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Difficulty: HardPhysics Fundamentals and Everyday Applications

During a quiet night, distant sounds such as train whistles or acoustic announcements are often heard much more clearly and over greater distances than during a hot afternoon. Which of the following physical phenomena primarily accounts for this atmospheric acoustic effect?

  1. Refraction of sound waves caused by a vertical temperature gradient in the lower atmosphereAnswer
  2. B
    Total internal reflection of acoustic waves at the boundary between atmospheric troposphere and stratosphere
  3. C
    Diffraction of sound waves around nighttime atmospheric pressure nodes
  4. D
    Polarization of acoustic longitudinal waves due to reduced humidity and thermal turbulence

Answer

Refraction of sound waves caused by a vertical temperature gradient in the lower atmosphere
The correct answer highlights atmospheric refraction resulting from a temperature inversion. At night, the surface air cools rapidly while higher air layers remain warmer. Because sound speed increases with air temperature, the upper portion of a sound wave front travels faster than its base. This causes sound rays heading upward to curve back down toward the ground, allowing sound to cover significantly greater distances with higher intensity.

Step-by-Step Solution

1
Analyze atmospheric conditions during nighttime versus daytime.
During the day, ground heating creates warmer air near the surface and cooler air above. At night, radiational cooling of the Earth makes the ground air colder than the air layer above it (temperature inversion).
Temperature distribution dictates the local speed of sound in air.
2
Relate temperature to the speed of sound.
The speed of sound in air is proportional to the square root of absolute temperature (v∝Tv \propto \sqrt{T}). Consequently, sound travels faster through warmer air layers than through colder air layers.
Wave fronts bend toward regions of lower wave speed according to Snell's law of refraction.
3
Determine the path of wave propagation at night.
As upward-moving sound wave fronts encounter warmer air higher up, the top part of the wave front moves faster than the bottom part. This differential speed bends (refracts) the acoustic wave path back downward toward the Earth's surface.
Downward refraction traps and redirects sound energy along the ground rather than letting it dissipate upward into the atmosphere.

Key Concept

Atmospheric Refraction of Sound Waves
Estimated Time:2m 0s
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Topics

Physics Fundamentals and Everyday Applications

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