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Hearing Research, 13 (1984) 221-228 Elsevier HRR 00466 221 Binaural masking-level differences in non-simultaneous masking Eberhard Zwicker and U. Tilmann Zwicker Institute of Electroacoustics, Technical University Munich, Munich, E R. G. (Received 13 September 1983; accepted 19 December 1983) Masking and binaural masking-level differences (BMLDs) were measured using short 400 and 800 Hz test tones masked by uniform masking noise in both pre- and post-masking conditions. The BMLD shows the same dependence on the temporal position of the test signal as masking itself. Additional data produced with interrupted broad-band masker and with low-frequency tonal masker lead to the conclusion that more information about temporal structure is transmitted towards higher levels of processing than can be seen in simple post-masking experiments. binaural, non-simultaneous masking Introduction Models describing the binaural cooperation of both ears in lateralisation, localization, or in bin- aural masked thresholds [3,4,7] have been devel- oped principally for simultaneous presentation of masker and test signal. Since the investigation of Small et al. [8] these models may need to be extended because BMLDs can also be found with non-simultaneous presentation. This effect had first been discovered by Detherage and Evans [2]. However, Small et al. have worked out the conse- quences for understanding the binaural processing quite clearly. Their results show that the binaural masked thresholds occur in both pre- and post- masking and that both conditions show BMLDs of approximately the same size. These effects have been confirmed in further experiments [1,5,6,9]. In the temporal configuration used in many of these experiments, either the masker and test tone were overlapping, or the maskers were only short sound impulses. Both pre- and post-masking conditions were used. It must be concluded from the results known so far, that the human binaural processing incorporates a storage with a relatively long stor- age time. In the present study apart from systematic in- vestigations using the same group of subjects in all conditions, the measurement of BMLDs for pre- and post-masking was the main goal in order to get more information about the storage used by the binaural processing system. The present study is an extension of an earlier investigation [17] and the test-tone frequencies of 400 and 800 Hz were chosen to be the same in both studies. The largest BMLD is found between a condition in which the binaural test tone is in-phase at the ears, S,, and a condition in which it is out-of-phase, S,; the masker is binaurally in phase, N,, in both condi- tions. This comparison was made throughout this study unless otherwise stated. Apparatus and Method The experimental setup and method described in our earlier paper [17] was used again. Four subjects took part in the experiments and each subject made two measurements per stimulus con- figuration. In a BCkesy tracking procedure a motor driven attenuator with a speed of 1.5 dB/s was used. The mean of at least 12 recorded extrema was defined as threshold. Medians and interquar- tile ranges obtained from the eight data points are given in the figures. The measurements for the N,S,, and N,S,, interaural phase relations were taken immediately after one another in order to 0378-5955/84/$03.00 0 1984 Elsevier Science Publishers B.V.

Binaural masking-level differences in non-simultaneous masking

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Page 1: Binaural masking-level differences in non-simultaneous masking

Hearing Research, 13 (1984) 221-228

Elsevier

HRR 00466

221

Binaural masking-level differences in non-simultaneous masking

Eberhard Zwicker and U. Tilmann Zwicker Institute of Electroacoustics, Technical University Munich, Munich, E R. G.

(Received 13 September 1983; accepted 19 December 1983)

Masking and binaural masking-level differences (BMLDs) were measured using short 400 and 800 Hz test tones masked by

uniform masking noise in both pre- and post-masking conditions. The BMLD shows the same dependence on the temporal position of

the test signal as masking itself. Additional data produced with interrupted broad-band masker and with low-frequency tonal masker

lead to the conclusion that more information about temporal structure is transmitted towards higher levels of processing than can be

seen in simple post-masking experiments.

binaural, non-simultaneous masking

Introduction

Models describing the binaural cooperation of both ears in lateralisation, localization, or in bin-

aural masked thresholds [3,4,7] have been devel- oped principally for simultaneous presentation of masker and test signal. Since the investigation of Small et al. [8] these models may need to be extended because BMLDs can also be found with non-simultaneous presentation. This effect had first

been discovered by Detherage and Evans [2]. However, Small et al. have worked out the conse- quences for understanding the binaural processing quite clearly. Their results show that the binaural masked thresholds occur in both pre- and post- masking and that both conditions show BMLDs of

approximately the same size. These effects have been confirmed in further experiments [1,5,6,9]. In the temporal configuration used in many of these experiments, either the masker and test tone were

overlapping, or the maskers were only short sound impulses. Both pre- and post-masking conditions were used. It must be concluded from the results known so far, that the human binaural processing incorporates a storage with a relatively long stor- age time.

