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# Propagation of Outdoor Sound - Partial Barriers

## The transmission of outdoor sound through and around barriers - the Fresnel Number.

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 δ /λ                                                       (1)

where

N = Fresnel number (dimensionless)

δ = 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 reduced for moving sources (like vehicles etc.) compared to stationary sources.

### Example - Attenuation by an Outdoor Highway Noise Barrier

The distance A from a highway to the top of a barrier is 20 m, the distance B from top of the barrier to a person is 30 m and the distance d between the highway and the person is 43 m. The d value can be calculated as

δ = (20 m) + (30 m) - (43 m)

= 7 m

The Fresnels number for noise with frequency 500 Hz ( wavelength 0.69 m) can be calculated as

N = 2 (7 m) / (0.69 m)

= 20

The barrier attenuation for noise with frequency 500 Hz can then be estimated from the diagram above to approximately

17.5 dB

The Fresnels number for noise with frequency 2000 Hz ( wavelength 0.17 m) can be calculated as

N = 2 (7 m) / (0.17 m)

= 82

The barrier attenuation for noise with frequency 2 000 Hz can then be estimated from the diagram above to approximately

20 dB

## Related Topics

• ### Acoustics

Room acoustics and acoustic properties. decibel A, B and C calculations. Noise Rating (NR) curves. Sound transmission through walls. Calculate sound pressure, sound intensity and sound attenuation.
• ### Noise and Attenuation

Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems.

## Related Documents

• ### Noise from Machines - Enclosures and Sound Pressure Level Reductions

Barriers and enclosures to reduce noise sound pressure levels from machines.
• ### Noise generated by Road Traffic

Estimate noise level generated by traffic.
• ### Outdoor Ambient Sound Pressure Levels

Outdoor ambient sound pressure levels (dBA) in rural and urban business and industrial environments with or without limited traffic.
• ### Propagation of Sound Outdoors - Attenuation vs. Distance

The propagation of outdoors sound vs. distance and attenuation.
• ### Signals - Adding Decibels

The logarithmic decibel scale is convenient when adding signal values like sound power, pressure and others from two or more sources.
• ### Sound - Attenuation and Speed vs. Sound Frequency and Air Humidity

The speed and attenuation of sound in moist air varies with sound frequency and air humidity.
• ### Sound Pressure

Sound Pressure is the force of sound on a surface perpendicular to the propagation of sound.

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## Citation

• The Engineering ToolBox (2003). Propagation of Outdoor Sound - Partial Barriers. [online] Available at: https://www.engineeringtoolbox.com/outdoor-sound-partial-barriers-d_65.html [Accessed Day Month Year].

Modify the access date according your visit.

7.8.15

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