II. Method.

In a Beech Bonanza rented for the purpose,
three loudspeakers were mounted across the rear
of the cabin ceiling (Figure 1). One loudspeaker
was at either side of the cabin, and one was in
the center; all faced forward. Each loudspeaker
was wired to a control panel that permitted the
selection of which loudspeakers were used at a
given time, and also permitted the inversion of
the phase at any loudspeaker (Figure 2). With
this arrangement, it was possible to produce sym-
metrical or asymmetrical signals with any com-
bination of phase relations for listeners in either
front seat of the plane. For example, for the
person riding in the co-pilot’s position, the center
and right-hand loudspeakers, used together, give
& symmetrical signal; the right and left loud-
speakers are asymmetrical.

The noise source for these tests was the aircraft
itself. The cabin-noise intensities are represent-
ative of those to be found in most light, single-

Figure 1.

3

engine aircraft.* The signal was one of four
modified-rhyme tests that were developed by the
National Academy of Science-National Research
Council Committee on Hearing, Bioacoustics, and
Biomechanics (CHABA) for use in testing
aviators. Standardization of materials has been
under way for several years at various govern-
ment laboratories, primarily the Naval Aero-
space Medical Research Laboratory at Pensacola,
Florida (private communication, Carl Williams
and Ronald Robertson). CHABA-furnished
tapes were played on a battery-operated Nagra
TII tape recorder (which features exceptionally
stable speed), then through an Amplivox S-402
amplifier (also battery operated) and a Hewlett-
Packard 350B attenuator, and finally, via the
phase-reversal and switching arrangement,
through the appropriate loudspeaker.

Subjects were 32 men and 5 women, all selected
for norma] hearing, for age between 18 and 30
years, and for willingness to ride in a light air-

Loudspeaker-mounuing bracket in rear of test airplane.



g

. Durlach, N.

. Jeffress, L.

References

Binaural Signal Detection: Equaliza-
tion and Cancellation Theory. In: Tobias, J. V. (Ed.)
Foundations of Modern Auditory Theory. Volume IT,
New York, Academic Press, 1972.

. Hanson, R. L., and W. E. Kock: Interesting Effect

Produced by ‘Two Loudspeakers Under Free Space
Conditions, JOURNAL OF THE ACOUSTICAL
SOCIETY OF AMERICA, 29:145, 1957.

. Hirsh, I. J.: Binaural Summation: A Century of In-

vestigation, PSYCHOLOGICAL BULLETIN, 4 193—
206, 1948.

. Hirsh, I. J.: Binaural Summation and Interaural

Inhibition as a Function of the Level of Masking
Noise, AMERICAN JOURNAL OF PSYCHOLOGY,
61 :205-218, 1948.

. Hirsh, I. J.: The Influence of Interaural Phase on

Interaural Summation and Inhibition, JOURNAL OF
THE ACOUSTICAL SOCIETY OF AMERICA,
20 :536-544, 1948.

. Hirsh, I. J., and F. A. Webster: Some Determinants

of Interaural Phase Effects, JOURNAL OF THE
ACOUSTICAL SOCIETY OF AMERICA, 21:496-501,
1949,

A.: Method for the Study of the Locali-
zation of Airborne Sound, JOURNAL OF THE
ACOUSTICAL SOCIETY OF AMERICA, 27:208,
1955.

8.

10.

i.

&

13.

14.

15.

Jeffress, L. A.: Binaural Signal Detection: Vector
‘Theory. In: Tobias, J. V. (Ed.) Foundations of
Modern Auditory Theory. Volume II, New York,
Academic Press, 1972.

. Levitt, H., and L. R. Rabiner: Binaural Release from

Masking for Speech and Gain in Intelligibility,
JOURNAL OF THE ACOUSTICAL SOCIETY OF
AMERICA, 42 :601-G608, 1967.

