Contains reports on one research project.U. S. Air Force Contract AF19(604)-7459, monitored by Operations Analysis Office, Air Force Command and Control Development Division, Bedford, MassachusettsNational Science Foundation (Grant G-21807

Green, D. M.

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XVI. SIGNAL DETECTION BY HUMAN OBSERVERSProf. J. A. SwetsProf. D. M. GreenLinda E. Branneman P. D. DonahueSusan T. SewallA. MASKING WITH TWO CONTINUOUS TONESOne of the earliest studies in the modern era of psychoacoustics was the paper ofWegel and Lane. 1 Their data show how a single continuous pure tone at some fixed fre -quency masks 2 other pure tones. One of the main findings reported by Wegel and Lanewas that a masking sinusoid of low intensity influences the detectability of the other tone280Fig.+ 40 db \300 400 500300 400 500700 +700 1000 2000 3000 5000 10,000FREQUENCY IN C.P.S.Fic. 1. The curves are masking audiograms of a pure tone of 400 c.p.s. The number under each curve is the sound-pressure level(dccibels re 0.0002 dyne/cm2 ) of the masking tone (0 clb sensation level corresponds to 15 db S.P.L. at 400 c.p.s.). The abscissa repre-sents the frequency of the masked tone and the ordinate represents the shift in decibels of its absolute threshold. Note how asym-metrical the curves become as the intensity, of the masking stimulus is increased.XVI-1. Results of Egan and Hake using a single sinusoidal masker [reprintedwith permission from J. Acoust. Soc. Am. 22, 622 (1950)].This work was supported in part by U. S. Air Force Contract AF19(604)-7459,monitored by Operations Analysis Office, Air Force Command and Control DevelopmentDivision, Bedford, Massachusetts, and in part by the National Science Foundation(Grant G-21807).283(XVI. SIGNAL DETECTION BY HUMAN OBSERVERS)only in a narrow range of frequencies centered about the frequency of the masking tone.Figure XVI-1, taken from a paper by Egan and Hake 3 shows one of these typical maskingpatterns.Presumably, the dip in the function in the region of 400 cps arises because of beatsbetween the masking tone and the signal. The asymmetry seen at the high level for themasking stimulus was presumed to represent, in part, distortion of the middle ear andthe generation of harmonics of the masking frequency. Except for the dip in the curvewhen the signal and masking frequency are very nearly the same, the general shape ofthe curve, especially at low masking levels, is reminiscent of frequency response ofa bandpass filter. Fletcher 4 pursued this same thought when he introduced the conceptof a critical band. The critical band was, in effect, a hypothetical auditory filter.Fletcher's estimates of the width of this critical band, based on other experiments inwhich noise was used as a masker, are listed in column 2 of Table XVI-1.Recent studies5, 6 have reported data repeating parts of the old Wegel and Lane data.While, in general, these studies confirm the earlier findings, two exceptions should benoted in detail. While both Ehmer's and Small's data show irregularities in the maskingcurves obtained at high masking levels, these irregularities are not at the harmonicsof the masking frequency. This result is at variance with the nonlinear distortion hypoth-esis. More important for our purpose is Small's result that the masking patterns,obtained at different frequencies and low intensity levels of the masking stimulus, areall very similar if one plots the ratio of the masking frequency to the signal frequency.Thus, if one tries to estimate "critical bandwidth" from Small's data, whether one usesa 3-, 6-, or 10 -db definition of bandwidth, one still will conclude that the critical band-width is proportional to center frequency. This variation of bandwidth with center fre -quency is quite different from the estimates suggested by Fletcher (see Table XVI-1).Table XVI-1. Estimates of the width of critical band in cps based onseveral different experiments.Zwicker 8Center 4 Flottorp PresentFrequency Fletcher Stevens Bkesy Experiment250 51 95 190 200500 51 110 3501000 63 160 760 6002000 98 295 15003000 141 450 24004000 204 680 3000 2200284(XVI. SIGNAL DETECTION BY HUMAN OBSERVERS)Fletcher's estimates come from experiments in which noise, rather than a sinusoid, isused as the masker. A variety of experiments have employed noise, or noiselike stimulias the masker. Stated briefly, the plan of these experiments is to measure the detecta-bility of a sinusoidal signal located in the center of a noise band. The noise band