that each partial in the original instantaneous
speech spectrum is replaced by a steep-skirted
band of noise, 1200-Hz wide, centered on the
partial. In the pulse-modulated procedure, all
that is retained of the original waveform is the
time and polarity of axis crossings; infinitely
peak-clipped signals look to have only that same
information (Licklider and Pollack, 1948), but
they are generally much more intelligible (Ains-
worth, 1967), even when experienced listeners
adjust the levels of both types for maximum in-
telligibility. Clipped speech sounds harsh;
pulsed speech sounds harsher. Noise-modulated
speech sounds very noisy, but is generally re-
ported to be much clearer than one would expect
with “that much noise” present.

The masking level and the modulator band-
width were selected to produce approximately
the same maximum intelligibility score (80% cor-
rect) for highly trained listeners as unmasked
pulsed speech does. Clipped speech is a bit “easier
to understand, and maxima near 90% are common.

B. Subjects. Each segment of these studies
used six university students as listeners. All sub-
jects had normal hearing, and none had any
previous experience with this kind of task. A
total of 18 series of experiments used 78 subjects.
Everyone was trained in shadowing before being
exposed to the distorted or masked signals. Most
subjects were then simply instructed to shadow
whatever they could hear. Several groups,
though, received special treatment: some shad-
owed for a total of only eight minutes in a 54-
passage session; another few shadowed every-
thing, but were informed that they would be
given a monetary incentive to do well.

III. Results and Discussion.

A. Speech Learning. The basic outcome of all
these experiments is perfecly predictable: intelli-
gibility starts at a low level and improves with
listening practice up to a plateau value. Figure
2 shows a learning curve for each of the four
kinds of signal. The rates of change are fairly
similar from one condition to another, although
the plateau values vary somewhat. The imme-
diately apparent point to note about all of these
data is that learning seems to be complete within
15 or 20 minutes. The auditory system makes
its analysis of the signal-plus-noise, determines
how to extract the maximum information, and

makes whatever modifications are necessary in
order to perform the extraction—and it does all
that in less than half an hour. The listeners are
probably not especially conscious of what they
are doing in order to get this analytical proc-
essing under way; most of them report no special
effort to get better, and generally they have little
recollection of how well they performed.

Although the curves are similar in shape, it is
inappropriate to try to draw inferences and con-
clusions about the speech-learning mechanism
from that fact. Learning curves simply look
alike. That does not necessarily demonstrate any-
thing about similarities or differences in the anal-
ysis of modulated and pulsed-speech signals.

The curves that represent what happens in this
learning mechanism do have one particularly
fascinating aspect, though (Figure 3). They
show that subjects returning after one or two
weeks away from the task start the first couple
of passages with scores slightly lower than their
previous maxima, but then, almost immediately,
they rise to a higher plateau than the one they
attained during their first test session. The
change from the first to the second plateau is
statistically significant at better than the .05 level ;
final scores on session two are 12 to 15 percentage
units above those on session one, making a total
improvement that is equivalent to about an 8-dB
shift in signal-to-noise ratio. During their time
away from the laboratory, the subjects had no op-
portunity to listen to the kinds of distortions that
were used, so it is unlikely that any conscious
rehearsal could have influenced the results. The
phenomenon is similar to the psychological entity
called reminiscence (Buxton, 1943), in which
some previously measured ability improves fol-
lowing an interval of time during which the sub-
ject is not permitted to practice. But even in
that context, a question remains: why didn’t sub-
jects continue to improve throughout their first
hour’s practice? We may speculate that the im-
provement does indeed continue, but at a much
slower rate—too slow to measure with this method.
However, we do not know how to determine if
the curves are biphasic—if they have two sep-
arate slopes.

Whatever the solution to that question, though,
one thing is clear. The brain, once it has or-
ganized itself to determine the transformations
necessary for analyzing difficult speech messages,



formation of useful hypotheses about how to
listen.

