backward masking for tones in narrow-band noise

THE JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA VOLUME 36, NUMBER 11 NOVEMBER 1964

Backward Masking for Tones in Narrow-Band Noise

H. N. W•m•rT*

Laboratory of Sensory Communication, Syracuse University, Syracuse, New York (Received 22 June 1964)

Appropriate transformations based on the theory of temporal summation were used to determine backward masking as a function of the time interval between the onset of 800-, 1000-, 1200-, and 1750-cps tones and the onset of an 80-dB narrow-band noise centered at 1000 cps that partially masked the tones. Backward masking was found to consist of at least two phases separated by a transition region inwhich threshold did not appear to be determined by the duration of the probe tone. Masking at 800, 1200, and 1750 cps was found similar to that at 1000 cps, but did not extend as far back in time. The results are interpreted to mean that masking first occurs for those tones within a noise band and then spreads to those frequencies outside the band until the pattern commonly observed under simultaneous masking is obtained.

ASKING has been shown to be constant as a function of the time interval between the onset

of a masking noise and the onset of the masked tones. 1,a Moreover, the intensity required to obtain threshold for tones within and outside a narrow-band noise as a function of the duration of each tone has been found to

be equivalent to that required in the steady state? Both the steady-state masking pattern and temporal-summation characteristics associated with masking, a-9 there- fore, appear established either before or at the instant that a masking sound is presented.

* Also' State University of Xew York, Upstate Medical Center, Syracuse, N.Y.

• J.P. Egan, "Independence of the Masking Audiogram from the Perstimulatory Fatigue of an Auditory Stimulus," J. Acoust. Soc. Am. 27, 737-740 (1955).

2 E. Zwicker and H. N. Wright, "Temporal Summation for Tones in Narrow-Band Noise," J. Acoust. Soc. Am. 35, 691-699 (1963).

a R. Feldtkeller and R. Oetinger, "Die H6rbarkeitsgrenzen yon Impulsen versch!.edener Dauer," Acustica 6, 489-493 (1956).

4 H. Scholl, "Uber die Bildung der Horschwellen und Mithor- schwellen von Impulsen," Acustica 12, 9t-101 (1962).

• W. R. Garner and G. A. Miller, "The Masked Threshold of Pure Tones as a Function of Duration," J. Exptl. Psychol. 37, 293-303 (1947).

• W. R. Garner, "Auditory Thresholds of Short Tones as a Function of Repetition Rates," J. Acoust. Soc. Am. 19, 600-608 (1947).

* J. E. Hawkins, Jr., and S.S. Stevens, "The Masking of Pure Tones and of Speech by White Noise," J. Acoust. Soc. Am. 22, 6-13 (1950).

8 j.p. Egan and H. W. Hake, "On the Masking Pattern of a Simple Auditory Stimulus," J. Acoust. Soc. Am. 22, 622-630 (1950).

s R. Plomp and M. A. Bouman, "Relation between Hearing Thresholds and Duration for Tone Pulses," J. Acoust. Soc. Am. 31, 749-758 (1959).

The masking produced by a narrow-band noise with a width of one critical band and centered at 1000 cps apparently extends backward in time by as much as 200 msec, at least at 1000 cps (Ref. 10). By using appropriate transformations based on the theory of temporal summation, TM the instantaneous backward masking produced by this narrow-band noise was found to consist of a short phase immediately preceding the onset of the masking sound, in which threshold is dependent upon the level of the masker; a long phase, in which threshold is independent of the level of the masker; and a transition region that lies between the short and long phases. •ø The purpose of this investigation was to extend these findings at 1000 cps to tones outside the noise band so that additional information would be

obtained on the way in which simultaneous masking is acquired.

I. PROCEDURE

The procedure used in the following experiments was essentially the same as that used previously at 1000 cps (Ref. 10), with the exception that the level of the narrow- band noise remained constant at 80 dB (Ref. 12) and the frequency of the masked tone was varied. The specific paradigm is shown in Fig. 1. The narrow-band noise had a constant duration (t•) of 600 msec and a

•0 H. N. Wright, "Temporal Summation and Backward Mask- ing," J. Acoust. Soc. Am. 36, 927-932 (1964).

• J. Zwislocki, "Theory of Temporal Auditory Summation," J. Acoust. Soc. Am. 32, 1046-1060 (1960).

•'-Measured with a true rms voltmeter, Ballantine model 320.

2217

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2218 H.N. WRIGHT

width equivalent to one critical band 1• at 1000 cps (A f= 162 cps). Such being the case, this noise exhibited a simultaneous masking pattern equivalent to that for a 1000-cps tone without the irregularities produced by beats.14,15

Monaural thresholds were measured in 2-dB steps as a function of the time interval (ti) between the onset of the tones and the onset of the noise, using the bracket- ing method. 2,1ø Tone and noise were alternated with noise alone at 3-see intervals so that a reasonably constant criterion could be maintained for the detection

of the tones when presented with the noise and also the variability among subjects kept at a minimum? When presented with the noise, the tones were always terminated 100 msec before the noise itself was terminated.

