Refraction: When sound waves pass from one medium to another, their velocity changes. This change in velocity often causes the wave to change direction, a phenomenon known as refraction.
Frequency Constancy: A critical principle is that the frequency () remains constant across boundaries. The source determines the frequency, and the medium cannot alter it.
Wave Equation Adaptation: Since and is constant, the wave speed () and wavelength () are directly proportional. If speed increases, wavelength must increase.
Less Dense Denser (e.g., Air to Water):
Velocity: Increases (molecules are closer).
Wavelength: Increases (wavefronts stretch out).
Frequency: Unchanged.
Denser Less Dense (e.g., Water to Air):
Velocity: Decreases.
Wavelength: Decreases (wavefronts compress).
Frequency: Unchanged.
Check the Medium: Always identify the state of matter first. If the question involves a solid, sound travels faster than in air. This is the opposite of light waves (which usually slow down in denser media).
The Frequency Trap: Examiners often ask what happens to frequency when sound enters water. The answer is ALWAYS "stays the same".
Proportionality Check: Use the formula . If you calculate that speed doubles, ensure your wavelength also doubles in your answer.