Relationships between Voice Variables and Speech Intelligibility in High Level Noise

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PURDUE UNIVERSITY

THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION

by

Gayland L. Drae^ert________________________________

e n t it le d

RELATIONSHIPS BBTWKËN VOICK VARIABLES

AND SPEECH INTELLIGIBILITY IN HICjH LEVEL NOISE

COMPLIES WITH THE UNIVERSITY REGULATIONS ON GRADUATION THESES

AND IS APPROVED BY ME AS FULFILLING THIS PART OF THE REQUIREMENTS

FOR THE DEGREE OF

Doctor of Philosophy______________________________

P ro fesso r

H ear

of

S

in

Charge

chool or

of

Th e s is

D epartm ent

August________ ie50 _

TO THE LIBRARIAN:-----

49THIS THESIS IS NOT TO BE REGARDED AS CONFIDENTIAL.

PKOFESSOB rsr OHAHGB

GJÏAD. SCHOOL F OHM O—3 . 4 0 —1M

RELATIONSHIPS BETWEEN VOICE VARIABLES AND SPEECH INTELLIGIBILITY IN HIGH LEVEL NOISE

A Thesis Submitted to the Faculty of Purdue University by Gayland L* Draegert

In partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy August, 1950

ProQuest Number: 27714165

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uest ProQuest 27714165 Published by ProQuest LLC (2019). C opyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C o d e M icroform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 - 1346

ACKNOWLEDGMENTS This investigation was undertaken at the Voice Science Laboratory, Purdue University, in conjunction with the office of Naval Research, Contract N6ori-104, Project 20-K-l. The Purdue Research Foundation sponsored the local project under the designation PRF #339.

The author is indebted

to all the members of the project staff for their assis­ tance in the conduct of the investigation.

Of especial

aid in planning and conducting the experiment was Dr. M. D. Steer, Director of PRF #339.

Valuable criticisms were

also contributed by Dr. Alan H. Monroe, Dr. T. D. Hanley, and Dr. L. M. Baker.

Dr. Irving W. Burr has given fre­

quent counsel regarding statistical procedures.

In addi­

tion to those specifically named the author is indebted to colleagues in the department who assisted in obtaining experimental subjects.

VITA Gayland L. Draegert Born October 7, 1914 Education Graduated from Academy, Western Illinois State College, Macomb, Illinois, June, 1931. B. Ed. Degree from Western Illinois State College, Macomb, Illinois, June, 1934. M. A. Degree from State University of Iowa, Iowa City, Iowa, August, 1937. Experience Instructor in Speech and English, Normandy High School, St. Louis, Missouri, one year, 1937-1938. Instructor in Speech and Technical Director of Theatre, Duquesne University, Pittsburgh, Penn­ sylvania, two years, 1938-1940. Instructor in English, Colona High School, Colona, Illinois, one semester, 1942. Instructor in Speech, Radio, and Voice Communications for US Army Air Force, two years, 1942-1944, stationed at Sioux Falls Army Air Field and St. Louis University Research Associate In Voice Communications for Psycho­ logical Corporation, one year, 1944-1945, Waco Voice Communication Laboratory, Waco, Texas. Instructor in Speech, Ohio University, Athens, Ohio one year, 1945-1946. Instructor in Speech and PRF Fellow, Purdue University Lafayette, Indiana, four years, 1946-1950. Publications Intelligibility Related to Articulation, Speech Mono­ graphs, Vol. 13, No. 2, 1946. The Purdue Pitch Meter — A Direct-Reading Fundamental Frequency Analyzer, with Dempsey, M. E., Siskind, R* P and Steer, M. D., Journal of Speech and Hearing Dis­ orders, June, 1950.

TABLE OF CONTENTS ABSTRACT----------------------------------------------- i INTRODUCTION-------

1

PURPOSE AND SCOPE OF INVESTIGATION--------------------- 5 HISTORY OF THE PROBLEM---------------------------------- 7 Intelligibility Testing-------------------Voice Variables in Noise—

— ---------- ------— -— 12

Loudness-

— ------------------------

Time— — — —

—

—

—

--7

— —

— —

12

----------------is

Pitch--------------------------------------------- 20 Quality-----------

— 23

Articulation— — -------

23

EXPERIMENTAL SUBJECTS--------------------------------- 26 PROCEDURE WITH SUBJECTS------------------------------- 28 METHODS FOR MEASUREMENTS------------------------------ 34 Interphone Trainer--------------------------

-34 — ---36

Speaker Rating----------

Speaker Intelligibility----------------------------- 36 Recording---— Intensity----

---------------

37 — ----

Time--- — ------------— ---------------------

40 ---40

Quality------

43

PitEh-----------------------------------------

43

Voice Control------------Vital Capacity

—

-44

------------------------------ 45

RESULTS-----------------

46

CONCLUSIONS AND DISCUSSION--------------------------- 65 APPENDIX

------------------------------------------- 68

Appendix I — Appendix II

Speaker Rating Sheet

Sample Exercise Blank----- ------- --70

Appendix III -- Answer FormsAppendix IV —

-------------- 68

--------------- --72

Speaker Word Lists— ------------

Appendix V -- Prose Passage-------Appendix VI —

89

Procedure for Recording onDiscs

Appendix VII -- Procedure Used for Playing RecordedTape BIBLI0GRAPHY-----------------------

85

90

------91

' -----------92

LIST OF FIGURES AND TABLES

Figure

Page

1.

Schematic diagram of arrangement of subjects and equipment employed in experimental intelligibility measurements---------- ----- ----— 29

2.