The time interval between onset of the tones and the

onset of the noise was specified at the half-power points when they were at equal amplitude. The rise-fall time of the noise and tones was 10 msec to avoid contami-

nation of the results by switching artifacts. •6.•7

II. PRELIMINARY EXPERIMENT

An indication of how the intensity required for threshold increases as the time interval between the onset of the tones and the onset of the noise decreases

was obtained by measuring threshold at 19 frequencies and 5 time intervals. Threshold was first measured at

1000 through 1200 cps in intervals of 50 cps; 1200 through 1500 cps in intervals of 100 cps; and at 1750, 2000, 2500, and 3500 cps. Threshold was then repeated at 1000 cps and measured through 800 cps in intervals of 50 cps and from 800 through 500 cps in intervals of 100 cps. During each threshold series, the time interval

TIME

PURE TONE

Ftc. 1. Tone and noise sequence for threshold measurement. ti is the time interval between tone and noise onset; tp is the duration of the probe tone; t, is the duration of the noise. Tone and noise were alternated with noise alone every 3 sec.

18 E. Zwicker, "Subdivision of the Audible Frequency Range into Critical Bands (Frequenzgruppen)," J. Acoust. Soc. Am. 33, 248 (L) (1961).

14H. Bauch, "•ber die Sonderstelling periodischer kurzer Drukimpulse bei der Empfindung der Zautstarke," Acustica 6, 494-511 (1956).

15 ' "" ' E. Zwmker, Uber psychologmche und methodische Grund- lagen der Lautheit," Acustica 8, 257-258 (1958).

• H. N. Wright, "Audibility of Switching Transients," J. Acoust. Soc. Am. $2, 158(L) (1960).

• H. N. Wright, "Switching Transients and Threshold Deter- mination," J. Speech & Hearing Res. 1, 52-60 (1958).

between tone and noise onset was held constant at

either 500, 100, 30, 25, or 0 msec. Under these conditions, threshold at ti = 0 msec represents simultaneous masking for a 500-msec tone. Similarly, thresholds at ti=500 msec were assumed to represent thresholds for 500-msec tones in quiet. This assumption seemed reasonable in light of previous findings at 1000 cps (Ref. 10) and was further tested in the main experiment.

A. Results

The smoothed curves shown in Fig. 2 represent the results for a single subject. Only in a few rare instances did the actual data deviate from the smoothed curves

shown here by more than 2 dB. These results indicated that backward masking should indeed provide some information on the acquisition of the simultaneous masking pattern observed for this type of noise. Apparently, the intensity increase required to obtain threshold first

u_

•u õ PARAMETER tn o t i in msec.

•" '• 60 zo -o

o3 o /

.J,- 20 og

.-•- 0 •---

500 I000 2000 4000 70O 1500

FREQUENCY IN CYCLES PER SECOND

FIG. 2. Threshold shift as a function of frequency for a single subject. Parameter is ti in msec. All values referred to those obtained at ti-- 500 msec. (ti = 100, 30, 25, and 0 msec; tp--600, 530, 525, and 500 msec; t• = 600 msec.)

occurs at the center frequency of the noise and then spreads to adjacent frequencies. The results shown in Fig. 2 are only qualitative. There are not a sufficient number of time intervals to determine exactly how much of the threshold shift was due to the effective

shortening of the tones and how much can be attributed to backward masking. The theory of temporal sum- marion n has been found useful to resolve this inde-

terminancy at 1000 cps (Ref. 10) and was the procedure followed in the next experiment in which instantaneous backward masking was determined at discrete frequencies.

III. MAIN EXPERIMENT

The foregoing has shown that acquisition of the masking pattern associated with a narrow-band noise can be demonstrated by measuring backward masking for tones outside the noise band. The present experiment


BACKWARD MASKING IN NARROW-BAND NOISE 2219

concerns itself with specifying backward masking at discrete frequencies over a greater number of time intervals so that instantaneous backward-masking functions could be determined at each frequency. Threshold was measured at 500, 800, 1200, 1750, and 3500 cps when the time interval between the onset of the tones and

the onset of the noise was varied over 11 values between

500 and 0 msec. Threshold was also measured at each

frequency when the duration of the tones was 500 msec.