Licklider, J. C. R.: The Influence of Interaural Phase
Relations Upon the Masking of Speech by White
Noise, JOURNAL OF THE ACOUSTICAL SOCIETY
OF AMERICA, 20:150-159, 1948,

Schubert, E. D.: Some Preliminary Experiments on
Binaural Time Delay and Intelligibility, JOURNAL
OF THE ACOUSTICAL SOCIETY OF AMERICA,
28 :895-901, 1956.

. Stevens, K. N., and A. S. House: Speech Perception.

In: Tobias, J. V. (Ed.) Foundations of Modern Au-
ditory Theory. Volume II, New York, Academic Press,
1972.

Tobias, J. V.: Cockpit Noise Intensity : Fifteen Single-
Engine Light Aircraft, AEROSPACE MEDICINE,
40 963-966, 1969.

Tobias, J. V.: Auditory Processing for Speech Intel-
ligibility Improvement, AEROSPACE MEDICINE,
41:728-733, 1970.

‘Pobias, J. V., and J. F. Curtis: Binaural Masking
of Noise by Noise, JOURNAL OF THE ACOUSTI-
CAL SOCIETY OF AMERICA, 31:127, 1959.



describe this same phenomenon is to consider the
case in which a signal is turned on in one ear,
and while it is there, enough noise is added to
the same ear to mask out the tone. Then, when
noise is added at the opposite ear, the tone again
becomes audible. (A further implication is that,
if the tone is also added at the opposite ear, it
can no longer be heard.)

Other high-MLD situations are those in which
the signal is presented to both ears identically,
and the noise, presented to both ears, has its phase
inverted (flipped, or shifted 180°) on one side.
The converse situation is also a good one for pro-
ducing release from masking. In it, the signal is
flipped 180° and the noise is in phase at the two
ears.

Both tonal and noise-band signals show large
shifts in detectability under appropriate binaural
conditions, but tests using speech signals?" “1
suggest quite a different pattern of analysis. In
the best possible earphone-listening condition, a
test of the binaural improvement in speech intel-
ligibility (the intelligibility-level difference or
ILD), translated into the amount of release from
masking, turns out to be only 5 or 6 dB. Two
reasons for this kind of lessening in signal-pro-
cessing capability seem possible, and in all prob-
ability, both of them are active in producing the
result. First, a listener who is asked to respond
to the intelligibility of a signal rather than to
its presence or its absence is operating at a sound
pressure well above the masking noise, where the
brain’s ability to decrease the effectiveness of a
masker may not be nearly so great or so impor-
tant—the usual MLD study concerns itself with
a threshold phenomenon, but since intelligibility
is measured at suprathreshold levels, [LDs must
be expected to result from a somewhat different
sort of analysis. Second, the task of “under-
standing” rather than “detecting” is relatively
quite complex; many workers have interpreted
data on speech-signal reception to mean that a
grossly different variety of mental function is
used than is called for with tones, noise, clicks,
or other non-intellective or non-semantic sig-
nals.??_ For most writers, the difference is par-
tially explained by the additional mass of neural
tissue required for speech analysis. The concept
is sensible and easy to agree with. Most of the

auditory nervous system (and perhaps large por-
tions of the rest of the brain as well) seems to
shift gears as soon as an acoustic input is
recognized as speech; the analysis problem for
intellective material is far more complex than
for any other kind of signal that human observers
receive.

A recent paper from this laboratory’ reported
a replication of the earphone findings for intel-
ligibility-level differences, but the signal presen-
tation system used pairs from a circular array
of loudspeakers in an anechoic space. Despite the
obvious differences between manipulating the
phase of signals presented via earphones, and
manipulating signal attributes with loudspeakers,
each of whose outputs must reach both ears, at
least to some degree, ILDs representing about a
5-dB shift in noise level are common. Thus, in
this particular series of tests, it was found that
one may produce nearly as much binaural effect
on speech with loudspeaker presentations as with
earphone presentations. This finding resulted
from comparisons of the results from pairs of
symmetrically placed loudspeakers that had both
signal and noise in phase, with the same loud-
speakers when they had noise in phase, but sig-
nals 180° out. For most subjects, the phase-
reversed signals appear to originate inside the
listener's head; this percept was first described
in detail by Jeffress’; shortly afterward, Hanson
and Kock? published an independent description.
A number of investigators have experimented
with the process since, and almost all of the work
has been done in anechoiec, semi-anechoic, or re-
lated sorts of sound-treated, low-reverberation
chambers.