In the present study apart from systematic in- vestigations using the same group of subjects in all

conditions, the measurement of BMLDs for pre- and post-masking was the main goal in order to get more information about the storage used by

the binaural processing system. The present study is an extension of an earlier investigation [17] and the test-tone frequencies of 400 and 800 Hz were chosen to be the same in both studies. The largest BMLD is found between a condition in which the

binaural test tone is in-phase at the ears, S,, and a condition in which it is out-of-phase, S,; the masker is binaurally in phase, N,, in both condi- tions. This comparison was made throughout this study unless otherwise stated.

Apparatus and Method

The experimental setup and method described in our earlier paper [17] was used again. Four subjects took part in the experiments and each subject made two measurements per stimulus con-

figuration. In a BCkesy tracking procedure a motor driven attenuator with a speed of 1.5 dB/s was

used. The mean of at least 12 recorded extrema was defined as threshold. Medians and interquar- tile ranges obtained from the eight data points are given in the figures. The measurements for the N,S,, and N,S,, interaural phase relations were taken immediately after one another in order to

0378-5955/84/$03.00 0 1984 Elsevier Science Publishers B.V.

Page 2: Binaural masking-level differences in non-simultaneous masking

50 1ooms2ocl

Fig. 1. (a and b) Post-masking thresholds, I.,, of 10 ms test tones masked by uniform masking noise bursts of 300 ms duration

(I.,$, = 70 dB) as a function of the delay time, cd, between the end of the masker and the end of the test signal. O------O, N,S,

configuration of test signal and masker. 0. . . .O, the N,,S,, configuration. a----- 0, individual BMLDs with AL+ on the ordinate.

AI1 symbols are medians of g measurements with four subjects; interquartile ranges are also given. THQ indicates threshold in quiet.

(a) fr = 400 Hz; (b) fT = 800 Hz. (c and d) Pre-masking thresholds, for which the time difference Al between the onset of the test

signal and the onset of the masker is abscissa. (c)fr = 400 Hz; (d)fr = 800 Hz.

d8 60

achieve relatively accurate values for the BMLD. If not otherwise stated, uniform masking noise, low-pass filtered at 20 kHz with 70 dB overall SPL, was used as the masker. The test-tone dura- tion was 10 ms and a masker duration of 300 ms was chosen for the pre- and post-masking experi-

ments. The temporal position of the test signal for post-masking is defined by the time, t,, between the end of the masker and the end of the test tone (see sketch in Fig. lb). For pre-masking, At is taken as time between the beginning of the test- tone burst and the beginning of the masker (see sketch in Fig. Id).

For the results given in the last part of this paper a temporally structured masker was used. Test-tone position did also differ from the values given above and is described separately.

Results

Post-masking BMLDs In Figs. la and b both binaural post-masking

thresholds, L*,, and post-mas~ng BMLDs, AL*,, are presented for test-tone frequencies of 400 and 800 Hz. The dependence of the post-masking

threshold on the deIay time, t,, is very similar to that measured monaurally. Both binaural post- masking and binaural post-masking level dif- ferences decrease with increasing delay time. This holds for both 400 and 800 Hz test-tone frequency. At 800 Hz, the magnitude of the BMLD is 2-3 dB less at short delay times. The values of simulta- neous BMLDs (12 and 9 dB, respectively) are approximated asymptotically for very short delay times.

