If a soundproof door is almost shut but still leaves a small opening, what wave phenomenon is responsible for someone being able to hear sound from the room while standing by the wall to the side of the opening?

Sound waves spread out and bend around obstacles or through openings, a phenomenon called diffraction. Because the doorway has a small gap, sound can spread through it and reach someone standing beside the opening. Absorption would reduce sound, while reflection and refraction do not explain this spreading around the opening.

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