Brightness and Loudness As Functions of Stimulus Duration
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Brightness and loudness as functions of stimulus duration 1 JOSEPH C, STEVENS AND JAMES W. HALL LABORATORY OF PSYCHOPHYSICS. HARVARD UNIVERSiTY The brightness of white light and the loudness of white (1948) and by Small, Brandt, and Cox (1962) for white noise were measured by magnitude estimation for sets of noise. White noise provides a more suitable stimulus stimuli that varied in intensity and duration. Brightness and for this kind of problem than pure tones, because the loudness both grow as power functions of duration up to a intensity of a white noise (unlike tones) can be rapidly critical duration, beyond which apparent magnitude is es modulated without effecting a material change in the sentially independent of duration. For brightness, the critical sound spectrum. Thus a prominent click is heard at duration decreases with increasing intensity, but for loudness the onset of a tone but not at the onset of a noise. the critical duration is nearly constant at about ISO msec. How the loudness of white noise grows with intensity Loudness and brightness also grow as power functions of in has been the subject of several investigations (S. S. tensity. The loudness exponent is the same for all durations, Stevens, 1966a). Like brightness, the loudness of white but the brightness exponent is about half again as large for noise obeys the general psychophysical power law short durations as for long. The psychophysical power func proposed by S. S. Stevens (1961) tions were used to generate equal-loudness and equal-bright IjJ ~ k¢.8 (1) ness functions, which specify the combinations of intensity E and duration T that produce the same apparent magnitude. where if; is subjective magnitude, and ¢ is the physical Below the critical duration ET equals k for equal brightness, magnitude. Of special concern here is how the constants and Era equa Is k for equal loudness. The value a is about of the power function depend on the duration of the 0.7 for threshold and about 1.25 for supraliminal loudness. stimulus. Apparafus for Producinc Visual Flashes The present paper concerns the way apparentbright The flash source was a fast-decaying 4-wattfluores ness and apparentloudness vary with duration throughout cent lamp (Sylvania's "Deluxe Cool White") that could the dynamic range of vision and hearing. Interesting be activated at a high intensity level for almost any similarities and differences between these two senses duration longer than about 0.5 rnsec, The operation and come to light from a comparative study of brightness circuitry are described by Gerbrands and J. C. Stevens and loudness over wide variations of time and intensity. (1964). Reasonably square pulses of light can be pro The principal method was to obtain magnitude esti duced at durations as short as 0.5 msec. and at mates of a matrix of stimuli covering a wide range of luminances as intense as 5 or 6 lamberts (L), about 10 intensity (50 dB for brightness, 73 dB for loudness) and 107 dB re 10- L. of duration (2000-fold for brightness, 100-fold for From a distanceof 35 emtheobserverviewed foveally loudness). From the numerical estimates of a group of a portion of the lamp (diameter 1.25 em, visual angle observers it is possible (1) to state how brightness and 203') from inside a dark booth (see Fig. 1). On a signal loudness grow with duration and with intensity, and (2) from the experimenter the observer pressed a key, to generate a family of equal-sensation functions that which, after a delay of 0.5 sec., triggered the flash. specify combinations of duration and intensity that The brief delay between pressing and seeing was found arouse a given level of apparentbrightness orloudness. to favor concentration on the flash. Just to the left of Except for certain procedural details, the investiga the target appeared a reel fixation spot. Throughout the tion of visual brightness was a repetition, with similar experimental session the observer viewed this spot outcome, of an earlier experiment by Raab (1962). The with his right eye while holding his head steady in a range of intensity was extended 10-fold (10 dB) in the chin rest. A black mask prevented his left eye from present study. Alba and S. S. Stevens (1964) used the seeing the target. method of brightness matching in order to study the The intensity of the flash was controlled by means of same problem, and S. S. Stevens (1966b) has proposed neutral density filters. a simple model to show how brightness may depend on intensity and duration. Procedure for Scaling Briglltness An analogous model for loudness has not,apparently, Before an experimental session, the observer was been worked out. Data on supraliminal loudness arenot dark-adapted with red goggles for 10 min., and then for very numerous, and there appear to be some puzzling another minute or two he sat in the dark booth. discrepancies. Of most relevance to the present study The observers were instructed to assign numbers are the equal-loudness functions measured by Miller proportional to the subjective brightness of the flashes Perception & Psychophysics, 1966, Vol. I comment 1966, Fsucnonomic Press, Goleta, Calif. 