Right answer: 1. Most emergency telephones are sound-powered phones, meaning they operate from the sound pressure of the speaker’s voice, with no power cable or battery required.
Sound-powered phones were initially developed as emergency communication devices for ships. Used widely during World War II, they are still common today. Because they can work even when there’s a power outage, they are ideal for disaster situations or just about anywhere reliable communications are essential.
Sound-powered phones are also a safe option for hazardous areas where explosion-proof equipment is needed. Such areas include arsenals, gas plants, chemical plants, oil refineries, mines and quarries, ballistic missile sites and nuclear power plants.
In regular telephones, a microphone picks up the sound waves of a speaker’s voice and turns them into an electric current. This current is turned back into sound vibrations at the other end of the line to transmit the sound to the listener. But all that requires a power source. In sound-powered phones, the microphone itself generates enough electricity to transmit the current – no battery is needed.
According to Hervé Lissek, an acoustics researcher at EPFL, “Sound-powered phones use a ‘balanced armature’ type of electrodynamic microphone. These microphones have a lighter diaphragm that is more sensitive to air vibrations. And the diaphragm is surrounded by a compact magnetic circuit, which makes it more effective in turning sound vibrations into electric signals.”
However, sound-powered phones have a limited range. “The signals generated by these phones are fairly weak. They can travel through cables for a few dozen kilometers, but that’s about it. I think the term ‘sound-powered phone’ is slightly misleading because a power source is still needed to carry signals over long distances,” says Lissek.
Photo caption: A sailor using a sound-powered phone in an ad for US company Western Electric, 1945
Où trouver les telephones spéciaux. Brochure by ae&t http://www.audin.fr/pdf/documentations/aet/alarmes_optiques_sonores_et_vocales/produits_de_communication/produits_de_communication.pdf
How do sound-powered telephones work? Dynalec Corporation http://soundpoweredtelephone.com/how-sound-powered-works.html
Sound-powered telephone. Wikipedia https://en.wikipedia.org/wiki/Sound-powered_telephone
Does the US Coast Guard, and Navy ships, still use sound-powered phones? Quora https://www.quora.com/Does-the-US-Coast-Guard-and-Navy-ships-still-use-sound-powered-phones