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223

0 03 0.2 0.3 QL 0.5 a6 07 08 (19 1 NT----- t/T p

Fig. 2. Masking-period patterns produced by a broadband-noise masker, 50 ms on and 50 ms off. Ordinates as in Fig. 1; relative

temporal position r/T within the 100 ms period T of the masker is abscissa. (a) f7 = 400 Hz, (b) fT = 800 Hz.

Pre-masking BMLDs Binaural pre-masking thresholds and corre-

sponding MLDs are shown in Figs. lc and d for 400 and 800 Hz test-tone frequencies, respectively. Again the thresholds behave similar to those mea- sured monaurally, and for At = - 20 ms they come as close as 5 dB to the threshold in quiet. The BMLDs, too, show a clearly steeper drop when compared to the values measured in post-masking. At At = - 10 ms only a small BMLD of 5 dB can be measured, at Ar = - 20 ms BMLDs are not much bigger than 2 dB.

On the other hand, in post-masking the amount of threshold shift in dB has been reduced by only about 60% at t., = 20 ms; in pre-masking it has almost disappeared at At = - 20 ms. Just as with monaural presentation, pre-masking clearly lasts for a shorter period of time than post-masking, the latter showing effects up to about t, = 100.. .200 ms.

The results obtained here are somewhat differ- ent from those of Small et al. [8] who report post- and pre-masking to have similar influence on the BMLD.

Masking-period-pattern BMLDs The mutual influence of pre- and post-masking

on the binaural masking threshold shift is most easily measured with masking-period patterns [ll]. To obtain these patterns, the masker is presented periodically and masked thresholds of the 10 ms test tones are measured at different points t/T within the period of the masker, T, i.e. simulta- neously as well as in both pre- and post-masking

conditions. The 10 Hz repetition rates of masker and test tone were equal. The time-structure of the masker (TM = 50 ms) is given in outlines in the lower part of Figs . 2a and b. In the same figures, the masking-period patterns for the N,,S, and N,S, phase configurations are shown as well as BMLDs for both 400 and 800 Hz test-tone bursts.

The results indicate that for masking-period patterns, post-masking again has a stronger in- fluence than pre-masking. The decay of the curves in the 50 ms gaps between masker impulses is longer than the relatively steep rise of pre-masking seen before the onset of the next masker pulse. This tendency is also seen in the corresponding BMLDs.

It is clear, then, that post-masking decays more flat than pre-masking rises and therefore post- masking has a greater influence on BMLDs.

Injluence of the masker’s envelope In the foregoing sections it was shown that both

pre- and post-masking result in BMLDs. This implies that our hearing system retains the infor- mation about the temporal structure of the signals in both ears (masker and test-tone) throughout the pause until the test tone is presented. Two experi- ments were performed to find out whether the envelope of the masker or its fine structure plays the dominant role in producing the results.

During the first experiment the burst of masker impulse derived from the uniform masking noise had additional temporal modulation. The 90 ms burst was divided into five 10 ms impulses each 10 ms apart. There was a gap of 110 ms duration

Page 4: Binaural masking-level differences in non-simultaneous masking

a2 a3 a& a5 a6 a7 a8 a9 i ' 0 0.1 Cl2 Q3 (14 Q5 0.6 ll7 0.6 IN 1 NT -

t/T- Fig. 3. (a) Masking-period patterns produced by a temporally interrupted masker as indicated on top. Coordinates as in Fig. 2. For

clarity, interquartile ranges are not given. Note the shortened test-tone duration of 2 ms! Parameters as indicated. (b) Same as (a) but

without interruption of the masker and L& = 60 dB. (c) Same as (b) but Lb = 70 dB.

between bursts of the masker. This configuration is illustrated by the sketch on top of Fig. 3a. The test signal was shifted over the entire period of the

masker. To measure the time-structure of the masked threshold more accurately, the duration of the test tone was shortened to 2 ms with 1 ms Gaussian rise and fall times. Measurements were

taken only at 800 Hz, in order that the width of the spectrum of the test signal should barely ex- ceed two critical bands.

