Useful Ceil

REFLECTIONS i

Figure 17.7 Reflections from a Flat Ceiling Section

Design of Rooms for Speech 585 Figure 17.8 Reflected Sound from a Segmented Ceiling (Doelle, 1972)

Figure 17.9 Reflected Sound from a Stepped Flat Ceiling

illuminate the front and middle portions of the space but much of the energy falling on the rear portion of the ceiling is grounded out on the absorptive rear wall.

To improve the design, the ceiling can be segmented as in Fig. 17.8 or the seating raised and the ceiling stepped as in Fig. 17.9. Note that only about half of the ceiling provides useful specular reflections in both Fig. 17.7 and 17.8 since the ends of the segmented reflectors are diffusive.

The energy distribution is dependent on the location of the talker, which may vary, so slightly convex panels may be used to provide additional flexibility. Panels should not be used to reflect sound directly down, or back to the listener from behind, since this shifts the perceived source location overhead.

Reverberation

Reverberation can be the boon or the bane of the acoustical performance of a room. In general, the more speech content there is to the sound, the lower the ideal reverberation time. For classrooms and small lecture halls times at or below one second are preferred. Longer reverberation times are desirable for music; the ideal length depends on both the room size and the type of music. For light opera such as Gilbert and Sullivan, where understanding the complicated play of words is critical, a time of 1.0 to 1.2 seconds would not be too low. For a Mozart opera preferred reverberation times might range from 1.2 to 1.5 seconds. A Wagnerian opera is ideal in a 1.5 to 1.6 second room, and romantic symphonies sound best in a 1.7 to 2.1 second hall. For organ concerts and chant, reverberation times between 2.5 to 3.5 seconds

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