Specifications

One of the most common uses of

distributed lines is to power ceiling

speakers in office, retail, and com-

mercial buildings. With ceiling-

mounted loudspeakers, one common

rule of thumb is to make the center-to-

center distance between them no

greater than twice the floor-to-ceil-

ing distance. Used with loudspeakers

that have a 90° angle of coverage, this

is often suitable for background music

systems, but for paging and public

address applications it tends to have

somewhat erratic coverage at normal

sitting and standing positions, as the

illustration below shows.

The Inverse Square Law

The Inverse Square Law says that as a sound wave travels away from its source, its intensity, or sound

pressure level, is inversely proportional to the square of

, the distance from its source.

Therefore, the intensity of a sound wave when

= 2 meters is only 1/4 of what it was at 1 meter;

at 3 meters, it’ll be 1/9, and at 4 meters, 1/16. In decibels, 1/4 power is equal to -6 dB; that’s

why the Inverse Square Law is often stated as ”-6 dB every time you double the distance.”

Sound waves travel outward from a source, such as a loudspeaker or a person’s mouth, in all

directions. The sound might be stronger in some directions than others, but it all travels at the

same velocity. Therefore, sound waves tend to be spherical, or partially spherical, around the

source. The area of a sphere is proportional to the square of its radius (analogous to

); the

energy of the sound wave is distributed over this area. If you think of sound intensity as

force

area

, such as dynes per square centimeter, you can see how the increasing spherical area causes

intensity to diminish accordingly.

For those unafraid of a little math, the difference in SPL at one distance, D1, and a reference

distance, D2, is equal in decibels to

dB = -10 × log(D1/D2)

A better technique is to space the ceiling speakers at twice the distance from the listeners’ ears to the ceiling.

This requires more loudspeakers spaced closer together but provides greater intelligibility through better, more

uniform coverage at realistic listening positions. In a room where people are standing, you’ll need a little closer

spacing than if they are sitting, simply because their ears are closer to the ceiling. For example, a company

lunchroom needs a sound system; it has a ceiling height of 2.9 meters (9.5 ft), and you determine the height of

an average listener’s ear, when seated, is about 1.1 meter (3.5 ft) above the floor. That puts the ceiling-to-ear

distance at about 1.8 meters (6 ft); therefore, you should space the loudspeakers no more than 3.6 meters (12 ft)

apart.