. if a signal is emitted by a sound source
the two ears, the resulting sound wave
perpendicuiar bisector of thl Iine between
thus inducing a temporal disparity' As the
reachesone ear before it reaches the other,
and'
interaural temporal differences is generated
sound source moves. a changein these
differtemporal
of the sound source' This
thus, is specificto aspectsof the trajectory
(d) by the
difference in distancetraveled
the
ence \rrcan be.ur.urui.Jby determining
in air
sound
(v)
of
(d, - d,), and dividing this by the velocity
soundwave to the two ears
o C ) .T h i s c a n b e e x p r e s s e a
ds:
{ 3 4 3 . 5m s - 1 a t 2 0
t :
d'-A
a sound source emits a signalevery half
If. for example{and anticipatingour method)'
of
betweenthe two ears' passesdirectly in front
second.runs paraliel to the line-created
t h e o b s e r v e r a t a d i s t a n c e o f l 5 . 2 4 m ' a n d m a i n t a i n s a c o n s t be
a n calculated
tvelocityof
temporal differencescould
48.28 km h-,, a specifi..hunge in interaural
(seetable 1)'
across.sav'fifteen signals
signal numbers)
tabie I (where the first column contalns the
of
The secondcolumn
of course'
disparity'
onset
of
in
terms
two ears
shows the temporal differencesat the
interauraiphasedifferences(9)couidjustaseasilyhavebeenshownbecausethetwo
are directlYrelated:
g = ltt,r,
where,/.is the frequencyof the signal'
. \ s e c o n d t v p e o f s t i m u l u s v a r i a b l e t h a t c a n r e l i a b l v i n d i c a tlorward
emovem
e n t olsf aas o u n d
there
the Doppier effect. As a source moves
source has been ,....J
o
wavelength f waves
o f r v a v e sl n f r o n t o f i t a n d a n i n c r e a s ei n
of *,avelength
shortenrng
alter rhe frequencvof the signal at a statlonarv
behind it. These .il"n!., in wavelength
'rhe
following formuiae can be
from the ,outt"'
point of observatron"-, ,orna tlistance
at a distal point at a given moment during
used to determinethe frequencyof the signal
thetraiectorr,oftheSourceasit.respectivelv.approachesandrecedes:
t
\
j
i
\
/t 'D : f'l \
it':''l
t
'
*
r
'
.
c
o
s
8
/
r'-..cc,sdl
r'
-
r
I
I
lr\
-
|
I
r r ' h e r e t . i s t h e o b s e r \ , e d f r e q u e n c Y . ' / , i s t h e o r i s t n a i f r e q u emnocvvi n
o gf t'ahnedsgo u
i sr c e . } l S t n e
w h i c h t h e s o u n d s o u r c ei s
r e i o c i n ' o f s o u n di n a i r . r ' .i s t h e v e i o c i t va t
the
of the source and a vector drawn from
rhe angieberweenihe strarghtiine trajectory
moves'
source
the
As
observation'
the point of
source at a siven foint in"lt, path) to
at th. point of observation' Thus' the
f..qu"ncv
ih"
does
as
this angle changes.
177
Locatrnga movtngsouno source
frequency transformation at the observer is characterized first by a gradual decrease
in fiequency as the source approaches,followed by a quick downward shift as it passes
the observer. and finally a gradual decreasein frequency as the sound source recedes'
Based upon the same parameters that were used to calculate the interaural temporal
difference transformation, an example of a Doppler effect has been outlined in the third
and fourth columns (for a high-pitch and a low-pitch signal,respectively)of table 1.
Amplitude change is the final variable that we will consider. It has been shown that
the intlnsity of a familiar sound can help observers judge the relative distance between
themseivesand a stationary sound source (von Bekesy 1949; Coleman 1961). The
intensity of a signal at a distance r from a source is inverselv proportional to r2. Every
doubling of distance. therefore, diminishes sound pressure level by 6 dB. As might be
inferred from the above, if a sound source of constant amplitude is moving at a constant
velocity and direction relative to a point of observation, then a change in amplitude
specifVingtime of closest passageis generated at that point. An example of an amplitude change transformation is mapped out. based upon the same parameters that were
used for the interaural temporal difference and Doppler effect examples, in the fifth
column of table 1.
It should be noted that the rhree types of stimulus variable work together in a very
particular way. In the example given, when the sound source is directly in front of the
obr.ru"r, .u.it typ. of stimulus variable is at a critical moment. At this point the signal:
{i) reachesboth ears simultaneously (and has a time difference of zero), (ii) is involved
in its most significant pitch change relative to the observer. and (iii) is at its peak
amplirude for the observer. Thus, all three types of variables are not only related
invariantlyto the point of closestapproach,they are related invariantly to one another.
Therefore,althougheach type of variable should be effectivein isolation,we expectthe
mosl accuratejudgementsto occur when all three work together'
a soundsource
of signalssimulating
of a sequence
and locationbparameters
Tablel. Acousrico
The soundsource
to the line of sightof the observer.
