How do plastic cup phones work?

The string must be pulled taut in order for the phone to work. Longitudinal waves travel through a medium by a process of compression and rarefaction, which in this case alters the tension of the string. If the string is not taut, the alterations in tension will not vibrate the listener's cup. Similarly, elastic strings will not work properly, because the wave will just stretch the string without vibrating the listener's cup.

Don't let the string come into contact with any other objects, as this will partially divert the sound wave into the object, causing a weaker signal on the receiving end. Speaking into the cup transmits the sound of the speaker's voice into the bottom of the cup. The bottom of the cup acts as a diaphragm and vibrates with the sound of the speaker's voice. As the bottom of the cup vibrates, it transmits the vibrations into the taut string. The sound travels along the string as a longitudinal wave and ultimately vibrates the bottom of the receiving cup. The cup transmits the sound into the air around the listener's ear, allowing her to hear the speaker. Because the sound travels through solid mediums--the cup and the string--it travels more effectively than through air, allowing the users to communicate across large distances with volumes that would be inaudible if spoken through air