THE
JOURNAL
OF THE
ACOUSTICAL
SOCIETY
OF AMERICA
VOLUME
26, NUMBER
4
JULY,
1954
Some Further Experiments upon the Recognition of Speech,
with
One and with
Two Ears
E. COLIN CHERRY*AND W. K. TaYLoR•
Departmentof ElectricalEngineering,Imperial Collegeof Scienceand Technology,
London,England
(ReceivedJanuary 21, 1954)
The experimentsdescribedhere continuea seriesreportedin this Journalearlier. A numberof objective
testsare described,directedtowardsthe generalproblemconcerningthe faculty we possess
of perceiving
only oneacousticworldundernormalconditions(in spiteof havingtwo ears)and yet, if our earsbe stimulated by differentsignals,artificially, we can attend to one or the other of them. The testshave been made
with continuousspeech(readingsfrom light fiction) and the resultsassessed
statistically.The first tests
aim at measuringthe reactiontime r requiredto "switch the attention" from oneear to the other, as assessed
by perceptionof the wordsof the message.
A secondset of testsshowthat we perceiveonly onespeaker,as
a "gestalt," when the earsare stimulatedby similarmessages
but with a delay betweenthem exceeding20
times that ever experiencedin real life by virtue of binaural directivity.
I. INTRODUCTION:
EARLIER
MENTAL
CORELATION
EXPEIMENTS
OF SIGNALS
EARS
ON
relation of such test conditionsto those experienced
in real life are hereregardedasof secondaryimportance.
AT THE
TWO
The reader is referredto this earlier paper for further
experimental conditions. The work described here
that we have two of themmand yet we hear only forms a continuationof these earlier tests, especially
one acousticworld; only one voiceper speaker.
of those tests in which di•erent messagesare fed to
In an earlier issueof this Journal, one of the authors the two ears (samespeaker,two tape recordings).The
has reportedsomepreliminaryobjectiveexperiments presentobject is to exploretwo problems'
(a) How to measurethe averagedelay time r in
uponthefacultywepossess
of (a) listening
and"attending to" messages
in either our R.H. (right-hand) recognizingcontinuousspeechmessagesintothe extent
or our L.H. (left-hand) earsat will, when the messages of being able to repeat the words.
theyreceive
arearranged
to bedifferent,or (b) listening (b) To observewhat factors in spoken messages
to a single integrated message,when the ears are control our ability to "attend to" one ear only (i.e.,
stimulated identically.x Roughly speaking, our ears how "close" can the two messagesbe made, and in
appear to be independentunder certain conditions, what aspects,beforea "gestalt" operates,making them
but not under others,and theseconditionsmay readily into one field of speech?).
Again, our earlier tests had illustrated the different
be controlled experimentally; in all cases,the final
act of recognitioncan only be performedupon one levels at which aural recognitioncan take place. For
example, under certain conditions a subject can
message
at a time.
In theseearlier experiments,and in thosereported recognizeonly the existenceof "human speech" as
here, the human "subject" is regardedonly as a opposedto other types of continuousnoise; in other
transducer;the tests are essentiallybehavioristic. conditions he recognizesthis speech as being, say,
Messages,
consisting
of tape recordings
of continuousEnglish or French, male or female, but nothing more.
Such might be called "statistical recognition."Under
readingsfrom literary texts, are applied through
different conditionshe can recognizewords, in conheadphones
to the subject'sears;the R,H. and L.H. nected chains, but may be unaware of the semantic
messages
are arrangedto differ in variousspecified contentof the messages
(and it is at this level that our
ways.The listeneris requiredto respond
by repeating experimentshave been directed). Finally, full recogniwhat he hears,in one ear or the other as demanded. tion involvesinterpretationand responseactions,such
Success"scores" have been based upon percentage
as the carrying out of instructionsor orders."Recogwords correct. Thus the tests are all of a statistical
nition" is then not one simple thing, but is a multicharacter. However, subjectiveaspectsconcerning layered phenomenon.
