Handbook for Sound Engineers

1348 Chapter 35

An important parameter for direct sound coverage is
the angle of radiation ) (beam width angle). It stands
for the solid-angle margin within which the directivity
gain drops by a maximum of 3 dB or 6 dB (or another
value to be specified) as against the reference value.
The curves of equal directivity gain are marked
or generally )n; the higher the directivity
the smaller the angle of radiation, Fig. 35-10.

Because of the curves of equal directivity gain and the
sound distribution loss, the impact of direct sound of a
loudspeaker on a surface may produce elliptic curves that
represent a calculated SPL isobar area of the direct sound

coverage. These isobar areas are important in the plan-

ning of sound reinforcement systems as coverage areas.

For combining the influence of the directional effect

as well as that of the distribution between directional

and omnidirectional energy, one uses, in acoustics, the

directivity deviation ratio:^10

(35-19)

This quantity is also of high importance to sound

reinforcement engineering. It characterizes the reverber-

Figure 35-9. Polar plot of the angular directivity gain of a
sound column with indication of the radiation angles.

)3–, )6–,

if

Figure 35-10. Plot of the directional effect of the radiator

DML-1122 (Electro-Voice) frequency 2 kHz; front to

random index 15 dB; maximum sound level at 1 m

distance: 125 dB.

A. Directivity characteristic.

B. Coverage area.

vertical horizontal

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