F. Retention. Most subjects were retested in
a third session one month following the second
(six weeks after the first). Third-session curves
and scores are similar to second-session curves
and scores. One group was retested after a year
had passed with no known intervening practice.
Their latest perfomances are similar to their
second sessions, too.

G. Non-Normal Hearing. Normal-hearing sub-
jects learn to understand badly mangled speech
after a short period of practice. There is no evi-
dence that people with pathological hearing do as
well, but it is certainly possible—even reasonable
—-to conclude that they do.

The plateaus of subjects in their second ses-
sions, no matter what the conditions of their
training (except those who are able to receive
only small amounts of information during the
first session), and no matter what kinds of signals
they were trained with, all are improved, and all
look similar. They crowd together. That fact
may partially explain why hearing-aid users are
reported to do much better at speech discrimina-
tion after they have used an aid for a week than
they did when the instrument was first tried.
The early listening presents them with a kind of
sound that is somehow different to them; they
have to learn about it before they can get maxi-
mum sense from it. If this learning process
works with some kinds of pathological ears as
well as it does with normals, we might also ex-
pect to find that, for those ears, experience with
any hearing aid will transfer to any other.

The hard-of-hearing person may have a greater
problem learning to understand distorted speech
than the normal-hearing listener, though, for the
very reason that he cannot hear enough of the
signal to work out an appropriate analysis strat-
egy. He could be like the low-plateau subjects
in the masked-speech experiments. However,
usually, the overall sound pressure level of his
work environment will be high enough to over-
come much of the problem caused by an elevated
threshold, so he can learn as well as iis colleagues.
This likelihood leads to the interesting possibility
that the results of some audiometric tests of the
ability to understand speech that is immersed in
noise may be more a function of learning than
of -hearing.

H. Training. Even two minutes of listening
can improve the ability to understand a talker
(Peters, 1955). Six to eight hours may be needed
to teach people to understand speech sounds that
are transformed by a spectral inversion (Beadle,
1970), and even then, it takes longer to learn
from an unfamiliar talker. But for optimum
training for the reception of non-inverted speech,
about an hour is needed.

How should the time be spent? If you want
to improve your reception of distorted speech, it
is not enough to listen to the right kind of in-
terfering noise. It probably will not help to be
exposed to non-speech sounds that are subjected
to the same sorts of distortions that affect the
speech; the analyzing activities of the brain are
quite different for speech and for non-speech
signals (Stevens and House, 1972). Student
pilots take far longer than half an hour of flight
time to learn to understand air traffic control
communications; factory workers do not begin to
understand what is said to them in noise until
days of listening have passed, not minutes. Both
groups commonly hear speech-plus-noise for only
short moments at « time, and then return to lis-
tening to noise alone. The requirement for rapid
learning, though, is that the listener be able tc
hear the combination of signal and noise at 2
signal-to-noise ratio that is high enough to permit
him some success in interpreting the message:
that are being transmitted. If his motivation tc
learn is high (or heightened), he can reach his
maximum capacity in an hour. He will probably
learn still faster if his training is done by ¢
talker whose voice is familiar to him.* Anc
when he has found the knack of how to listen
he will keep it for a long, long time.

Once, after a static-filled, phase-distorted
narrow-band, whistling short-wave radio broad
east of a concert, Sibelius is reputed to hav.

* Schubert and Parker (1955) reported in a paper 0
a speech intelligibility study, “A puzzling phenomeno
occurs with three of the subjects, who were wives ¢
the talkers, In euch case the wife exhibits only a ver
slight dip in intelligibility, if any, . . . when her ow
husband is the talker, but shows about the averag
dip for either of the other two speakers. This ol
viously falls beyond what has previously been considere
the boundary of auditory theory and the authors, wh
were two of the talkers, are relieved of the risk of di
cussing it further.” So are ‘Tobias and Irons.


