functions of a standard Békésy audiometer, and
so such an audiometer (Grason-Stadler model
E-800) was used as the foundation of the system.
Figure 1 shows the essential components that:
permit the modified use. The motor-driven
oscillator and the recording attenuator are part
of the Grason-Stadler audiometer; everything
else is extra. Several amplifiers, attenuators, and
switches are not shown—they were necessary to
handle gain and impedance problems, in this
particular case, but would not have been neces-
sary had different components been available.
‘At any rate, their inclusion would have cluttered
rather than clarified the figure.

The motor-driven oscillator from the audiom-
eter can be switched either directly to the record-
ing attenuator—for standard audiometry—or
into an amplitude-modulation system (such as
Grason-Stadler model E3382B) where its output
serves as the carrier for the probe in masking
or noise-audiometry measurements. The modula-

tion signal is a low-frequency noise source, reach-
ing, at the lower end, as close to de as possible;
the prototype uses a noise band of 5-75 Hz. The
output of the modulator, then, is a band of noise,
150-Hz wide, centered at the carrier-oscillator
frequency. Because the carrier is continuously
variable, the noise band can be shifted contin-
uously through the frequency range that is to be
tested. The test signal from the modulator is
then returned to the recording attenuator, and
automatic noise-band audiometry becomes pos-
sible.

(The construction is less complicated with the
Grason-Stadler 1701 audiometer, which has pro-
vision for modifying the tone before it enters the
recording attenuator without rebuilding or modi-
fication. However, the system can be put together
from individual components almost as easily as
it can be constructed even with the 1701 audiom-
eter’s special features, so one need not be deterred

Switch i
Switch
#\ #2
Motor- 94? { of
Driven
Oscillator |
t Recording
| [> jAttenuator
7 Electronic
Low- Carrier Input "| Modulator -—>| Switch |
Frequency #1 i}
Noise Modulation Input |
Source
\ M
Timer | |
|
| X
Timer | E »
J | a Switch R
t)
Electronic | #3 a
Mosker Attenuator Switch | )
#2 “a
b

Figure 1, Block diagram of noise audiometer.

3





tween tone and noise was less severe than that
between tone and tone, so the masking-pattern
curves lacked the irregularities and discontinui-
ties that had been nécessary to include in the
earlier study.

Wegel and Lane used a tone selected from one
of several frequencies available to them, and pre-
sented it at one of several constant sensation
levels. Then, in the presence of this fixed, tonal
masker, another tone was introduced—the probe
tone or test tone—and its threshold was deter-
mined. In the Egan and Hake procedure, the
tonal masker was replaced by a narrow-band-
noise masker, and in its presence, a probe-tone
threshold was measured. In each case, the
amount by which the test tone’s threshold was
elevated from a measurement made in the quiet
was taken as the masking effect of the original
tone or noise band on a tone of the test frequency.

For tonal maskers, letting the probe approach
the masking frequency leads to beats, and because
the beats are far easier to hear than the tone
itself, thresholds are improperly lowered in the
very range in which accuracy is more-than-
usually important. Because subjects cannot
separate the tone from the beat, measurement
loses precision throughout the frequency ranges
where beats occur.

The problem is both simplified and exaggerated
when the two tones are made identical in fre-
quency and phase. The beats vanish, to be sure,
but the listener can no longer differentiate be-
tween the test tone and the masker in any way.
At the time that this difficulty was most trouble-
some, George Miller‘ had not. yet noted that a
differential threshold is equivalent to 2 measure-
ment of the masking of a signal by itself.

When Egan and Hake? substituted a narrow
band of noise for the fixed masker, they overcame
most of the difficulties inherent in the older pro-
cedure. Still, they were limited by their equip-
ment to a single center frequency—410 Hz—for
their noise, which was 90-Hz wide at the half-
power points. They also thought that they had
a problem because their noise band was less wide
than a critical band, but that concern turns out
to have been unnecessary and inappropriate.

