The effects of seating on the acoustics of auditoria
The two main attributes of seating in auditoria have been investigated. Tle first is random incidence absorption. The second is the low-frequency selective attenuation which seating can impart to sound travelling over it at grazing incidence: the so-called "seat dip" effect. It was found that there was a need for a more accurate laboratory measurement method to predict auditorium seat absorption. The traditional method tended to overpredict the absorption of the exposed front and sides of seating blocks. A new method was studied which involves the use of barriers to obtain realistic measurements of front and side absorption. The new method was validated by comparing measurements of seats made in a reverberation chamber with in-situ absorption data for the same seats, calculated from reverberation time measurements in ten auditoria with and without the seats present. The accuracy of the new method was found to be satisfactory in all cases, although a severe lack of diffusion in two of the halls hindered the validation process. The important physical factors affecting seat dip attenuation were investigated by measurements in a concert hall and on scale model seats. A scheme for reducing the attenuation with resonant absorbers was evaluated, and a simple theoretical model developed. 'Ibe subjective significance of the effect was established with a panel of ten subjects and a fully simulated auditorium sound field. The absolute threshold of perception of the seat dip effect was found to be 7.1 ± 0.6 dB attenuation in the 200 Hz octave band of the early field. It was found that seat dip attenuation might be made less audible in a hall by: (i) supplying early energy along paths remote from the seating, (ii) increasing the vertical angle of incidence of the direct sound and (iii) installing resonant absorbers in the floor between seat rows.