NEofthe
most
striking
facts
about
our
ears
is
what the subject"feelslike," his difficulties,or the
II.
* D. Sc.,A.M. I. E. E. Readerin Telecommunication,
Imperial
College,
Universityof London.Part of thisworkwasperformed
SWITCHING
OF A MESSAGE
TO ANOTHER
FROM
ONE
EAR
at the Research Laboratory of Electronics, Massachusetts Our earlier experimentshad suggestedthat a finite
Institute of Technology,Cambridge,Massachusetts,
and was delay elapsesfor the attention to "catch onto" one ear
supported
inpartbytheSignal
Corps,
theAirMaterielCommand,or the other, when stimulated differently--a kind of
$ B. Sc.,M. Sc.ImperialCollege,
University
of London.
Now mental time constant r. It was suggestedthat the
NuffieldFoundationScholar,AnatomyDepartment,University average
value of r might be assessed
by periodicswitchCollege,Universityof London.
ing of the messagefrom one ear to the other; then, at
• E. ColinCherry,J. Acoust.
Soc.Am.25,975(1953).
and the Officeof Naval Research,of the United States.
554
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RECOGNITION
OF
SPEECH
WITH
low rates of switching, the attention also must be
alternated between the ears, but lagging in phase. At
a certain rate, one might expect the acousticswitching
and the mental switchingto becomeout of phase, so
that no recognitiontakes place. This was found to be
the case, but with the significantcomplicationthat
the syllabicrate of speakingaffectsthe results.
In the first experimentsa continuousreading from
contemporarylight literaturewasrecordedon magnetic
tape; a singlespeakerwas used throughout,and his
words/minute were observed and recorded.• This
message
wasappliedto oneear or the otherof a subject,
through headphones,being switchedelectronicallyat
periodswhichcouldbe varied between10 seconds
and
0.!
millisecond.
Thus
each ear received
sound and
silencealterr/ately,with 50 percenton-offratio; the
switchingwas periodicand in no way dependentupon
the speechsyllabicperiods.The subjectwas instructed
to repeat the wordsas he heard them and a count was
• ii01
I
I
,.,I00.
0
80
:-
60
I
ß
I
I
•
• 40
•
ONE
AND
TWO
EARS
555
• 16
•12
j
0 60
I
I
I
I0
I00
I000
I
ß
• •o
•,,• 20-0.1
1.0
I0.000
FREQUENCY- GPS
Fro. 2. Subject; J.K. Date; March 11, 1953. Articulation score
for continuousspeech,switchedperiodicallyfrom one ear to the
other. For each ear the proportion of the period occupiedby
speechis 50 percent, the remainder being silent. Voltage across
telephones0.087-volt rms when speechis uninterrupted.
starting with a complete switching cycle of !0 sec.
At the end of 120 words, a new textual passagewas
taken, and a secondtest made at a faster switching
rate ... and so on, with different passages,until a
wide range of switching rates had been covered.
Figures!-4 showa plot of the resultsfor somedifferent
subjects;here, since the words per minute differed
slightly for each successiveexperimental point, this
rate is indicatedat the top of eachcurve.The average
rate, about !30-!40 words per minute, is a normal
rate of speakingfor a southernEnglishman.
All the curves taken under such conditions showed
2o
the sharpdipping towardsa very low word-identification score, at a switching (complete cycle) period
between 0.3 and 0.2 sec, approximately, depending
Fro. 1. Subject;J.K. Date; March 4, 1953.Articulation score uponthe subject.When a test wasset at sucha point,
for continuousspeech,switchedperiodicallyfrom one ear to the the subjectwould be observedto be making syllabic
other. For each ear the proportion of the period occupiedby "mutterings," with only an occassional
word correct.
speechis 50 percent,the remainderbeingsilent.Voltageacross
The
second
dip,
which
is
seen
to
occur
at
about
!500-cps
telephones
0.087-voltrmswhenspeechis uninterrupted.