100 var Sesh nd 100 - -
90F st Session nd Session 90F Ist Session 2nd Session
Bor sob
TOF MASKED TOF PULSED
— 60F 60F
sor sob
40F 40
3 30 30
ao 20 20-
8 1OF 1or
Pr [an ee ee jo ee oe ee {4
0 10 20 30 40 50 0 10 20 30 0 10 20 30 40 50 0 10 20 30
>
La
2 100 100
@ 90f Ist Session 2nd Session sob ae 2nd Session
© sof MODULATED sor
am) L
4 70 zob
w 60F 60
z 50 50F
= 40 40
30F 30F
20r 20r
1OF 1OF
1 L L 1 1 L 1 1 1 1 1 1 i 1
© 10 20 30 40 50 0 10 20 30 Oo 10 20 30 40 50 0 10 20 30
TIME IN MINUTES
Figure 3, Mean learning curves for second experimental session, Note the improvement following a week or more
without practice.

Although pulsed speech lacks most of the spec-
tral information of the original waveform, and
modulated speech lacks most of the temporal in-
formation, the transfer of ability from one to
the other seems complete. The suggestion is
strong that human observers, once they have
learned how to listen to difficult speech, can suc-
cessfully understand almost any form of it (for
another example, see Beadle, 1970). Perhaps
that idea helps to account for the fact that some
people are able to understand English spoken
with many kinds of dialects, but that others can-
not get much intelligibility out of what is said
to them by talkers with just moderately variant
speech.

A practical result of this finding is that a
person probably does not need to be trained to
listen to distorted or masked signals that are
of the precise form that will occur in his work.
Once he has mastered some types of speech learn-
ing, he will be able to assimilate others rapidly.

100r 2nd Session

| MODULATED pyisep

70r r

Ist Session
sob

80 MODULATED
6or E
50 t
40 E
3ob t
20b r

to

INTELLIGIBILITY SCORES (%)

a oe ee poi
10 20 30 40 50 0 10 20 30 40 50
TIME IN MINUTES

°

Ficure 4. Mean learning curves for subjects who re-
ceived all of their listening experience on modulated
speech. When tested on pulsed speech, their scores
were almost immediately at the maximum expected
for subjects experienced with pulsed-speech listening.



100

90 Ist Session

80Fr
TOF

MASKED

100
90Fr
80Fr
TOF
60Fr
SOF
404,

Ist Session

PULSED

20

Oo 10 20 30 40 50

Oo 10 20 30 40 50

INTELLIGIBILITY SCORES (%)

oo L Ist Session M0 L Ist Session
80Fr MODULATED 80F chipere
TOF TOF
60F 60
5OF, 50F
40 40F
30F 30F
20-F 20
lOF 1OF

l l i 1 1 1 n ! ! i

0 10 20 30 40 50
TIME

Oo 10 20 30 40 50

IN MINUTES

Ficvre 2, Mean learning curves for each type of signal.

continues to refine the analyzing process. These
changes and refinements are the kinds that allow
the listener to generalize or transfer the tech-
niques of decoding one sort of distortion tothe
interpretation of other sorts.

B. Transfer. In tests of the transfer of speech
learning, subjects were trained during the entire
first session with material that had been treated
with one variety of distortion. For the first half
hour of the second session, two weeks later, they
continued with that same distortion; by the end
of the 30 minutes, it was certain that they had

reached their new, higher asymptotic intelli.
gibility score. Then, for the last half hour of
that session, they were given material that had
been subjected to a different distortion. For
example, subjects who spent all of their hour and
a half of practice time on modulated speech were
tested on pulsed speech (Figure 4). Within three
to five minutes, they reached plateaus that were
at least as high as those reached by similar sub-
jects who had listened to nothing but pulsed
speech during both sessions. Transfer is equally
good in the opposite direction,



100F Ist Session

INTELLIGIBILITY SCORES (%)

40 / —— PULSED
30 E/ — — MASKED
20H]
1OF
4 1 1 L 1
° 10 20 30 40 50

TIME IN MINUTES

Figure 6, Mean learning curves for subjects who re-
ceived monetary incentives.