movrnson a strargh!line perpendicuiar
directlyin front of the observerat a
movesat a constantvelocityof 48.28km h-t and passes
d i s t a n coef 1 5 . 2 4m '
Signal
t/ms
I
l
-)
+
5
:
E
irl
l l
ll
L-l
i+
i,<
Location parameter
Acoustic parameter
fo,^/Hz
0 . 5 5 7 804.0
804.0
0.547
0.533 803.4
802.6
0.509
(1.46
800.9
7
( ) . 38 7
79 8 . 0
" 92.0
{).3
26
784.0
0.000
0.236 r15.7
770.5
0 . 3 87
t ) . 4 6 1 t-67.7
{).0
59
76 6 . 3
- 6,s.0
{ 1 .353
t).5-17
l6 5.0
r ) . 5 , i 7 ,-61.6
fo,/Hz
P/dB'
602.9
601.9
601.5
600.7
599.1
59 7 . 5
_i93..1
587.0
580.ri
-_i
76.9
,<74.8
573.rJ
,i73.1
57 1 . 0
57 2 . 0
65 . 0
66.0
67.5
69.0
70.5
t'2.5
-.1.0
75.0
rl.0
12.5
70 . 5
69.0
6 1. 5
66.t1
6 5.i)
trl 5
3.5
1.0
2.5
2.0
1.5
1.0
0.5
0.0
(1.5
1.0
1.5
1.0
t.0
i.5
d,/m
d"/m
16.94
10.23
33.53
?6.82
20.12
13.41
6.71
0.00
6 . ,11
1 3 . ;I l
20.12
26.82
33.53
-10.23
+6.94
49.35
13.02
36.83
30.85
25.24
20.30
16.65
15.24
16.65
:0.30
25.21
30.85
-16.83
13.02
-19.35
. T h e a c o u s t i cp a r a m e t e rasr e t h e i n r e r a u r aol n s e td i f f e r e n c e, r r .h i g h - p i t c ha n d l o * - p i t c h s i e n a ti r e r. and amplitude of the srenalLP .
quenclessimuiatrnethe Doppier effect t lon and /o,, respecttveiy
''The
r
r
,
r
.
d
i
s
t
a
n
c
ef r o m i n t e r c e p ti d ,l . a n d d t s t a n c e
r
n
t
e
r
c
e
p
t
f
r
o
m
a
r
e
t
t
m
e
p
a
r
a
m
e
t
e
r
s
iocation
from observert d. r.
'dB SPL,soundpressurelevel referredto 20 pPa.
C Carello,R E Pastore
L D Rosenolum,
/6
relative effectivenessof the three
lsolation condrtronscan be used to evaluate the
to be associatedwith rhe most accurate
,yp.. "i variabies.The most effective ought
effective.then this compa.son may
..iporrr.r. But if. as is expected.all three types are
in addition to isolation conditions we
p.o'* ,o be insufficientiy sensitive.Therefore,
moments for each type of variables
used competltlon conditions. in whrch the critical
At issueis whether or not listenersrespondto one type of
do not occur simultaneously.
r a r i a b l eo v e r t h e o t h e r s .
the time at which a sienal-emitting
I n a l l c o n d i t i o n s t. h e s u b i e c t ' st a s k w a s t o indicate
siren) sounded as if it was .;ust
source iin this case, a simulatedtwo-tone ambulance
oassinghim or her.
2 Method
2.1 Subiects
psvchology course at the state
Thirteen undergraduatesenrolled in an introductory
experimentas one meansof
this
in
participated
universitv of New York at Binghamton
heartns'
normal
having
reported
tuttlttinga courserequirement'All subiects
2.2 Stimuli and aPParants
high-pitch and low-pitch signals)
A European ambulancesiren {consistingof alternating
Each simulation containedfifteen
rvas simulatedwith a Wavetek 132 VCG generator'
steady sine-wavetones approximately
signalsof 500 ms each alternatedbetween two
(a modulation rate of two signalsper
19,-Hz apart with no time betweeneach signal
rise and decay times and care was taken
second).Each signai possessedinstantaneoui
Three properties of the signal-interaural
to start and end each pulse at zero crosslng.
Doppler effect. and amplitude-were
onset iand hence.ongoing temporal) differences.
t r a v e l i n ga t ' + 8 " 2 8k m h - r a l o n g
m a n i p u l a t e d , nu I n u n n . r - u p p r o p r i a tteo a n a m b u l a n c e
puter
a l i n e 1 5 . 1 - 1m i n f r o n t o f t h e p l a n e o ft h e l i s t e n e r . A N o r t h s t a r H o r i z o n m i c r o c o m
difference' Doppler effect' and
rvasused to applv the appropriate interaurai temporal
i 1 2 - b i t a t 1 0 k s a m p l er a t e t o f 5 8 7
a m p l r t u c i ter a n s r o r m a t i n noi n d i e i t a lr e p r e s e n t a t l o n s
in table 2 (I)'
be
seen
uni zS+ Hz tones.Thesetransformationscan
Fivedifferentt},pesofconditionweretested(seetable3).Inthefirstcondition
i n d i c a t e dp a s s a g ea t t h e . s a m e
c o n t r o l s e q u e n c e st )h e t h r e e t y p e so f v a r i a b i e st o s e t h e r
t i m e L d u r i n g t h e f i f t h . e i g h t h . o r e l e v e n t h s i g n a l ) . l n t h e S e c o n d c owere
n d i t held
ion(isolation
other two tvpes
r each ua.rJle iuas presentedin isoiation. and the
sequences
alternated between 804 and
constant: amplitude was set at 69 dB SPL; frequency
was zero'
602 Hzonlv: and the interauraltemporal difference
w
e r e u s e di s e et a b l e 3 ' I n c o m p e t t c
o
n
d
i
t
i
o
n
T h r e e d i f f e r e n rr v p e so f c o m p e t i t i o n
indicatine passaqeat the same
tion t1'peA nvo r,ariablesrvere kept congruent {both
e i t h e r t h r e e s i q n a l sb e f o r e o r
s i g n aIl $ h r l e t h e r e m a i n i n gv a r i a b l ei n d i c a t e dp a s s a g e
including the critical moment of
l f t e r i h a t r - \ . t h e t r a n s f o r m a t l o no f o n e v a r i a b l e .