Simplified functional block diagram of port­ able interphone trainer. Device 8-1, arranged for testing--— -----

35

3.

Major components in initial recording system—

---- 39

4.

Schematic diagram of method used to obtain speech signal from carbon microphone line for recording------------------

5.

Equipment arranged for copying from disc to tape----------

39 —

— — -39

6.

Equipment arranged for intensity measurements— ----41

7*

Equipment arrangement for speech sound time measurements— — — — ----

--41

Intelligibility scores from recorded lists for selected population----------------------

--47

8. 9.

Intelligibility scores for unselected population--------- -----------------------------— 49

10.

Speaker ratings for unselected population------- ---49

11.

Mean syllable duration for two types of reading material----------------------------------- 51

12.

Mean syllable intensity for two types of reading material-----------------

52

13.

Intensity variability for two types of reading material----------------------------------- 52

14.

Mean fundamental frequency for word lists---------- 54

15.

Voice quality indicated by low frequency intensity minus high frequency intensity using 800 cps division point — -------------------- 54

16.

Reading rate for prose---------

-56

17.

Proportion of speech time for prose reading—

18.

Voice control —

19.

Vital capacity in liters--------------------------- 58

--- 56

ability to sustain phonation— --— 58

Table 1. Coefficients of correlation between measures------- 59 2.

3a.

3b.

Means and standard deviations for intelli­ gibility test and voice variables for 88 speakers-----------------

62

Comparison between speakers of superior and inferior intelligibility reading test --word lists

63

Comparison between speakers of superior and inferior intelligibility reading a prose passage-- — ---------------

64

i RELATIONSHIPS BETWEEN VOICE VARIABLES AND SPEECH INTELLIGIBILITY IN HIGH LEVEL NOISE ABSTRACT

An undergraduate population numbering 555 men students at Purdue University was tested for intelligibility In high level noise.

Of this number, 88 were selected -- 59 from

the highest quart!le and 49 from the lowest quartile — further investigation.

for

Selection was on the basis of both

the Waco VOL 24-word multiple-choice intelligibility test and a combined rating score provided by 6-9 circuit mates listening to impromptu speech of the speaker.

Selected

subjects were recorded reading two types of material into a carbon microphone while 106-108 db of noise was in their earphones.

Recorded test lists were later used to estab­

lish relative intelligibility for each subject. Variables Investigated for readings of both a prose passage and a word list were mean syllable duration, mean syllable intensity (loudness), and intensity variability. Mean fundamental pitch and voice quality as indicated by predominance of energy in frequencies above or below 800 cps were investigated for the test word list.

Rate

in words per minute and proportion of speech time were measured for the recording of the prose passage.

Ability

il to sustain phonation and vital capacity were also measured for each individual.

Inter-relationships between all pairs

of variables and recorded intelligibility, initial intelli­ gibility, and initial rating were computed. Results indicated that the group which was superior in intelligibility as determined by the recorded test lists : 1,

was superior on the initial test;

2,

was superior

on the initial rating;

3,

used greater

mean syllable duration for both

prose and word list reading; 4,

used greater loudness for both prose and word

list reading; 5,

phonated during a greater proportion of the total

reading time for a prose passage; 6, was not significantly different (at the 5$ level of confidence) from the group which was of inferior intelligibility on any of the other variables. All Intelligibility testing employed a portable inter­ phone trainer. Device 8-1.

Speaking for recording was

accomplished with the same noise barrier in the earphones of the speaker. Instrumentation included use of the Purdue Speech Sound Timer for speech time measures, a Sound Apparatus

Company HPL-E for intensity measures, a Western Electric Filter Set RA-358 in conjunction with the HPL-E for the voice quality measures, a Vibrograph (Phonellograph) for fundamental pitch measures, and a wet spirometer for vital capacity measures.

1 INTRODUCTION One function of speech has been described as "the communication of ideas to impart knowledge and secure cooperative action#n (34# 27)^

In military communications

the exchange of knowledge has been accomplished in a variety of ways -- visual signals such as flaghoists# blinkers, pyrotechnics# and audible signals such as radio and wire telegraphy and telephony#

The telephony methods

employing voice generally provide the fastest means and are widely used in communications# Voice communication, while generally faster, is subject to failure from a variety of sources:

the system,

the physical environment, language difficulties, listener performance and speaker performance.

As reported by Steer

and Hadley (45), a decade ago military organizations recog­ nized the existence of problems in the area and took appropriate action to set up investigative and training installations.

Medical personnel, speech scientists,

psychologists, engineers, and other specialists contributed to investigations, expérimentation, and reporting of the factors involved.

Much of the background information

contributing to this report is the result of programs at Harvard Psycho-Acoustic Laboratory (PAL), Pensacola Naval Air Station Acoustic Laboratory, and Waco Voice Communi-

^Number In parentheses refers to item in bibliography.

cation Laboratory (VCL).

Current experimentation is being

conducted under contracts with the Office of Naval Research, Special Devices Center, Human Engineering Branch, and much of it is being rëported in related technical journals. Each of the major elements in the total situation has been investigated*

Typical of the research into the

electrical system is the measurement of response charac­ teristics of microphones, headphones, and amplifiers com­ monly used in military communication situations. (40) The noise environment was described in a report from the Electro-Acoustic Laboratory (43) at Harvard University* Effects of temperature and altitude pressure on both elec­ trical and human systems were investigated in military installations* (j?)

Improvements in language usages were

suggested on the basis of experimental evidence obtained. (4 and 36)

Miller (^33) reported listener improvement over a

period of 4-7 hours of practice.

Mason (31) reported that

11listening training alone was relatively ineffective" with student pilots.