The results (Figure 6) are similar to those for
the passive listeners: curves continue to rise for
a longer period of time during the first session,
and, within the first hour, they reach values that
are comparable to second-session plateaus. This
relation between passive-listening results and
motivated-listening results certainly suggests that
the passive subject continues to improve because
he is more interested in the task than the active
subject is. He is able to relax a bit, and he can
actually attend to what the talker is saying (re-
member that his retention is better than the active
subjects).

Second-session scores for these subjects are in-
distinguishable from those of any other subjects.
Changes in ultimate peak scores, if they occur
as a result of monetary reward, are not ‘large
enough for us to measure with these techniques.

B. Masked Speech. Experiments with masked
speech at a —3 dB signal-to-noise ratio show one
kind of quantitative difference from the other
experiments: first-session subjects reach two quite
different kinds of asymptotes, apparently as a
function of their earliest scores. Listeners who
do well in the first few minutes are like most
listeners; they improve rapidly to plateaus of
80% or so. But those who start with intelli-
gibility scores of approximately 10% reach peaks
in the neighborhood of only 50 or 55% (Figure
7). The intention had been to set 2 signal-to-
noise ratio that would give a first-minute intelli-

gibility score of 20 or 30%, but the selection was
not right for these subjects; their initial scores
actually ranged from 5 to 29%. The low-plateau
subjects show greater variability than might be
accounted for by the relatively unrestricted ‘range
in which they were working. Their learning
curves rise comparatively slowly, sometimes tak-
ing 35 or 40 minutes to get the asymptotic value.
The curves are unlike any others we have seen.

An explanation may lie in an evaluation of the
learning experience that each group receives: the
people who start off well get exposed to large
numbers of correctly heard words; those who
start poorly receive relatively little information
that helps them in organizing an attack upon
the problems of learning to understand difficult
speech. Indeed, they hear very little that they
recognize as being any kind of speech.

We cannot be certain that the apparent separa-
tion of subjects into exactly two groups is correct,
although it looks to be. And we have no testable
explanation for why the original differences in
intelligibility exist. But it seems clear that some
listeners do start off with higher scores. Are they
the sort who have learned to listen to dialects,
perhaps? Whatever the initial reason, after a
listener has some success in pulling intelligence
out of noise, he can magnify that success into
higher scores. The listener who starts low may
spend most of his training time struggling to hear
anything at all. His decoder never gets enough
samples of the difficult sounds to permit the

100 Ist Session

90F A
gor c
70
6oF

50

amo

4oF
30F,
20

10 Fy

INTELLIGIBILITY SCORES (%)

1 L 1 1 ni
° 10 20 30 40 50

TIME IN MINUTES

Individual learning curves for subjects who
listened to masked speech.

Fiecre 7.



TECHNICAL REPORT STANDARD TITLE PAGE

T. Report No. 7%. Government Accession No. 3. Recipient's Catalog No.
FAA-AM-73-13
% Title and Subtitle 5. Report Date

RECEPTION OF DISTORTED SPEECH December _1973

6. Performing Organization Code

7 Author(s) 6.

}. Performing Organization Report No.
Jerry V. Tobias, Ph.D.
F. Michael Irons, M.Ed.

. Performing Organization Name and Address 10. Work Unit No.

FAA Civil Aeromedical Institute

P. 0. Box 25082 TI. Contract or Grant No.
Oklahoma City, Oklahoma 73125

13. Type of Report and Period Covered

. Sponsoring Ageney Name and Adres

Office of Aviation Medicine OAM Report
Federal Aviation Administration
800 Independence Avenue, S.W. 14. Sponsoring Agency Code

Washington, D. C. 20591

s

Supplementary Notes

This research was conducted under Tasks No, AM-A-71-PSY-16, AM-A-72-PSY-16, and
AM-B-73-PSY-16,

2

Abstract

Noise, either in the form of masking or in the form of distortion products,
interferes with speech intelligibility. When the signal-to-noise ratio is

bad enough, articulation can drop to unacceptably--even dangerously--low levels.
However, listeners are capable of learning to listen to such speech and to
improve their comprehension of it. In the experiments described here, the
nature of this learning and the necessary amounts of time for maximum improve-
ment are explored. The effects of several types of signal degradation are
discussed, as are suggestions for training listeners to understand them.

