Why do doors stop sound traveling?

Answer 1

Fundamentally, a door is a moveable continuation of a wall. So it should have the same properties as the wall - sound, thermal, maybe opacity but maybe not, and so on.

A sound barrier is the construction product that minimizes the sound from travelling from one place to another. So it should be as massive as practical, and most important, there should be only tiny gaps round the edges.

A small crack round the edge of a door will allow the sound to get past the door. Equivalent to a one-quarter wavelength each side of the crack, multiplied by the length of the crack! So ensuring those cracks are sealed as much as possible is a good start. Most doors naturally have a labyrinth path at the edges, and that does aid.

Answer 2

Doors can stop sound from traveling through because they are dense and rigid, which helps to reflect and absorb sound waves. The materials and construction of the door can block sound vibrations, preventing them from passing through. Sound waves are also disrupted by the seal around the door, which reduces sound leakage.