A sound signal is a term used to describe ATONs that produce an
audible signal designed to assist the mariner in fog or other periods of
reduced visibility. Sound signals can be activated by several means,
such as manually, remotely, or by a fog detector device. It should be
noted, however, that in patchy fog conditions, a fog detector may not
always activate the signal.
Sound signals are distinguished by their tone and phase characteristics.
The tones are determined by the devices producing the sound, such as a
horn, bell, or gong. Phase characteristics are defined by the signals
sound pattern, or the number of blasts and silent periods per minute
when operating. In the case of fixed structures, sound signals generally
produce a specific number of blasts and silent periods every minute;
buoy sound signals generally do not because the sound signal is
generated by wave action.
The characteristic of a sound signal can be found in column 8 of the
Light List. For example, for Chesapeake Light (LLNR 355) it reads
"Horn: 1 blast ev 30s (3s bt)." What this means is that 30 seconds is the
time required for one complete cycle to occur. During this 30-second
cycle, there are 27 seconds of silence and 3 seconds of blast. You can
time this cycle with a stopwatch just like a light. Timing a sound signal
is another method of positively identifying an ATON. Unless it is
specifically stated that a sound signal "Operates continuously," or the
signal is a bell, gong, or whistle on a buoy, it can be assured that the
sound signal only operates during fog, reduced visibility, or adverse
CAUTIONS TO OBSERVE IN USING SOUND SIGNALS: Sound
signals depend upon the transmission of sound through the air. As
ATONs, they have certain inherent limitations that you must consider.
Sound travels through air in a variable and unpredictable manner. At
times, these signals may be completely inaudible even when close by.
At other times, they may appear to be coming from a direction quite
different than the actual bearing of the signal source. Mariners should
not rely on sound signals to determine their positions.