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Exterior Noise Propagation





Acoustic Propagation in Exterior Environments

Acoustic propagation in outdoor environments involves the transmission of sound waves from a source to a receiver over a distance. Several factors influence this process, such as the atmospheric conditions, ground surface characteristics, and the directivity of the sound source. The following formula is commonly used to calculate the acoustic sound level from the acoustic power level of sources at a given distance:

\[ W = \frac{I \cdot 4 \pi r^2}{Q} = \frac{\left\langle p^2 \right\rangle \cdot 4 \pi r^2}{\rho c Q} \]

Formula Parameters

Where:

  • \( W \): Acoustic power of the source (in Watts)
  • \( p \): Acoustic pressure of the source at a distance \( r \) (in Pascals)
  • \( Q \): Directivity factor, considering the proximity of the source to surfaces (dimensionless)
  • \( r \): Distance from the source to the receiver (in meters)
  • \( \rho \): Density of the medium, typically air (in \( kg/m^3 \))
  • \( c \): Speed of sound in the medium, typically air (approximately 343 m/s at room temperature)

Assumptions in Acoustic Propagation

This formula is based on several assumptions about sound propagation in exterior environments:

  • The ground or surrounding surfaces do not significantly affect the acoustic power level.
  • The sources are acoustically incoherent, allowing for an energetic summation of sound levels.
  • The sources are positioned close to surfaces compared to the distance between the source and the receiver.
  • The distance between the source and reflective surfaces is less than one-tenth of the wavelength of the emitted sound.

Acoustic Propagation in Outdoor Environments

In outdoor environments, sound propagation is affected by various factors, including atmospheric absorption, wind, temperature gradients, and terrain. The attenuation of sound over distance in an exterior environment can be modeled using this formula, which takes into account the directivity and distance between the source and receiver. As sound travels through the air, it experiences energy loss due to absorption, diffraction, and scattering.

Understanding these factors is critical in fields such as environmental noise control, urban sound planning, and outdoor event acoustics. Accurate prediction of sound levels in outdoor settings helps professionals assess noise pollution, design sound barriers, and optimize sound systems for large open spaces.

References: Engineering Noise Control Theory and Practice - David A. Bies


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