For speaker training Black (7) reported

that in the AAF

the "training program brought about

general improvement in voice intelligibility through short periods of training*11 Kelly (26), in an investigation of the effectiveness of different methods of training speakers, found some superiority for a training situation less severe than an expected operational situation.

Kelly (26) and

Hanley (22) reported retention of the effects of speaker

3 training over a period of 12 weeks.

Kelly (j27) found a

trained group to be superior to a control group even one year after training* Training in speech intelligibility has been effective. According to Kelly (26) the training environment Is not critical.

Shoup (jkl) described four-hour training courses,

Kelly (26) used a somewhat similar four-hour program. Steer and Hadley (.45) described one eight-period syllabus and another twelve-day course for instructors*

The author wit­

nessed an aircrew training program at one AAF installation2 in 1944 where small crews were being trained in two one-hour periods.

Instructor personnel varied from civilian special­

ists and commissioned officers to enlisted men with minimum service school training in the intelligibility program* Course content in the various training programs has had various phases of emphasis.

Practice was, of course, an

important part of each program.

Five of the eight periods

listed by Steer (ji5) were headed “Speech intelligibility practice.“

Shoup (4^) included “drill11 as a part of each

of the four periods and stated:

“The training procedures in

voice communication were characterized by directed drill over service equipment in the presence of high-level noise." Drills were often of the type to be encountered in actual

2B arks dale AAF, Shreveport, La.

4 operating circumstances later —

weather reports, position

reports, flight plans, and simulated flights.

Such drills

were designed to serve the multiple purpose of teaching procedure words and phrases, practice with equipment, and drill in proper use of the voice. Points of emphasis, according to Black (7), 11repre­ sented the best guesses of professional speech teachers and took into account the recommendations of flying instructors, experience with communication in flight, researches of other laboratories, and earlier experience with communication problems over service equipment.n

Shoup (jll) listed loudness,

rate, articulation, and accustomed patterns of speaking as factors singled out for attention.

Steer and Hadley (45)

included slowness, rhythm, phrasing, distinct enunciation, and effective pitch and loudness levels of voice as voice factors deserving special attention.

In another report

Steer (j|4) stressed the necessity for special voice characteristics for effective communication in high level noise.

5 PURPOSE AND SCOPE OP INVESTIGATION It is the purpose of this investigation to measure the relationships of voice variables to speech intelligi­ bility in high level noise when talkers and listeners use service equipment»

This is not a new area for investiga­

tion as may be seen in the following section on History» The techniques of analyzing the performances of several speakers and relating those analyses to the effectiveness of the same performances in terms of intelligibility were used to provide measures with practical significance* The voice variables measured were aspects of intensity, time, pitch and quality*

The subjects were adult male

undergraduates enrolled in Purdue University*

Speech in­

telligibility measurement was by means of a 24-word multiplechoice intelligibility test developed at Waco VOL (21) and used in other research and training programs since*

Speaker

performances were recorded for repeated measures of the same sample*

Speech samples from which voice variable

analyses were made were recorded immediately adjacent in point of time*

Intensity of the speech samples was measurëd

from graphic recordings of intensities used by the speakers. Intensity variability measures were obtained from these same graphic traces.

Time measurements were obtained by

using a stop watSh for prose reading rate, and the Purdue Speech Sound Timer (39) with modifications indicated by Han­ ley & Steer (23) for syllable duration and ratio of speech

6 time to total time measures*

Mean fundamental pitch, was

determined for a sample of actual test words from phonellograms of the words as spoken by the experimental subjects. An indication of quality was obtained from the ratio of the level of low frequency energy to the level of high frequency energy where the division point was 800 cycles per second. Additional measures, one indicating voice control and the other physiological development, were obtained as possible predictors of success in speech intelligibility. The voice control indicator was the length of time during which the individual could phonate an "ah" sound Judged acceptable by a speech clinician.

The physiological

development measure was vital capacity measured by volume of air exhaled into a wet spirometer. These variables were related to the speech intelligi­ bility scores obtained by playing the recorded test lists to panels of listeners.

Wherever a high correlation exists

between a variable and intelligibility that variable may be presumed to be an important concomitant of intelligibility and worthy of investigation in an experimental training pro­ gram. It is hoped that findings of this experiment may con­ tribute to formulation of more effective and efficient in­ telligibility training programs.

HISTORY OP THE PROBLEM Intelligibility Testing Since the desirable result of voice communication is the comprehension of a message by a listener, efficient performance of all portions of the communication chain is desirable.

More than twenty years ago Dr. Harvey

Fletcher (19) and his colleagues investigated the perfor­ mance of telephonic equipment under conditions of use. When talker-listener variables and message variables had been reduced or eliminated, resulting variation in perfor­ mance was ascribed to variations in telephone equipment. Dr. Fletcher used spoken syllables and spoken words as test items.

The quotient obtained by dividing the number

of correct responses by the total number of possible correct responses was called ^percent articulation.”

This articu­

lation or Intelligibility score served as an indication of the efficiency of the variable in the situation.

Lists

could be formulated to represent any communication activity. Harvard PAL continued these lines of investigation in test­ ing the efficiency of military equipment under simulated operating conditions as well as speaker or listener effic­ iency. Abrams (1), in testing effectiveness of microphone position, used test lists of 84 nonsense syllables composed of combinations of phonemes representing all the sounds common in American speech.

Four talkers read a total of

8 122 lists to seven well-trained listeners*

In another

report Abrams (2) described the construction of eleven 100-word lists of equal difficulty*

These lists were com­

piled from 1100 common English monosyllables and dissyll­ ables after non-discriminating items had been removed.

