Edwards Signaling Catalog

3. Ambient Sound Conditions: The frequency of the
signaling device selected should be as different from
the background noise as possible. The signal should
override this noise by a minimum of six decibels. Usually,
the noisier the area, the louder the signal required and the
greater the number of signaling devices required. Over the
years, horns and bells have been the most popular signals
because their varied tones can be heard over most ambient
conditions. See the table on page 27 for typical dB levels
associated with various activities.
4. Size of Area to be Covered: The larger the area, the
louder the signal required or the greater the number of
units required. As a rule of thumb, sound output drops by
approximately 6 decibels each time the distance between
the sound source and the human ear is doubled. For
example:
Source in dB
Feet from Source
116 dB 10
110 dB 20
104 dB 40
98 dB 80
92 dB 160
If a signal is rated at 116 dB at 10 feet, then a signal twice
as loud would be rated at 122 dB at 10 feet. In an application
where the ambient noise level was 92 dB, the signaling
device in the above example would adequately cover a
distance of approximately 80 feet. Better coverage is
frequently obtained by the use of several signals of lower
dB rating than by using one very loud signal, (providing the
signals can be positioned to advantage).
5. Mounting Considerations: The surface that the signal is
mounted to can markedly affect the signal’s performance.
A horn, for example, should be rigidly mounted to a sturdy
surface so that all of its energy is directed into the sound
output and not absorbed by the wall. A buzzer, however,
can sound much louder if mounted on a resonant surface
because it will act as a “sound board” for the buzzer.
TM
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