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A sound powered phone is usually very similar in appearance to an old-style handset phone. It has a receiver and a transmitter and is hardwired into a network of other sound powered phone stations. Unlike cell phones or modern landline phones, sound powered phones do not rely on an electronic exchange to connect users to each other. This is because the sound powered phone is physically connected in an electronic circuit with one or more sound powered phones.
The microphone of a sound powered phone often uses a flexible diaphragm that reacts to the pressure of sound waves created by the speaker. The motion of the diaphragm converts the pattern of sound waves into an electronic signal which is then converted back into sound by a reverse process in the receivers of all the other phones on the circuit.
On a sound powered phone system, only one person can speak at a time or the signal will become hopelessly garbled. For this reason, most sound powered phones feature some kind of push-to-talk button similar in function and importance to that used by walkie-talkies and ham radios. Another challenge of using a sound powered phone is the need to speak loudly and clearly. The speaker's voice is literally what is powering the communication. If there are many listening circuits with active receivers, or if the farthest listeners require long wires to connect them, communication can become degraded simply because the speaker is not loud enough--i.e. his voice does not have enough energy--to overcome all the potential losses and disruptions throughout the circuit.
Relying on the speaker's volume and clarity sounds like a weakness, but it is also one of the key advantages. Sound powered phone technology has found a place of special importance for emergency communications networks in industry as well as for everyday use in shipboard applications. Being voice powered means that when regular telephones go down or the ship loses power, there is still a way to get those vital messages to the stations that need them.
Sound powered phones are perfect example of an old technology that may be marginalized, but will never quite go out of date. Given time and development, sound powered phone could however come to mean something completely different. It is theoretically possible for sound to power your cell phone one day; advances in nanotechnology and piezoelectricity bring us closer to this being a reality every day.