Sound-powered telephone
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A sound-powered telephone is a communication device that allows users to talk to each other with the use of a handset, similar to a conventional telephone, but without the use of external power. This technology has been used for at least six decades for both routine and emergency communication on ships to allow communication between key locations on a vessel even if power, including batteries, is no longer available. A sound-powered phone circuit can have two or more stations on the same circuit. The circuit is always live, thus a user simply begins speaking rather than dialing another station. Sound-powered telephones are not normally connected to a telephone exchange.
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[edit] Operation
The headset microphone transducer converts sound pressure from a user's voice into a minute electrical current, which is then converted back to sound by a transducer at the other end. The most significant distinction between ordinary telephones and sound-powered telephones is in the operation of the microphone. Since the more common carbon microphone used in most telephones does not convert acoustic energy to electrical energy (it only modulates the supplied electrical current) it cannot be used in sound-powered transducers. Rather, most sound-powered telephones use a dynamic microphone. A common approach to transducer design is the balanced armature design because of its efficiency. The number of simultaneous listeners is limited because there is no amplification of the signal.
A sound-powered telephone circuit can be as simple as two handsets connected together with a pair of wires. More complex circuits include magnetos, selector switches and bells to allow one user to select and call another. The voice communication circuit is completely separate from the call circuit[clarification needed], allowing communication to take place without external power.
[edit] Usage
Sound-powered telephones are widely used on naval vessels. A typical example on a U.S. Navy ship is the "JL" circuit which is used by the lookouts to report visual contacts to the pilot house and the Combat Information Center (CIC). In this case there would be five stations on the circuit (stern lookout, port lookout, starboard lookout, pilot house and CIC).
U.S. Coast Guard Regulations require this emergency type communication in most vessels today and dictate where phones should be located. A dial telephone system with a battery backup will not meet the USCG Regulations as they currently exist.
Other uses for sound-powered telephone technology today include emergency communications systems for high-rise buildings, draw bridges, ski lifts, and temporary locations where reliable communication is necessary. These types of systems allow for two or more parties to be able to talk to one another in areas that experience loss-of-power or when radio communication is hampered by RF signal losses and/or limitations.
Many different types of equipment have attempted to replace sound-powered telephones on ships, however, due to the rugged, reliable and power-free nature of this form of communication equipment, it is still in use on all U.S. military vessels, commercial vessels and work boats.
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