It is not very easy for the subjects to determine the masked threshold. Therefore the test tone, which was presented once per masker period, was also time-structured. With a repetition rate of 5 Hz, three test-tone impulses were presented, then three test-tone impulses omitted, and so on. In this way the masked thresholds could be measured more exactly. The time structure of the test signal was triggered by the masker’s time structure, and, in addition, the test tone itself was synchronized with the masker pulse so that the time pattern of the whole test stimulus always stayed the same.

Fig. 3a shows the masked thresholds for both the A#, and P&S,, phase relations and the BMLD

as a function of the temporal position of the test

signal during the masker period. This test-tone position was adjusted so that the test signal was

either presented simultaneously with a masker-im- pulse or non-simultaneously during a pause. Dur- ing the 110 ms gap relative distances, t/T, within the period of the masker were chosen as in the earlier test series.

The results (Fig. 3a) show a very strongly struc- tured time pattern of the masked threshold and of the BMLDs during the masker impulses. In the gaps, the masked threshold decays rapidly and in this range BMLD has a nearly constant value of the only l-2 dB.

Because of the short 2 ms test signals, a direct comparison of these results with those outlined in Fig. 2 is not possible. Therefore two more experi- ments were carried out. In the first, the masker level was set to 60 dB (see Fig. 3b). In this way, the non-simultaneous masked thresholds in the gaps of the masker burst (Fig. 3a, t/T = 0.15 or 0.85) reach approximately the same values as at the corresponding t/T-values in the simultaneous presentation of Fig. 3b. In the next experiment,

Page 5: Binaural masking-level differences in non-simultaneous masking

the masker level was increased to 70 dB (see Fig. 3c) so that it was possible to match the simulta- neous masked thresholds (at t/T= 0.1; 0.2; 0.8; 0.9) from Fig. 3a to approximately the maximum of the masked thresholds in Fig. 3c.

Comparing the results from Figs. 3b and c with those from Fig. 3a it can be seen that the shapes of the masked thresholds roughly agree; only one test-point (at t/T = 0.25, the first test-point after switching off the masker) differs from the expected result: while with the pulsed masker the masked thresholds in both NJ,, and N,S,, phase configura- tions and also the BMLD are very low (AL*, approximately 3 dB), clearly larger values are yielded for both L, =60dBandL,=70dBfor non-pulsed masker (AL*, approximately 5-6 dB). This effect is possibly connected to two facts: (a) the strongly structured masker produces an over- shoot effect [lo] which leads to a rise of the simultaneous masked threshold (Fig. 3a, maximum values) and (b) the decay of post-masking is shorter with short impulses than with longer masker im- pulses [13,15]. Moreover, there seems to be no influence of the masker’s time structure on the masked threshold during the masker pause.

In any case the 10 ms on-off time-st~cture of the envelope of the masker is not imposed as a zig-zag curve on the decay process. The latter seems to follow the same laws known from normal monaural and binaural post-masking.