V a r i a b l e s rA' l l p o s s i b l ev a r i a b i e
p a s s a s cr. i a s d i s p l a c e d1 5 0 0 m s f r o m t h e o t h e r t $ ' o
e i g h t h s i g n a l s ir e s u l t e di n s l x
p a r r i n u sa n r l p a s s a u es i g n a i si i n t h i s c a s e t h e f i f t h a n d
v
a r i a b i e s$ e r e p r e s e n tw i t h i n a
s e q u e n c eos t t h t s n ' p e . I n c o m p e t i t i o nt ) ' p e [ 3 . a l i t h r e e
r
t h a ti s ' . n e a t t h e f i f t h ' o n e a t
r
i
m
e
p a s s a g ea t a d i f f e r e n t
. ' * i u - .tnr i . i b u t e a c hl n d r c a t e d
t h e e i g h t h . a n d t l n e a t t h e e l e v e n t h s i g n a l r . A g a i n . a l l p o s s i b l e o r d ecroi m
n gpse "t t' -' | . _ . l l : . .
c involled direct
r a r i a b i e sp r o d u c e ds i x s e q u e n c e sF.i n a l l v .c o m p e t i t i o nt l p e
parts ot
1,lt shouid bc noted that. oivrng ro the iimitations ot the computer slstem used' smail
t
r
a
n
s
i
O
r
m
a
t
l
o
n
SO f t h e
a
C
t
U
a
i
t
h
e
p
C
f
i
e
C
t
i
r
r
c
O
l
t
C
a
t
c
l : c s e s r n t n e s r Z e dt r a n S l o r m a t t o n sC o u l t l n o t
'i-tai-*orid erent. i'or exampie.rve couid oniv manrpuiate the oneoing temporai variable-sotnat
und not r"rthin sienais However' table 2 shows
.nsnges tn tnts parameter occurred onlv across.
cvent \\ere mtntmai lr'loreover' glven tnat
r
c
a
l
*
o
r
id
t
h
e
f
r
o
m
d
e
v
i
a
t
r
o
n
s
t
h
e
g
e
n
e
r
a
i
.
,i.,o,.ln
s
e
q
u
e
nces.it ts reasonable to assume that our
subtects \\ere accura{e rn tudging ali isoiarton
v a r r a b i e st e s t e d
s
r
t
m
u
i
u
s
t
h
r
e
e
a
l
l
r
e
p
r
o
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s
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v
,strmuii
t/3
l-ocatrnga movtngsounclsource
tion between two of the three variables on a given trial (the remaining variable was held
constant as in the isolation conditions). Here again a temporal disparity of three signals
f the fifth and eighth) was used so that the two variables wouid each indicate passage
1500 ms apart. The permurations and orderings of the three variables produced an
additional six sequences.
The threestimulusvariables
Table2. Stimuiusvariableparametersusedfor a control sequence.
simulatedan ambulancetravellingat 48.28 km h-t alonga line 15.24m in front of the pianeof
thelistener.
Signai
1
2
3
I
5
6
7
8
9
r0
t l
LZ
I J
1 4
t f
,/ms
f6/Hz
P/dB
0.6
0.6
0.5
0.5
0.4
0.3
0.2
0.0
0.2
0.3
0.4
0.5
0.5
0.6
0.6
804.0
602.0
803.0
601.0
801.0
59 8 . 0
792.0
5t7.0
716.0
577.0
768.0
5 74 . 0
765.0
572.0
765.0
65.0
66.0
b /.)
69.0
70.5
'72.5
74 . 0
75.0
74 . 0
72.5
70.5
69.0
o/.)
66.0
6).U
(l), and Doppler
Trble 3. Signal during which amplitude (P)' interaural temporal difference
condition.
function
of
effect{D ) indicatepassageas a
Conditton
Sisnai number in trtai
1l
Control
PDI
PD
I
PI
D
DI
r
Competrtion tvPe B
Competitron type C
PDI
P
D
I
Isolation
Competition type A
or
I
PD
D
PI
P
DI
P
I
t)
D
P
I
P
Y
l)
I
I
P
D
P
i
D
I
D
r
I
P
I
D
I
P
I
D
D
P
180
L D Rosenblum,C Carello,R E Pastore
Each sequencewas presentedfour times. For sequencesinvoiving interaural temporal differences. the siren was heard to be moving right to left twice and left to right
was coming straight
twice tsequencesnot involving this variable sounded as if the siren
at the listener).ln order to preclude signalcounting, two sequencesconsistedof fifteen
signais and two consisted of thirteen signals. The ninety-six trtals were randomly
blocked into twelve blocks of eight trials each, and these blocks were presentedin a
different random order for everv two subjects.Each trial was presented directly from
rhe computer through two Ithaco 4302 filters set to low pass the signal at 1000 Hz.
The signals were then attenuared (with charybdis programmable attenuators)and
pr...n*d through TelephonicTDH-49P headphonesto subjectsin a sound attenuating
chamberr:).When a subject made a button-push response,the computer recorded the
signainumber within the giventnal.
2.3 Procedure
would pass
Subjectswere told that they would hear an ambulance-typesiren which
when
the
ambulance
from different directions. They were asked to press a button
from the
was
coming
it
as
if
soundedas if it was just passingthem. If the siren sounded
facing
a
sidewalk
on
themselves
rieht or the left. subjectswere instructed to imagine
on,
was
straight
coming
if
it
as
pJrpendicularto the flow of traffic. If the siren sounded
traffic
t-low
of
the
over
a
bridge
on
it"u,r,.r. told to imagine themselvesstanding
iacing in a direction paiallel to the flow). To indicate better what was meant by this task
The
the expertmentertraced the imaginary path of the ambulance rvith his hand'
Before
all)'
(four
in
trmes
blocks
three
everv
instructions were repeated between
hearing the critical conditions. subjectsheard thirtv practice trials including several
instancesof the control sequenceand sequenceswith two of the variablespresentand
of
congruenr.These trials \\'ereused to familiarize subjectswith the task and the sound
the
of
completion
analvsis.