switchingfrequencyis due to a purely physicalcause'
made of the percentagecorrect.õAll questionsas to the severe distortion of the speechsound spectra at
his methodof executingthis task, whetherhe "guesses"
or not, are here regardedas subjectiveand irrelevant
_ 150
to our presentobject; so alsowas the questionas to
o
o.I
,
I.O
IO
IOO
FREQUENCY - GPS
IOOO
, ,,i
IO•OOO
whether he understood the "sense" of the text. All
subjects
werewithinthe agegroup25-40 andof similar
educationalbackground (University). Test runs of
!20 consecutivewords were used for assessingthe
results, but the first 20 words were ignored, being
•110
I
I
I
1
/"
80
regardedonly as a meansof gettingthe subjectover
his initial difficulties and starting him on to steady
conditions of behavior.
With a given subject,a seriesof tests was made,
:1:
It was thought that prior familiarity with the text might
influencea subject'sbehavior. All subjectswere questionedon
this point, but it wasfoundto exertno noticeableeffecton his
0
•1
I I Ill
1.0
I0
I00
FREOUENCY-
I0•
I0,•
CPS
Fro. 3. Subject; E.M.T. Date; March 5, 1953. Articulation
scorefor continuousspeech,switchedperiodically from one ear
õ As a matter of interest,all subjectsthoughtit was easierto to the other.For each ear the proportionof the periodoccupiedby
storeup a few wordsor phrases
and repeatthemquickly,before speechis 50 percent, the remainderbeing silent. Voltage across
0.087-voltrms whenspeechis uninterrupted.
proceeding
to storeup the next,ratherthanspeakcontinuously. telephones
responses.
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556
E.
C.
CHERRY
AND
W.
K.
TAYLOR
listenswith bothearsto what now appearsto be speech
interrupted with a square wave. But such a test
corresponds
somewhatto Miller and Licklider's test on
140
interrupted
speech,
and it is not surprisingthat their
I
I
1201
I
I
I
results correspondcloselywith ours, but only so far
I00 .• ............................. e..
as the R.H. flanks of these curves, Figs. 1-4, are
80 _
"-. ß
ß ß
concerned.An additional set of tests was made, simu60
lating Miller and Licklider's, but using continuous
40
speech.This was applied to both ears simultaneously,
and switchedon and off periodically;again the subject
\
was instructedto repeat what he heard, while listening,
0 i I ,,:1 ,; : i,I I , ,•1 I ,•,1 : , :•1
0.1
1.0
I0
I00
I000
I0,000
and a count was made of the percentof wordscorrect.
FREQUENCY- CPS
Figure 5 showsa typical result; the R.H. flank of the
Fro. 4. Subject; A.C.L. Date; March 5, 1953. Articulation main dip correspondsclosely to those in Figs. 1-4,
scorefor continuousspeech,switchedperiodicallyfrom one ear
to the other. For each ear the proportion of the period occupied while the L.H. flank, of course,differs. At very low
by speechis 50 percent,the remainderbeingsilent.Voltageacross interruption rate (0.! cps) only half the' speechis in
telephones0.087-voltrms when speechis uninterrupted.
fact availableto the subject,and so his scorerisesto
50 percent only. A further set of tests was made,
this high interruption rate. Miller and Licklider2 have applying interruptedspeechto one ear, with backward
published some results similar to ours as shown in interrupted speechto the other ear, the two being out
Figs. 1-4. However,their testshad a differentpurpose, of phase in their switching;[[this simulatesmore
and were based upon lists of separate words (not
continuoustext) repeated at unknown intervals at
Ii1100
unspecified
syllabicspeeds;factorswhichare important
o
o 8o
to our presentpurpose.These authors' resultsshow a
o 60
seconddip similar to that seenin our Figs. 1-4, caused
by spectrumdistortionthroughthe switchingsquare- 40
wave modulation.ô
•- 20
We are primarily interestedin the first sharpdip, at
0
3-5-cps switchingrate. It is suggestedthat this dip
0,1
1.0
I0
I00
I000
i0,000
FREQUENCY- CPS
is a goodobjectivemeasureof the recognitiontime r
for an individual subject, with continuous speech.