A. Results

The mean threshold shift for three subjects as a function of the time interval between tone and noise onset

at each frequency is shown in Figs. 3 and 4. The results at 1000 cps are those obtained previously for this noise under identical experimental conditions. TM The threshold

8O FREQUENCY IN CP•

70 - I000 0 800 /%

J 500 X L• 60 I:D ß

,", ,50-

I- 40

m 30-

o :z 20

:z IO-

0-& • • I

500 400 300 200 IO0 TONE-NOISE INTERVAL IN MILLISECONDS

Fro. 3. Threshold shift during backward masking as a function of time interval between tone and noise onset. Parameter is fre-

quency in cps. Closed symbols represent simultaneous masked threshold for 500-msec tones. (ti varied between 500 and 0 msec; tp varied between 1 sec and 500 msec; t• =600 msec.)

80 FREQUI•NCY IN CIPS

3500 0 _ 1750 V

1200 []

-- _

_

_

I I

7O

,,'d 60

c• 50

• 4o 30

.J o ::[: 20

• •o

o

500 400 300 200 ioo o TONE-NOISE INTERVAL IN MILLISECONDS

Fro. 4. Threshold shift during backward masking as a function of time interval between tone and noise onset. Parameter is frequency in cps. Closed symbols represent simultaneous masked threshold for 500-msec tones. (ti varied between 500 and 0 msec; tp varied between 1 sec and 500 msec; t•=600 msec.)

moved from the simultaneous masked threshold; and a transition region between the two.

Figure 6 shows the first step necessary to determine the effective duration of the tones at each frequency. The expected function owing to the shortening of the duration of a tone is derived from the theory of temporal summation. TM The threshold shift as a function of the tone-noise interval at each frequency is shown with the intervals set equal to the duration of the tones. If there were no backward masking, we would expect the threshold shift at each tone-noise interval to equal that of the expected summation function. Because of backward masking, however, the duration of each tone is less than the tone-noise interval indicates. By relating the amount of threshold shift at each interval to the ex-

pected summation function, the effective duration o the tones can be determined at each tone-noise interval

in quiet for 500-msec tones was equivalent (4-2 dB) to those obtained at ti=500 msec. Consequently, all _•ao thresholds are referred to those obtained when ti= 500

m60 msec.

Subsequent analysis of these data showed that the results obtained at 500 and 3500 cps were not useful •4o because their effective duration could not be determined

at each tone-noise interval. This was not anticipated, but did indicate that the approach used here to determine the effective duration of the tones at each • o interval is limited by the amount of threshold shift observed under simultaneous masking.

The results shown in Fig. 5 for 800, 1200, and 1750 cps are similar in form to those obtained previously at 1000 cps. That is, there seems to be an interval over which threshold is approximately the same as that obtained under simultaneous masking; an interval re-

FREQUEN'CY IN CPS I 800 A

IOOO 0 1200 [] 1750 •

500 250 I00 50 25 I0 TONE - NOISE INTERVAL IN MILLISECONDS

Fro. 5. Threshold shift during backward masking as a function of time interval between tone and noise onset. Parameter is frequency in cps. Closed symbols represent simultaneous masked threshold for 500-msec tones. (ti=500, 400, 300, 200, 100, 50, 25, 20, 15, 10, and 0 msec; tp--1000, 900, 800, 700, 600, 550, 525, 520, 515, 510, and 500 msec; t•=600 msec).


2220 H. N. WRIGHT

,o j, • 2o

z_ 15

•o •o

.I I I

• EXPECTED

FREOUENCY IN CPS •< - - 800

iooo o 't 12oo [] _ 1750

I I0 I00 500 DURATION IN MILLISECONDS

Fro. 6. Expected threshold elevation required by the theory of temporal summation if there were no backward masking (ti-- tone duration) and the values obtained between time intervals of 500 and 25 msec. Parameter is frequency in cps.

The threshold shift for a 1750-cps tone at ti = 50 msec, for example, is 12 dB. Taking this threshold shift, we can determine the effective duration of this tone under

the present conditions by determining what duration tone would produce a threshold shift of 12 dB. It turns out that, when a tone is shortened from 500 to 12 msec, 12 dB more intensity will be required to obtain threshold. Similar calculations at each frequency for each tone-noise interval yield the transformations shown in Fig. 7.

A few remarks about these transformations are now

in order. The threshold obtained at 500 and 3500 cps (Figs. 3 and 4) could be transformed into effective durations only over a limited number of time intervals. This was because the intensity increase required to obtain threshold due to the effective shortening of these tones exceeded the threshold shift observed under simul-

taneous masking. Such being the case, there was not a clear delineation between backward and simultaneous masking. These results indicated that the method used here for determining the effective duration of backward- masked tones and subsequent specification of instan-

5OO

• •50

• • •oo

z•_• 5o

25

Fro. 7. The effective duration of the tones because of backward masking as a function of tone-noise interval in msec. Parameter is frequency in cps.

taneous backward masking is appropriate only when the threshold shift produced by the masking sound exceeds approximately 30 dB.