Upon
in
the
inciuded
were
not
rhe presentatronsand
p r u . i i . . s e s s i o ns. u b j e c t sh e a r d f o r t v - e i g h tc r i t i c a l t r i a l s .A t t h e h a l f w a vp o i n t s u b j e c t s
, " . . . n r " . n a - i m i n r e s t .T h e n . b e f o r e t h e c r i t i c a l s e q u e n c e sw e r e c o n t i n u e d .s u b j e c t s
h for each
agarnheard sixteenpractice trials. The entire experiment took less than i
subject.
3 Results
the
For each t-"-peof stimulus variable in each triai the number {in the sequence)of
was
subtracted
sisnal at rvhlch a siven variable specifiedpassageof the sound source
rbv a button press)
fr-omthe number 6i the stgnaltjurinq u'hich the subject indicated
h e a r i n gr h e s o u r c ep a s s .I n t h i s w a v . a ' m e a n d i f f e r e n c ef r o m t h e e x p e c t e d ' ( M D F E )
. i x t e e ns u c h
c o u l d b e c a l c u l a t e da c r o s s t h e f o u r t r i a l s c o n s t i t u t i n ge a c h s e q u e n c eS
w
a
s
i
n
v
o
l
v
e
d )w e r e t h e n
v
a
r
i
a
b
l
e
s c o r e s o n e f o r e a c h s e q u e n c ei n w h i c h a n v o n e
v
a
r
i
a
b
l
e
.
p o o i e dt o f i n d a n a v e r a g es i g n e dM D F E p e r s t i m u l u s
+3 for one
B e c a u s eo I t h e n a t u r eo t t h e s ea v e r a g e ds i S n e d< i i f f e r e n c ersa n N I D F E o f
r
t
(
)
t
.
1
t
h
e)'are not
.
m
e
a
n
d
i
f
f
e
r
e
n
c
e
i o r a n o t h e rc o n d i t i o n v i e l d s a
c,ndrtion and
m
e a s u r et s
t
h
i
s
H
o
w
'
e
v
e
r
'
a
n
d
a
c
c
u
r
a
c
v
.
u s e f u li n a n a n a l v s i so f r e l a t i v ee f f e c t r v e n e s s
e
i
t
h
e re a r l i e r
w
h
i
c
h
r
e
s
p
o
n
d
e
d
s
u
b
j
e
c
t
s
t
o
t
h
e
d
e
q
r
e
e
i n i o r m a r i v ei n t h a t i t c a n p o r t r a v
i
s
o
l
a
t
i
o
n .T h e s e
i
n
v
a
r
i
a
b
l
e
e
a
c
h
p
a
s
s
a
g
e
l
o
r
o
f
m
o
m
e
n
t
.r later rhan the specified
t
e
mporal
i
n
t
e
r
a
u
r
a
i
a
n
d
a
m
p
l
i
t
u
c
l
e
t
o
q
u
r
t
e
$
i
t
h
r
e
s
p
o
n
s
e
s
a
v
e
r
a
g
e
lori,.
\cores\\ere
r H e a o o h o n e p r e s e n r a t l o n h a s n o t a b l e l i m r t a t i o n s . C . l e a r i v .a n v r o i e t h e p r n n a ( ) f t h e e a r m l g h t
s i l U n d i : l O s t t n s u C h l n S t a n C e S . . { l S t ll.n r C n i l n g e si n h e e d
;iat rn \truClurtngthe amOtenl
t t l t l r e s o u n d 5 9 g 1 g g* o u l d
a,r,,t,nn $nrch mrgnt rnfiuence the lOcatronof the two ears relattve
i . c a b s e n t * . i t h h c a d p h o n e s . . \ n d . a s w e d r s c u s sl n s e c t r o n - 1 .l l l n a t u r a i l v o c c u r r l n g r e v e r b e r a n t
are presented
\!)und $ hrch mrsht aid locaiization tudgements rs lost u'hen svnthettc sllmult
s
'
v
s
t
e m a t l ct n v e s t l p
e
r
m
t
t
s
i
t
b
e
c
a
u
s
e
u
s
e
f
u
i
i
s
t
h
e
t
e
c
h
n
i
q
u
e
N
o
n
e
r
h
e
l
e
s
s
.
tnrough headohones.
gatron of spectftc aspecls of the stimulus variables
locatrnoa movtngsounosource
td I
differencesinitiated less than one-haif signal later than the moment of passage\0.442
and 0.333 respectiveiyl.and responsesto the Doppler effect stimuius variable occurrine just over one-half signal before the moment of passage(yielding a mean of
-i).5571.
In order to evaluatereiative effectivenessand accuracv.two other indices of judgements were considered.First, an averageabsolutemean differencefrom the expected
was determinedfor each variable.This metric was calculatedin much the sameway as
the signed means except that the absolute difference betweenthe expectedvalue and
acruairesponsewas used (table 4). Unfortunately,the distribution of this index failed a
Cochran test for homogeneityof variance, thus violating a major assumptionof the
analvsisof variance.The standard deviationslor each of the three variablesfor each
subjectwere aiso caiculatedfor the competition conditions and thesescoreswere then
pooled acrossail subjectsltable 4t. Here, low reiativevanability in responsesto a given
ranabie can be raken as an index of high reiative strenglh.That is, becausethese are
differencesfrom an expectedvalue of zero. the standarddeviationsinform about more
than simpiv the consistencyof the responses.Becauseof this. and the fact that this
index did pass the Cochran test for homogeneityof variance.it was decided that the
statlsticalanalysiswould be carried out on thesescores.
