The wave front model of refraction on an interface makes it clear why the direction of the sound propagation changes.
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The graphic illustrates the case when the speed of propagation becomes slower at the transition from the first to the second medium: The wave is broken at the perpendicular of the boundary surface.
An explanation of this phenomenon is provided by the Huygens' Principle: Every point on a wave front is the starting point for a new wave, known as an “elementary wave". The enclosing end of the elementary wave creates the new wave front.
Information and ideas:
Refraction at boundaries also occurs with sound waves (for example, in the atmosphere at the transition from warm to cold layers of air).
Relevant for teaching:
Vibrations and waves