On Aug 17, 2009, at 3:33 AM, Henrik Møller wrote:
I would like to point out that Guttman and Julesz ("Lower Limits of Auditory Periodicity Analysis", Letters to the Editor, JASA, Vol. 35(4), p. 610 (1963)) used periodic signals, consisting of repeated wide-band noise with repetition periods corresponding to frequencies in the low/infrasonic region. There was no fundamental in their signal, thus no low-frequency/infrasonic exposure at all.
I agree that repetition frequency and frequency contents can easily be mistaken. In assessments of environmental noise, it is not unusual that a signal of higher frequency repeated with an infrasonic frequency is - erroneously - attributed to infrasound. Examples are broadband ventilation noise that is amplitude modulated by a low frequency, or sources of tonal noise with close frequency that cause beating at a low frequency. However, the sensation is completely different, whether the fundamental is there or not.
Richard M. Warren skrev:
The term infrasound can be misleading; while the frequency threshold for hearing sinusoidal tones (at a reasonable amplitude) is roughly 20 Hz, holistic frequency perception of complex waveforms (e.g., iterated noise segments) continues for another five octaves of “infrapitch” below 20 Hz. Guttman and Julesz (1963) found that a percept they called “whooshing” occurred from roughly 0.5 to 4 Hz, “motorboating” from 4 Hz to 20 Hz. A noisy pitch is heard from 20 to 100 Hz, and a pure noiseless pitch with interesting timbres from 100 Hz up to 16 or 20 kHz. Thus, infrapitch and pitch form a seamless continuum of perceptual “iterance” extending for 15 octaves for stochastic waveforms, subserved by partially overlapping neural mechanisms of periodicity detection and place detection.
Studies of infrapitch, since the pioneering study of Guttman and Julesz, have been conducted by Irwin Pollack, Christian Kaernbach, and myself. Infrapitch detection has also been studied in nonhuman vertebrates.
Infrapitch periodicity is quite different from “broadband ventilation noise that is amplitude modulated by a low frequency.” Acoustically, a Repeated Frozen Noise segment (RFN) has a line spectrum consisting of all integral multiples of the repetition frequency, each harmonic having a randomly determined amplitude and phase. Only harmonics within the range of roughly 20 Hz to 20 kHz can contribute to audibility. However, RFNs having repetition frequencies from 0.5 Hz to 20 Hz can be heard as patterns based on unresolved harmonics. For example, a 2 Hz RFN has thousands of harmonics in the audible range (20, 22, 24, 26, …20,000 Hz) which cannot be resolved because of their close spacing. But they do create different periodic patterns within each critical band. Each pattern has the identical repetition frequency of 2 Hz (as can be heard when a 1/3-octave is swept through the spectrum). RFNs above 20 Hz can be considered as stochastic complex tones without the special amplitude and phase characteristics of specific complex tones such as pulse trains, voices, and musical instruments. For example, RFNs with a repetition period of 5 ms are 200 Hz complex tones having no hint of noise; each of these individual RFNs has a unique exceptionally rich timbre based upon its particular amplitude and phase spectrum. Jim Bashford and I have used RFNs to study pitch and infrapitch perception starting in 1981 (“Perception of acoustic iterance: Pitch and Infrapitch,” Perception & Psychophysics, 29, 395-402).
Richard M. Warren
and Distinguished Professor Emeritus
Department of Psychology
University of Wisconsin-Milwaukee
PO Box 413
Milwaukee, WI 53201