319 and to disregard any apparent variation in the duration of the flashes. For the first flash presented to him, the observer was asked to decide on an appropriate number to stand for the subjective brightness; to all subsequent booth neutrol woll stimuli he then assigned numbers proportional to sub filters jective brightness. I mosk Altogether, 60 stimuli (i.e., combinations of peak clear gIOSS~ ~ luminance and duration) were presented in a series of six experiments (see Table 1). Approximatelyone-sixth I fixation lamp of these stimuli were used in each of the six experi I ments, but in order to tie the results of all six experi I ments together, one particular stimulus (200 msec. at 85 dB re 10-10 L) was presented in every experiment. I I The sequence of the various experiments differed from one observer to another. left eye mosk \. I Each stimulus was presented twice in an experi ~~kright eye mental session, except that the first stimulus was chin rest ~ presented three times with only the last two judgments counted. This procedure was adopted because extensive experience with the method of magnitude estimationhas Fig. 1. Schematic diagram of the visual apparatus (not to scale). shown that the original number assigned as a "modu lus," whether by the experimenter or by the observer variation, such as duration or quality (qualitative himself, is not always exactly the same as the "effec variation as a function of duration constitutes a promi tive" modulus implicit in the subsequent judgments. nent feature of the shorter bursts). Sixty combinations The order of the stimuli was irregular with respect of physical magnitude (peak SPL) and duration were to brightness and differed from one observerto another. presented in a series of six experiments as specified The dimmest stimulus was neverpresented immediately by Table 2. One stimulus, 80 msec. at 81 dB peak SPL, after the brightest, and all the stimuli were presented was presented in each of the six experiments. The once before any was presented a second time. The time order of these experiments differed from one observer between presentations was about 15 sec. If an observer to another. felt that he had not seen a given flash properly, he could Each stimulus was presented twice in a session, except ask for another presentation. for the first stimulus (always one of intermediate loud Altogether 21 persons (graduate students, staffmern ness), which was presented three times altogether; only bers, and employees) served as the observers. Someof the last two judgments of this stimulus were counted them served in only one experiment, others in 2, 3,4, as magnitude estimates. In each session the order of 5, or all 6 of the experiments, but 10 persons served in the stimuli was irregular with respect to loudness, each of the six experiments. No person ever served in except that all the stimuli were presented once before more than one experiment on the same day. any was presented a second time. The order of stimuli also differed from one observer to another. Apparatus for Producing Bursts of Noi se Twelve persons served as observers in all six An electronic switch (Grason-Stadler Type 829-S-14) experiments. Ten of them were undergraduate students, was used in conjunction with an interval timer (Grason one a graduate student, and one a staffmember. Before Stadler Type 471) to gate the filtered output (low-pass the first session the observers were asked to make 20,000 Hz) of a white noise generator. The gated signals magnitude estimates of the lengthofafewline segments were then passed through a decade attenuator to a pair in order to given them familiarity with the method and of PDR-8 earphones. The duration and the intensity of the bursts were monitored throughout the experiment Table 1. The combination of physical magnitude and duration with an oscilloscope and a voltmeter. used as stimuli in Experiments 1 through 6 on brightuess. For The interval timer was set to deliver repeatedbursts Experiments 1 to 3. the first stimulus was 200 msec, 85 dB. The of noise, separated by 900-msec. intervals of silence. asterisks mark the first stimuli for Exp eriments 4 to 6. On a given stimulus presentation the observer pressed a switch for as long as he wished to hear the repeated Peak luminance Duration in milliseconds in dB re 1O-10L o.s 1 2 5 10 20 50 100 200 500 1000 bursts. The observer sat in a sound isolation booth and listened binaurally.