Pour

talkers reading all words twice and twelve listeners con­ tributed to the final selection*

Sentence tests were also

constructed containing key words for measurement*

Twenty

sentences, each with five key words, provided 100 scoreable items*

Correlation between two performances for each talker

was .85. In another investigation Abrams (3) reported that estimates of intelligibility and articulation scores cor­ related .83 to .99.

After listeners became practiced,

estimated scores and obtained scores for talkers in noise approximated one another very closely.

In this same ex­

periment it was found that a standard speech interview appropriate to the requirements of a college course in speech was of limited validity In selecting talkers for noisy situations of the military type.

Abrams (4) also

reported the development of word and sentence tests for testing of speaker intelligibility in noise.

The sentences

each contained four monosyllables and one dissyllable making a total of five items per sentence or 100 items for a 20-sentence test.

Test lists are approximately equiva­

lent in difficulty.

For both sentence tests and word tests

9 inadequate discriminât ion oeeur red unless the communication was made difficult by use of a noise barrier.

Reliability

of word tests is reported as .89 for 23 talkers reading two 100-word lists to 13 listeners.

A reliability of .88

was obtained for 80-word lists. A test easily administered to groups of subjects for rapid evaluation of one of the elements of the communica­ tion chain -- the talker -- was felt desirable in the program at Waco VOL.

Haagen (20) described the development

of a write-down intelligibility test in which 8-12 subjects could participate as speakers and listeners.

Test lists

as finally used were equated and intelligibility scores (percent correct responses) assigned by groups of listeners were reported to have coefficients of reliability from .68 to .83.

Ease in scoring was felt to be a desirable charac­

teristic of such a test for widespread usage.

For this

reason a multiple-choice test (.21) was developed.

In the

24-word multiple-choice test listeners marked one of four similar-sounding words as a speaker read each test item. This test was widely used in the training programs described by Shoup. (41)

The same test was also used by Kelly (26) as

a criterion test in a training experiment*

Hanley (22) uséd

the same test in measuring retention of intelligibility training on a college population not enrolled in speech classes.

Doyne (13) used an adaptation of the test in

10 measuring speech reception ability of listeners in a freefield situation*

Reliabilities reported range from «58

to *93. Other methods of testing have been described for use in other situations*

Snidecor & Hanley (42,) developed a

series of operational type messages for speaking and repeat­ ing as a means of selecting talker-11steners for special ship-board duty*

Impromptu or extempore speaking on non­

standard material over a communication system in noise was described by Steer, Hadley, and Kelly (46) as an intelli­ gibility test*

Average ratings assigned by circuit mates

correlated *68 with word-test scores achieved within a few minutes of connected discourse evaluation. Tolhurst (48) used consonant-vowel-consonant nonsense syllables in an experiment to determine thresholds of detect­ ability and intelligibility.

Ansberry (j5) used a word test

to determine the effect of frequency band attenuation on intelligibility.

Mason (31) used recorded word tests and

found test-retest correlation in high level noise to be .83*

Listener improvement found in this manner was small

when training was general, greater when Intervening practice was on test items.

Speaking tests of the type described by

Haagen (20) were administered to a large number of student pilots at Waco AAF.

Test results and personal data infor­

11 matIon for 1913 men were analyzed "by Mason (30) to investi­ gate speaker differences or listener differences concomi­ tant with region of origin.

Region of origin was taken as

service command area from which the individual entered the army.

The test indicated that some regional speech

peculiarities detrimental to universal understandability did probably exist. Syllable tests* word tests, and sentence tests have been developed for testing the efficiency of the elements in a communication system.

The equipment, the conditions,

the listener, or the talker may be tested and compared with other similar elements.

Test reliabilities, both test-re­

test and split-half correlations, indicate that carefully constructed and administered tests have acceptably high reliabilities (.68 for 12-word write-down tests with four listeners to .89 for 100-word write-down tests with 13 listeners).

Either standardized items or non-standardized

connected discourse may be used as test material.

The 24-

word multiple-choice test is easily administered and pro­ vides measures with reliability reported to range from .72 to .85.

12 Voice Variables in Noise

Loudness Loudness refers to the magnitude of an auditory senaation.

Its chief physical determinant is intensity.

In intelligibility measurement in high level noise loud­ ness has usually been indicated on the basis of related signal voltage in the electrical system.

Comparison be­

tween loudness measures may be expressed in terms of differences, ratios, logarithms of ratios, volume units (VU), or decibels (db). Moore (35) reported 11that

loudness of voice is an

important factor" in voice communication.

In an experi­

ment at Waco VCL involving 24 student pilots who were trained to maintain specific loudness levels it was found in one series of measures that a mean peak word intensity level of -2.7 db re 10 volts was accompanied by a word intelligibility score of 36.9^.

In the same series, speech

Intensity of 6.0 db resulted in a score of 50.0^.

In

another series of measures using low impedance ear-phones with smooth rubber ear seals speech signal intensity of -3.0 db re 3 volts resulted in a mean intelligibility score of 52.1^ while an increase to 5.4 db was accompanied by an intelligibility score of 61.3^.

These two series involved

aircraft-type interphone amplifiers and T-17 handheld carbon microphones.

A third series of measures using the same type

13 microphone with an aircraft radio also indicated super­ iority for the louder signals.

A voice signal of -2.5 db

re 3 volts resulted in intelligibility of 59.5^ while a sig­ nal of 5.8 db resulted in a score of 66.3^.

All the intelli­

gibility scores reported are group means for groups of 24 speakers.

In every series of measures the condition of

higher signal intensity was accompanied by higher intelli­ gibility scores.

