Selecting Siren Locations
Decibel dropoff rate and minimum acceptable range used to plot area coverage and determine most suitable sites
In early America, fire companies were brought together by the ringing of bells, gongs, shouting and word of mouth. In contemporary America, large cities with full-time fire companies are equipped with direct line or radio signaling systems, while suburban companies alert volunteers with a loud audible signal produced by a siren or a horn and radios. Since a poorly placed fire siren will produce no better results than an early American shout, we will look at some of the factors to be considered in fire siren placement.
The ear hears sound by responding to minute pressure variations in the atmosphere. The rate, or number of cycles per second, at which these pressure variations occur determines sound frequency. The amount of pressure variation determines sound intensity, or loudness. The familiar siren wail is produced by rhythmically interrupting a column of air pumped through the siren.
Sound intensity is measured in units called decibels. The minimum hearing level is about 10 decibels and the maximum tolerable level is approximately 130 decibels. Since sound waves radiate in all directions, sound intensity, or loudness, falls off very rapidly with distance. In clear air, sound intensity drops off about 6 decibels each time the distance from the source is doubled. Since the decibel is a logarithmic term, this constitutes a considerable dropoff.
Baffles control sound
When sound waves strike an object, they are either reflected or absorbed, depending on the object’s acoustical properties. Siren manufacturers take advantage of this property to eliminate the sound from unwanted areas by directing it with baffles. The area immediately above the siren is blanked out to save siren power while the area immediately below is usually blanked out to prevent the sound level from exceeding our tolerance level.
In addition to being reflected or absorbed, sound waves can also be affected by air temperature, humidity, and prevailing wind or can be blanked out by background noise. Civil defense officials take these factors into account by assuming an 8-decibel dropoff rate each time the distance from the siren is doubled.
Siren horsepower is a gross indicator of sound output, but it ignores such factors as siren directional characteristic, baffling, and efficiency. A more valid indicator of siren performance is the manufacturer’s stated output in decibels 100 feet from the siren.
Plotting sound intensity
Using this catalog data, a sound dropoff rate of 6 or 8 decibels for each distance doubled, and a sheet of semilogarithmic graph paper, we can construct a simple plot of sound intensity vs. distance from the siren. The chart shows plots of sound intensity for standard 5, 7½, and 10-hp sirens. Note that a dropoff rate of 6 decibels is used for the first 100 feet of distance, where there will probably not be any obstructions, whereas a dropoff rate of 8 decibels is used out to the maximum range to account for some of the previously mentioned factors.
A level of 70 to 80 decibels should make a siren clearly audible. The lower level is satisfactory for unobstructed areas in a mild climate, while the upper level is better for areas of high background noise, large buildings, trees, enclosed dwellings, or cold weather.
To determine the maximum range of a given siren, pick a desired minimum sound level and draw a vertical line on the chart until it intersects the 8-decibel dropoff line. For our example, using 80 decibels as the minimum intensity, the maximum ranges for 5, 7½ and 10-hp sirens are 900, 1200, and 1600 feet.
Note the sound intensities at distances of 30 and 50 feet from the siren. Mounting sirens 30 to 50 feet above the ground, in conjunction with baffling, makes the sound level in the immediate vicinity of the siren quite high but still acceptable.
Armed with the chart developed above and starting out with a general idea of siren locations (dictated by existing power or signal circuits, available property, local exposures, etc.), we can proceed to lay out the system.
The first step is to obtain an appropriate scale map of the area to be covered. Next evaluate obstacles, such as tall structures, heavily treed areas, areas of high noise levels and any natural obstructions. Then pinpoint either the existing or proposed siren locations. Lay out circles of sound intensity, using the maximum siren ranges obtained from the chart.
Now rearrange siren locations to avoid overlap and work around trouble areas. Fill in trouble areas with a directional horn if required. Finally, locate siren heights in relation to surrounding terrain and obstacles. The ideal last step would be a trial installation with the actual equipment to check the results of your effort.