The data from Egan and Hake’s work is
smooth rather than discontinuous, but otherwise
shows strong similarities to the Wegel and Lane
work, and fills in some questionable segments

with sensible and reasonable values. If there are
flaws in the work, they are only that the available
filters limited the number of testable frequencies,
and that the measurement of discrete-frequency
thresholds is time consuming. The idea of using
noise bands for the measurement of tonal mask-
ing, though, is very good. If the Egan and Hake
technique could be reversed, it would be even
better. That is, if the test tone (rather than the
fixed masker) were a narrow-band noise, then
masking audiograms for any interfering signals
could be made without concern for the confound-
ing interactions of nearly matched tones. Ideally,
such a procedure would also incorporate contin-
uous frequency and sound-pressure variation
(rather than discrete), both for complete cover-
age of the ranges under investigation, and for
ease of testing. In other words, the method
would be the sort of Békésy-audiometric device
that this paper describes.

Ill. Design.

A masking pattern is a graph of the inter-
fering effects of a signal on other signals,
throughout the acoustic spectrum. If a noise-
band audiometer is to be used for measuring
masking patterns, it must meet a number of
criteria (and, as it turns out, if it meets the
criteria for masking audiometry, it also meets
the criteria for a number of other diagnostic and
research uses) :

1. it should allow the threshold of the inter-
fering signal to be measured,

2. it should allow probe-threshold determina-
tions both in the presence and in the absence of
the interfering signal,

3. it should permit continuous frequency vari-
ation,

4. it should not allow either the interfering
signal or the probe (the test band of noise) to
fatigue the listener,

5. it should not allow beats or difference tones
between the probe and any component of the
interfering signal, no matter what that signal
might be, and

6. it must permit the subject to distinguish
easily between the interfering signal and the
probe.

In constructing a prototype, it was considered
that the major functions were related to the


from trying the device on grounds of equipment
complexity.)

One problem that becomes obvious in the study
of masking patterns is that listeners can some-
times show fatigue effects from continuous ex-
posure to the masker. To overcome that difficulty,
two gates or electronic switches must be added.
One, in the masking channel, interrupts that
signal for 200 msec out of every 800 (Fig. 2).
The other turns the probe on for 400 msec, cen-
tered in the on-cycle of the masker. The program
thus starts with the masker coming on for 100
msec, Then, while it is still on, the probe tone
comes on for 400 msec, and goes off while the
masker continues for another 100 msec. Finally,
after 200 msec of silence, the cycle is repeated.
Subjects always find this to be an easy way to
distinguish the masker from the test tone, and
additionally, the masker serves as a kind of
pedestal upon which the test tone appears. The
pedestal becomes a reference signal, and the sub-

in position a, a masking-signal threshold can be
plotted; (2) with Switches #1, 2, and 3 in posi-
tions b, b, and a, respectively, a noise-band
audiogram is plotted, and with all the Switches
at b, a masking audiogram with a noise-band
probe is produced; (3) continuous frequeney
variation results from the use of a motor-driven
beat-frequency oscillator; (4) fatigue is kept
minimal by the gating arrangement; (5) beats
and difference tones are no problem because the
probe is a noise band; and (6) the introduction
of the pedestal permits the listener to distinguish
between the masker and the probe with little
difficulty.

TaBLe 1—Functions of the system shown in Fig. 1.

Switch Positions Function.

Switch Switch Switch

. we Pr #1 #2 #8
ject’s task is simply to discriminate whether a
change occurred. If it did, he responds; if it any oa any | Masking-signal threshold
. , Tonal audi
did not, he waits for the change to happen. , iM : Noiseban 4 udiogram
a ob b Masking audiogram with tonal
IV. Results. probe
The six criteria for a masking audiometer are poe > eee with nolse-
met, as listed in Table 1: (1) with Switch #2
200
msec.
MASKER |__| 600 msec ——>}eTM2°55]¢___ 600 msec ——>
ON
PROBE OFF J<— 400 msec—>|<400 msec>]< 400 msec->|
n 1 1 1 1 1 1 1 1 1 1 r 1 J
ver)
100 msec TIME >

Ficure 2. Switching program for noise audiometer.