It must, however, be kept in mind that

the barrier was a constant 108-110 db ambient noise of controlled spectrum. In another experiment at the same installation Curtis (11) Investigated microphone positions and observed relations between Intensity and intelligibility in the same direction as those reported by Moore.

For the T-17 microphone the

least effective position reported was the one-inch distance. Intelligibility measured for a group of 16 student pilots was 31.4$ and average peak word intensity was 9.6 db above one volt for the least effective position and 46.4% with intensity of 14.7 db above one volt for the most effective position.

Improvements in intelligibility with increased

signal intensity were also noted for the T-30 throat microphone worn in various positions.

Scores of 17.3% and

35.5% were observed for signals of -6.2 VU and -2.2 VU respectively, where the 0 VU reference was 4.2 volts. Abrams (1) also reported the effect of handheld microphone

14 position on intelligibility.

With eight trained listeners

and three experienced talkers intelligibility scores ranged from 88# at 11zero distance,M the most effective position, to 70# at one-half inch distance.

Data showed speech

signal to increasë as the microphone distance was decreas­ ed.

There was a simultaneous increase in intelligibility

score as speech signal increased. In an investigation of several attributes determining the intelligibility of speech in noise Miller (35) reported a correlation of .28 between intensity and intelligibility for 47 men.

Removing those with speech defects and dialect

deviations from the group served to raise the correlation to .45 for the remaining 34 men. Harris (24), in an experiment involving 14 male uni­ versity students, reported a mean intensity of 28.76 db above an arbitrary reference and a mean intelligibility score of 77.2# under normal conditions.

The correlation

between intensity and intelligibility was .84.

The same

people under stress conditions showed decreases -- in intensity to 27.72 db, in intelligibility to 74.9#.

Cor­

relation under stress conditions was also reduced, re­ ported as .54.

The speakers under stress conditions exhib­

ited lower intelligibility and lower intensity.

Lightfoot

and Morrill (29) reported for 16 speakers and 12 listeners that signal and noise reduction in the speakersf earphones was accompanied by increased vocal intensity and improved

15 intelligibility*

Black (8) reported that listeners tend

to answer at a loudness level which is a function of the stimulus level. It is interesting to note that untrained speakers tend naturally to acxsommadate their vocal intensity to increases in the level of barrier noise.

Hanley and Steer (23) re­

ported that the uninstructed mean intensity for 48 men in­ creased 5.2 db when headphone noise was increased from 100 to 122 db re 10"*^ watt/cm^. Mason (32) reported a series of experiments involving the relation between very loud speech signals and intelli­ gibility.

For one group of 87 student pilots divided into

classes on the basis of loudness, pre-training intelligi­ bility scores ranged downward from 78.4% for the loudest quintile to 67.2$ for the least loud quintile.

Associated

signal levels were 4.7 volts and 1.6 volts, respectively. Post-training scores indicated no clear superiority for the loudest class.

Loudness classes and concomitant mean

intelligibility scores were:

5.9 volts, 82.2$; 5.0 volts,

84.6$; 4.2 volts, 83.7$; 3.6 volts, 84.3$; and 2.4 volts, 80.2$.

This experiment involved the use of a noise-shielded

microphone with speakers and listeners all in high level ambient noise.

In another experiment with radio transmission

for a group of 23 subjects a mean intelligibility score of 37.8$ was associated with an average signal voltage of 2.5

16 volts, and a mean score of 44*3$> was associated with a signal of 4.5 volts.

With another type of equipment, a

group of 24 student pilots obtained mean intelligibility scores of 55.1% and 65.8% for signals of 2.5 volts and 4.5 volts respectively.

In each situation duplicate ex­

periments were conducted with ear plugs in listeners1 ears, and improved group intelligibility was noted in each case.

Group means increased from four to nine percentage

points when ear plugs were used*

Every group improvement

was significant at the 1% level of confidence.

In another

experiment involving constant signal-to-noise ratios, in­ creased signal loudness was accompanied by decreased in­ telligibility for a given signal-noise situation.

With

215 listeners a signal increment of 30 VTJ was accompanied by an intelligibility decrement from 61.2% to 48.2% for a signal/noise ratio of 2/1.

When the constant signal/noise

ratio was 3/2 for a panel of 101 listeners, a signal in­ crease of 30 VU was accompanied by an Intelligibility de­ crement from 40.5% to 20.7%.

According to Mason, 11these

experiments show that loud signals in the earphone circuit decrease intelligibility when they do not result in a more favorable signal-to-noise ratio.11 The results of these experiments indicate that with constant barrier noise the louder speech signal should be the more intelligible.

Considering the limitation imposed

above, the assumption that 11the primary requirement of a

17 good talker is an ability to speak loudly” (35) is sup­ ported. In the investigation by Abrams (2) cited earlier in which the relation between voice level and intelligi­ bility was reported, mean vocal intensity for each talker was determined by noting the intensity indication on a VU meter for each word*

Variability for each talker was com­

puted as the variance of his word peaks.

For the 23 talkers,

each reading three times (with different microphones), the variability in intensity was found to have no significant relationship with either intelligibility or mean Intensity. In the array of variables reported by Miller (33) he found the aspect of steadiness (of level and emphasis as well as rate) to add nothing to the accuracy with which the intelligibility of the voices could be predicted on the basis of the other variables.

Harris (524) found that trained

speakers under stress conditions exhibited increased vari­ ability accompanying decreased intensity and lower intelli­ gibility.

An intensity variability score for each man was

derived from the standard deviation of the distribution of syllable intensities obtained from a graphic intensity trace of the speech sample (a 24-word list).

