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Psychoacoustic Evaluation of Musical Sounds

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Psychoacoustic Evaluation of Musical Sounds Perception & Psychophysics 1978, Vol. 23 (6), 483-492 Psychoacoustic evaluation of musical sounds ERNST TERHARDT InstituteofElectroacoustics, Technical University ofMunchen Arcisstrasse 21, D-8000 Miinchen 2, WestGermany A review is given of several basic aspects of musical sounds, i.e., the perception of dura­ tion, the perception of sound impulses as events within temporal patterns, timbre, the equiv­ alence of sensational intervals, roughness, and the pitch qualities of complex sounds. Selected examples illustrate how psychoacoustic results can contribute to the evaluation of musical sounds and to the understanding of the perception of music. Music is a "language" whose acoustic realization DURATION OF TONES usually is prescribed much more precisely and rigidly than is the case with other auditory signals, including A simple parameter of a single musical sound is speech. In particular, the frequencies of musical its duration; corresponding to the physical duration tones and the time pattern in which they are realized is a psychoacoustic attribute, called subjective are essential and significant carriers of information, duration. The relation between these two parameters But the ultimate receiver of this information is the is not as simple as may be expected at first sight auditory system. It is not the physical sound param­ (cf. Zwicker, 1970). See Figure 1 for an example in eters as such (e.g., frequency and temporal envelope some experimental results ofBurghardt (1973). of tones) that are the decisive criteria of musical per­ In one experiment, Burghardt presented alternately formance, but the corresponding auditory qualities a I-kHz tone and a 200-Hz tone in endless succession, (e.g., subjective duration, rhythm, timbre, roughness, and made the subjects adjust the physical duration of pitch, harmony). Hence, the quantitative relations one of the tones in such a way that successive tones between stimulus parameters and auditory sensations were perceived as having the same apparent duration. are of particular importance in the theory as well as Care was taken to eliminate methodological effects. the realization of music. It is particularly in the elec­ The relation between the physical durations of the tronic realization of music, where the physical two tones (T, kHz and Tf, with f = 200 Hz) under parameters of musical stimuli in many cases are the condition of equal subjective durations is shown specified numerically without auditory feedback, by the curve marked f = 200 Hz. The departure of that it is desirable to know as precisely as possible this curve from the dashed line indicates that the how acoustic stimuli are transformed into auditory 200-Hz tone has to be presented with a distinctly sensations. Moreover, careful analysis of those greater physical duration than the I-kHz tone to be psychoacoustic transformations may provide val­ perceived with the same apparent duration. uable theoretical insight into such basic musical In another experiment, a I-kHz tone and a 3,200 Hz phenomena as, e.g., consonance and harmony. tone were matched in duration in the same way. The The present contribution reflects the author's result is shown also in Figure 1. Clearly, subjective endeavor to proceed into that part of musical per­ duration of pure tones depends not only on physical ception. Several examples of psychoacoustic evalua­ duration, but also on frequency. When the physical tion, selected from the author's and his colleqgues' duration of a pure tone is held constant, its subjective own work, will be discussed. The study may be useful duration is distinctly diminished by decreasing fre­ as an introduction into an interesting aspect of quency. It should be noticed that the range of dura­ psychophysics, a guide to many experimental results, tions in which the effect is significant is almost iden­ and a stimulus for further research. It is not intended tical with the range of durations covered by the to present a comprehensive review of contemporary majority of musical tones (durations of less than results. about 800 msec). The essential content of this contribution was orally presented With some caution, these results may be applied at the Workshop on Physical and Neuropsychological Foundations to real musical tones, which usually are complex of Music, Ossiach/Austria, July 18·22, 1977. The described tones, i.e., tones composed of several harmonics. research of the present author was carried out in the Sonderfor­ Experimental data on the subjective duration of schungsbereich Kybernetik, Munchen, supported by the Deutsche complex tones are not available as yet, but one may Forschungsgemeinschaft. The author is indebted to H. Fastl, who read the manuscript and made valuable suggestions. Also, the infer rather safely that the subjective duration of a helpful comments given by the two referees of Perception and complex tone will be determined by its dominant Psychophysics on a first draft are gratefully acknowledged. harmonics. For example, it may be predicted that a 483 484 TERHARDT .1000.