Technical Report Documentation Page 

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 

FAA-AM-76- 3 

4. Title and Subtitle 5. Report Date 

MASSED VERSUS DISTRIBUTED PRACTICE IN LEARNED March 1976 IMPROVEMENT OF SPEECH INTELLIGIBILITY 6. Performing Organization Code 

--:-· 

8. Performing Organization Report No. 

7. Author's) 

Jerry V. Tobias, Ph.D. 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

FAA Civil Aeromedical Institute 

P.O. Box 25082 11. Contract or Grant No. 

Oklahoma City, Oklahoma 73125 

13. Type of Report and Period Covered 

12. Sponsoring Agency Name and Address 

Office of Aviation Medicine OAM Report 

Federal Aviation Administration 

800 Independence Avenue, s.w. 14. Sponsoring Agency Code 

Washington, D.C. 20591 

15. Supplementary Notes 

This research was conducted under Tasks AM-D-74-PSY-51 and AM-B-75-PSY-50. 

16. Abstract 

Student pilots or new air traffic controllers have two ways to learn to understand 

the noisy and distorted communications common to aircraft operations: they may 

learn as they are working on other aspects of the activity, or they may learn 

by devoting a continuous period of time to speech--intelligibility improvement. Work 

completed at this laboratory indicates that two listening sessions of less than 

1 hour each can accomplish the improvement and, under some circumstances, a single 

session will do. The current experiments are intended to reveal some of the microstructure of this learning process. For the first experiment, subjects were 

divided into five groups: One was permitted 2 minutes of listening practice; 

another, 4· ' another, 8· ' another, 16; and the other, 32, Retests 2 weeks later 

showed intelligibility improvement as a function of the number of minutes of 

previous experience up to 16; the 2- and 4-minute subjects appeared to be no 

different from subjects with no previous experience on this task, 8-minute subjects 

were somewhat better, and 32-minute subjects were indistinguishable from ones 

with a full hour of experience, Subjects for the second experiment were similarly 

divided except that each one returned to the laboratory three times a week until 

every listener had heard 32 minutes of distorted speech. The five groups are 

nearly indistinguishable from each other, which suggests that the nervous system 

stores even small amounts of information gained from practice with distorted 

speech to use later for comparisons and improvements, 

17, Key Words 18. Distribution Statement 

Communication Document is available to the public through 

Learning the National Technical Information Service, 

Noise Springfield, Virginia 22151 

Speech 

Speech Reception 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21· No. of Pages 22. Price 

$3.00 PC 

Unclassified Unclassified 6 .95 MF 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized 

Acknowledgments 

This work could not have been successfully completed without the 

valuable assistance of F. M. Irons, V. Kidd, and vV. E. Morris. 

MASSED VERSUS DISTRIBUTED PRACTICE IN LEARNED 

IMPROVEMENT OF SPEECH INTELLIGIBILITY 

I. Introduction. 

Distorted speech becomes more intelligible 

following listening experience of various kinds. 

Until recently, not much else could be said to 

describe this intelligibility-learning process; certainly no quantification of it was available. Now 

Tobias and Irons5 have provided some data. 

Gathering such information helps both the applied and the theoretical worker; obvious applications exist for student pilots, air traffic controllers, and those who have to communicate 

with non-native speakers (and a less obvious 

application may exist for students of foreign 

languages) ; some of the theoretical implications 

are related to reminiscence, rehearsal, attention, 

memory, and motivation. A survey of all these 

matters is included in the Tobias and Irons 

paper, together with descriptions of and data 

from the previous experiments. 

In the Tobias and Irons experiments, subjects 

listened to highly distorted speech for 1 hour 

at a time and learning curves were plotted to 

describe the measured intelligibility of the speech 

during each minute of the hour. In general, 

except where additional confounding factors are 

part of the test condition, intelligibility scores 

rise to a plateau in about 20 or 30 minutes. 

Subjects who return to the laboratory after an 

absence from the task (usually a week or more) 

start at a slightly lower intelligibility level than 

the one they reached at the end of the first session, but within a few minutes the scores rise 

to a level significantly higher than the firstsession plateau. This delayed improvement suggests that, since scores increase even in the 

absence o£ further experience, subjects might 

also improve with far less experience in listening to difficult speech material. The present 

paper is concerned with two experiments designed to investigate this question. As with all 

1 

the experiments on learned speech intelligibility, 

these two look at the way in which people can 

manipulate signal-to-noise ratios mentally. The 

signal-selection mechanisms of the listener's brain 

seem to search out the best method for processing 

speech-plus-noise, and, after a time, they produce improved intelligibility. 