4


TECHNICAL REPORT STANDARD TITLE PAGE

T. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
FAA-AM-71-1 :
4. Title and Subtitle 5. Report Date

NOISE AUDIOMETRY January 1971

6. Performing Organization Code

7 Auto". Jerry V. Tobias, Ph.D. 8. Performing Organization Report No.
9. Performing Organization Name and Address 10. Work Unit No.

FAA Civil Aeromedical Institute, P. 0. Box 25082,

Oklahoma City, Oklahoma 1. Contract or Grant No.

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. 20590

15. Supplementary Notes

16. Abstract
The displacement of a threshold from its measured-in-the-quiet value to the value
it takes in the presence of another sound is masking. Measurement of that dis-
placement is masking audiometry. And the measurement of displacements at a large
number of frequencies produces masking patterns. This paper concerns itself
with a procedure that produces masking patterns with good precision, sensitivity,
and rapidity without the problems of tonal interference and beats that normally
interfere with the determination of masking patterns. Several applications of the
techniques are suggested, including one for determining the auditory effects
produced by aircraft noises, and one for testing hearing protectors.

17. Key Words TB Diapapenigy Sereent
1

Availal ity is unlimited. Document may
Audiometry, Hearing, Instrumentation be released to the Clearinghouse for
Masking, Noise

Federal Scientific and Technical Infor-
mation, Springfield, Virginia 22151,
for sale to the public.

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

Unclassified Unclassified $3.00

Form DOT F 1700.7 (e-69)



Thresholds can still be measured in the usual
ways (Table 1), so standard audiometry is pos-
sible without tearing down the equipment. But
most important of all, the system allows any
kind of signal to be investigated for its masking
properties, no matter how complex, and no matter
how filled with tonal components, without par-
ticular influence from beats. The same device
can be used to study the masking effects of tones,
noise bands, recordings of vowels, of aircraft-
engine sounds, and indeed of whatever signals
one wishes,

Further, the system allows the option of using
a tonal probe, as Wegel and Lane did. It also
allows the use of discrete frequency variation by
simply disconnecting the oscillator motor, or of
limited-frequency-range testing by operating the
motor slowly.

Vv. Application.

As a diagnostic device, the noise audiometer
is at least as accurate as pure-tone audiometers,
although its major value in this function may
only be that it produces a signal that is unlikely
to confuse naive listeners about what is expected
of them. Further, it solves the problems of
audiometry in patients with tinnitus and with
monaural diplacusis. It is usable both for dis-
crete-frequency and continuous-frequency tests.
It can discriminate between patients with Jer-

ger’s? Type II audiometric patterns and all
others.* When it is used to operate a bone-
conduction receiver, it offers for the first time a
Rainville-Jerger (SAL) technique producing
data throughout the audible frequency range.

The noise audiometer also permits rapid test-
ing of ear-protective devices (for this function,
it is used to drive a loudspeaker), and it allows
the tests to be made without concern about
standing-wave patterns that would destroy the
validity of the results. It allows rapid deter-
mination of the immediate effects of any environ-
mental (or other) noise on hearing, and it does
so without confounding the data with acoustic-
beat artifacts. The most obvious application of
these techniques is to the measurement of the
ways in which aircraft noise can interfere with
normal auditory processes.

The method is quite precise despite the inherent,
variability of Békésy audiometry. And what
loss in precision one does have to accept is made
up for in the ease of test administration.

The cost of incorporating a noise audiometer
into an audiological armamentarium can vary
over a wide range. For most physicians, the
price would probably be excessive, but for a
facility with advanced equipment already in use,
the device can be added at fairly low cost and
minimal time.

REFERENCES

”

. Egan, J. P. and Hake, H. W. On the masking pattern
of a simple auditory stimulus. Journal of the Acous-
tical Society of America, 22:622-630, 1950.

2, Greenwood, D, D. Auditory masking and the critical
band. Journal of the Acoustical Society of America,
83 1484-502, 1961.
Jerger, J. Békésy audiometry in analysis of auditory
disorders. Journal of Speech and Hearing Research,
3 :275-287, 1960.

