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Direct Sound Diffused Ref lection

Direct Sound Diffused Ref lection of incidence. The temporal response is constrained to a narrow band determined by the path length. Some temporal smearing is present due to the after vibration of the reflecting surface and the diffuse behavior of the material.

A reflection by a purely absorptive material is also specular, however the amplitude is decreased by an amount characterized by the absorption coefficient. The temporal behavior is narrowed since there is less reradiation by a porous absorber. The behavior of panel absorbers can be expected to show more temporal smearing.

The pattern generated by an ideal diffuser is spatially uniform. There also can be temporal spreading depending on the type of process. For example with quarter-wave tube diffusers, energy is stored in the wells and released over time. A diffuser such as a half cylinder or pyramid would produce a temporal pattern much like that of a specular reflector.

Diffusion in listening rooms and studios can be helpful by contributing to a sense of spaciousness and envelopment. It can help fill in the gaps between specular reflections. Where there are flutter echo problems a diffusive surface can be used to break up the flutter loop without adding excessive absorption. Diffusion can also make absorptive materials more effective by creating a reverberant field, which assures that the acoustic energy will interact with the absorbers. Diffusers do not need to be perfect scatterers to be effective.

Imaging

Imaging refers to the preservation of the spatial relationship in the original sound field perceived by the listener. In home listening rooms, designed for high-end audio, the stereo format with a subwoofer is the most common. Stereo loudspeakers are arranged at an included angle slightly greater than 60°, and the listener is about 10 to 12 ft (3 to 3.5 m) away, slightly closer than the separation distance between the loudspeakers. Many loudspeakers are very sensitive to placement, especially separation distance and toe-in angle. By varying these parameters one can often increase definition and detail in listening rooms. Space around the loudspeakers can be helpful in contributing to the soundstage. Where the dimensions allow it, loudspeakers should be placed at least 1.5 m (5 ft) from any walls. Reflections from a nearby wall can color the sound, particularly in the lower registers. Patches of absorptive material on side and rear walls help control rogue reflections. Transondent wall finishes such as wood slats are common in control rooms where cloth-covered panels can be located.

The design of professional listening rooms tends to be format driven. A sampling of possible loudspeaker arrangements is given in Fig. 12.13. Clearly there are so many possible arrangements that not all can be accommodated in a single room. A working room is designed for one principal format, with 5.1 being the most common for film and stereo the most common for TV and recorded music. The number and position of the surround loudspeakers is not always fixed but is fitted to the size and arrangement of the seating in the room.

The arrangement of a listening room can influence the audio image. Reflections from the side walls tend to broaden the phantom stereo image. Conversely side-wall absorption narrows it. Diffusion behind and to the side of the listener increases the sense of spaciousness. Too much diffusion behind the listener has been found to generate an acoustical fog or clutter (Toole, 1998).

Experiments were carried out in Japan by Kishinaga et al. (1979) to assess the best listening environment in which to evaluate home audio products. Four audio professionals did these experiments at Nippon-Gakki (Musical Instruments of Japan) for four different

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