Propagation of Sound Outdoors and Partial Barriers

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If a barrier is interposed between a sound source and a receiver, part of the sound will be

• reflected back from the barrier
• transmitted through the barrier 
• diffracted around the barrier

The attenuation can be expressed with the help of the Fresnel number expressed as: 

N = 2 d / λ        (1)

where

N = Fresnel number (dimensionless)

d = A + B - d, can be calculated from the figure below (m or ft)

λ = wavelength of the sound (m or ft)

Note! The attenuation depends on the wavelength of the sound. For short waves (high frequencies) the Fresnel number limits infinitely. For higher wavelengths the Fresnel number limits to zero.

The outdoor barrier attenuation for different Fresnel number can be estimated with the diagram below.

Note! The attenuation is lower for moving sources (vehicles etc.) than stationary sources.

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