Which type of noise-cancelling device uses the theory of entropy to reflect the sound waves against each other?

• A. Straight-through
• B. Muffler
• C. Cross-flow
• D. Barrier

Answer: (C)

The main idea is using how sound waves interfere with each other to reduce noise. In a cross-flow design, sound paths are arranged so waves collide and bounce between opposing surfaces as the air streams cross. These reflections create phase differences that produce destructive interference, so the waves cancel out more of the sound energy instead of passing through unchanged. This spreading and mixing of the wave energy also makes the sound field more disordered (higher entropy), which helps dampen the audible noise.

A straight-through path just lets sound go straight with little interaction, a muffler mainly relies on absorbing energy to reduce noise, and a barrier blocks direct transmission without promoting the kind of wave reflections needed for cancellation. The cross-flow setup is the one that harnesses reflected wave interactions to cancel noise.