It can be seen from table 4 that lor all conditions the order of the scoresfor the
d i f f e r e n t v a r i a b l e sw a s t h e s a m e b o t h f o r a b s o l u t em e a n a n d f o r s t a n d a r dd e v i a t i o n
indices.Amplitude change dominated inleraural temporal differences,which, in turn.
dclmrnatedthe Doppler effect when these variables were put into competition.This
-ordering was found to be significantin an omnibus analvsisof variance(F2.21 12.71,
p < 0.002) and three simple effects analvses i amplitude versus Doppler effect.
F , . : = - 1 , 3 . 6 - 1p. < 0 . 0 0 0 1 : a m p l i t u d e \ e r s u s i n t e r a u r a l t e m p o r a i d i f f e r e n c e s .
F , ., : : - 1 . 8 5 . p < 0 . 0 4 8 : i n t e r a u r a l t e m p o r a l d i f f e r e n c e sv e r s u s D o p p l e r e f f e c t .
F ::8.37.p < 0.014i.
R e c a l l t h a t t h e r e w e r e t h r e e t v p e s o f c o m p e t i t i o nc o n d i t i o n r A , B , a n d C ) . T h e
a v e r a g e da b s o l u t e m e a n s a n d a v e r a g e ds t a n d a r d d e v i a t t o n sf o r e a c h o f t h e s e a r e
presentedin table ;1.Becauseeachof thesecondition typesvieldedthe samedominance
erdering of variables. no formal analvsis of the interaction of condition type and
. h e s e r e s u l t si n d i c a t et h a t t h e r e l a t i v ei m p o r t a n c eo f
v a r i a b l eo r d e r i n qw a s c o n d u c t e d T
a pariicular stimulus variable holds over a number of different competition-tvpe
conditions.
T h e m e a n s t a n d a r dd e v i a t i o n so f t h e M D F E s f o r t h e t h r e ei s o l a t i o ns e q u e n c e\s\ ' e r e
t h e a c c u r a c vo f r e s p o n s etso a v a r i a b l eo f a q i v e n
r r l s oc a i c u l a t e dt.h i s t i m e r e p r e s e n t i n q
(
t v p e .I t r v a sf o u n d t h a t t h e t h r e et ) ' p e s ) t v a r i a b l ew e r e o r d e r e di n t h e s a m ew a v a s w h e n
r c c u r a c vi n r e s p o n s etsh a n
p u t r n t oc o m p e t i r i o ni s e et a b l e . 1r :a m p l i t u d ep K r d u c e dg r e a t e a
m
o
r
e a c c u r a t er e s p o n s e tsh a n
w
h
i
c
h
.
i
n
t
u
r
n
.
r
'
i
e
l
d
e
d
d
i
f
f
e
r
e
n
c
e
s
.
t
e
m
p
o
r
a
l
interaurai
table .t. \1eans rrr rhe ab-soiute ntern .irffcrence rr()m cxpected
i , r q e t h re r r r t n t h e m e a n s t a n d a r cdi c l ' i a t i t l n .: D . o i t h e \ I D F E
( L)nrlltlon
lnteraurai differenccs
mean
( ( j m D e u t l ( - r nt v p e - ' \
( ' L r m D e t l t l o nt r l e B
( ' ' , m p c t l t l o nl \ n e ( '
( . r m p e t r t t l r np o o r c d
l.hl--i
l.+7i
:.151
i.!r;5
i.il{l(l
r).716
i)oppler rttecl
\lDFE r ior each conditton.
A m p l i t u d ec h a n u e
meiln
:D
mean
SD
95(.1
:ll
.: lll
r.)r5-l
l.()l-l
: ..;16
l { )1 6
._+-
897
187
501
5il
().912
t ) . 72 6
l.u0l
l.3tl
1.5-1
L.-<I -l
1r.69.1
rr650
\D
is-l+
' -
1 :
i.s5s ..5+S
;;5
I.oiation
rt.il5 r.650
) . 6 , i( )
L ontroi.
' Becausethe three variabies are congruenl this caiculatron i: the same for each
l6z
L D Rosenblum,C Carello,R E Pastore
theDoppiereffect.SinceonlyonetypeofisolationSequenceforeachvariablewas
deviations across subjects
to each subject, however, no average standard
;;11t!i
for each variable were pooled across
iould be calculated. Instead, the signed means
scores yielded the values presented in
subjects and the standard deviations of these
(only three standard deviations
table 4. since an analysis of variance was rmpractical
(lochran test of homogeneity of variance was used for analysisof
were so produced), a
An omnibus cochran test was shown to
tt " u*rug. standard deviations of the MDFEs.
:0'6457'p
< 0'05)' Three simple effect tests with the ratios of
be significant(C.r.u
performed. The.amplitude versus Doppler
these three va.ances used for an F test were
<
0'01) along with the Doppler effect versus
5'66' p
.f"., .o.parison (F,:.,::
= 2.65, p. < o.05)were found to be
interaurai temporardifferencescomparison Ftz.rz
temporal differences test was not'
significant tonly the amplitude versus interaural
amplitude generally elicited more
which
aithough their means indicated a trend in
variable)'
u."urui" responsesthan the interauralstimulus
(in which all three variables congruently indiFinally. the three cont.ol sequences
and yielded the lowest averageabsolute
cated passageof the sound source)were pooled
(0.650)' Thus, the most accurate
deviations
..un, l0.ii6) and lowest averagestandard
the most-and the most
contained
that
average responses were to the sequences
consistent-stimulation'
4 Discussion
of responses to the isolation
The low absolute means and standard deviations
stimulus variablesdo, in fact'
of
types
these
that
notion
sequenceslend support to the
when all three types of
reliabiv indicate the time of closest approach. Furthermore,
congruentlv'subjectswere
variabiescombined to indicate passagsof the sound source
to the adequacvof our stimulus
most accuratein their judgements.This result attests
'-ambulance'did sound as if it was passingwhen all of the
simulation: the syntheiic
variablesindicatedthis event'
able to respondaccuratelyto
A comment should be made on how subjectsmight be
moment of closestapproachis
these stimuli. on first reflection.it might seem that the
has passed'For example' in the
not indicated bv the variablesuntil afier the moment
that observerscannot know which
case of the amplrtude variable. one might claim
hear subsequentsignals of lower
signal is rhe most lntense of the series until they
for eachisoiationcondition'
intensitl-.However,as is indicateciby the low signedmeans
later than the moment of passage'Furthermore'in
subjectsdid not respondsignificanti,v
at the moment of closest
real-world situatlonsactions must be performed accurateiv
are auditoraily guided)'
that
animais
approach{for example.the predatory behavior of
the isolatedacoustic
not
it
is.