Among the processes investigated are the transfer of listening experience with
one kind of signal to the analysis of another kind, the effects of actively
participating in the listening task, and the results of manipulating motivation.
Some inferences are drawn regarding speech learning by listeners with non-normal
ears.

7. Key Words 18, Distribution Statement
Learning Availability is unlimited. Document may
Noise be released to the National Technical
Speech Information Service, Springfield, Virginia
Speech Reception 22151, for sale to the public.

5. Security Classif. (of this report) 20. Security Classif. (of this page) Zi. No. of Pages | 22. Price
Unclassified Unclassified 9 $3.00

vm DOT F 1700.7 (8-69)





yointed at the radio receiver and said, “I can’t
derstand how anyone but a musician could
mjoy listening to that thing.” In the same way,

one who is not trained to listen to speech will not
enjoy it. Indeed, in many circumstances, he may
not even be able to hear it.

REFERENCES

1, Ainsworth, W. A.: Relative Intelligibility of Different
"Transforms of Clipped Speech, JOURNAL OF THE
ACOUSTICAL SOCIETY OF AMERICA, 41:1272-
1276, 1967.

2. Beadle, K, R.: The Effects of Spectral Inversion on

the Perception of Place of Articulation. Doctoral

Dissertation, Stanford University, 1970.

Buxton, G. E.: The Status of Research in Reminis-

cence, PSYCHOLOGICAL BULLETIN, 40:313-340,

1943,

4, Cherry, E. C.: Some Experiments on the Recognition
of Speech, With One and With Two Ears, JOURNAL
OF THE ACOUSTICAL SOCIETY OF AMERICA,
25 975-979, 1953,

5. Kryter, K.D.: Effects of Ear Protective Devices on
the Intelligibility of Speech in Noise, JOURNAL OF
THE ACOUSTICAL SOCIETY OF AMERICA,
18 :413-417, 1946.

6. Licklider, J. C. R., and I. Pollack: Effects of Dif-
ferentiation, Integration, and Infinite Peak Clipping

i

Upon the Intelligibility of Speech, JOURNAL OF
‘THE ACOUSTICAL SOCIETY OF AMERICA,
20:42-51, 1948,

Peters, R. W.: The Effect of Length of Exposure to
Speaker’s Voice Upon Listener Reception. Joint
Project Report No. 44, U. S. Naval School of Avia-
tion Medicine, 1955.

Pierce, L., and H. R. Silbiger: Use of Shadowing in
Speech Quality Evaluation, JOURNAL OF THE
ACOUSTICAL SOCIETY OF AMERICA, 51:121,
1972.

9. Schubert, E. D., and C. D. Parker: Addition to
Cherry’s Findings on Switching Speech Between
the Two Ears, JOURNAL OF THE ACOUSTICAL
SOCIETY OF AMERICA, 27 :792-794, 1955.

10, Stevens, K, N., and A. S. House: Speech Perception.
In J. V. Tobias (Ed.), Foundations of Modern Audi-
tory Theory. Volume II, New York, Academic Press,
1972.