Fro.6. Subject;
E.M.T.Date;April24,19•3.
Articulation
On the L.H. flank of this dip, the subjectis switching score for continuous speech, switched periodically to the right
his attention from oneear to another synchronouslybut ear whilst continuousbackward speechis switched periodically
to the left ear. For eachear the proportionof the period occupied
laggingin phasewith the switchingcycle.At the dip by sound is 50 percent, one ear being switched on whilst the
itself he is frustrated. On the R.H. flank however, he other is silent. Voltage acrosstelephones0.087-volt rms when
z
signalsare uninterrupted.
closely the conditions of our first tests, but again
(Fig. 6) showedresultssimilarto Fig. 5.
• II0
I
I
III.
IOO
EFFECT
OF SYLLABIC
UPON
RATE
OF SPEAKING
lIECOGNITION
•: 80
o
z 60
• •o
C- 20
o
o.I
I.O
10
ioo
Iooo
io,ooo
FREQUENCY- CPS
The facultiesof speechand hearingnormallydevelop
together within one individual. When we speak we
hear and may monitor the syllabicflow of sound.Is it
unreasonableto assumethat the rate of productionof
syllables,by any one speaker,is set not only by anatomical dynamics, but by this monitoring process
also, at least as regardshis normal maximum rate of
speaking?Perhaps,then, the syllabicrate of speaking
and the rate of recognition are interdependentto
Fro. 5. Subject; K.R.F. Date; March 7, 1953. Articulation
scorefor continuousspeech,switched periodically to both ears
simultaneously.The proportionof eachperiod occupiedby speech some extent.
is 50 percent,the remainderbeingsilent.Voltageacrosstelephones
Although not intended as a direct test of such an
0.087-volt rms when speechis uninterrupted.
•' G. Miller and j. C. R. Licklider, j. Acoust. Soc.Am. 22, 167
(1950).
ô This hasbeenconfirmedby oneof the authors(W.K.T.) by
Fourier spectracalculationswhich will not be reproducedhere.
hypothesis,the followingexperimentwas suggested
by
it; this is to repeatthe earliertestsbut usingrecordings
I[Backwardspeech
washereusedasunintelligible
noisehaving
approximatelyLthestatisticalcharacteristics
of normalspeech.
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RECOGNITION
OF
SPEECH
WITH
ONE
AND
TWO
EARS
557
made at various average syllabic rates of speaking.
With somepractice it was found that it is possibleto
read texts at average speedsof 85 and 36 wordsper
minute, in a tolerably uniform way (normal speed
130-140 words per minute). The marked effect upon
•40
o o138
WORDS/MINUTE
the principal dip in the word-scorecurve is shownby
C-- 20
ß $6 WORDS/MINUTE
two examplesof Figs. 7 and 8 (the 138-words/min
curve in Fig. 7 correspondsto the curve of Fig. 2;
(•.!
•.o
•o
•oo •ooo •o,ooo
FREQUENCY- CPS
that in Fig. 8 correspondsto the curve of Fig. 3).
At 36 words/minute the subject is able to repeat
Fro. 8. Subject; E.M.T. Articulation scores for continuous
substantiallythe whole of the text; the dip virtually speech,switchedperiodicallyfrom one ear to the other. For each
•f
vanishes.
At normal rates of speaking(here 138 words/minute
or, roughly,6 syllables/sec)the sharpdip (e.g., Fig. 7)
occurs at a complete switching cycle (both ears)
period of 3 per sec, that is, when each ear receives
about 1 syllableat a time. But at 36 words/minute
(roughly 1.6 syllables/sec),and at the sameswitching
rate, each ear receivesat least 3 or 4 distinct samples
of each syllable, on an average. Such samples are
presumablymemorizedand associated,
therebyassisting
the faculty of syllable and word recognition.Notice
howeverthat the switchingcyclefrequencyof the dips
x85
WORDS/MIN
eartheproportion
of theperiodoccupied
by speech
is 50 percent,
the remainderbeingsilent.Resultsfor threeratesof speaking
word, but rather proceedsat many levels. Thus our
method of measuringr and our interpretationof this
quantity are essentiallystatisticalin character;they
are averagesfor the whole complex speech-hearing
process.