In the sound stimulus ~nfiguration described, the masker’s temporal envelope seems to be the only relevant value.

~~~~nce of masker’s fine structure To investigate the influence of the masker’s fine

structure on the BMLDs, experiments with 40 Hz tonal maskers were made. During all the measure- ments, the pressure-time function in each of the two headphones was checked with two l/Zinch microphones (Bfiel and Kjaer 4133), coupled to the air under the cushions via two tubes (5 cm long) and connected to the oscilloscope. The spec- tral width of the test signals was decreased by using 800 Hz test-tone bursts of 4 ms duration and Gaussian rise and fall times of 2 ms. Detailed preliminary investigations showed an unexpected and quite troublesome effect: the masking impact of low-frequency sounds is so different in the two

2. “UTZ”

+nght

10. x left

t- f- f- Fig. 4. Masking of 4 ms test-tone bursts (fr = 800 Hz) by 40 Hz maskers. Masker level L, chosen to produce in each ear the same monaural masking-period pattern as indicated in (a). Binaural presentations with test signal in and out-of phase indicate no BMLDs as shown in (b). Note the shifted zero for ALf ordinate! Effect of reversal of the masker is shown in (c). The time functions of the maskers are indicated on top of each part. Abscissa represents time f at which the peak of the test signal is reached. One subject.

ears of most subjects that quantitative statements about binaural interaction can only be made after careful adjustments of the resulting masking-period patterns. For this reason the results shown in Figs. 4 and 5 are taken from only one subject. The test-tone impulses were sync~o~zed to the 40 Hz masker in such a way that they were presented with a repetition rate of 1.25 Hz at the same position in the masker period and with identical time functions.

The results of the efforts to produce masking- period patterns as identical as possible for both ears are shown in Fig. 4a. The masker’s temporal pattern is sketched at the top. Its masking effect is most pronounced during the rarefaction phase [ll]. For approximately equal masking of the test sig- nal - in spite of nearly identical thresholds in quiet - different masker sound pressure levels L, are necessary in the two ears; left: L, = 100 dB; right: L, = 106 dB. With simultaneous presenta- tion of these two maskers the binaural masked thresholds shown in Fig. 4b can be measured. Circles represent the in-phase test signal, dots stand for the out-of-phase condition. The BMLDs occur- ring in these configurations are illustrated in the lower part of Fig. 4b. They are very small and, within the measurement accuracy, do not differ systematically from zero; there is no BMLD for the ~nfiguration chosen.

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226

Another extreme configuration results from keeping the test tone in-phase while inverting the masker at one ear. Under these conditions differ- ent masking-period patterns arise in each ear: on the left, the normal one as in Fig. 4a; on the right, the same pattern but shifted by 12.5 ms. At the instant of the condensation maximum in the left ear there is a rarefaction m~mum in the right ear and consequently a higher masked threshold on the right. Fig. 4c shows the masked thresholds measured in N&!$, and N-,.S, configurations (the corresponding time functions of the maskers are shown on top). The N&S0 configuration makes clear that the low& monaural masked threshold becomes the binaural masked threshold. This ob- servation is consistent with Durlach’s model and corresponds well with the subject’s iateralizations: with the interaurally phase-reversed masker the test tone is perceived only in the ear receiving condensation maximum. Consequently, it is not possible to speak of BMLDs in the sense that the binaural masked threshold lies below the two monaural ones. Rather, one monaural threshold determines binaural threshold; phase inversion of the masker actually decreases the binaural masked threshold only down to the lower of the two monaural thresholds.

Still the masking-period patterns are quite in- teresting for studying BMLDs. It could well be possible that the masker’s sound pressure-time function can be found in the post-masking threshold, in other words that the storage effect necessary for non-simultaneous BMLDs can be measured in monaural post-masking-period pat- terns.

To investigate this possibility, a burst of five periods of the 40 Hz tone switched on and off at zero crossing was used as a masker. After a pause of 275 ms the presentation was repeated, leading to a total period of 400 ms. The test signal was presented with every second masker burst. The masker’s sound pressure-time function had to be equalized with additional care, because a small ringing of the pressure-chamber system consisting of the transducer, circumaural cushion and ear displayed a weak decay process dependent on the subject. With the addition of an adequately shaped Gaussian impulse, it was possible to keep the sound pressure during the first 30 ms after the

*c__-._~~._ --~..---~_--~ I@ THQ

-37.5 -25 -12.5 0 12.5 25 37.5 50ms t---..

Fig. 5. Monaural post-masking of 4 ms test-tone bursts (f7 =

800 Hz) by 40 Hz masker impulses. Two levels L$, and two

phase conditions as indicated. One subject. Note: only sta-