If this laboratory's findings are to be appli-
cable to a practical listening situation in which
the intelligibility improvement might serve a
safety function, it must be demonstrated that the
ILD can still be created when the loudspeakers
are mounted in reverberant surroundings. Fur-
ther, natural situations need not have precisely
correlated noise at the two ears. In order to test
the effects of both of these deleterious conditions
on the intelligibility-improving techniques pre-
viously investigated, an extremely difficult listen-
ing situation was selected: tests were performed
in a light airplane, in flight.


SPEAKERS — LEFT CENTER RIGHT
OUT OUT OUT
PHASE © B&K 2203
REVERSE OFF OFF OFF r-@ ham (as) SOUND-LEVEL
SWITCHES? IN IN IN °O° METER
ote adi
q 4 (for calibration only)
8. 8
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® OOO: ome) 2 00 00
NAGRA It HEWLETT-PACKARD AMPLI-VOX
TAPE RECORDER 350-B $-402
ATTENUATOR SET AMPLIFIER

Fieure 2. Diagram of signal-presentation equipment.

plane. The experimenter sat in the back seat
with the equipment; subjects rode in the right-
hand front seat, and carried a clipboard to which
answer forms were attached. For each word
on the test tape, the answer form listed six
rhyming words, only one of which was correct.
Subjects were instructed to mark which of the
six words they thought they heard for each
presentation. -Instructions for the task were
given before takeoff. Total time in the plane for
each subject averaged less than 45 minutes.

The four lists of recorded test words were used
in random order. In addition, four conditions
were randomly assigned to the four lists: (1) the
center and right-hand Joudspeakers were used
in phase with each other, (2) the center and right-
hand loudspeakers were used out of phase with
each other, (3) the left-hand and right-hand
loudspeakers were used in phase, and (4) the
left-hand and right-hand loudspeakers were used
out of phase.

Signal levels were adjusted so that, when the
plane was in level flight at cruising speeds, the
signal-to-noise ratio was approximately the same
as CHABA called -2dB on comparable record-

ings in noise. At this intensity, average scores
varied between 30 and 50% for these subjects.

III. Results.

An analysis of variance was performed on the
percent-correct intelligibility scores (Table 1).
The lowest scores occurred for in-phase condi-
tions, and the highest occurred for out-of-phase
conditions. In statistical analyses, this difference
in effectiveness in combating the masking effects
of noise is significant (p<.01).

TABLE 1
Mean Intelligibility Scores

Symmetrical Asymmetrical
In-phase Out-of-phase In-phase Out-of-phase
33.68 46.43 87.41 40.92
SD=9.56 SD=9.83 SD=8.57 SD=7.06

On a Newman-Keuls test, four comparisons of con-
ditions show significant differences (p<.01) : sym-
metrical in-phase is significantly different from
symmetrical out-of-phase ; symmetrical in-phase is
significantly different from asymmetrical out-of-
phase; asymmetrical in-phase is significantly dif-
ferent from symmetrical out-of-phase; and
asymmetrical out-of-phase is significantly different
from symmetrical out-of-phase. Other comparison
were not significant.




TECHNICAL REPORT STANDARD TITLE PAGE

T. Report No. Z. Government Accession No. 3, Recipient's Catalog No.
FAA-AM-72- 31
4. Title and Subtitle 3. Report Date
September 1972
BINAURAL PROCESSING OF SPEECH IN LIGHT AIRCRAFT 6. Performing Organization Code
7. Authors) 8. Performing Organization Report No.

Jerry V. Tobias, Ph.D.

9. 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

12. Sponsoring Agency Name ond 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 No, AM-A-71-PSY-16 and AM-A-72-PSY-16.