a

e



Should we decide to make a set of recordings to _ are consistently higher than the active group’s
train aviators to listen to radio transmissions, (second-session curves are alike). Second, active
those records need not contain precisely the same — shadowers have almost no retention of the ma-
kinds of signal degeneration that actually arise terial that they listen to, but passive listeners
in the cockpit; anything similar—or maybe even remember a great deal of what they hear. One
dissimilar—ought to do as well. interpretation of this finding is that our active
C. Passive Listening. Must such training in- __ listeners are giving us scores that do not represent,
volve continuous speech-related activity on the improvements in shadowing technique, but rather
part of the listener? To find out, we ran an real improvements in the ability to understand
experiment on the learning that accrues to sub- difficult speech signals. The higher passive scores
jects who hear the distorted signals, but who do might be interpreted to mean that shadowing
not have to shadow them (Figure 5). If the somehow interferes with speech learning, and we
shadowing activity is contributing to the learning, cannot refute that possibility. However, it is
then levels of performance ought to be higher possible too that the listener who can understand
for those who are thus employed during their and retain what he listens to is better motivated
listening. Twelve subjects were used to test this to learn. That possibility can be tested.
idea. Before their first exposure to the distorted D. Motivation. Two groups of subjects were
speech, they were trained in shadowing, just as tested—one group on masked and one on pulsed
all the subjects had been, using clear speech. speech—but, unlike previous listeners, these were
Then they were asked to shadow the first pair, given monetary incentives to do well. After a
the last pair, and two equally spaced interme- subject had worked through the first three pas-
diate pairs of distorted passages. For the rest sages, the three scores were averaged, and he was
of the time—46 passages—they were silent. told that for each 1% by which the 54th passage
When these passive listeners’ responses were com- _was better than that beginning average, he would
pared with the responses of active, continuously be paid a bonus of $.05. Also, in order to keer
shadowing subjects, two differences were apparent him working at a high level during the entire
from the data and from the subjects’ reports. test session, he got an additional $.10 for each
First, the passive group’s final first-session scores passage on which the score was above 90%.

~ 100 Ist Session 2nd Session
& sof
oO 80-r a—e_—- oo
Ww A
a
oO 7OF
S Y
 6O0Fr
> SOF
Ee PASSIVE PARTICIPATION — — GROUP A ACTIVE
2 40+, GROUP A AND B ——— GROUP B PASSIVE
a
© 30}
4
a 20-7
rt)
Ee L
z 10
n 1 n n 1 L n 1 1
oO 10 20 30 40 50 Lo) 10 20 30 4

TIME IN MINUTES

Fiaure 5. Mean learning curves for passively listening subjects. In the second session, half were asked to shadov

6


signals seems not to help. But practice listening
to the combination of speech and its intelligibility-
destroying noise leads to rapid improvement. The
available data cover studies of both masked and
distorted speech; the results from experiments
with one kind of signal are similar to the results
from experiments with the other.

Subjects were all taught to shadow (Cherry,
1953) while listening to recorded speech. In
shadowing, the listener immediately repeats every
word he hears, even as he is hearing new ma-
terial. Although the idea may sound difficult,
subjects are quite adept at learning it, and in-
telligibility-test scores measured by shadowing
are similar to scores earned in other kinds of
tests. Indeed, if anything, shadowing is a par-
ticularly sensitive measuring tool (Pierce and
Silbiger, 1972). Most subjects reach 95-100%
intelligibility scores on clear, continuous speech
within a few minutes. Our subjects were trained
in shadowing until they had scored higher than
95% in five successive one-minute intervals,

The speech used for the speech-learning experi-
ments was not the same as that used to teach
shadowing. The experimental speech is a series
of easy-to-understand 120-word passages, read by
a male talker who monitored himself during the
recording session in order to insure a constant
speaking level. Later, slight variations in level
were made from passage to passage in order to
insure that all would be equally intelligible in a
simple masking experiment. Each passage is ap-
proximately 50 seconds long, with a 10-second
pause between passages. A total of 54 such one-
minute segments was available, and the segments
were spliced together in many randomized orders.