is madeprogressively narrower until a point is reached at which the detectability of the signalis first influenced. This point is taken as the critical bandwidth. It is clearly importantto use noise bands that are very sharp so that the width of the critical band is clearlydefined.Greenwood7 has used a heterodyne technique to achieve very sharp frequencyresponse characteristics for his noise signal. Greenwood also estimates critical band-widths, and these estimates of critical bandwidth are in general agreement with a recent8paper in which a variety of experiments is summarized. It can be seen fromTable XVI-1 that Zwicker, Flottorp, and Stevens' estimates of critical bandwidths arelarger than Fletcher's estimates by almost a factor of three.The present experiment used two continuous sinusoidal signals as the masking stim-uli. The width between the two sinusoids is varied and a critical band is inferred fromthe amount of masking that such a complex produces on a third sinusoid, the signal,which is located in frequency midway between the two maskers. By comparing theamount of masking produced by the two sinusoids in combination and separately, theexperiment also provided a means of assessing the masking effectiveness of combinationsof sinusoidal stimuli.ProcedureTwo continuous sinusoids, each of 77-db sound pressure level, (SPL) were themasking stimuli. The two sine waves were separated in frequency by an amount denotedAf. Midway between these two sinusoids, Af/2 above the frequency of the lower masker,a gated sinusoid is presented as the signal. The signal duration was 100 msec with a25-msec rise and fall to the signal to minimize switching transients. A two-alternativetemporal forced-choice procedure was used to measure the detectability of the signal.The signal occurred at random in one or the other of two temporal intervals, markedby lights, for the observer. The intensity of the signal needed by the observer to detectthe signal 75 per cent of the time was the dependent variable of the experiment.This signal level was estimated by presenting the signal at five different intensitiesand estimating the percentage of correct detections at each level. The resulting functionrelating the percentage of correct detections to the level of the signal is called a psycho-metric function. Two hundred observations were used to estimate each point on the psy-chometric function. The 75 per cent point was determined by visual interpolation aftergraphing the results. 9285(XVI. SIGNAL DETECTION BY HUMAN OBSERVERS)Three center frequencies were used for the signal: 250, 1000, and 4000 cps. At eachcenter frequency the separation in frequency between the maskers was varied from aminimum width of 4 cps to a separation large enough to either drive the lower frequencymasker to near zero frequency or until a complete curve relating Af to signal level wasobtained. In Fig. XVI-2 the experimental data are presented.Let us first discuss the data for Af in the range 4-100 cps. It appears that theamount of masking produced by two sinusoids symmetrically located in frequency aboutthe gated signal is independent of the center frequency of the complex. This is a pecul-iar and unexpected result for two reasons. First, it is unlike the data on the detectionof gated sinusoids in wideband noise. Data from Green, Licklider, and McKey10 showthat from 250 cps to 4000 cps the signal level must be changed by 8 db in order to main-tain some constant level of detectability. The Fletcher estimates of the critical band-width as a function of center frequency reflect, in part, exactly this variation.Second, such independence of center frequency is unexpected from data on single -sinusoid masking or from an experiment measuring the detectability of an increment inMEANS FOR 3 OBSERVERSEACH MASKER = 77 DB SPLNo = 12 DB SPLt = 124 MSECSIGNALf CPS2f CPSSINGLE MASKERAT 80 DB SPL~1111^ I I I 1 1 I I I I I 1 I I I I II IFREQUENCY DIFFERENCE BETWEEN MASKERS (Af)Fig. XVI-2. Results of the experiment using two sinusoidal maskers. Open pointsindicate results with incoherent relation between masker and signal.Solid points indicate results when signal was added in phase to masker.28660-A50-N4000 CPSIO000 CPS250 CPS100 1000I I I I(XVI. SIGNAL DETECTION BY HUMAN OBSERVERS)the intensity of a sinusoid. At the left-hand side of Fig. XVI-2, above the abscissa valueof zero, we have plotted the intensity of the gated sine wave that is needed for 75 percent correct detection in the presence of a single sinusoidal masker. The