The ability to hear or understand signals that 

are inaudible or unintelligible to the untrained 

or inexperienced observer can be improved by 

listening practice. The use of such an approach 

for distorted as well as for masked sounds is 

based in the idea that both masking and distortion cover up part of the otherwise available 

information to make the signals "noisy." Both 

decrease the intelligibility of speech. The effect 

of an intelligibility-decreasing distortion can be 

nearly indistinguishable from that of masking. 

For example, Kryter3 showed that the measured 

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 masked nonreverberant speech 

raised 80 dB ; the reverberation has a masking 

effect similar to that of an extra 20 dB of noise. 

II. Method. 

Sixty college-age subjects with normal hearing were divided into two groups-one to participate in each of the two experiments. Each 

group was further subdivided into five grou~s 

of six subjects each. No subject had previous 

experience with this kind of task. 

Subjects were all taught to shadow1 while 

listening to recorded speech. In shadowing, 

listeners quietly speak what they hear at the 

same time that they are listening to it. A person 

who has not tried it might assume that shadowing would be difficult, but it is really quite easy 
to do and is a particularly sensitive measuring 

tool.' Subjects are trained in shadowing normal, 

undistorted speech until they reach intelligibility 

scores of 95 percent or higher in five successive 

1 minute intervals; then they continue trainingor overtraining-for approximately 50 minutes 

longer. In the training period, the speech materials used are different from those that are 

later subjected to distortion for the experimental 

conditions. 

The speech, both practice and experimental, is 

a series of easy-to-understand 120-word passages, 

read by a man who monitored himself during 

the recording sessions to maintain a constant 

speaking level. Later, small amounts of attenuation were inserted from passage to passage to 

insure that all passages were essentially equal 

in intelligibility in a simple masking experiment 

-that is, they were "homogenized" according to 

their masked intelligibility. Each passage is 

approximately 50 seconds long, with a 10-second 

pause between passages. Sixty passages were 

available for practice. Another 123 experimental passages were available with the segments 

spliced together in many randomized orders. A 

full test session comprised 3 practice passages 

and 54 test passages. 

Because previous experiments had indicated 

that learning to listen to one kind of distortion 

is much like learning to listen to any other (in 

fact, transfer of learning between types of distortion is also nearly perfect5 ), only one variety 

of distortion was tested. This distortion is produced by replacing the original speech waveform with 0.1-msec pulses, all of the same height, 

whose spacing and polarity are determined by 

the instant and direction of axis crossing in the 

original waveform (see Figure 1). Pulsed speech 

sounds very harsh. In the earlier experiments, 

subjects selected their own signal levels: A 

1000-Hz tone adjusted to the same peak VUmeter reading as the speech had a sound-pressure 

level (SPL) of 75 -+-4 dB, which is within the 

optimum range according to preliminary tests 

of the relation between level and intelligibility. 

In the current experiment, the level was preset 

to 75 dB SPL. For highly trained listeners, 

pulsed speech is 85-90 percent intelligible. 

2 

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SPEECH SIGNALS 

TIME 

CLEARUNDISTORTED 

PULSE 

MODULATED 

FIGURE 1. The upper curve represents a short sample 

of an undistorted-speech waveform. The lower 

curve, drawn from the same sample and subjected 

to the distortion used in these experiments, illustrates how the waveform information was discarded except for the time and direction of axis 

crossings. Each pulse has the same amplitude and 

width as each other pulse. 

Intelligibility scores are obtained by having 

a trained observer score the subjects' shadowing 

while listening to the same signal before it undergoes the pulsed distortion and while following 

a written transcription of the text. The last 

100 words of each 1-minute segment are scored 

to produce a "percent correct" number. 

III. Results and Discussion. 

A. Experiment I. Following their training 

period, subjects in the first experiment went 

right on to shadow the difficult pulsed speech. 

One group of six subjects listened to it for 2 

minutes; another group listened for 4 minutes; 

another, for 8; another, for 16; and another, for 

32. Then they were dismissed and scheduled to 

return 2 weeks later. In their second sessions, 

subjects were given 5 minutes of shadowing practice on clear, undistorted speech and were then 

tested for a full hour on pulsed speech. 

Figure 2, a composite curve for all five groups, 

shows the first-session results. The subjects in 

all five groups began with intelligibility scores 

of about 45 percent. The 2-minute subjects im-
proved a little in the second minute and then 

were dismissed. Subjects in the other groups 

continued to improve with additional practice. 

