TEMPORAL ASPECTS IN THE EVALUATION OF ENVIRONMENTAL NOISE

The frame of reference has a significant effect on the evaluation of environmental noise. Hearing is the perception which conveys information along the temporal stream and temporal aspects play an important role in the evaluation of noise. In this paper the effect of frame of reference was discussed in relation to the temporal aspects dividing the time span into three categories; perceptual moment, perceptual present and auditory memory. 1 INTRODUCTION Psychological evaluation of environmental noise is affected not only by the physical properties of the noise but by the frame of reference in which the noise is evaluated. The frame of reference consists of various factors such as the context of the noise and social and cultural background of the listeners [1-3]. Hearing is the perception which conveys information along the temporal stream and temporal aspects play an important role in the evaluation of noise. Therefore, the temporal aspects also contribute to forming the frame of reference in the evaluation of noise. In this paper, the evaluation of environmental noise will be discussed in relation to the temporal aspects of the frame of reference. Subjective temporal stream is quite different from physical one. Physically time runs from the past to the present and to the future at a constant speed. On the other hand subjectively the speed of elapse of time is not constant and sometimes it goes back even from the present to the past. The time resolution is not constant either. Our hearing is very sensitive to a minute temporal difference and we can detect a very short gap in the sound stream [4], while we roughly grasp the whole range of a long-term event by neglecting insignificant portions. When the environmental noise is evaluated, different frame of reference is used according to the duration of the noise and to the task given to subjects. Subjective temporal span can be divided into the three categories; perceptual moment, perceptual present and auditory memory. 1.1 Perceptual moment If two sounds, A and B with very short duration, e.g. a few milliseconds, are presented successively with a short interval, we cannot identify whether A is followed by B or vice versa. The time span where the temporal order of sounds cannot be identified is called ”perceptual moment” [5]. The duration of perceptual moment may be less than about 100 msec, which is different depending on the attribute or method of measurement [5, 6]. In this time span, though the temporal order of the sounds cannot be identified, we can identify the sound by the difference of sound quality. Also their localization can be detected as indicated by Haas effect [7]. 1.2 Perceptual present Physically ”present” is a point, however, psychologically there is a time span in which all the events generated are perceived simultaneously. Woodrow [8] said, ”If one listens to the ticks of a clock, he may notice that he is aware of several clicks at once. It may be asked, therefore, how many ticks before the last one, or how many successive ticks, can be heard simultaneously. More generally the problem may be stated as that of the physical time over which stimuli may be spread and yet all be perceived as present. ” Woodrow defined this time as the threshold of unitary duration or the upper limit of the perceptual Copyright SFA InterNoise 2000 2 (psychological) present. According to Fraisse [9], two close sounds belong to the same perceived present when the interval is approximately 1.8 2 sec. It is also reported that maximum duration of perceptual present is 7 to 8 sec [10]. It seems that the limit of the perceptual present may be affected by various psychological factors, e.g, materials (speech, music, or noise), redundancy, expectancy, interval between sounds, duration of elements, individual difference among subjects, etc. Therefore, the duration of perceptual present cannot be fixed. However, there is no doubt that the perceptual present has a certain duration. It is not a point like the physical present. In the time span of perceptual present, the temporal order or stream can be identified. That is, whether sound A is presented before or after sound B can be identified and therefore the meaning of speech or the melody of musical performance can be understood. 1.3 Auditory memory When listening to a sound longer than perceptual present, the preceding portion of the sound is being stored in the memory with the elapse of time and the overall impression of the sound is judged on the basis of memory. Many studies have been conducted concerning auditory memory [e.g.11, 12]. Various factors may affect the impression of long-term sounds. It may be considered that the sounds and/or the portions of the sounds which can easily be recalled may have greater effect on the judgment of overall impression than the other sounds and/or the other portions. 2 EVALUATION OF ENVIRONMENTAL NOISE The evaluation of environmental noise will be discussed in relation to these three time spans. In the evaluation of noise, it is necessary to measure subjective responses to noise and to find physical indices which show good correspondence with subjective responses. In these measurements, it is important to use appropriate tools according to the purpose of the measurement. 2.1 Perceptual moment in the evaluation of environmental noise When the duration of a sound is shorter than perceptual moment, the impression of loudness, pitch and timbre can be perceived, but temporal sequence is not clear. Fundamental research of perceptual moment indicates that the duration of perceptual moment is less than about 100 msec. From the viewpoint of the evaluation of environmental noise, perceptual moment is considered to be the period during which our impression of the noise is integrated. In this meaning the impulsive noise is a typical example of an event in perceptual moment. The physical duration of impulsive noise may become about 1 sec when it is generated in a reverberant place. However, subjective duration is not equal to physical duration [13]. The perception of on-set and off-set of a sound is not clear when the sound has a gradual rise and/or decay. Subjective duration differs depending on the envelope pattern of the sounds. The duration of steady state sounds and sounds with long rise time and short decay is perceived longer than physical duration owing to the after effect after the cessation of the sounds. On the other hand, the duration of sounds with short attack and long decay as found in most impulsive noises is perceived shorter than physical duration since the latter low level portion may be masked by the preceding high level portion [14]. Therefore, perceptual moment may possibly become longer than 100 msec. It is considered that listeners would perceive an impulsive noise as a single event which occurs in perceptual moment. (1) Loudness of impulsive sounds The event in perceptual moment may be integrated as a single event. In our former studies of the loudness of impulsive sounds [15], LAE (sound exposure level) was found to show a good correspondence with the loudness. The results of our former eight experiments conducted since 1968 are plotted together in Fig. 1 [16]. High correlation can be seen between the loudness and LAE. A round robin test on the evaluation of impulsive sounds was conducted in Japan and similar results were found [17, 18]. These results suggest that in the perception of impulsive sounds the energy of impulsive sounds is integrated as a unit and that LAE is a good index of the loudness of a single event of an impulsive sound. (2) Effect of temporal pattern Though the temporal sequence in a single event of an impulsive sound cannot be identified, the difference of the temporal pattern may be perceived as the difference of sound quality. Examples of the temporal change of calculated sharpness [19] of sounds generated by hitting a golf ball by a golf club are shown in Figs. 2 and 3 [20]. Closed circles in these figures indicate the time when a ball was hit. The ”sharpness” was calculated every 20 msec with a work station (Cortex Psychoanalyzer CF90). It is clear that there is a difference in the pattern of the temporal change of calculated sharpness in these two examples. Close examination of the temporal change of sharpness suggested that sharpness during 60 ms after hitting seems to have an effect on the impression of sound quality. The difference between sharpness at the point of hitting (S0) and that of 60 ms later (S60) was calculate and related to the impression of sound quality. Copyright SFA InterNoise 2000 3