is
that
How might thrs be accomplished?One suggestion
the time of closest
for
information
as
uatt
tttit
structureat the exact moment of passage
of
transformations
acoustic
overall
in
the
changes
approach as much as the particuiar
j u s t b e f o r e c l o s e s ta p p r o a c h 't h e r a t e o f c h a n g e
r h e s es r i m u l u sv a r t a b l e sF. o r e x a m p l e .
to the rate of changegeneratedwhen the sound
relative
rtf amplitu<lebegrnsto decrease
total rate changepatterningof
source is further lrom the point of observationlfor the
b a s et h e i r i u d q e m e n t so f
a m p l i t u d e .r e f e r t o t a b l e 1 , . t t u r . i t m i g h t b e t h a t o b s e r v e r s
variablesthat occur
passageon changesin the rate of changeof thesedifferent stimulus
interpretation and
an
such
just before passageof the sound rou*". IFor examplesof
(
1
9
8
1
)
i
'
this way'subjects
I
n
Todd
a n a l v s i so f o p t r c a lr n f o r m a t i o n s. e eL e e \ l g ' 1 6 \ a n d
to make their
order
in
approach
need not hear the srgnalsafter the moment of ciosest
and
interaural'
ampiitude'
one
juduements.Clearlv,luture researchin which more than
use
do
subjects
not
or
whether
Doppier transtormationis used couid help to clarifv
with
familiarity
their
(rather
say,
than,
chansesin the rate of changeof these variables
judgements'
the ciangesof particularstrmuii)in makine their
Locatinga movlngsoundsource
What might account for the obtained dominance ordering of variables that we have
shown to be so consistent in our results? One conjecture is that subjects rely on
whichever stimulus variable is most discriminable within a given presentation.This can
be evaluated for each stimuius variable by scaling the total change in the relevant
parameter (time, frequency, or intensity) to the difference limen for that parameter'
betails of the evaluationsfor each variable are given in the appendix. It should be noted
rhat these discriminability indices are necessariiyapproximate in that the discrimination
functions used for their calculation were taken from experiments in which hig$y
trained subjects and simple stimuli under conditions of minimal stimulus uncertarnty
were used (unlike in this experiment).Furthermore. the existenceof differential practice
(Owing to the
effects for the three types of stimulus variables was not considered here.
of discrimidetermination
limitations of the apparatus used to generate our stimuli,
nation functionsfor the sdmuli usedhere proved impossible')
In any event, these calculations indicate that. for the stimuli presented to subjectsin
our explriment, interaural temporal differences have the highest index of discriminabiliry, followed by the Doppler effect, and, finaily, amplitude change.if discriminability
is the governing principle. then in both isolation and competition conditions the
not
ordering of the variables should follow this pattern. The results, however, do
potentiai
fewest
the
have
to
variable
calculated
the
fact
supporithis interpretation. ln
(JND) per sequence-amplitudechange-dominatedin all of
lusinoticeable difierences
ihe co-petition analyses and elicited the most accurate responses in the isoiation
But it must be reiterated that, although many aspectsof our stimuli are
anaiysesi:1.
taken lnto account rn these calculations(see the appendix),the discriminabilityinterpretationcannotbe evaluatedfully at this time.
It should be noted that all signalsused in the present experiment involved easilv
discriminablechanges.lt could be argued that, as long as this was true, finer discriminability wouid be inconsequential.Thus, as an alternative interpretation regarding
diffeiential effectiveness.it might be worthwhile to consider the usefulnessof the
variables as they are produced by reai-world sources that exist in environmental
situations involving different veiocities, paths, and sound sources(a)'If use of one type
were
of variable is generalizedto a wide variety of circumstances-that is, if one type
to
generalizability
considers
If
one
dominate.
it
should
situations-then
in
more
useful
bv
be the principle underiying effectiveness,an ordering different from that predicted
the discriminabiiity interpretation is suggested'In this view, reliance on the Doppier
effect (in terms of detectablepitch change)would be underminedbecauseit is likely to
be undetectablefor certain types of sound sources(thosethat produce clicks or squeaks
lacking the requisitepitch changes)or at extremelvslow velocitieslwhere the changeof
mavbe so slightas to t'allbelow the differencelimen (seeWier et al 1977)1.
freque-ncy
In contrast. interaural temporal rJifferencesare usefui regardlessof the nature of
ms
most (real-world) sound sources.For example. it has been reported that even I
s
lae
v
e
n
M
o
r
e
o
v
e
r
.
1
9
7
7
j
.