----,-----,--------.. tions (5-400 msec), and combinations of envelope shapes; the tone frequency was around 3 kHz. Under the condition of subjective equability, the ms time interval between the onsets of two successive tones was in many cases not T12 as might be expected, but systematically different from this value by a cer­ r 100 tain departure M, as indicated and defined in Figure 2. With rectangularly shaped tones of different dura­ tion (as in Figure 2A), M was maximally found to be 7f 20 msec. With tone combinations similar to the 10 example in Figure 2B, the average M value attained about 60 msec. It was found that these values were almost independent of T. Moreover, the results were almost independent of sound pressure level and tone 21L.------'-------'--------' (or sound) spectrum (cf. also Schutte, 1978a, 1978b). 2 10 100 ms 1000 Similar results, which agree well with these observa­ 71kHz- tions, have recently been published by Morton, Figure 1. Duration T, of a tone with frequency f = 200 and Marcus, & Frankish (1976). 3,200 Hz, respectively, as a function of the durauon of T1kHz of a I-kHz tone, for equal subjective duration; SPL 60 dB (from TIMBRE OF SLIGHTLY Burghardt, 1973). INHARMONIC COMPLEX TONES complex tone of a certain physical duration, with The timbre of a steady sound is to some extent 200-Hz fundamental frequency, and many strong dependent on the spectral envelope (i.e., the gross harmonics up to about the 20th will possess approxi­ spectral distribution of intensity), for example the mately the subjective duration typical of a high pure third-octave-band spectrum (cf., e.g., Plomp, Pols, tone (frequency of some kHz) instead of the (shorter) & Van de Geer, 1967; von Bismarck, 1974). But a subjective duration typical of a pure 200-Hz tone. considerable part of timbre is not specified by the spectral envelope. Rather, the precise frequencies of EQUABILITY OF TONE SEQUENCES the part tones covered by a given envelope also play a role. In electronic organs, for example, synthesis In music, the rhythmic relations between successive of complex tones by superposition of pure tones tones are an important (and usually carefully pre­ which are taken from the equally tempered scale is scribed) feature. A tone sequence is defined as being a usual method. In these synthesized complex tones, subjectively equable when the successive events only those partials which are one or more octaves which are determined by the single tones are per­ above the fundamental are "true" harmonics; the ceived as being uniformly arranged in time. It is other partials are slightly inharmonic. For example, usual to consider a tone sequence as "equable" or "uniform" when the physical onsets of the single impulses are arranged equidistantly on the physical 1-------- T time scale. This is an arbitrary definition of equabil­ Tl2~T12 ity, which is fairly compatible with common auditory f experience. Yet it was considered as sensible to check its psychoacoustic validity (Terhardt & Schutte, 1976). Figure 2 illustrates the experimental procedure with two examples (A, B). A sequence of tones was presented, alternately composed of two tones with different durations and/or temporal envelopes. In case A, both successive tones were rectangularly shaped, but different in duration. In case B, the two tones additionally were different in shape of envelope. The time interval between each pair of identical tones, T, was constant. The time interval between the suc­ cessive (i.e., nonidentical) tones was varied until the Figure 2. Two examples (A, B) of tone sequences which are perceived as equable (uniform); schematic amplitude-time pattern, criterion of subjective equability was fulfilled. The The sequences are periodic with period T. The departure lit experiments were carried out with various T values indicates that perceived equability is not identical with equal time (0.2 sec ~ T ~ 2 sec), combinations of tone dura- intervals between physical onsets. PSYCHOACOUSTIC EVALUATION OF MUSICAL SOUNDS 485 the 3rd partial differs from the true 3rd harmonic there is no timbre difference, because both complex by - 2 cents, the 5th partial from the true 5th har­ tones of pair A are identical in this case. The inhar­ monic by + 14 cents. Though the relative amplitude monicity effect comes into play when n = 3. With of each partial may be controlled in a wide range, n > 3, the difference increases rapidly, attaining thus providing many distinctly different spectral saturation. envelopes, the sound of those electronic organ tones In electronic organs, there usually is n ~ 6. With is somewhat uniform and specific. The timbre of a n = 6, the effect of inharmonicity has reached complex tone which is synthesized in the described almost its maximally possible magnitude, which is way is more or less different from that of a truly equivalent to the timbre difference between a single harmonic complex tone comprising the same number pure tone and the same tone "enriched" by six addi­ of partials with the same amplitudes.
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