On the assumptionof sucha simplehypothesis,that
the time interval
r has the nature of a dead-time
constantduringwhichnothingof the signalis perceived
at eitherear,because
the "attentionis beingtransferred
from oneear to the other," a simplecalculationmay
be made.Thus r is a dead-timeinterval;at any switching frequency
f, eachear receivesthe signalfor periods
1/2fat a time;hence
theremainder
ofthetime(1/2f- r)
corresponds
to the time during which the signal is
_•6o
perceivedat each ear. The fraction of each cycle of
•
/
o I,.'38
WORDS/MINUTE
switching,duringwhichperceptiontakesplaceis then
2f(1/2f--r) or 100(1-- 2fr) percent.As we seefromthe
curvesFigs. 1-4, the sharp dip occurswhen f•3
o•.•
•.0
•0
•o0
•000
•0,000
cycles/sec,
when little perceptionoccurs.Hence we
FREQUENCY- CPS
assume,from the above formula, that (1- 2fr)-- 0
Fro. 7. Subject; J.K. Articulation scoresfor continuousspeech, givingr= • sec.
switchedperiodicallyfrom one ear to the other. For each ear
The curve100(1--2f/6) is plottedas a dottedline on
the proportionof the periodoccupiedby speechis 50 percent,
the remainder being silent. Results for three rates of speaking Fig. 4; it may be seento correspond
quite closelyto
are given.
the experimentalcurve (and those others, such as
Figs. 1-3) on theL.tt. flank. Sucha theoreticalcurve,
remainsnearlyconstant(Figs. 7 and 8) as the syllabic of course,takesno accountof the syllabicstructureof
rates of speakingare varied.
speech;it may be regardedasa limitingcurve,towards
which
the otherstend asymptoticallyas the syllabic
IV. RELATION BETWEEN "RECOGNITION TIME" •,
0
/
-
SWITCHING
ß woRs,.UT
RATE BETWEEN
PERCENT
ARTICULATION
THE EARS, AND
SCORE
We have earlier made the observation that the shard
periodsgo to zero.
As we have already remarked, the R.H. flanks of
such curves correspondto a different mental process
on behalf of the subjects.Here, the switchingrates
dip in the percent-articulation
scorecurvecorrespondshave become so fast that the attention cannot "transfer
to an average"word recognitiondelaytime," calledr, betweenthe ears"in synchronism;
the subjectrather
wherethis idea is given a rather naive interpretation. listensto one ear, but the syllabic rates are such that
It is beingassumedthat the time r we are measuring he is able to obtain a number of samplesof each
(about-} second,
asjudgedfrom the dip in the curves)
syllable, thereby ensuring an increasedperception
has the generalcharacterof a dead-timeconstant,
(and articulation) score. The dead-time r is then
as thoughit takesa listenera time r to "switchhis
assumed
to be ineffectiveover the R.H. flanks, and a
attention" from one ear to another. Of course,speech
different
hypothesis is required to account for this
itself does not consist of a chain of statistically inpart
of
the
perception(articulation) scorecurve. It
dependentsyllables,or words;redundancyarisesat
will be assumed that this score increases as the number
manylevels(syllableandwordtransitionprobabilities,
phrasestructure,syntax,etc.) Recognition
too, cannot of samples of each syllable, reaching the one ear,
be said to proceedsyllableby syllable,or word by increases.On an average,let d be the syllabicduration.
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558
E.
C.
CHERRY
AND
• 60
K.