The 4-minute subjects were dismissed after their 

fourth minute, the 8-minute subjects were dismissed after their eighth, and so on. Predictions 

from the previous experiments were confirmed : 

Only the 32-minute subjects reached any kind of 

plateau. 

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100 

80 

60 

40 

20 

1st SESSION 

32 

0 10 20 30 

TIME IN MINUTES 

FIGURE 2. Intelligibility of pulsed speech as a function 

of time spent shadowing. Each mark across the 

curve represents the final score for one of the five 

groups of subjects. 

Figure 3 illustrates the second -session performance of each group. The small horizontal 

line crossing each curve represents the average 

final score reached during the first session by 

that group. 

The curves for 2-minute and 4-minute subjects are not detectably different from similar 

curves obtained from subjects with no previous 

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......... 2 min 

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--- 8 min 

-·-16min 

--32min 

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0 10 20 30 40 50 

TIME IN MINUTES 

FIGURE 3. Second-session performance of the subjects 

shown ln Figure 2. The mark that crosses each 

curve Indicates the average final score reached by 

that group during Its first session . 

experience. Figure 4, from an earlier study, 

shows mean first-hours and second-hour performance for subjects whose first-session training 

time was 54 minutes; comparison with Figure 3 

shows the similarity between the current 2- and 

4-minute subjects' second-hour performance and 

the fully trained subjects' first-hour performance. 

The curve for 8-minute subjects rises more 

rapidly than curves for inexperienced subjects, 

but the plateau value at the end of their second 

session is essentially the same as that for a normal, full-hour, first session. The rising slope of 

the second-hour curve for these 8-minute subjects is beginning to show differences from the 

slope for untrained listeners, though. 

The 16-minute group's performance is clearly 

transitional: Like those of the 8-minute subjects, 

their scores rise for the first 10 minutes or so 

to an early (but otherwise normal) first-sessionplateau value. However, instead of stopping 

there as the 8-minute curves do, they slowly 

continue to improve with additional second-session practice until, by the end of 40 or 45 

minutes, they have reached the 90-percent secondsession-plateau value expected of highly experienced listeners. 
100 I st SESSION 2nd SESSION 

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TIME IN MINUTES 

FIGURE 4. Average performance on pulsed speech for subjects who were trained for 54 minutes. 

The 32-minute curve, like the 8- and 16-

minute curves, rises less rapidly than curves for 

subjects who have listened for a full hour during the first session (see Figure 4). Ho"·ever, 

the plateau has increased to the usual secondsession value, although it has done so in 10 to 

15 minutes rather than 3 to 5. 

B. Ewperiment I Discussion. Consider that 

the brain's distorted-speech analyzer may incorporate a kind of time constant such that unless 

it receives at least that much input information. 

it cannot function well. Certainly a qualitative 

difference in the shapes of learning curves exists 

for subjects who have been trained for Hi or 20 

minutes rather than for much longer or much 

shorter periods, and the connotation of a discontinunity in the analyzer's response pattern 

is clear. One might conclude that the process 

is associated with memory such that permanent 

storage of the template against which a listener 

measures incoming distorted speech depends on 

his receiving a quarter hour or more of rehearsal. 

This experiment determines the duration of that 

4 

rehearsal but does not speak to whether it must 

be continuous or whether it can be broken into 

shorter segments separated by comparatively 

long periods of other activity. Experiment II 

attempts to answer that question. 

C. Ewperiment II. Following their training 

period, subjects in the second experiment went 

right on to shadow pulsed speech, just as did 

the subjects in the first experiment. On that 

first clay, again as in Experiment I, one group 

of six subjects listened to distorted speech for 

2 minutes; another group listened for 4 minutes; 

another, for 8; another, for 16; and another, 

for 32. However, instead of being sent away for 

2 weeks, the first four groups were brought back 

sooner. .Members of the 2-minute group were 

first tested on a Monday (or a Tuesday) and 

then returned each \Vednesday, Friday, and 

Monday (or each Thursday, Saturday, and 

Tuesday) until they had completed 16 sessions 

for a total of 32 minutes of experience with 

pulsed speech. Similarly, members of the 4-

minute group returned for eight sessions; the 
8-minute group, for four sessions; and the 16-

minute group, for two. 'When a subject completed his 32 minutes of listening, he was dismissed to return 2 weeks later for retesting. In 

second sessions, subjects were given 5 minutes of 

shadowing practice on clear, undistortecl speech 

and were then tested for a full hour on pulsed 

speech. 