The average standard

deviation for the group under normal conditions was 1.55 db (77.2% Intelligibility) and 2.85 db (74.9% Intelligibility) under stress conditions.

Differences In Intelligibility,

18 Intensity, and intensity variability were significant at the 1$ level of confidence based on 14 replications. Time Another voice variable which may be a contributor to intelligibility is time and its various aspects.

Increased

syllable duration was stressed in a training record used by Kelly. (j26)

The specific instruction was:

"to keep your

message clear, hold on to all of your sounds."

In this

same investigation trained speakers used a mean syllable duration of .317 second while untrained speakers achieved a mean syllable duration of .189 for similar prose material. The prose material was of the type: In Meadowlands, Minnesota, the last killing frost comes on June third, on the average. The first one of winter comes on September tenth. This gives a growing season of ninety-nine days. Instructions in the manner of reading indicated that listen­ ers would write the pertinent details.

A listener* s obser­

vation of the speaking was that duration was generally greater than for conversation or other connected speech in noise.

A measure of intelligibility was obtained from

recordings of these prose messages, and it was found that trained speakers were superior to the untrained on this non-standardized test as well as on the standardized intelli­ gibility test.

Harris (24) found the mean syllable duration

19 or 14 trained speakers to be •652 second for test words under normal conditions and •451 second under stress con­ ditions.

Correlation between syllable duration and intelli­

gibility was .65 under normal conditions and .13 under stress conditions.

Hanley and Steer (23) found that the uninstructed

syllable duration of 48 untrained speakers increased from .109 second to

.167 second as barrier noise increased

from 100 db to122 db.

These

subjects were instructed to

read a prose passage 11as if you were communicating to some­ one at the other end of the line.** A second aspect of time is rate measured in words per minute.

In an experiment reported for OSRD (18) no clear

superiority was found for any rate from 90 to 210 words per minute, although a slight decrement in intelligibility was noted at rates of 180 and 210 words per minute.

The cri­

terion used was a sentence type intelligibility test.

Han­

ley and Steer (23) reported that speakers tend to reduce speaking rate as the noise barrier becomes more intense. The mean rate for 48 subjects was 183.2 words per minute at 100 db and 165.4 (35) found tha louder speech.

words per minute at 122 db.

Miller

same trend of slower rate to accompany Judges estimated optimal rates under dif­

ferent conditions and agreed on 120 wpm, although a mean rate of 141 wpm was used by the talkers.

Relationships

between rate and sentence intelligibility showed some de-

20 crease in intelligibility at faster rates.

Word duration

was also judged in the same experiment, but ,frate, duration, and steadiness” contributed nothing to increased accuracy of predicting intelligibility.

Lightfoot and Black (28)

reported that response rates are functions of stimulus rates when listeners repeat or respond to spoken messages. Proportion of time spent in vocalization is another aspect of time as a voice variable.

Reported investigations

of this variable regarding connected speech in high levël noise are rare.

Kelly (26) indicates that trained speakers

spend essentially the same proportion of the total time vocalizing that untrained speakers do —

66.0% for trained

and 66.8% for untrained. Pitch Pitch is a third voice variable which has been inves­ tigated with respect to its relation to intelligibility of speech in high level noise.

In an experiment reported by

Miller (.35) in which pitch was controlled by the speakers, superior intelligibility scores were obtained by those using higher pitches; but it was also noted that higher intensity accompanied the use of higher pitch.

The tentative con­

clusion drawn was that the improved intelligibility noted as accompanying the higher pitch may well have been due to

21 increased loudness•

In another comparison the effects of

variables other than pitch were held constant through statistical manipulation.

Results showed a slight negative

correlation between pitch and intelligibility.

The con­

clusion to this experiment was that if any difference exists in favor of pitch, the speaker maintaining sufficient loud­ ness will be more intelligible if the pitch is lower. Brackett (10) reported an experiment performed at Waco VCL which investigated the relation between pitch and in­ telligibility.

A group of 40 student pilots was tested in

noise at uninstructed pitch levels with controlled loudness. Subjects then practiced at higher or lower pitch levels, still with controlled loudness, until performance was con­ sistent.

another intelligibility test was performed at

this stage.

The process was repeated until intelligibility

at four pitch levels had been measured, all by means of recordings.

A non-significant advantage was observed in

favor of uninstructed pitch over low pitch, and uninstructed pitch was accompanied by intelligibility scores significantly superior to high and very high pitch performances.

An

analysis of variance was applied to groupings by frequen­ cies, and no significant advantage was found for any class of pitches.

Even among the samples of uninstructed pitch

the rank difference correlation between pitch and intelli­

22

gibility for the 14 subjects was only ♦04,

Frequency was

measured for each speaker on the basis of 3-5 samples of the word “number” which was used as part of the carrier phrase.

The mean fundamental frequency used at the unin­

structed pitch level was 137,8 cps with a range of from 106-191,

In another study 27 subjects normally employing

average pitch were recorded at four loudness levels from soft-conversational to shouting.

Intensities were -6, -3,

0, and 3 VU for the four level#.

In each case of increased

intensity the pitch of the voice was raised, as determined by both judgment and frequency measurement of 11 speakers. Mean frequencies indicated for the four intensity levels of the 11 speakers were 119,4 cps at -6 VU, 136,0 cps at -3 VU, 154.8 cps at 0 VU, and 201.8 cps at 3 VU.

The find­

ings stated by Brackett for the situation indicate that conscious raising of the pitch is detrimental to intelli­ gibility, increased loudness of voice Is accompanied by higher pitch, and no correlation exists between measured frequency and intelligibility. Additional inference that fundamental pitch is not related to intelligibility Is presented by Abrams (2) in comparing women talkers to men talkers.