V
i
s
k
o
v
a
n
d
r
A
l
t
m
a
n
c l i c k s , r - i e l dg o o d a p p a r e n tm o t i o n
(
Rosenzwetg
differences
interaural
throueh
localized
tionary sound sources can be
interl g 6 1 ) . A n a n g l eo f a p p r o a c hi n t h e s a g i t t a lp l a n e "h o w e v e r .w o u l d p r o d u c e n o
for
wav
both
same
the
ordered
,r) However.it shouldbe noredrhatthe findingthat the variables
with the discriminabilit-t" lnterisolatron and Ior competttion condittons rs, in itself. consistent
pretallon.
,tiRegarding the nature of the sound source as a sttuation condition' it is acknowledged that
and frequencv propertlCs'
reli-worid \r)und \Llurce5 iltl€Il dtr not havc constant tntensitv
within a reiattvelv limited ranqe' lt
t
o
c
n
a
n
q
e
t
e
n
c
i
p
r
o
p
e
r
r
i
e
s
t
h
e
s
e
s
r
t
u
a
t
i
o
n
s
m
o
s
t
r
n
However.
chanee of this range, occurrlng as a
*ign, U" the case that.in naturai conditions it is rhe overail
d
e
t
e
c
r and base their judgements on in
s
u
b
j
e
c
t
s
t
h
a
t
s
o
u
r
c
e
.
t
h
e
s
o
u
n
d
o
I
m
o
v
e
m
e
n
t
o
f
resuit
d
i
f
f
e
r
e
ntrallv by this aspect ol the nature
a
f
f
e
c
t
e
d
n
o
t
a
r
e
thls wav. tne tnree sumulus variables
of a sound source.
.,84
C Carello,R E Pastore
L D Rosenblum,
of confusion would
aural temporal differences,whereas an approach along the cone
path of the sound
the
throughout
differences
temporal
oroduce the same lnteraural
type.
this
stimulus
of
the
seneralizability
,ou... therebvlimiting
interaural temporal
In the foregoing circu-stances where the Doppler effect and
It is specific to a source's
differencesare equivocai, ampiitude change is reliable.
to the velocity of the
trajectorv irrespectiveof the angle of approach.It is indifferent
of von Bekesy(1949) with
sound sourcepassuggestedby the early localizationstudies
amplitudel'Also, it
,ingi" p..r.ntations of a famiiiar sound sourcewith constantsource
for amplitude
limens
is useabiefor any type of sound source:owing to the difference
so rapidly as
decreases
iyost and Neilson 1985)and the fact that intensity of a source
at all,
be
detected
can
one moves awav from it. for all intents and purposesif the signai
amplitudechangecan also be detected'
that amplitude
Basecion the above anaiysisour secondinterpretationwould predict
condiin
competition
variable
changeshould be the most dominant type of stimuius
isolation
in
variable
tionsl Also. from this perspective.responsesto the amplitude
presentationsthat include
should. on the whole, be more accurate than responsesto
variables'
oniy interauraltemporal differencesor the Doppier effect
In all cases'
Clearlv.the resultsare consistentwith the generalizabilityinterpretation'
we
considered
that
conditions
the stimulusvariable independentof the three situation
(interaural
temporal
condition
rampiituder dominated the type dependent on one
contwo
situation
on
dependent
6ifferencest, rvhich dominated the stimulus variable
judgeaccuracy
for
was
found
ditions iDoppler effect).Furthermore.this sameordering
ments$ hen eachvariablewas presentedin isolation'
relative contribuThese results provide a successfulfirst pass at understandinethe
m
o
v
i
n es o u n d s o u r c e s '
o
f
i
o
c
a
l
i
z
a
t
i
o
n
f
o
r
t i o n s o i d i f f e r e n tt v p e s o f s t i m u i u sv a r i a b l e s
listeners would
what
mimic
faithfullv
not
The rariables used here. however. do
source are
a
sound
of
position
of
the
L'ncounterin natural circumstances.Judgements
of the
surfaces
reflecting
bv
the
srructured
most often based on stimuii that are
H
ence'
1
9
8
2
)
'
i
t
s
e
l
f
i
W
a
r
r
e
n
e
v
e
n
t
s
o
u
r
c
e
s
o
u
n
d
e n v i r o n m e na
t s well ason the moving
comare
variables
effect
Doppler
and
difference.
the amplitude. inreraural temporal
or
Whether
here'
used
stimuli
the
synthetic
of
was
tme
monlv much more lntricate than
variables
stimulus
the
of
dominance
relative
the
influence
wouid
not this consideratlon
ought to be usefulin
remainst. be seen.Nonetheless,the samemethodologicalstrategy
understandingof how
evaluatingother tvpes of acoustic information so that a better
h u m a n sl o c a l i z em o v i n gs o u n ds o u r c e sc a n b e p u r s u e d '
\1'assupported in part bV National Science
Acknowledgements. I'he preparation of this paper
pasrore. we gratefully acknorviedge the assistance
E
t
o
R
t
s
N
S
b
o
t
j
:
l
o
+
Foundarron cranr
L l f G c o t f B i n q h a m . R i c h a r d B u r r l s h t . L e o n a r d K a t z . B r u n o R e p p . \ l a r k S c h m u c k l e r . C a' rr il
(rt thl\ mJnu\crlpt l hc commenrs
\lJnnirru. :nJ \lrchaci furvev tn the prcpararr()n
BrianLj\{oclreandananony.mousrevtewerwereespeciaiivheiptul.