For some subjects, the passages were masked
with a wide-band Gaussian noise; for others, the
speech was infinitely peak clipped ; for still others,
the signal became a pulse train whose spacing
was determined by the line-crossings of the speech
wave; and finally, in one series of tests, the
speech became a carrier that was amplitude-
modulated by a band of noise. Subjects selected
their own signal levels; for a 1000-Hz tone ad-
justed to the same peak level as the speech, the
sound-pressure level was 75-4 dB, which was
near the optimum choice according to preliminay;
tests of the relation between level and intelli-
gibility. Figure 1 illustrates the kinds of dis-
torted signals that were used. In the masked
condition, the speech wave is simply added to the

noise. In the modulated condition, though, a
multiplication transform is used, with the effect

SPEECH SIGNALS

AMPLITUDE

Clear-Undistorted

nN] |

K- AMPLITUDE >

Infinite peak-clipping

i

Pulse modulated

KEAMPLITUDE>| =} AMPLITUDE 3

Noise modulated

I TIME >|

Ficure 1. Waveforms of test signals used.


RECEPTION OF DISTORTED SPEECH

I. Introduction.

Noise has direct physiological, psychological,
and social consequences. It also has indirect con-
sequences that are associated with public health
and that certainly are not limited to damage to
the auditory physiology, to the psyche, or to the
community’s acceptance of loud sounds. Con-
sider the effect of a bit too much noise on an air-
line pilot’s reception of an air traffic control
message: the physical well-being of hundreds of
passengers and of unknown numbers of people
on the ground can be changed by the inaccurate
understanding of an instruction. A missed warn-
ing in a steel mill can produce frightening—even
deadly—effects on personnel; the physiological
results are not confined to the temporal bone.

We know ways to measure speech interference,
and we know something about the acoustic factors
that determine how well a listener will be able to
understand masked speech. However, there is
another kind of influence on speech intelligibility
that all of us have had experience with, but that
no one has measured before: people can learn to
manipulate signal-to-noise ratios mentally as well
as acoustically. It only requires that the lis-
tener’s brain be adequately exposed to the masked
signal. This exposure allows the signal-selection
mechanisms to search out the best methods for
processing the speech-plus-noise, and, after a time,
produces greatly improved intelligibility. One
of the questions that has not been answered before
is how much time it takes to learn that new an-
alyzing process.

Here is a practical illustration of what the
phenomenon is. People often whistle or hum or
sing while they work. Many nod their heads or
tap their fingers to keep time with their music.
In offices, you can sometimes see three or four
people, each tapping out a different rhythm, ob-
livious of the tempo being strummed on the next
desk. Sometimes, though, in a noisy work en-
vironment, a bizarre variation of this behavior
appears: a group of employees who could not
possibly hear each other’s humming because of

nearby loud machinery all move in time to the
same invisible drummer. The first time we saw
such a thing, we asked one of the workers what
he was waving at. He said, “It’s the music,”
and we pretended to understand. Of course, when
we went into a storeroom a little distance from
the machinery, there was music. Whether for
morale or for entertainment or for setting a work-
ing pace, the company pumped recorded music
into the factory. The workers heard it even
though a visitor could not make it out above the
din of the equipment.

Similar stories can be picked up from anyone
who measures noise. All the anecdotes lead to
the same conclusion: the ability to hear masked
signals that are inaudible or unintelligible to the
untrained or inexperienced observer can be im-
proved by listening practice. The anecdotal evi-
dence has been overwhelming. The laboratory
evidence has been non-existent.

II. Methodology.

A. Signals. Masking and distortion are sim-
jlar kinds of operations; each covers up part of
the otherwise available information with extra-
neous matter to make the signal “noisy.” They
both decrease the intelligibility of speech signals.
The effect of an intelligibility-decreasing distor-
tion can be nearly indistinguishable from that of
masking. For example, in a masked-speech ex-
periment, Kryter (1946) showed that the meas-
ured intelligibility of highly reverberant speech
that is masked by enough noise to raise it to a
level 60 dB above threshold is comparable to the
intelligibility of non-reverberant speech raised
80 dB. In that study, the reverberation had a
masking effect similar to the effect of an extra
20 dB of noise.

A learning process permits man to overcome
the change that the noisiness produces. Prac-
ticing listening to the speech without the noise
(or without the distortion), however, seems not
to help intelligibility much. Exposure to the
noise alone or to the distortion of non-semantic