Sound  Abatement vs. the Distance from Source
The disruption of the sound pressure wave and the reduction of noise is called attenuation  Sound Pressure Level vs. distance calculator.
The sound pressure from a source is reduced with distance from source.
Spherical Distance
Sound pressure in spherical distance from a noise source can be calculated as:
p^{2} = ρ c N / (4 π r^{2}) (1)
where
p = sound pressure (Pa, N/m^{2})
ρ = density of air (kg/m^{3})
c = speed of sound (m/s)
N = sound power (W)
π = 3.14
r = distance from source (m)
Half Spherical Distance
Sound pressure in half spherical distance from a source can be expressed as:
p^{2} = ρ c N / (4 π r^{2} / 2)
= 2 ρ c N / (4 π r^{2}) (2)
A more generic expression for sound pressure in distance from source can be formulated to:
p^{2} = D ρ c N / (4 π r^{2}) (3)
where
D = directivity coefficient (1 spherical, 2 half spherical)
The directivity coefficient depends on several parameters  the position and direction of the source, the room and the surrounding area, etc.
The Sound Pressure Level  L_{p}  can be expressed logarithmic in decibels as:
L_{p} = 20 log (p / p_{ref})
= 20 log ((D ρ c N / (4 π r^{2}))^{1/2} / p_{ref})
= 20 log (1 / r (D ρ c N / (4 π))^{1/2} / p_{ref}) (4)
where
L_{p} = sound pressure level (dB)
p_{ref} = 2 10^{5}  reference sound pressure (Pa)
Note!  a doubling of the distance from a sound source  will reduce the sound pressure level  L_{p}  with 6 decibels.
Sound Pressure Level Calculator
Example  Sound Pressure from a Wood Planer
The sound power generated from a wood planer is estimated to 0.01 W. The sound pressure in distance 10 m from the planner can be calculated as
L_{p} = 20 log ((D ρ c N / (4 π r^{2}))^{1/2} / p_{ref} )
= 20 log (2 (1 kg/m^{3}) (331.2 m/s) (0.01 W) / (4 π (10 m)^{2}))^{1/2} / (2 10^{5} Pa))
= 71 dB
Related Topics

Acoustics
Room acoustics and acoustic properties, decibel A, B and C, Noise Rating (NR) curves, sound transmission, sound pressure, sound intensity and sound attenuation.
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