Miller, G..A. Sensitivity to changes in the intensity
of white noise and its relation to masking and loud-

-

»

ness. Journal of the Acoustical Society of America,
19 :609-619, 1947.

Palva, T., Goodman, A., and Hirsh, I. J. Critical
evaluation of noise audiometry. Laryngoscope,
68 :842-860, 1953.

6. Simon, G. R. and Northern, J. L. Automatic noise-
band audiometry. Journal of Auditory Research,
6 403-407, 1966.

Wegel, R. L. and Lane, ©. E. The auditory masking
of one pure tone by another and its probable relation
to the dynamics of the inner ear. Physical Review,
23 :266-285, 1924.

a

2





NOISE AUDIOMETRY

I. Introduction.

The basic components of a common, general-
purpose audiometer are a tone generator, an
attenuator, perhaps a switch, and something to
transduce the generated signal into an acoustic
signal. The resulting test tone is almost invari-
ably the stimulus chosen for clinical measure-
ment, and frequently is picked for laboratory
tests. This paper describes an audiometric de-
vice whose test signal is not a tone, but a narrow
band of noise.

The concept of a test signal that is, in reality,
a band of noise is certainly not new. Palva,
Goodman, and Hirsh® reviewed much of the his-
tory of this sort of testing. However, previous
applications were to specific research tasks, were
cumbersome to use, and were slow in providing
data—far too slow for the clinic, and often less
than ideal for the laboratory.

A. noise audiometer differs from narrow-band
audiometric masking devices in that the noise
band serves as the stimulus to the ear being
tested rather than as something to interfere with
responses at the opposite ear. Too, the band-
width is narrower (but variable according to the
needs of the user), and the intensity of the sound
is continuously variable so that audiometric
thresholds can be measured. In the CAMI proto-
type, the center frequency of the noise band is
also continuously variable. In its diagnostic
configuration, the device is a Békésy audiometer
that uses a noise-band rather than a tonal signal.
Thus, it overcomes problems of complexity of
design and of use, it eases the interpretation of
results, and it provides rapid testing and repeat-
able results.

Such an instrument is advantageous because
it is resistant to the problems of testing patients
with tinnitus, because it is capable of permitting
rapid testing of hearing protectors in sound
fields where standing-wave patterns would de-
stroy the validity of the results, because it is
essentially impervious to monaural-diplacusis

effects, and because it can allow the rapid deter-
mination of masking patterns in normal or
pathological ears without confounding the data
with acoustic-beat artifacts,

Il. Background.

The displacement of a threshold from its
measured-in-the-quiet value to the value it takes
in the presence of another sound is called mask-
ing. Measurement of that displacement can be
called masking audiometry, and the measurement
of a series of such displacements across the
acoustic spectrum produces graphs that are rep-
resentative of mashing patterns—pictures of the
auditory system’s responses to various interfer-
ing signals. These patterns indicate something
about the function of the basilar membrane and
of the auditory nervous system, and they are
critical to an understanding of how we perceive
pitch, so masking patterns have been a matter
of interest to auditory theorists for a long while.

In 1924, Wegel and Lane’ made their first
systematic explorations of the masking effects of
one tone on another. Their procedure, although
tedious, was thorough, and permitted them to
describe almost everything in the universe of
sensations produced by tonal interactions. How-
ever, every one of their measurements included
conditions in which these very interactions inter-
fered with adequate measurement of the mask-
ing. The greatest problems arose when their
two tones were close enough in frequency to beat,
although further difficulties occurred at harmonic
partials when combination tones and subjective
harmonics interfered similarly.

Later investigations tried in several ways to
overcome these confounding and obscuring sen-
sations, and the best of these techniques came
from the 1950 work of Egan and Hake. They
replicated parts of the Wegel and Lane study,
but they did it using a narrow band of noise in
place of the tone whose masking effect was being
investigated. Necessarily, the interaction be-