76, Abstract

Laboratory studies have shown that the human binaural auditory system can extract
signals from noise more effectively when the signals (or the noise) are presented
in one of several interaurally disparate configurations. Questions arise as to
whether these laboratory studies in anechoic or semi-anechoic spaces can be
generalized to more reverberant listening conditions. In the current study, tests
were conducted in the cabin of a light airplane, in flight. For symmetrical
signal sources, loudspeaker transmissions of intelligibility-test materials
produce higher intelligibility scores for speakers out-of-phase than for speakers
in-phase.

17. Key Words 18. Distribution Statement

Noise Aircraft, Light | Availability is unlimited. Document may

Hearing Communication be released to the National Technical

Speech Intelligibility Information Service, Springfield, Virginia

Masking Level Difference 22151, for sale to the public.

19, Security Classif. (of this report) 0. Security Classif. (of this page) Zi. No. of Pages | 22. Price
Unclassified Unclassified 6 $3.00

Form DOT F 1700.7 (s-69)



No significant difference was found between
symmetrical and asymmetrical presentations for
in-phase conditions. However, a. comparison of
symmetrical with asymmetrical presentations for
out-of-phase conditions shows the symmetrical
pattern to be slightly better than the asym-
metrical (p<.01). Clearly, a symmetrical, out-
of-phase signal can be expected to lead to higher
intelligibility than the asymmetrical, in-phase
signal that is commonly available to light-aircraft
pilots.

Preliminary to gathering the data, there was
some concern that scores might improve as sub-
jects learned the tasks. Tests of this possibility,
though, indicate no measurable order effects.

IV. Discussion.

Numerically, the aircraft tests show a much
smaller increase in intelligibility scores than was
noted in the laboratory tests. Two kinds of rea-
sons account for this difference—the first and
more obvious reasons are those associated with
those characteristics of the test environment that
are relatively detrimental to the auditory sys-
tem’s ability to analyze binaural signals; the
other reasons are a bit more subtle and are asso-
ciated with differences in the test materials. The
primary differences between the aircraft and the
laboratory environments are reverberation and
noise sources. In the laboratory tests, reverbera-
tion is kept to a level that is too small to measure;
in the airplane, the bulkheads are highly reflec-
tive, which permits more of the signal from the
contralateral loudspeaker to reach each ear, and
that in turn must be expected to decrease the
perceptual effect. In the laboratory, the noise
source was arranged so that identical noise wave-
forms reached both ears simultaneously; in the
airplane, noise sources include the combustion
noises of the engine, exhaust noise, the sounds
of air streaming past the fuselage, propeller noise,
airframe rattles, and so on. Thus, unlike the
situation in which an interaurally correlated,
laboratory-produced noise produces the lowest
possible baseline (as is the case for most MLD
and ILD experiments), the relatively uncor-
related noise in an aircraft-listening situation
produces a somewhat elevated baseline.

In the laboratory studies, the test materials
were long passages of continuous discourse. In

the aircraft study, the test lists comprised single
words, which are much more difficult to under-
stand because no context is available to offer cues
as to what is coming next. Therefore, a given
improvement in apparent signal-to-noise ratio
produces a smaller measured change in intelligi-
bility scores for the modified-rhyme test than for
the continuous-discourse test. In order to com-
pare the results of the two kinds of experiments,
it is not appropriate to investigate ILDs, since
they are so susceptible to variations in the test
materials. Instead, those intelligibility values
must be transformed into MLD values. In this
case, the unpublished data from the Naval Aero-
space Medical Research Laboratory on the modi-
fied-rhyme test show that the 12-15 percent
increase in intelligibility score with the inversion
of phase at one loudspeaker is equivalent to a
noise decrease of about 2 dB. For the kind of
speech material used in air-to-ground and ground-
to-air transmissions, 1 2-dB improvement might
be expected to produce, instead, an increase of
20 percent or so in intelligibility. In the labora-
tory situation where the baseline values are deter-
mined for highly correlated noise, and where the
reverberation is low, such a symmetrical, out-of-
phase presentation might produce an increase of
40 percent or more.