tionary post-masking; no difference in post-masking for re-

versed phase of masker for 12.5 ms < I -C 37.5 ms.

masker’s offset approximately at zero. During this time period the remaining pressure was more than 30 dB below peak-SPL of the masker.

Investigations were carried out monaurally at two masker levels. Additional measurements were made with phase-reversed masker. Again, 800 Hz tone bursts of 4 ms duration and 2 ms Gaussian rise and decay time served as the test signal. Fig. 5 displays the results, showing the relevant time range of the last masker periods and the adjacent part of the pause. The masked thresholds during the masker represent, as expected, the masking- period pattern. After the masker is switched off, the masked threshold decays during 12.5 ms down to a nearly steady level slightly above the threshold in quiet. If the masker ends with a condensation half-period, it appears that an ongoing excitation in the inner ear is created which elongates the rise of the masked ~eshold up to 6.25 ms. Between t = 12.5 ms and t = 37.5 ms, phase reversing of the masker does not result in a change of the post- masking threshold. This means that the time struc- ture of the 40 Hz masker is not found in the post-masking threshold. The stationary increase compared to the threshold in quiet is probably connected with the threshold rise, recently de- scribed for low-frequency maskers [16].

Discussion and Conclusions

From the results presented above the most astonishing effect is the fast decay of masking

Page 7: Binaural masking-level differences in non-simultaneous masking

227

after switching off the masker. For L, = 107 dB (dots in Fig. 5), between t = - 6.25 ms and t = 0 there is a slope as steep as -28 dB/6.25 ms, that is -4.5 dB/ms. It should be noted that the rather coarse time scale chosen here is incapable of indi- cating steeper decays during shorter time intervals. That steeper slopes can actually occur within the masking-period pattern has been shown in a lot of studies. Independent of masker frequency, maxi- mum values for rise and decay of about 10 dB/ms have been found ([12], Fig. 9b); these amounts are a lot larger than those measured with normal post-masking here. Although the steepness of the post-masking function is dependent on the dura- tion of the masker [13,15], no gradients faster than 3 dB/ms have been measured, while with long masker durations (see also Fig. 2 of this study) values of 1 dB/ms are found. Thus, our hearing system can follow the time function of a low- frequency masker (as measured by masking ef- fects) much better than the envelope of a broad- band noise masker. It seems reasonable to con- clude that the masking measured with masking- period patterns using low frequency maskers is created more peripherally than post-masking. This assumption is confirmed by objective data from evoked oto-acoustic emissions in suppression period patterns 1141. This means, that post-mask- ing is not peripherally created in the inner ear but on a higher level. From the periphery more infor- mation about the time structure of a masker is transmitted towards higher levels than can be de- rived from the post-masking thresholds.

The informations arriving from both ears ap- parently are processed for the BMLDs. Thus, the storage necessary for BMLDs with non-simulta- neous masking has to be looked for in the same higher processing levels responsible for post- masking. Because of this, post-masking and BMLDs may result from a similar, if not the same, information processing level. From the present study and our earlier work on BMLDs the follow- ing conclusions may be drawn:

(a) Post-masking caused by broadband-noise maskers creates BMLDs which decrease with in- creasing time distance from the masker in the same way as the post-masking thresholds.

(b) The same result occurs in pre-masking. (c) BMLDs resulting from pre- and post-mask-

ing superimpose in the same way as pre- and post-masking in corresponding masking-period patterns (MPPs).

(d) The dependence of the post-masking decay on the impulse duration of the broadband masker is also found in the BMLDs.

(e) The envelope-time structure of broadband maskers plays no role for BMLDs apart from the effect mentioned in d.

(f) BMLDs measured in both simultaneous and non-simultaneous masking are directly related to the amount of masking: more masking produces larger BMLDs.

(g) The time function of low-frequency maskers (causing MPPs) does not give rise to BMLDs. The lower monaural masked threshold of both ears is decisive for the binaural masked threshold.

(h) The peripheral processing, i.e. up to and including the first synapse, transmits more infor- mation about the temporal structure of the signal to higher processing levels, than is evident from post-masking thresholds. It is supposed that this additional information is used for the formation of both the simultaneous and non-simultaneous BMLDs.

Acknowledgements

The authors thank Dr.-Ing.habil. H. Fast1 and Dr. Bruce Henning for their very valuable sugges- tions impro~ng the manuscript. Dipl.-Ing. W. Bauer executed parts of the measurements. This investigation was carried out within the Sonder- forschungsbereich 204 ‘Cehor’, Miinchen, sup- ported by the Deutsche Forschungsgemeinschaft.

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