Bose Panaray MA12 기술 기반 및 토론 - 페이지 18

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Bose Panaray MA12에 대해서도 마찬가지입니다: 사용자 설명서 (2 페이지), 설치 매뉴얼 (16 페이지), 설치 매뉴얼 (8 페이지), 설치 매뉴얼 (7 페이지), 서비스 매뉴얼 (11 페이지)

페이지 인쇄

Figure 9. The radiation pattern of a long, thin source is wedge-shaped.

The sound radiates in two dimensions: in and out and left and right, but

not up and down.

How does sound intensity fall off as a function of distance for such sources and such

waves? The answer lies halfway between spherical waves and plane waves, as shown in

Figure 10. The spherical source radiates in three dimensions and falls off as 6 dB per

doubling of distance, the cylindrical wave radiates in two dimensions and falls off as 3 dB

per doubling of distance, and the plane wave radiates in one dimension and falls of as 0 dB

per doubling of distance.

For the reader interested in understanding the underlying physics of these differences, imagine a sound-

intensity-meter ten feet in front of a source producing each kind of wave: spherical, planar, and

cylindrical. Furthermore, imagine that at this distance, each source produces the same level. Now,

consider a small area of the sound wave at the location of the meter.

- As the spherical wave spreads out, the wave must expand over the surface of a sphere. When the sphere

doubles in diameter, the small area of the sound wave must expand over a proportionately larger area at

the doubled distance. As a result, the intensity of the original area of sound diminishes. Since the area of

a sphere increases as the square of the radius, increasing the distance from the source by a factor of two

(doubling the distance and therefore the radius) means reducing the sound intensity by a factor of four

(two squared). A factor of four in sound intensity corresponds to 6 dB.

- As the cylindrical wave spreads out, the wave must expand over the surface of a cylinder. When the

cylinder doubles in diameter, the small area of the sound wave at the closer distance must spread over a

proportionately larger area at the doubled distance. As a result the intensity of the original area of sound

wave is reduced. Since the area of a cylinder increases proportionately with only the radius (rather than

as the square of the radius as in the case of a sphere), increasing the distance from the source by a factor

of two (doubling the radius) means reducing the sound intensity by only a factor of two. A factor of two

in sound intensity corresponds to 3 dB.

- As the plane wave progresses, the wave does not expand. When the wave reaches a distance that is

double the original, the small area of the sound wave at the closer distance has not spread at all, and as a

result the intensity of the original piece of sound wave is the same. A factor of zero in sound intensity

corresponds to 0 dB.

Bose

MA12™ Modular Array: Technical Foundation & Discussion

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