V. Conclusions.

Free-field tests of binaural unmasking show
that the human auditory system is capable of
improving speech intelligibility for symmetri-
cally placed, out-of-phase loudspeakers by an
amount equivalent to a 4-dB shift in signal-to-
noise ratio. Similar tests performed in the cabin
of a light aircraft show the improvement to be
equivalent to approximately 2 dB. The differ-
ence is attributable to a situation in which the
noise is inhomogeneous, and in which both the
signal and noise are subjected to modification by
reverberant walls. Yet, even in this difficult
listening situation, the brain’s ability to select
wanted from unwanted sounds can improve the
intelligibility of ground-to-air transmissions by
about 20 percent. In critical situations, such an
improvement in apparent signal-to-noise ratio
could make the difference between a safe and an
unsafe flight.





BINAURAL PROCESSING OF SPEECH IN LIGHT AIRCRAFT

I, Introduction.

Pilots of large jet aircraft do not often face
noise levels that will interfere significantly with
speech reception, but those who fly light planes
frequently do. The noisiest times can be the most
critical; for example, during takeoff, a tower
transmission may be heard but not understood.
Perhaps it is a message to someone else, but the
pilot cannot always be sure.

For most flyers, the use of earplugs or other
hearing protection would increase the speech in-
telligibility enough to keep the message clear,
but for those who cannot or will not wear such
protection, there has been no useful alternative
to missing an occasional message. However, there
are theoretically good approaches to message im-
provement under the borderline condition in
which the noise is just a bit too much for signal
clarity.

In this noisy situation, pilots commonly listen
to a single sound source (the cockpit loudspeaker)
immersed in a field of fairly homogeneous noise
(the engine and wind noises outside the plane
and the rattles and squeaks inside). The noise
masks the signal to a greater or lesser degree.
Typically, such a situation is no better than one
in which both signal and noise are heard with
only one ear (monaural listening). Yet, the
brain offers certain two-eared (binaural) ad-
yantages for appropriate signals, This study was
devised to test whether the inexpensive introduc-
tion of an unusual binaural signal into the cock-
pit would in fact improve speech intelligibility by
a significant and useful amount.*

* In these tests, an in-phase binaural signal was com-
pared with an out-of-phase binaural signal; both signals
were generated in two loudspeakers. In many airplanes,
the communication s one speaker, but the
effect of two in-phase loudspeakers is perceptually the
same as the effect of one loudspeaker located midway
between the two.2* We used the paired speakers in order
to insure that the radiated acoustic energy would remain
constant.

Monaural masked thresholds are relatively
poor, for a given amount of masking noise, com-
pared to certain binaural masked thresholds. The
binaural manipulations of signal and noise that
lead to these differences in masking level (mask-
ing-level differences or MLDs) have come under
systematic investigation since the late 1940s, when.
Hirsh,‘ ® Licklider,? and Hirsh and Webster®
first noticed a peculiar kind of what they termed
“binaural summation.” Since then, nearly every
study that has concerned itself with quantifying
MLDs has used earphones rather than loud-
speakers.in order to insure independent control of
each aspect of the signal at each ear.

In these earphone studies, the binaural ability
to select wanted from unwanted input signals
improves (over the monaural ability) by amounts
that are equivalent to an increase in level of a
tonal signal by as much as 15 or 20 dB (which
is equivalent to decreasing the noise by approxi-
mately the same amount), even though no actual
change in signal or noise sound pressure occurs
at either ear. The size of the change is striking.
Such large shifts in masked threshold, though,
only occur for non-intellective signals that con-
tain low-frequency energy. For high-frequency
signals, the changes are small, even under the
most effective of binaural conditions. (That
there is any MLD at all at high frequencies is
still somewhat mysterious; for a theoretical view
of the reasons, and also for an overview of mask-
ing-level-difference phenomena, see the papers
by Durlach* and Jeftress.*)

The maximum MDLs—the maximum changes
in masked threshold—occur for the binaural con-
ditions that are the most unusual and the least
likely to be encountered in the real world. For
example, one of the greatest improvements over
monaural masking is found in the situation in
which the same noise appears at both ears but the
signal appears at only one ear. Another way to