resultsobtained for the single masker are obviously dependent on the frequency region tested.When only one sine wave is used as the masker, the level was set at 80 db to equalin power the intensity of the pair of sinusoids used in the main experimental condition.The signal was either added coherently, in phase, to the continuous background signalor the signal was derived from an independent oscillator and gated on without regard tothe phase of the continuous masker. This later arrangement is denoted as the incoherentcondition in the legend of Fig. XVI-2. Note again that the results are clearly frequency-dependent; the highest frequency signals are easier to hear than the low frequency sig-nals. The difference between the coherent and incoherent conditions are about the sameat all three frequencies, namely 5. 5 db. This difference is presumably explained onthe simple basis that in one case (coherent conditions) the signal always causes an incre-ment in the power of the sound, whereas in the other case (incoherent condition) thesound may increase, decrease, or stay roughly the same in power.The difference in masking effectiveness between a single sine wave and two sinewaves, 4 cycles apart and of the same power, is approximately 10 db at 250 cps, 15 dbat 1000 cps, and 20 db at 4000 cps. It has long been known that noise is a much moreeffective masking agent than a sinusoidal stimulus. This difference in effectiveness ona power basis is ~12-20 db, the difference depending on the exact circumstances and con-ditions of the experiment. The difference just cited between single and double sinus-oidal maskers would suggest that there is little if any difference in the maskingeffectiveness of noise and two closely spaced sinusoidal maskers. As this experimentshows, masking with two sinusoids is independent of signal frequency, whereas maskingwith noise is not; thus any more rigorous pursuit of this comparison would appear tobe impossible.Finally, let us consider the data obtained with large frequency separations of thetwo maskers. We have arbitrarily chosen a drop in masking effectiveness of approxi-mately 20 db from the maximum masking, at Af = 4, to use as our estimates of our crit-ical band.Were we to change our arbitrary 20 db to some lower figure, our estimates of crit-ical band would be independent of center frequency.Were we to increase the figure to 30 db, all of the estimated bands would increaseroughly by the same fractional amount. These estimates are extremely large for anycritical-band estimate based on psychoacoustic data. They are not far different fromthe width of the envelope of the travel wave as reported by von B6k6sy. 11It would appear that two general mechanisms are involved in these results. One,the extent to which the basilar membrane is vibrated by a sinusoid, is probably287(XVI. SIGNAL DETECTION BY HUMAN OBSERVERS)responsible for the rapid fall in masking at the very large frequency separations (Af).The other mechanism appears to be a temporal one that is clearly place- or frequency-independent, and probably is basically the same type of mechanism investigated by thoseinterested in periodicity pitch.We are at present trying to develop a more quantitative theory to explain the data.These results and their interpretation also suggest a reevaluation of some of the oldermasking data.D. M. GreenReferences1. R. L. Wegel and C. E. Lane, Phys. Rev. 23, 266-285 (1924).2. Masking is defined as "the amount by which the threshold of audibility of a soundis raised by the presence of another sound" (Z24.1, 1951) American Standard AcousticTerminology (American Standards Association, New York, 1951).3. J. P. Egan and H. W. Hake, J. Acoust. Soc. Am. 22, 622 (1950).4. H. Fletcher, Revs. Modern Phys. 22, 47-65 (1940).5. R. H. Ehmer, J. Acoust. Soc. Am. 31, 115 (1959).6. A. Small, Pure tone masking, J. Acoust. Soc. Am. 31, 1619 (1959).7. Greenwood reports an off-band attenuation of 200 db per octave. (SeeD. D. Greenwood, J. Acoust. Soc. Am. 33, 484-502 (1961).)8. E. Zwicker, G. Flottorp, and S. S. Stevens, J. Acoust. Soc. Am. 29, 548(1957).9. More sophisticated analysis is now being made.10. D. M. Green, J. C. R. Licklider, and M. J. McKey, J. Acoust. Soc. Am. 31,1446-1452 (1959).11. G. von B6kesy, Experiments in Hearing (McGraw-Hill Book Company, Inc.,New York, 1960).288

Signal Detection by Human Observers

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Signal Detection by Human Observers

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