Physics 36 / Music 36 Duke University Spring 2008 Handout 17
Have the room to yourself. Make the noise conditions match those of the anticipated performance (amount of traffic, air conditioning equipment configuration, stage lighting equipment (which may include noisy cooling fans backstage), house lighting settings (some house light dimming systems produce audible tones at low settings), etc.). Inquire about any episodic noise (train schedules, deliveries, adjoining rest rooms, special event nearby, etc.). Also consider sources of noise in the lobby (which may enter the hall with any latecomer or early departer).
Stand on stage, down front, center. (a) Listen for outside noise, HVAC or other interior noises. (b) Clap your hands sharply or burst a toy balloon and listen for any distinct echoes. If any are heard, try to identify the source(s). Repeat from various locations in the audience, looking for parallel highly reflective side walls (flutter echo problems) and concave reflecting surfaces (potential sources of focusing effects).
Pace off the floor dimensions, estimate an average ceiling height, and calculate an approximate volume in cubic feet or cubic meters. Be looking for diffuse reflecting surfaces for all audible wavelengths. Check for surface materials (like thin plywood or glass, or too much carpet or drapery) that will absorb sound strongly in certain frequency ranges.
Back on stage, use a balloon or starter's pistol to excite the room's modes and estimate reverberation time by measuring the extinction time. (Using a watch, think a tempo of 5 beats per second. This will allow you to determine extinction time to better than 0.2 s accuracy.)
If possible, excite the room with loud sustained tones of limited frequency range for at least twice the reverberation time to saturate room modes. Then measure the time from cutoff to extinction for such tones in several different frequency ranges. (Alternatively, produce hand clap excitations with different spectra by altering the cupping and contact area of your hands.) Listen for uneven decay, a symptom of uneven distribution of sound in the room. Listen also for any change in spectrum as the reverberant sound decays. In these ways form an impression of the frequency dependence of the reverberation time of the room.
Calculate the empty room's total absorption using the Sabine equation and the volume and reverberation times you have determined. Estimate the effect of an audience on that total absorption and, in turn, the reverberation time. (It is best to do this in terms of the area occupied by an audience, but a rough rule of thumb is that filling an upholstered seat adds about 1 square foot of absorption, while filling a wooden or metal seat adds about 5 square feet -- at mid frequencies and for areas of the audience that are 50% or more occupied.)
Check for prompt reflection surfaces (rear and side walls for stage, stage ceiling, forestage floor, side walls and side balcony fronts, clouds, main ceiling.) Consider how the height of the stage, the rake of the audience, and the depth of any balconies affect the amount of direct sound reaching the audience. Decide where the performers should be located on the stage and how any adjustments possible in the room should be made. If the audience seating is moveable, consider whether it might be arranged for better sound.
Consider the balance on stage for the performers. (In some cases it may be useful to think of the stage as a room with a proscenium-sized open door.) Consider alternative positioning of performers and/or furnishings to improve the balance on stage.
Identify any doors that should be kept closed and any areas that should be restricted during the performance to limit noise, and any noisy equipment that should be turned off during the performance.