Figure 5 shows that as the tone-noise interval is de- creased the intensity required for threshold at each frequency continuously increases. A region is reached, however, where the intensity required to obtain threshold not only remains relatively constant as a function of the tone-noise interval, but also approximates that obtained under simultaneous masking. This plateau was observed earlier in 40-, 60-, and 80-dB noise (Ref. 10) and was called the short phase of backward masking. In the present experiment, a short phase of backward masking also seems to occur. Its extent, however, appears to vary with frequency.

Returning to the transformations shown in Fig. 7, the analysis thus far permits us to determine the amount of instantaneous backward masking at 800, 1000, 1200, and 1750 cps. Specifically, by knowing the effective duration of a tone and also the amount of threshold

80

c• 40

z•20

• o

FREQUENCY IN CPS 800 A ! I iooo o I

/ 12_.00 [] /El 1750 V / /

//

//

500 200 I00 50 20 I0 TONE-NOISE INTERVAL IN MILLISECONDS

Fro. 8. Instantaneous threshold shift as a function of tone-noise

interval. Parameter is frequency in cps. Closed symbols represent thresholds obtained for 500-msec tones at ti= 0 msec.

shift at each time interval, the amount of instantaneous backward masking can be specified exactly. The 1750- cps tone presented at ti= 50 msec, for example, had an effective duration of 12 msec. This result is interpreted to mean that the masking effect of the noise apparently

i , , extended backward in time by 38 msec. The 12-dB - PARAM threshold shift is then the amount of instantaneous

backward masking for the 1750-cps tone at 38 msec. - Similar calculations at the other frequencies and

time intervals yield the instantaneous backward- masking functions shown in Fig. 8.

•ooO IV. DISCUSSION

The results of the foregoing experiments are inter- _ ,•o preted to mean that the masking pattern observed under

, , i simultaneous masking, at least for narrow-band noise, I0 I00 500

EFFECTIVE DURATION IN MILLISECONDS begins at the center frequency of the noise and then spreads to adjacent frequencies. It would appear that the growth of masking outside the noise band is related to the rate of growth at the center frequency, and not


BACKWARD MASKING IN NARROW-BAND NOISE 2221

the other way around. This interpretation may seem somewhat contrary to the conclusion reached by Scholl •s that the acquisition of masking has a time constant of 10 msec. Close examination of his experiment, however, reveals that this conclusion was based on the threshold

for a short tone presented between two narrowly spaced noises. It could well be that Scholl's results reflect the

interaction of two adjacent masking sounds, instead of the way in which masking is acquired by a single narrow- band noise.

The instantaneous threshold shift obtained for tones

outside the noise band are quite similar to those previously obtained at 1000 cps in different levels of noise. There seems to be a short phase, a long phase, and a transition region between the two. The broken portion of the curves shown in Fig. 8 represents the transition phase at each frequency.

The short phase is of particular interest, primarily because the time intervals over which it extends appear to vary with frequency. Because the amount of simultaneous masking also varies with frequency, it might appear as if the extent of the short phase of backward masking depends on the steady-state masked threshold. The earlier results at 1000 cps (Ref. 10), however, indicate that the extent of this phase of backward masking does not vary with different amounts of simultaneous masking. Consequently, the extent of the short phase of backward masking observed here for tones outside the noise band probably depends more on frequency than on the amount of simultaneous masking.

•8 H. Scholl, "f)ber die Bildung der H6rschwellen und Mith6r- schwellen von Impulsen," Acustica 12, 91-101 (1962).

If such is the case, these results suggest that time and frequency information are traded in the detection of backward-masked tones. Furthermore, the change in the extent of the short phase with a change in frequency can be interpreted to mean that the presumed relation between the perception of temporal order •ø.2ø and backward masking may only be the approximate 20-mesc time interval required for each under specific experimental conditions.

The long phase at 1000 cps has been shown not to vary as a function of the amount of threshold shift. •ø This finding was interpreted to mean that the long phase probably represents a more central effect than the short phase. Consequently, the long phase of masking observed here for frequencies outside the noise band may also reflect a more central process than the short phase.

In any event, the present results indicate that masking first occurs for those tones within a noise band and then spreads to those frequencies outside the band until the pattern commonly observed under simultaneous masking is obtained.

ACKNOWLEDGMENT

This research was supported in part by the National Institute of Neurological Diseases and Blindness, Na- tional Institutes of Health, U.S. Department of Health, Education, and Welfare.

•0 I. J. Hirsh, "Auditory Perception of Temporal Order," J. Acoust. Soc. Am. 31, 759-767 (1959).

•0 I. J. Hirsh and C. E. Sherrick, "Perceived Order in Different Sense Modalities," J. Exptl. Psychol. 62, 423-432 (1961).


backward masking for tones in narrow-band noise

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