Of the several ways available to graph the 

results, the most informative is to chart the intelligibility score in percent as a function of 

the number of minutes of listening experience, 

no matter how many days it took to get that 

experience. Figure 5 is such a graph. Except 

that the curve for the 32-minute group rises 

more rapidly, the groups are practically indistinguishable from each other during their 32 

minutes of practice and during their retest following 2 weeks. The result would probably not 

be predicted on the basis .of Experiment I. 

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D. Emperiment II Discussion. Listening to 

distorted speech for a long enough period of 

time, whether that time is continuous or intermittent, allows one to improve his ability to 

understand that kind of speech. Whereas Experiment I demonstrates that short periods of 

exposure to difficult speech contribute nothing 

to the ability to analyze and thus to understand 

it, Experiment II demonstrates that even for 

short exposure times, a. residue of information 

about the distortion pattern is retained, at least 

for a few days, to be summed with later samples 

of similar speech as if no hiatus had occurred. 

The analyzing system appears to use data from 

previous days as readily as it uses data from 

previous minutes. 

IV. Conclusions. 

Past experience in listening to badly distorted 

speech is stored by listeners for comparison with 

· ····· · ·· 2 min 

------- 4 min 

--- 8 min 

-·-16min 

---32 min 

RETEST 

10 20 30 40 50 

TIME IN MINUTES 

FIGURE 5. Practice and retE'st results of Experiment II. Each practice curve shows the result of 32 minutes of 

shado"·ing pulsed speech; the curve marked "2" repr0sents sixtet'n 2-minute sPssions and required about a 

month to complete, the curn.> marked "4" r0prcsents Pi~ht 4---minutP sessions and required about 2 weeks, and 

so on to the curve marked "32," which reprPscnts one 32-minute session. Retest curves for all five groups 

represent a single, 54--minute session. 

5 
other difficult-to-understand speech signals. Just 

as Durlach2 theorized (to explain a binaural 

masking process), a neural equalization and 

cancellation must be performed to improve the 

intelligibility of the difficult speech. The nervous system must somehow create a yardstick 

against which to gage the distortion and a 

replica of the distortion process that can be reversed to return the signal to its original form 

(as nearly as possible), once the process is well 

learned. A critical amount of exposure to distortion is required before the task can be done 

efficiently ; until the listener has heard at least 

15 or 20 minutes worth, the stored information 

on how to transform the speech into a more 

intelligible version is inadequate. At least 30 

minutes are needed before adequate information 

has been stored, and for the best level of performance, either a period of rest or an especially 

high level of motivation is necessary.5 The 

process is accelerated slightly if the original rehearsal period is an hour rather than a half 

hour, but the intelligibility level measured 2 

weeks later differs between 30-minute and 1-

hour exposures only during the first 10 or 15 

minutes of testing, suggesting that the extra 

exposure is not critical to later performance, nor 

even very efficient. 

In practical terms, the experiments reported 

here show that, although the person who learns 

to listen to radio communications over noisy and 

distorted systems generally complains that he is 

learning to understand speech transmissions very 

slowly, he really may learn as rapidly as if all 

his listening time were lumped together. His 

samples of distorted speech are infrequent, it 

takes many of them to add up to an hour of 

exposure, and the actual elapsed time may be 

weeks. The inverse interpretation may also be 

valuable: The student pilot or new air traffic controller can overcome this aspect of his communication problem by devoting an hour or two to 

doing nothing but listening to distorted transmissions (or recordings of them might be even 

more effective). Should he do so, his learning 

of radio-associated tasks might well be facilitated significantly. 

References 

1. 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. 

2. Durlach, N. I. : Binaural Signal " Detection: Equalization and Cancellation Theory. In J. V. Tobias (Ed.), 

Foundation of Modern Auditory Theory, Volume II, 

New York, Academic Press, 1972. 

3. Kryter, K. D. : Ef'tects of Ear Protective Devices on 

the Intelligibility of Speech In Noise, JOURNAL OF 

6 

THE ACOUSTICAL SOCIETY OF AMERICA, 

18:413--417, 1946. 

4. Pierce, L., and H. R. Silblger: Use of Shadowing in 

f'lpeech Quality Evaluation, JOURNAL OF THE 

ACOUSTICAL SOCIETY OF AMERICA, 51 :121, 1972. 

5. Tobias, J. V., and F. M. Irons: Reception of Distorted Speech. In W. D. Ward (Ed.), Proceedings 

of the International Congress on Noise as a Public 

Health Pt·oblem, 550/9-73--008, Washington, D.C., U.S. 

Environmental Protection Agency, 1973.