Sound-powered telephone
From Wikipedia, the free encyclopedia
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 five decades for both routine and emergency communication on ships to allow communication between key locations on a vessel when 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.
[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 do not convert acoustic energy to electrical energy (they only modulate the battery energy) they cannot be used in sound powered transducers. 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, allowing communication to take place without external power.
[edit] Usage
Sound-powered telephones are widely used on naval vessels. A typical example on a US Navy ship is the 'JL' circuit which is used by the lookouts to report visual contacts to the pilot house and the Combat Direction Center (CDC). In this case there would be five stations on the circuit (stern lookout, port lookout, starboard lookout, pilot house and CDC).
USCG 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, 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 US Military vessels, commercial vessels and work boats.
