Perception &Psychophysics 1981,29 (4), 395402 Perception of acoustic iterance: Pitch and infrapitch RICHARD M. WARREN and JAMES A. BASHFORD, JR. University of Wisconsin, Milwaukee, Wisconsin 53201 Detection of acoustic repetition is considered as a continuum extending from .5 through 16,000 Hz. Perceptual characteristics are mapped for the entire range, using repeated randomly derived waveforms (segments from Gaussian noise) as model stimuli. Contributions from tem­ poral domain (neural periodicity) analysis extend from about .5 through 5,000 Hz and from fre­ quency domain (neural place) analysis from roughly 50 through 16,000 Hz. Within the range of overlapping analyses (50 through 5,000 Hz), it is difficult to separate the effects of temporal cues from place cues. However, by using low-frequency acoustic iteration from 1 through 16 Hz, we were able to study temporal analysis in the absence of place cues to repetition. New perceptual phenomena are reported for the "infrapitch" produced by "infratones," some of which are analogous to phenomena observed for the pitch produced by tones. It appears useful for theory to consider pitch and infrapitch as a single topic: the perception of acoustic iterance. Waveforms excised from Gaussian noise and re­ has a line spectrum spanning all audible frequencies, peated without pauses are useful as model periodic with successive harmonics separated by 1 Hz. The stimuli. Such repeated noise segments (which will be ear's spectral analyzing power is limited (see Plomp, called recycled Gaussian noise or RGN) have no 1964) and cannot resolve such closely spaced compo­ a priori restrictions concerning power and phase nents. The interaction of the many unresolved har­ spectra, and thus represent a general case for de­ monics stimulating the same locus on the basilar termining general rules governing perception of acous­ membrane produces complex local patterns of ampli­ tic repetition or iterance. tude fluctuation that are repeated once each second. As we shall see, experiments with RGN have sug­ The change of amplitude with time differs at separate gested that perception of acoustic iterance represents loci, but each of these different patterns has the same a continuum extending well below the tonal limit, period. Information concerning this low-frequency ranging from about .5 through 16,000 Hz (repetition periodicity seems to be available along the entire periods from about 2 sec through .06 msec). Two length of the basilar membrane, for, when we swept types of neural analyses appear to operate within this a V3-octave filter through a broadband infratonal range: temporal domain or periodicity analysis from RGN, listeners could hear an infrapitch periodicity roughly .5 through 5,000 Hz, and frequency domain corresponding in frequency to that of the iterated or place analysis from roughly 50 through 16,000 Hz. acoustic waveform at all center frequencies of the Low-frequency RGNs from about .5 through 4 Hz filter. It should be noted that infratonal acoustic sound like periodic "whooshing," and from about iterance is detected solely through temporal informa­ 4 through 20 Hz sound like "motorboating" (Guttman tion without the concurrent place information char­ & Julesz, 1963). The noisy or hiss-like quality asso­ acteristic of the tonal range. ciated with lower frequencies disappears for RGNs We designed Experiment 2 to determine whether at about 100 Hz (Warren & Bashford, 1978). At such a "pure" neural temporal analysis in the infra­ higher frequencies, in keeping with other complex tonal range follows rules observed for the mixed tones, pitch is determined by the period, and timbre temporal and place analysis occurring in the tonal is determined by the particular harmonic structure range. But, before undertaking Experiment 2, it was of individual RGNs. considered desirable to complete the description of Let us look more closely at the RGNs below the RGNs of different frequencies by studying the un­ tonal range, or "infratonal" RGNs, I first studied by explored range from 20 through 100 Hz. Experi­ Guttman and Julesz (1963). An RGN of, say, 1 Hz ment 1 provides information to fill this gap. EXPERIMENT 1: This study was supported in part by a grant from the National Perceptual Categories and Boundaries ScienceFoundation (BNS 79-12402)and in part by funds provided for Low-Frequency Pitch by the Graduate School and the College of Letters and Science at the University of Wisconsin-Milwaukee. Requests for reprints should be directed to Richard M. Warren, Department of Psychol­ When we explored perception of RGNs between ogy. University of Wisconsin, Milwaukee, Wisconsin 53201. 