TAYLOR
presentto the "rejected" ear, during each switching
half-cycle,not completesilencebut a low-levelsignal
havinga fractionk of the powerof that reachingthe
opposite ear. It might be of interest to see how r
changeswith k; that is, to seehow our reaction times
in switchingattentionfromoneear to the otherdepend
upon the extent to which the attention is "primed"
(3
TIME
00NSTANT--O.025
mSEC
e:: 20 -
o SWITCHIN(3 TIME CONSTANT=5.0
roSE(;
_
I
•.1
W.
I ,•1
1.0
I
I III
I
I0
I III
I
I00
I III
I000
•
I III
I0,000
in advance.
FREQUENCY- GPS
Fro. 9. Subject; E.M.T. Date; March 12, 1953. Articulation
scorefor continuousspeech,switched periodically from one ear
to the other. Voltage acrosstelephones0.087-volt rms when
speechis uninterrupted. Average rate of speakingis 146 words
per minute for both curves.
VI.
ON THE
FACULTY
OR
OF LISTENING
WITH
WITH
ONE EAR
BOTH
As stated earlier, our experimentsdescribedhere
formpart of a series,somehavingbeenreportedearlier,•
all directedtowardsthat generalproblem:how is it
Then at switchingrate f, this number of samplesisfd. we are able to listen with one ear (when the two are
From previous considerations,we have seen that the stimulatedwith different signals)or with both (when
perceptionscoreapproacheszero percent whenfd--•-l. stimulated similarly), at will? How closedo the two
In this case, the simplest function representingthe stimuli need to be beforewe correlatethem mentally
perceptionscorewill be 100(1-1/fd). This has also into a "gestalt"?In our earlierpaper,someexperiments
been plotted in Fig. 4, as a dotted line (taking were describedin which two unrelatedmessages
were
1/d=5.98), and resemblesthe R.H. flank of the fed to the two earsof a subject,who was requiredto
experimentalcurve,in an approximatemanner,but of
coursetakes no accountof the subsidiarydip due to
the speechspectrum interferenceby the high-speed
square-wavemodulationeffectof switching.
V. EFFECT
OF LESS-ABRUPT
REVERSALS
BETWEEN
THE EARS
It was thought that the abrupt switchingof speech
signalsfrom one ear to anothermight produceserious
noise, by virtue of the square-wavemodulation it
producesupon the signal reachingeach ear. Simple
Fourier calculationsshowedhowever,that the energy
spectra of the speechsignalsare not appreciably
affectedat switchingrates up to and well beyondthe
critical region of 1-10 cycles/sec.A further experimental checkwas devised.This consistedof fading out
the signalat one ear while fadingin that at the other
ear; exponentialmodulationenvelopeswereproduced,
suchthat the totalspeechenergy (reachingboth ears)
remained constant. Now there is always somesignal
reachingboth ears, but the ratio of the two signals
---0•
I00
t 0 -0-f
_
20
1e.
-o.4
-o.3
-o.2
-o.•
o
o.i
o.•,
c•3
o.,•
TIME OISPLAGEMENTOF RIGHTEAR MESSAGE,RELATIVE
TO LEFT EAR ME$$AGE,-otSEO
Fro. 10(a). Binauralperceptionof speech
direction(right or left).
repeat one, concurrentlywith hearingit; then we have
shownthat he is unawareof any singleword of the
messagereachingthe "rejected" ear--only statistical
properties of it are perceived. Secondly, the two
swingsup anddownexponentially,
ratherthanabruptly messages
were made the same,but with a long time
as before. The results with one subject, using two delay between them; this time delay was steadily
different exponential time constants, are shown in reduceduntil the subjectwas aware of the similarities
Fig. 9; comparingthis curvewith that of Fig. 3 (same of the messages,at a delay between 6--•2 seconds,
subject)showsa similarity,thoughthe dip appearsto roughly.It then appearsthat there is a memoryof this
haverisenslightlyalongthe switching-frequency
axisJ duration which is operative under such conditions;
(this frequencyfor this particular subjectwas rather that is, a word or phrase is memorizedat one ear and
higherthan average).The exponentialtime constants identified later at the other.
