A string telephone can be made from two metal cans and some string. Suggest why -- ?

Question

a) the telephone works only when the string is stretched tight

b) the telephone does not work when a third person touches the string.

Answer 1

When you talk into the can the vibrations from your voice travel along the string into the other can, if the string looses tension or someone touches it, the vibes will not carry

Answer 2

Sound is a mechanical wave and needs medium for propagation. When the string isnt stretched tight the sound waves cant go that properly. Similar thing happens if it meets interference in the form of an object or a person.