John S. Bradley - (bradley@irc.lan.nrc.ca)
National Research Council
Ottawa, Canada, K1A 0R6
Gilbert A. Soulodre
Communications Research Centre
Ottawa, Canada, K2H 8S2
Scott Norcross
Physics Dept.
American University
Washington, DC 20016-8058
Popular version of paper 4aAA6
Presented Thursday morning, June 19, 1997
133rd ASA/NOISE-CON 97 Meeting, State College, Pennsylvania
Embargoed until June 19, 1997
A key focus of architectural acoustics research has been to determine the physical characteristics that make concert halls `sound good'. To do this we must identify the subjectively important aspects of acoustical quality and then learn how each of these is related to the physical characteristics of concert halls. The goal of the present work was to identify how bass (low frequency) sounds are perceived in concert halls.
In three experiments, subjects listened to pairs of simulated sound fields in which the level, the decay time, and the direction of arrival of the low frequency energy was systematically varied. The results offer new insights into the characteristics of concert halls that lead to the perception of a strong bass sound. In contradiction to most texts, low frequency reverberation time was not found to be important. However, increased levels of early and late arriving low frequency sound were shown to be essential for a strong bass sound. It was also shown that the addition of low frequency reflections can help to compensate for the seat dip attenuation of bass sounds.
Some History
Many texts[1] suggest that reverberation time (RT) should increase at low frequencies to ensure adequate bass sounds for musical performances. This is usually justified as compensating for the frequency response of our hearing system so that the time for a sound to decay to inaudibility is similar at all frequencies.
Beranek[2] suggested the Bass RT ratio as an indicator of the adequacy of the bass sound in concert halls. This is simply the ratio of the low-frequency to mid-frequency reverberation times.
Barron tested this and other measures in his subjective survey of British concert halls[3] and he found that the Bass RT ratio had the weakest correlation with subjective evaluations of bass balance. However, ratios of low frequency sound levels and early decay times were more strongly correlated with subjective evaluations.
More recently Soulodre and Bradley [4] showed subjective evaluations of bass strength to be significantly related to the level of the early arriving low frequency sound. This was thought to be related to the large reductions in bass sounds that can occur in concert halls when sound passes over audience seating at near grazing incidence[5]. This finding led to the more comprehensive study described here.
Experimental Procedure
The present tests were carried out using simulated sound fields in an anechoic test room with subjects sitting in the middle of an array of 8 loudspeakers. Each loudspeaker was used to simulate individual reflections from a particular direction as well as reverberant sound. The signal to each loudspeaker was modified by a computer controlled matrix of time delays, equalizers and reverberators. This allowed the characteristics of the sound field to be changed instantaneously, thus allowing the listener to compare pairs of simulated concert halls. By careful adjustment of the settings of these devices, the low frequency sound levels and decay times of the simulated sound fields were systematically varied. For example, one could switch between conditions where the low frequency reverberation time varied from long to short without varying mid-frequency reverberation times or other characteristics of the simulated sound fields.
A recording of Handel's Water Music recorded in an anechoic environment was used as the musical source in these tests because it was found to have adequate bass content.
In each test, subjects compared pairs of sound fields and provided a rating of the perceived strength of the bass sounds on a 5 point scale. Without interrupting the music, listeners could switch back and forth between two conditions representing two different concert halls. The pairs of sound fields were presented to the subjects in a randomized order so that neither the subject nor the experimenter knew the characteristics of particular simulated conditions.
Varied low Frequency Reverberation Time and Early Sound Level
Figure 1. Mean subjective rating versus early low frequency sound level G80(125) for long and short reverberations times RT(125) at 125 Hz.
It is commonly believed that the low frequency reverberation time has a strong influence on the perception of bass sounds in concert halls. The purpose of the first experiment was to determine the relative influence of the low frequency RT and the early sound level.
Ten subjects evaluated the 8 combinations of 4 different levels of early arriving low frequency sound and 2 different low frequency RT's (see Figure 1). The variations in levels and reverberation times were chosen to be representative of the range of these quantities found in real halls. Subjects' ratings of the strength of the bass sounds increased significantly with increases of the level of the early-arriving low frequency bass sounds. However, the large changes in low frequency reverberation time (from 1.3 to 3.3 s) had no significant effect on the subjective assessments.
Varied Early and Late Low Frequency Sound Levels
Figure 2. Mean subjective rating versus low frequency early sound level G80(125) for three levels of late arriving sound GL(125).
The results of the first experiment revealed the importance of the low frequency sound level. The second experiment was designed to determine whether the distribution of the low-frequency energy over time influences the perception of bass sounds. The subjective effects of sounds arriving within about 80 ms after the direct sound are usually quite different to those of later arriving sounds. Therefore, subjects listened to sound fields with combinations of varied early- and late-arriving sound levels. As illustrated in Figure 2, the levels of both the early- and later-arriving low frequency sound were found to significantly affect subjects' perceptions of the strength of bass sounds. Although early- and late-arriving sound levels were both important, subjective assessments were more sensitive to changes in the level of the later-arriving bass sounds.
Varied Low Frequency Early Sound Level and Direction of Arrival
In auditoria the low frequency content of sound passing over audience seating at near grazing incidence can be strongly attenuated[5], thus reducing the perceived strength of the bass sound. Added low frequency reflections could potentially compensate for this loss. The third experiment was intended to evaluate whether the direction of arrival might influence the perceived effect of such added reflections. (We know that the direction of arrival has a strong effect on the Apparent Source Width and Listener Envelopment components of spatial impression[6]). Therefore, the level of the early-arriving low frequency sound and the direction of arrival of this sound was varied.
Although the direction of arrival did have a small effect, changes in level had a much stronger influence and these results confirm that added low frequency reflections could be used to increase the perceived strength of bass sounds.
Conclusions
References
[1] Knudsen, V.O. and Harris C.M., Acoustical Designing in Architecture, Wiley, New York, (1965).
[2] Beranek, L.L., "Music Acoustics and Architecture", Wiley, New York, (1962).
[3] Barron, M., "Subjective Study of British Symphony Concert Halls", Acustica, vol. 66, pp. 1-14, (1988).
[4] Soulodre, G.A., and Bradley, J.S., "Subjective Evaluation of New Room Acoustics Measures", J. Acoust. Soc. of Am., vol. 98, pp. 294-301, (1995).
[5] Bradley, J.S. "Some Further Investigations of the Seat Dip Effect", J. Acoust. Soc. Am. vol. 90, pp. 324-333, (1991).
[6] Bradley, J.S., and Soulodre, G.A., "The Influence of Late Arriving Energy on Spatial Impression", J. Acoust. Soc. Am., vol. 97, pp. 2263-2271, (1995).