Continuing such tests, these earlier results were
of this switchingwere madevery short (<5 msec)so
that only the initial part of eachhalf-cyclewasaffected; confirmedby further trials and, subsequently,the
then the "unstimulated" ear, during each half-cycle behavior of a subject was observedwhile the delay
of switching, received energy very far below the betweenthe identical messages
reachinghis two ears
thresholdof hearing, exceptperhapsduring the first was steadilyreducedto zero. We wish to report one
few milliseconds.The slight increasein r may not be result of particular interest.
very significant,but it suggests
a further experiment, Continuouslyspoken, tape-recordedreadingsfrom
which has not yet been carriedout. This would be to light fiction were appliedto the two earsof a subject.
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RECOGNITION
OF
SPEECH
The only differencebetweenthe two signalswas a
time delay T, which couldbe varied from zero up to
50 msec.Naturally, suchconditionssetup some,though
not all, of the subjectiveeffectsof binauraldirectivity.a
The subjectwasinstructedto saywhetherthe "speaker"
appearedto be to his right- or left-hand side (i.e., a
logicaldecisionRH/LH). The subjectiveeffectof the
speaker'sappearingto move to the right-or left-hand
of the listeningsubject can be producedeither by
changingthe delay betweenthe aural signals,or by
changingtheir relative amplitudes.Over a wide range
the effectsare indistinguishable.But in our tests the
aural signalswere set to have equal powersand only
the delaywasvaried. The delay T waschangedsuccessively to random values and the results recordedas a
WITH
ONE
1400
-
1200
-
AND
TWO
EARS
559
-
•ooc -
800_
600-
400_
200-
o'• • f,,•:,
-OJ
I.O
, ,,-'• , •,il
I0
I00
FREQUENCY-
, • t,I ....
I000
•
I0,000
CPS
Fro. 11. Impedanceof onetelephoneearpiece
percent of correct guesseswith delays as high as
15 msec.
probability distribution. Figure 10(a) shows some
The surprising thing about such distributions is
typical results. Figure 10(b) showsthe same curve their widths. In normal,everyday binaural hearing, the
plotted on a much-compressedscale, showing the greatest delay we can experiencebetween our ears
correspondsto occasionswhen the sound source is
SIGNALS PERCEIVED
to our extreme right or left hand. In such casesthe
.
,/- ASONE
ACOUSTIC
delayis no greaterthan «-1 msec;how then have we
,oo
I
/ FIELD
(SAME
CURVE
learned to correlate the stimuli of our two ears, when
the delay is as great as 15 msecso as to perceivea
"gestalt"?If the delay T was increasedbeyond this
there would comea value suchthat the subjectwould
be unable to give a logical decisionR.H. or L.H.; he
would protest that he heard not one personspeaking,
but two. The delay is quite critical, with any one
subject;at onemomenthe hearsa speakerapparently
standing to one side of him, the next moment the
soundimage "tears apart" into two.** Figure 10(b)
: t••'•
• •'
F'G"ø(ø)
' ,/
SlGNALSPERCEIVED-x
illustrates
O0
5
I0
•5
80
25
30
TIMEDELAYOF RIGHTEARMESSAGE
RELATIVETOLEFT
Fig. 11 showsa plot of their impedance/frequency
EAR MESSAGE-mSEC
characteristics.The acousticphasingof the headphone
pair was found to make no observabledifferenceto
any of the results.
Fxo. 10(b). Binaural perceptionof one or of two acousticfields.
Positive delays are shown here; curves for negative delays are
quite similar.
a S.S.
this barrier.
On all the various diagramsan indication is given
of the rms signal voltage across the headphones;
Stevens and H. Davis, HearingsIts Psychologyand
Physiology
(JohnWiley and Sons,Inc., New York, 1938).
** Stevensand Davis report similar measurements,by themselvesand others, but basedon continuoustones or on "clicks";
such elementalsignalsdo not apparently show this remarkable
binaural coherencewhich is the casefor speech.
Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 192.87.79.51 On: Tue, 14 Jul 2015 12:18:33