100 and .5 Hz in a preliminary study, we could find Copyright 1981 Psychonomic Society, Inc. 395 0031-5117/81/040395-08$01.05/0 396 WARREN AND BASHFORD no distinct threshold value for pitch. The RGN be­ Procedure. The subjects were tested while seated in an audio­ came progressively more hiss-like with decreasing metric room. There were two sessions, each lasting about 30 min. frequency. Somewhere between 100 and 50 Hz, the In the first session, the subjects chose the RON repetition rate corresponding to their lower limit for pitch (i.e., they judged that RGN underwent a transition from a smooth, homo­ a decrease below this value did not produce a change in any pitch­ geneous signal to one that sounded rough and seemed like quality of the RON), and also selected the RON repetition rate to pulsate at the repetition rate. Within this transition within the pitch range corresponding to the transition from a range, it sometimes was possible to change at will smooth, homogeneous sound to a rough, pulsed sound. Each type of judgment was made twice: once with presentation of subranges from an "analytic" mode producing the rough sen­ by the experimenter in order of increasing repetition rates (ininal sation with a temporally fine structure to a "synthetic" subrange setting at maximum value) and once with presentation mode producing the smooth unitary percept. Further of subranges in the opposite order (with minimum initial settings). decrease in frequency caused the pulsing sound grad­ The subjects varied the repetition rates within successive subranges ually to lose its tonal quality. While Guttman and until the criterion value was reached by adjustment within a sub­ range. The experimenter then recorded the clock frequency driving Julesz (1963) described RGNs as sounding like "motor­ the delay line, which was used to calculate the transition-threshold boating" from 19 through 4 Hz and as sounding like judgment. Judgments of the pitch/infrapitch boundary were alter­ "whooshing" from 4 through 1 or .5 Hz, no formal nated with judgments of the smooth/pulsating boundary, and pre­ data were presented for placing this transition at sentation orders were balanced within the group of subjects. During the second testing session on a subsequent day, each subject 4 Hz. The listeners in our laboratory also heard the made the same judgments, with presentation orders reversed with boundary between motorboating and whooshing at respect to those received in the first session. about 4 Hz. However, the transition seemed to take place over an octave or more, and any values within Results and Discussion this range seemed acceptable to our listeners. All the subjects found it possible to make judg­ It should be noted that detection of RGN iterance ments of the pitch/infrapitch and smooth/pulsing requires no special training. It can be heard clearly category boundaries for RGNs. Each of the subjects within a few seconds by inexperienced listeners for reported that the qualitative changes corresponding any frequency greater than 1 Hz. to both types of transitions varied gradually with repetition frequency, with some perceptual ambiguity Method at the values selected for transitions. Subjects. Six subjects were used, each of whom had training in auditory research experimentation, music, or both. They all had The mean threshold values obtained in Experi­ heard RONs of different frequencies before the start of the experi­ ment 1 are shown in Table 1. These values, together ment. with earlier information for other RGN repetition Stimuli. The stimuli were produced by repetition of white-noise rates (data for the infrapitch . range reported by segments. Output voltage from a white-noise generator was sampled every 20 IlSeC and coded in l2-bit form by a digital delay Guttman & Julesz, 1963; data for the portion of the line built to our specifications by the Physical Data Company. pitch range above 100Hz reported by Warren & Maximum storage was 600,000 bits, corresponding to a l-sec delay Bashford, 1978) were used to construct Figure 1, when the delay line was operating at its maximum bandwidth of which describes the perceptual characteristics for the 16,000 Hz. By using a frequency synthesizer as an external clock, entire range of readily detectable RGN repetition it was possible to control the stepping rate of the delay line's shift registers. Input was bandpassed from 50 through 16,000 Hz. rates.' While boundaries separating the different per­ By closing a "recycle switch," input to the delay line was rejected, ceptual qualities are represented diagrammatically at and the signal was looped or recirculated indefinitely in digital single frequencies, it should be emphasized that all form. Digital-to-analog conversion produced an RON with a period transitions are gradual and occur in the vicinity of the determined by the number of shift registers placed in the circuit and the external clock rate. An internal antialiasing filter reduced indicated frequencies. Within the transitional range, spectral artifacts corresponding to digital-to-analog conversion, it is often possible to direct attention to one side of and reduced transients corresponding to the closing of the digital the boundary or the other. Tones above 5,000 Hz are loop (such transients could not be heard by listeners or seen on sound spectrographs).