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VOLUME OF ROOM - ftsx 1,000

VOLUME OF ROOM - ftsx 1,000

between 0.8 and 1.5 seconds, depending on the size of the room. Figure 21.10 gives a number of values based on room volume and the type of music. For small studios and control rooms, the preferred shape of the reverberation-time-versus-frequency curve is flat.

Scoring stages, designed for orchestral music, are like concert halls with reverberation times in the 1.7 to 2.2 second range, but can be furnished with heavy drapes or other variable absorption elements to reduce the reverberation as needed. In scoring stages a reverberation time that rises at low frequencies is desirable; however, too much low-frequency reverberation can reduce the clarity. Increases below 1 kHz (about half of those shown in Fig. 17.11) are appropriate.

Diffusion

A diffusive surface reflects incident sound in several directions rather than principally in one. Its reflections are termed nonspecular as contrasted to the specular or mirror-like reflections produced by a smooth flat surface. Reflectors such as cylinders can be diffusive in one plane and specular in another. Quadratic residue or quarter-wave slot diffusers can also behave in this way. Multifaceted reflectors can generate specular reflections in several directions and contribute to diffusion in a room although their behavior is not perfectly diffuse.

Figure 21.11 shows in a qualitative way the characterization of a sound interaction with a surface. The components of the process are described using absorption, transmission, and diffusion coefficients represented here by single-number values all less than one. This is a great simplification of the actual behavior, since there is a complicated relationship that depends on the type of incident wave, its angle (both incident and reflected), frequency, and the size of the reflecting surface.

Figure 21.12 illustrates more details of the interaction process. In a specular reflection from a hard surface, most of the energy rebounds at an angle centered on the angle

Figure 21.11 Sound Interaction with a Rough Surface

Scattered

Figure 21.11 Sound Interaction with a Rough Surface

Scattered

Figure 21.12 Comparison of the Spatio-Temporal Properties of Acoustical Interactions (D'Antonio, 1987)

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