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To calculate the acoustic sound level from the acoustic power level of sources at a certain distance, we use this formula:
$$ W = \frac{I \cdot 4 \pi r^2}{Q} = \frac{\left\langle p^2 \right\rangle \cdot 4 \pi r^2}{\rho c Q}$$
with :
- \( W \) the acoustic power of the source in Watt
- \( p \) the acoustic pressure of the source at a distance r in Pa
- \( Q \) the directivity factor when the source is closed to surfaces
- \( r \) the distance from the source in m \( i \)
There are some assumptions using this formula:
- The surface doesn't affect the acoustic power level
- The acoustic power of the sources are incoherent so that possible to do an energetic sum
- The sources are closed to the surface in comparison with the distance of the receptor
- The distance between the sources and the planes are less than on tenth of the wavelength of the radiated noise
References : Engineering Noise Control Theroy and Pratice - David A.Bies