For a noise barrier (such as a highway sound wall) to be effective, which physical phenomenon must be minimized to prevent sound from reaching the receiver?
- ARefraction
- BDiffractionCorrect
- CRarefaction
- DDoppler Effect
Solution
Concept
Sound diffraction over acoustic barriers
Formula
N = 2 × (A + B - d) / λ
Step-by-step solution
1. A noise barrier creates an acoustic shadow zone by blocking direct line-of-sight sound propagation. 2. Sound waves encounter the top and side edges of the barrier and bend into the shadow zone. 3. This wave bending phenomenon is known as diffraction. 4. To maximize barrier attenuation, diffraction over the barrier crest must be minimized. Answer: B
Explanation
Sound barriers attenuate acoustic energy by creating an acoustic shadow behind the barrier, but sound waves tend to bend over the top edge and around flanking edges. Minimizing this acoustic diffraction through optimized barrier height, cantilever tops, or absorbing crests is essential to preserve the acoustic shadow and shield downstream receivers.
Why the other options are wrong
A) Refraction is the curvature of sound rays resulting from atmospheric wind and temperature gradients. C) Rarefaction is the low-pressure phase of an acoustic longitudinal compression wave. D) Doppler effect is the perceived frequency shift caused by relative motion between source and receiver.
Shortcut
Sound bending over wall edges = Diffraction; Barrier effectiveness depends on minimizing edge diffraction.
Common mistake
Selecting refraction, which is bending caused by sound speed gradients in temperature or wind, not barrier edges.
Correct answer: B) Diffraction