The automation of fixed aids to navigation has had its effect not only on lighthouses but also on the fog signals sited at major lights. Fog horns have not only become automated, but are exposed to the acid test of efficiency that drives a budget-constrained Coast Guard.
One approach the Coast Guard has taken to improve efficiency has been to replace older air-powered fog horns with horns that use electronic circuits to construct the very low frequency air vibrations. Recently, the Coast Guard installed just such a horn on Alcatraz Island in San Francisco Bay. Local fog horn lovers, however, were not happy with the tone of the new horn. They complained loudly enough to get the Coast Guard to refurbish and reinstall the old-style air-powered horn. The reinstalled fog horn will be operated and maintained by volunteers.
While most boat owners are familiar with manually sounding a fog signal when the fog lowers, human control is, of course, impossible at an unmanned site. Coast Guard engineers have solved the problem turning the horn on and off by using an automatic fog detector.
Employing the principle of backscatter, a properly calibrated fog detector can sense when visibility lowers to a pre-set threshold value and will turn on its associated horn.
Inside the fog detector is a strobe that flashes at the rate of three Hz (three flashes a second). If the air is clear (low humidity, low dust, low pollutants), the vast majority of the light from this strobe will depart from the detector, never to return. If, however, fog particles are present (10 to 30 microns in size), some percentage of strobe light will be bounced back to the collecting lens. (The spherical drops act like small corner reflectors. For more on water droplet interaction with light, see “Luminous noise,” Issue No. 57). Behind the collecting lens is a photodiode that converts reflected light pulses to electrical pulses. These pulses are displayed on the unit’s “visibility meter.” (The best way to determine visibility is to have a separate light source and detector about 100 feet apart, with each component pointed at the other. This is the method used at airports. It is not used at lighthouses because there usually isn’t enough real estate to mount the separate components.)
Once the flash lamp and the photodiode are properly calibrated, the voltage of the electrical pulses can be directly related to visibility. In other words, a voltage of x indicates two-mile visibility, while y is 1/2-mile visibility. The local aids to navigation team sets the visibility level that turns on the fog signal.
That level is often dictated more by local politics than by navigational considerations. For example, when a large horn is located near a residential area, homeowners will often complain about the sound. Thus, a horn with neighbors will often be set to turn on when visibility has gotten down to 1.5 miles. A similar horn in a more secluded location might come on at three miles.
When the fog horn apparatus is energized, an electronic circuit produces an output frequency of 150 Hz. But, in order for mariners to hear anything, it is necessary to convert the 150 Hz AC electrical current into sound waves.
In previous decades, large fog horns were powered by compressed air or even by steam. Probably the most famous air horn was the F2T. These were called diaphones and had a distinctive high/low, two-tone sound produced by a slotted reciprocating piston. (Any old black and white movie set in San Francisco has one of these two-tone fog horns on the sound track to add atmosphere.) Compressed air makes for a deep, throaty horn, but it also requires plenty of machinery for compressing the air, storing it in tanks, removing the condensed water, and directing the air into the horn to vibrate either a diaphragm or a piston. This type of installation works well enough when there is a watchstander present to monitor things. When a lighthouse is automated, however, a compressed air system becomes too expensive to operate.
Because of this, the Coast Guard developed an electronically powered horn. This sounds impressive, but is ready nothing more than a heavy duty version of a stereo speaker. Inside the horn assembly of the CG-1000, manufactured by Automatic Power, Inc. of Houston, Texas, is a steel plate that acts as a movable diaphragm (a stereo speaker uses a paper cone). Affixed to the back of the plate is a magnet and encircling the magnet is a coil of wire. When current flows through the coil, a magnetic field is produced. This field attracts the magnet, drawing it into the coil and warping the steel plate. Because this is AC, the current suddenly changes direction, causing the magnetic field around the coil to collapse. Freed from the attraction of the magnetic field, the steel plate releases the energy it has stored by its initial warping and andquot;springsandquot; outward. Current now flows in the opposite direction, establishing a second magnetic field around the coil. Once again, the magnet is drawn into the coil, warping the steel plate. With another reversal of current, the magnetic field collapses and the plate releases energy, springing away from the coil. So, the steel diaphragm goes through two vibrations per cycle, thus, it vibrates at an acoustical frequency of 300 Hz (the normal range of human hearing is 30 to 16,000 Hz).
The CG-1000 has an input power of 1,000 watts and this produces an acoustic pressure of 143 decibels at the mouth of the horn (single emitter). Since this sound energy must disperse as it travels outward, the pressure falls off to roughly 126 db at 25 feet. This decibel level is louder than what one would hear in an airport parking lot when the Concorde SST was taking off. When one power supply and one emitter is used, the range is an average of three miles; two power supplies and two emitters together give a four-mile range. (Unlike lights, where a given strength of light can be equated with a nominal range, sound signals are more fickle in terms of range. Depending on atmospheric conditions, on the shape and type of obstructions near the signal, and the direction and strength of the wind, fog signals will be heard at different ranges. Even strong signals may have “dead spots” where the horn is not heard at all.)
Once the sound is produced by the diaphragm, it must be directed out to sea where it is needed. This is accomplished by the shape of the assembly. The metal andquot;bellandquot; structures are proportioned to efficiently transfer the vibrations produced by the diaphragm out into space. However, once the sound waves exit the horn, they are free to spread out in any direction. Since the bigger horns are fairly noisy, and can disturb people living nearby, various types of acoustical baffles are employed by the Coast Guard in an attempt to cancel the sound in the direction of inhabited areas.
The baffles are often a grid of steel plates that function as a series of acoustical pockets. These are designed to capture sound and resonate, effectively attenuating the fog signal in those areas where it is not desired.