The mean in­

telligibility score for 22 men was 47%, for 10 women, 48%,

Mean intensities were the same for both groups.

25 Quality Voice quality does not lend itself readily to physi­ cal measurement»

Erickson (15) said that quality or timbre

is determined by the number and strength of the overtones or the partials> or by the wave form.

A graphic repre­

sentation of intensity vs. frequency when measured by means of band pass filters is also a description of voice quality.

Miller (j33) described a method employing band

pass filters, noise meter, and graphic meter.

For 47

talkers studied the correlation between average energy level in mel bands and intelligibility was .31 for words and .32 for sentences, both for loud voice.

Examination

of speech spectra for the five best and five worst talkers indicated no significant difference.

Ratio of energy in

highest energy band to total energy, ratio of highest to lowest band levels, and irregularity of spectra were each related to intelligibility by correlations not signifi­ cantly different from zero. Articulation Articulation or precision of uttering sounds has been long felt to be a factor in intelligibility.

Training

centers and operational crews often gave the instruction ”speak clearly” or an equivalent.

Kelly (_S6) included the

24 instruction and a period of drill with emphasis on clear­ ness in a training experiment.

Miller (53) concluded

that advice to nspeak loudly, articulate correctly and strongly*' was most important*

In Miller's experiment with

47 talkers consonant strength and consonant precision were rated by a panel of judges•

When the ratings were analyzed

to determine which variables are most important in judging intelligibility scores, the order of importance of the factors was found to be:

"consonant strength, consonant

precision, dialect, and noise penetrating quality*" In an articulation training experiment in noise re­ ported by the author (14) it was found that stressing final consonants was accompanied by a significant increase in intelligibility when 30 talkers used handheld microphones, no increase for 32 talkers using throat microphones, and non-significant decrement for 27 talkers using noiseshielded microphones.

At another time 29 men were trained

to stress sibilant sounds*

Their intelligibility remained

essentially unchanged by the training and matched the mean score of a control group of 29 at both before and after stages in the specialized articulation training.

When 29

men were drilled in quiet in clear pronunciation and tested before and after in noise, significant improvement occurred. Training In noise using difficult words as drill material for articulation was followed by an Intelligibility gain

25 of 7,4 points while a matched control group gained only •7 point without the articulation training.

Judgments

of speech before and after training showed that articulation training did change the speakers1 performances as well as improve the mean intelligibility from 58.8/é to 66*2%*

It

seems evident that training was successful in changing articulatory patterns as well as in improving intelligi­ bility# Voice control per se has not been widely reported in relation to speech intelligibility in high level noise, but the training manual provided to accompany the portable interphone trainer (49) includes instructions relative to 11clearness,11 and ability to provide a good tone may overlap clearness as well as articulation patterns,

Vital capacity

measurement was shown by Barnes (^6) to bear little relation­ ship to ratings of oral reading for 124 male university students, and the author could find no investigations re­ lating vital capacity to speech intelligibility in high level noise. It seems desirable to approach the problem of relation­ ships of voice variables to speech intelligibility in high level noise from another direction -- one which may reduce sampling error due to small samples, one in which obtained results will be less influenced by training and instruc­ tion.

26 EXPERIMENTAL SUBJECTS Subjects for this investigation of relationships between the voice variables described and intelligibility were drawn from a university undergraduate population. All the subjects were males and were enrolled in a basic speech course at the time of participation.

Enrollment

in the course was not a requirement for participation in the experiment, but the classrooms served as an excellent source of subjects.

In the original population tested,

555 students received a combination of an intelligibility rating scale and the 24-word multiple-choice test discussed earlier.

There was no reason to believe these students

different from groups tested by Kelly (.26) or by Hanley (22) + The group described by Kelly was essentially the same with respect to intelligibility as military groups. Of the original group tested, 94 subjects were selected for further investigation by virtue of their scores on both the intelligibility rating and the multiple-choice test. The requirement for selection was that the individual be in either the top 25% on both measures or the bottom 25% on both measures. These 94 subjects were recorded and the records were used for the analyses.

The group of 94 subjects was fur­

ther attenuated to 88 by virtue of poor disc recording or almost complete failure of the subject to operate the micro-

phone switch properly#

The final group of 88 subjects for

whom records were made and analyses completed was comprised of 39 who had been superior speakers and 49 who had been inferior speakers in the initial situation. The initial group of students came from classes in such a manner that no known bias existed except hsevere speech defectives and severe foreign!sm cases" were not sent to the testing session.

Inasmuch as Kelly (26) found

in a similarly chosen college population that age, mili­ tary experience, major curriculum, and scholarship exhibit­ ed no close relationships with intelligibility, those factors were not investigated for this study#

28 PROCEDURE WITH SUBJECTS The initial testing session was accomplished in a manner similar to that described by the staff of the Office of Naval Research, Project 20-K-l, at Purdue Voice Science Laboratory, and used by Kelly (26)•

A circuit

was composed of one portable interphone trainer. Device 8-1, and the associated microphones and headphones for 10 men.

Subject stations were arranged in a line and

separated from one another by half-partitions.

A diagram

representing the arrangement may be seen in Figure 1.

On

each circuit 8-10 men participated on the party-line sys­ tem in the round-robin fashion described by Haagen (20) where each man in his turn spoke his message and all his circuit mates responded by writing or marking* When the subjects arrived for testing they were seated in the numbered stations.

They were told they were partici­

pating in a research program designed to investigate certain features of intelligible speech in high level noise.

