Distributed Sound Systems

J.W. Davis & Company

TECHNICAL NOTE # 78:

Distributed Sound Systems - Part 1 of 2

Overhead distributed sound systems are capable of delivering outstanding music and paging audio performance. Unfortunately, many do not attain their potential due to the installer’s failure to provide adequate coverage. This problem can be avoided in most instances by following the minimum guidelines suggested below.

First, do not assume that the speaker coverage will exceed a 90° angle. While some speaker systems do give greater coverage, these are exceptions that should be carefully documented by independent tests for their validity.

Second, a rough "rule of thumb" method of estimating speaker spacing is to determine the ceiling height, subtract 4' to allow for the height of the listener, and multiply by 2. For example, the space between speakers in a room with a ceiling height of 14' would be 20’ (14 – 4 x 2).

Third, the hexagonal spacing pattern shown in Figure 1 is a good compromise between end-to-end and overlapping patterns. Increasing the volume to compensate for an inadequate number of speakers will raise the loudness level but only at the expense of losing intelligibility and clean music reproduction.

Fourth, to find the coverage area of each speaker, determine the ceiling height, subtract 4' for the height of the listener, multiply that number by itself and then multiply the result by 3.1416. For example, if the ceiling height is 14', subtract 4' for the listener’s height (14 - 4 = 10), multiply 10 x 10 (100) then multiply by 3.1416 to find 314 square feet speaker coverage (pr², the area of a circle).

Fifth, after finding the coverage area of each speaker, calculate the number of required speakers by dividing the size of the room by the coverage area of an individual speaker. For example, a 40' by 40' room (1,600 sq. ft.) with an individual speaker coverage of 314 square feet would require 5 speakers (1600 divided by 314).

Sixth, the sound level of the speakers should be set to exceed the normal room noise level at least 3dB for music and 6dB for paging.

Seventh, adequate amplifier power is essential to produce satisfactory sound reproduction. This may be determined in 25V and 70V systems by simply adding the value of the taps on each of the transformers in the system and multiplying the sum by 1.43 for "headroom" to allow for line and transformer power losses. In the example above, assuming that each transformer was tapped at 5 watts, this would be 37.75 watts (5 X 5 X 1.43). Therefore, a 40 watt amplifier would be the proper selection.

NOTE: Larger, more complex installations with special circumstances such as ceilings over 30' or under 10' require more detailed information than is provided here. For additional assistance, call J. W. Davis & Company Customer Service or for comprehensive technical information see Sound Systems Engineering written by Don and Carolyn Davis published by Howard W. Sams & Company.

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