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( "i;*";;
;. i961
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9
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L)eutscn
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\lLorr-i-erm.\lemon.edD
s Deursch.J A Deutscn New
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9
8
6
\
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Florentrne
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a movtngsoundsource
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{NewYork: Holt.RinehartandWinstont
186
R E Pastore
C Carello,
L D Rosenblum,
APPENDD(
variable
inAi.". of discriminabililv for each stimulus
using the parametersprovided in table 2'
Indices of discriminabilitv were calculated
D i s c r i m i n a b i i i t y o f c h a n g e s i n i n t e r a u r a l t e m p o r a l d i f f e r e n cFor
e s i example'
s a f u n c t iMills
onofthe
the initial temporal disparity'
frequency of the signal al we[ as of
onset
interaural
signal with an initial baseline
io. a 70O1gg.z
i1958) has shown,n"i
an interaural temporal disparity of
pJ(yi"taittg
zz.5
disoaritv of 600 pr. u .iung" of
in
for highly trained listeners.to hear a change
Itui."'iit.i
*t-"r-siz.s ps] is needed
the
account
into
using this iriterion, and taking
the relative location "i ,rtri signal.
literal extent), inspection of table 2 revealsno
(or
Jimulated
changingbaseiinedisparitv
,1.4 (100 vsi22.5 ps :4.4) potential JNDs between
JND between signais i and 2,
s i g n a l s 2 a n d 3 , a n d s o o n f o r e a c h s i g n a i s t e p ( 3 t o 4 , 4 t o 5interaural
. . . . . 1 . 1 ttemporal
o15).Adding
potential discriminabilitvof
thesescoresyieids a rough index of the
was
used here, a score of 80.2 total potentialJNDs
differencechanges.t or ine stimuli
calculated.
T h e d i f f e r e n c e l i m e n f o r f r e q u e n c y o f s t e a d y s i n e - w a v e t o n eNelson
s d e p e 1986;
ndsonthe
of the signal (Freyman and
duration. intensitv, and initial frequency
been founJ that in simulated Doppler shift
Wier et aI l9i7\. Furthermore. it has
of
change can affect significantiv discrimination
contexts. rhe corresponding intensity
and
durations
equai
et al lg7g). Given
frequency changes(5)(Jora"szte82l nyffeit
increaseswith initial frequency'
frequency
for
limen
difference
intensities.however. the
of
highly trained subjects'an 800 Hz frequency
For example.it has been found that for
5 0 0 m s h a s a d i f f e r e n c e l i m e n o t l , 2 H z , * t , " ' " u ' a 6 0 0 H z t o n e i s deti sal
c nr977\l'
m i n a bKeeping
lewitha
proportional to /1 (after Wier
chanseof onlv r Hr'irritg rogA/
was calcuiatedfor the Doppler effect variable
this in mind. an inder of dlscnminabilitv
2. Becausethe stimulus presentationalternated
for the signal steps outiined in table
a scaled JND was found for every other signal
betweenhigh and low tones. however.
s t e p . H e n c e . t i , " . t u n g . , i ' o t 8 0 4 H z t o 8 0 3 H z ( s i g n a l s l a n d 3 l asnc o
d rf er o
0 e2dHaz t o
symi e6l d
,"pu.ut.iy. The.summed
6 0 1 H z r s i g n a i sz a n " a+ i w e r e e v a l u a t e d
been
the Doppler effect stimuli' Although it has
rough <iiscriminabilitf index of. 62.5 for
presenof pitch can be affectedby the
shown that both ...Jgnition and discrimination
t a t i o n o f i n t e r v e n t n g t o n e s b e t w e e n t h e t o n e s t o b e j u d g e d t D eHere'
utsch
lgT5),itisnot
aithough
with the stimuli used in this study'
clear whether this need be the case
e i t h e r t h e h i g h o r l o w t o n e s e t s c a n b e t h o u g h t t o ' i n t e r v e n e . a n dfrequency
t h e r e f o r at
e hthe
inderthe
be no[d that both sets chanqe
discriminationof the other. it should
in
aid
potentially
used by Deutsch),which could
same generalrate (unlike the stimuli
o v e r a l lP i t c hd i s c r i m i n a t i o n '
limen for amplitude changeis depenFinallv. it i,as ueen ,ho*r, that the difference
Yost
i n i t i a l a m p i i t u d eo i a s i g n a lr F l o r e n t i n e1 9 8 6 1
d e n ro n t h e d u r a t r o ni.r . q u . n . u . a n d
a n d N e r l s o n l g 8 5 l . I f u e s i m p l i f v a n d c o n s i d e r o u r 5 0 0 m s t oand
n e scalculate
t o b e a cthe
onstant
j00 Hz a clifferenceof 100 Hi at theselevelsmatterslittlet' however'
of
step. \\'e obtain a summed rough index
number oi potentralJNDs for each signal
:
1
o u r s t i m u l u ss e t '
f
o
r
a m p l i t u d e. i t " n g . d i s c r i m i n a b i l i tov f 3 1
wouid seem that these iindings mteht
t ' G i r e n t h e p o t e n t t a l d r s c r l m r n a b r i i t vt n r e r p r e r a r t o n ' l t
tn competrtion condr,"l"tru.,eitectrveness of prtch chanse
ha,'e Lmpircarion, tor"tt",'.'ioiantrui
r
e
i
e
r
a
n
t to our tindine about
n
o
t
a
r
l
t
h
e
v
t' rnvoivedi Ho*tut''
tions \\nere rntenslt\';l;;n;
c
i
r
anse was presenteo rn
on.n ti.r. Doopier eftect frequencv
rhe reiatrve accuracv ";',;;;;;;;,
t
h
e potential discrirnint
i
:
:
'
^
i
l
.
l
"
^
:
,
c
"
t
h
i
s
i
n
ts not tnuoluear'
isoiatron \\nere rntensttv change
a b i l i t v r n t e r p r e t a t l o n p l e o l c t s t h a t l u d g e m e i l s u s l n g t h i s t l ' p e o t ss ht iomwus i tahtat to tnhar sl oi sn ec sl ehaor luv l d
n obte
on lntensrty t-hongt' table'l
more accurate than juduements based
the case.