Clear-Com's MS-704 Encore Partyline....

For those that are new to the world of intercom, here are some terms you don't want to confuse with each other: Analog vs. Digital     Encore Analog Partyline HelixNet Digital Partyline Analog systems record waveforms as they are. Digital systems will convert the analog waveforms into sets of numbers and record the numbers instead. When played back, the numbers are converted into a voltage stream that approximates the original analog wave. Two-wire vs. Four-wire Four-wire ...

Scuba divers can use hand signals to communicate.

Learn how microphones work in the video and article below: "Microphones are a type of transducer - a device which converts energy from one form to another. Microphones convert acoustical energy (sound waves) into electrical energy (the audio signal)." 

Read full article here: How do Microphones Work?