J. S. Bradley
IRC, Natl. Res. Council, Montreal Rd., Ottawa, ON K1A 0R6, Canada
The effects of adding orchestra shells or modifying orchestra shells were measured in six different concert halls varying in volume from 3000 to 32 000 m[sup 3]. Early, late, and total sound levels were seen to increase with the addition of an orchestra shell. Effective orchestra shells were found to increase total sound levels by up to 2 dB at mid-frequencies in audience areas of halls. Average changes to measured clarity could be larger. The measured increases in sound levels varied with location in the halls, and in several cases sound level increases were greatest towards the rear of halls. In one case, adding an orchestra shell was found to decrease low-frequency levels due to a shift of the seat dip attenuation. On-stage, larger effects were measured. Support values increased by as much as 5 dB with the addition of an orchestra shell. Partial changes to shells generally produced much smaller effects. In audience seating areas the effects of partial changes and adjustments to orchestra shells were usually less than estimated just noticeable differences in the measured quantities.