J.S. Bradley - john.bradley@nrc.ca
Institute for Research in Construction
National Research Council
Montreal Rd.
Ottawa, Canada, K1A 0R6
Popular Version of Paper 2aAA3
Presented Tuesday morning, 14 May 1996
Acoustical Society of America, Indianapolis, Indiana
Embargoed until 14 May 1996
Over 100 years ago Sabine related reverberation time to the volume of a hall and the amount of sound absorbing material in it. Because in most auditoria the chairs and the audience make up most of the sound absorbing material, they are perhaps the most important surface in the room, and have large effects on the acoustical characteristics of the room. Although we do not completely understand all of the effects of chairs and audience on acoustical characteristics, new work has considerably expanded our understanding of these effects.
Some years ago Beranek[1] challenged the conventional approach to calculating the effect of chair absorption on the reverberation time of a hall. He showed that it was more correct to assign absorption coefficients to the surface of blocks of seats including some of the edges of the blocks. More recently the absorption coefficients of groups of chairs have been shown to vary with sample size as occurs for simple absorbing panels[2]. The absorption coefficients of groups of theatre chairs vary approximately with the ratio of perimeter to area of the measured groups of chairs (see Fig. 1). The groups of chairs used in reverberant test chambers are much smaller in area than the blocks of chairs in an auditorium, and measured absorbing properties in a reverberant test chamber would considerably over-estimate the properties of the chairs in a typical auditorium.
Fig. 1 illustrates the simple linear relationship between absorbing properties and sample perimeter to area ratio for both occupied an unoccupied chairs. One can now measure several different sized samples of occupied or unoccupied chairs in a reverberant test chamber and extrapolate the results to more accurately predict the effect of larger groups of chairs in an auditorium
Fig. 1. Measured absorption coefficients versus sample perimeter to area ratio, P/A.
The absorption of occupied chairs depends mostly on the occupants, and hence varies less among chair types[3]. It was thus possible to estimate the properties of average occupied theatre chairs as a function of the size of the block of chairs. Differences in the absorption of occupied chairs depend mostly on the properties of the parts of the chairs that are not covered by the occupants.
Theatre chairs and audiences have other more complex effects on the acoustical properties of an auditorium than simply influencing the total amount of sound absorbing material. Sound propagating over the audience at near to grazing incidence can be strongly attenuated at low frequencies. This reduction in the early arriving bass sounds has been associated with a perceived reduction in the strength of bass sounds in halls[4]. Measurements of this effect in real halls have shown that it can vary from hall to hall and with the angle of incidence of the sound[5]. The influence of an audience of this reduction of the bass sounds can vary with the type of chairs[6].
The effect of an audience on the sound levels in an auditorium depends on both the shape of the hall and the absorbing properties of the audience. Levels of the direct and early arriving sound can even increase with the addition of an audience due to shifts in the frequency of the grazing incidence bass attenuation. In halls with low ceilings a considerable portion of the total sound energy may arrive at the listener as ceiling reflections. Because the ceiling reflections are not influenced by the presence of an audience, the audience has less effect on sound levels in these halls. Perceived clarity is often related to the ratio of early arriving to later arriving sounds. The effect of an audience on these ratios, and hence on perceived clarity, can again depend on the shape of the hall as well as the absorption added by the audience. Measures of spatial impression, relating to the apparent width of the source and the degree of listener envelopment, are not changed greatly by the presence of an audience.
New studies offer improved techniques for estimating the sound absorption of occupied and unoccupied chairs. They show that the absorbing properties of both occupied and unoccupied theatre chairs vary with frequency and with sample size. The absorbing properties of partially occupied seating areas have not been extensively studied. Other effects of theatre chairs and their occupants are less well understood and are influenced by the shape of the hall as well as the absorbing properties of the chairs and occupants.
References
(1) Beranek, L.L., "Music Acoustics and Architecture", John Wiley and Sons (1962).
(2) Bradley, J.S., "Predicting Theatre Chair Absorption from Reverberation Chamber Measurements", J. Acoust. Soc. Am., Vol. 91, pp. 1514-1524, (1992).
(3) Bradley, J.S., "The Sound Absorption of Occupied Auditorium Seating", J. Acoust. Soc. Am., Vol. 99, pp. 990-995, (1996).
(4) Soulodre, G.A., and Bradley, J.S., "Subjective Evaluations of New Room Acoustic Measures", J. Acoust. Soc. Am., Vol. 98, pp. 294-300, (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. "On the Effects of the Audience on Auditorium Acoustics", Proc. 15th Int. Cong. on Acoust., pp. 389-392, Trondheim (1995).