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Updated: Mar 22, 2026

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Cylindrical spreading due to downwind refraction.

Rufin Makarewicz1

  • 1Institute of Acoustics, Faculty of Physics, A. Mickiewicz University, 61- 614 Poznan, Poland.

The Journal of the Acoustical Society of America
|April 24, 2016
PubMed
Summary

Low level jets (LLJ) alter sound propagation. Downwind, sound spreads cylindrically, not spherically, impacting noise from sources like wind turbines. LLJ parameters determine this transition distance and sound intensity.

Area of Science:

  • Acoustics
  • Atmospheric science
  • Fluid dynamics

Background:

  • Sound propagation near the ground is typically spherical.
  • Low-level jets (LLJ) are common atmospheric phenomena with wind speed maxima.
  • Wind turbine noise is a significant environmental concern.

Purpose of the Study:

  • To analyze downwind sound propagation influenced by low-level jets.
  • To determine how LLJ characteristics affect sound intensity and spreading.
  • To understand the transition from spherical to cylindrical spreading.

Main Methods:

  • Analysis of sound propagation from elevated point sources.
  • Characterization of LLJ parameters (wind speed, height).
  • Examination of wind turbine noise measurements.

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Main Results:

  • Downwind propagation transitions from spherical (6 dB/doubling) to cylindrical (3 dB/doubling) spreading.
  • The transition distance and sound intensity are dependent on LLJ parameters.
  • Non-coherent superposition of ground-reflected rays in turbulence is key.

Conclusions:

  • LLJs significantly alter sound propagation patterns.
  • Understanding LLJ effects is crucial for accurate noise modeling of wind turbines and aircraft.
  • Atmospheric turbulence plays a pivotal role in sound intensity variations.