They

were assured of the independence of this program from the classroom, especially regarding scores and grades.

The

equipment was described briefly and proper use was explained and demonstrated by the experimenter.

In an immediate follow-

up each student demonstrated for the experimenter the proper use of microphone and headphones.

After all students had

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30 demonstrated knowledge of use of the equipment, test blanks (see appendix) were Inspected and Instructions were given for the rating portion of the session.

When all men in­

dicated understanding of instructions, headphones were adjusted and the noise barrier was inserted.

Speaker num­

ber one then spoke into his microphone identifying himself, and proceeded with an impromptu speech on a topic presented to him.

The topics were of the type:

ever saw," “How to hitch-hike," etc.

“The best movie I At the end of 30

seconds speaker number one was signaled to stop and his circuit mates rated him on a 1-5 scale (1- "unintelligible"; 5- "wholly intelligible").

Speaker number two then repeated

the performance of speaker number one using another of the impromptu topics.

In this manner all students on the cir­

cuit participated as speakers and judges. At the close of the rating for the last speaker on the circuit the noise barrier waw eliminated and instructions for marking the mult ip le-choice test were given.

The instruc­

tions included the marking of a sample list to acquaint the students with the location of the possible responses* Speakers were cautioned to read so they could be heard and not to race from item to item. to read:

"Number one, —

seconds) —

number two —

The recommended pattern was

dashboard fort plain —

(pause two

shut mortar assist — ."

The two-

second pause was emphasized to allow the listeners ample

31 opportunity to mark their responses.

When all subjects

asserted their comprehension of the instructions, the noise barrier was again introduced into the earphones.

The first

man on the circuit then read his word list, the second speaker read list number two while the remainder, including the first man, marked their responses.

The succession con­

tinued in numbered rotation until each had read a list of 24 test words. Occasionally a talker forgot the instruction to keep the microphone button depressed, or to hold the microphone close, or to speak squarely into the face of the microphone. If the experimenter found no desired signal in the earphones he corrected the mispractice of the speaker immediately. This procedure tended to eliminate extremely low scores due to mishandling of equipment. Upon completion of the test session and before dismis­ sal the subjects were notified that individuals selected would be called for additional participation on an available day.

Informal observation by the experimenter indicated

that student interest in the results and the experiment was generally high. After the papers had been scored and the sample selected the specific students were called in small groups at their regular class times.

The second stage of subject parti­

cipation consisted of the individual* s reading from a sound-

32 proofed studio into a carbon microphone for recording. The situation was similar to the initial session in that the talker used the same type microphone, the same type earphones, and had the same type noise barrier in the phone line.

An experimenter was present in the room with

the subject to provide the desired reading material in the proper order and to interpret signals for starting and stopping from the recording room.

The experimenter also

refreshed the subjects on the topic of equipment usage, especially the microphone.

The reading material used by

each subject consisted of a prose practice paragraph to enable the recording operator to set attenuators at op­ timum values, a 73-word prose passage (see appendix for script), test list 1-A and one other test list.

Each

subject was instructed to read the material presented to him as if he were communicating to a group of circuit mates such as he had had on the previous test occasion.

At the

conclusion of the reading exercises the subject removed the earphones and performed two additional exercises —

the

phonated *ah" and the vital capacity measure. For the continuous phonation measure the subject was instructed to take as deep a breath as he wanted and say ”ahM continuously at whatever pitch and loudness he chose. The task was to see how long a good tone could be sustained.

53 The experimenter timed three trials while two trained speech correctionists judged the acceptable tones•

The

score used was the mean of the three trials. Vital capacity was measured in the traditional manher using a wet spirometer.

The subject was instructed to in­

hale as deeply as possible, then exhale into the spirometer. Three trials were measured and the mean of the three trials was used as the vital capacity score. The second and final participation of the speaking subjects was thus completed by the reading for recording of a prose passage and two test lists, demonstrating ability for continuous phonation, and making vital capa­ city measures. Recordings of intelligibility test lists were later played to groups of naive listeners to obtain actual in­ stead of inferred scores for the recorded test lists.

For

these measures each list was played to a panel of ten listeners.

There were four series of lists and four panels

of ten listeners were used.

The panels of listeners were

instructed in the marking of the response blanks; and then the series of test lists was played through the interphone trainer where noise was mixed with the speech signal from the recording.

Subjects for the final measures were ob­

tained from speech classes similar to those from which the speaking subjects were chosen.

METHODS FOR MEASUREMENTS Interphone Trainer Initial testing was accomplished in a manner similar to that described by Kelly (26) for his 11standard conditions The equipment consisted of a portable interphone trainer. Device 8-1, and the associated dynamic headphones, type ANB-H-1A, and carbon microphones, type T-38.

The inter­

phone trainer served as a combination speech amplifier and noise generator*

The simplified block diagram. Figure 2,

indicates the electrical arrangement•

The specific output

impedances and their loads were those recommended by Nordyke (37) in a report on the interphone trainer* Calibration of the amplifier and noise channels pro­ vided a speech channel gain of 17 db and noise at the lis­ teners* ears of 106-108 db re l O ~ ^ watt/cm^.

Calibration

procedure described In a report (12) from the Purdue Speech Science Laboratory was followed.

The level of noise

was determined by measuring the acoustic output of a typical earphone coupled to an artificial ear, Ballantine, model 505.

Secondary checks were provided by measuring the

voltage across the headphone line under the calibration conditions and checking daily operation at this voltage (•67v)•

Optimum speech channel output had been found by

Kelly (26) and the amplifier gain in decibels was determined

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