Propagation of Sound Outdoors  Attenuation vs. Distance
The propagation of outdoors sound vs. distance and attenuation.
The energy in the propagation direction of the sound is inversely proportional to the increasing surface area the sound propagates through and can be expressed as
L_{p} = L_{N} + 10 log [(Q / 4 π) (1 / r_{1}^{2}+ 1 / r_{2}^{2}+ ...+1 / r_{n}^{2}) + 4 / R] (1)
or modified for a single source:
L_{p} = L_{N} + 10 log (Q / (4 π r^{2}) + 4 / R) (1b)
where
L_{p} = sound pressure level (dB)
L_{N }= sound power level source in decibel (dB)
Q = directivity coefficient (1 uniform spherical, 2 uniform half spherical (single reflecting surface), 4 uniform radiation over 1/4 sphere (two reflecting surfaces, corner)
r = distance from source (m)
R = room constant (m^{2})
Single Sound Source  Spherical Propagation
With uniform spherical radiation (1) can be modified to express the sound pressure level from a single sound source as
L_{p} = L_{N} + 10 log (1 / (4 π r^{2})
= L_{N}  10 log (4 π r^{2}) (2)
since
Q = 1
R ≈ ∞
(2) can also be expressed as:
L_{p} = L_{N}  20 log (r) + K' (2b)
where
K' = 11 (single sound source and spherical radiation)
Single Sound Source  HemiSpherical Propagation
When the sound source propagates hemispherically with the source near ground, the constant can be set to
 K' =  8
Note! When the distance  r  from a power source doubles, the sound pressure level decreases with 6 dB. This relationship is also known as the inverse square law.
Other factors that affects the radiation of the sound are the direction of the source, barriers between the source and the receiver and atmospheric conditions. Equation (1) can be modified to:
L_{p} = L_{N}  20 log r + K' + DI  A_{a}  A_{b} (3)
where
DI = directivity index
A_{a} = attenuation due to atmospheric conditions
A_{b} = attenuation due to barriers
Linear Sound Source
With a linear sound source, like a road or highway with heavy traffic, (1) can be summarized (integrated) to express the sound pressure as
L_{p} = L_{N}  10 log (4 π r) (4)
Note! When the distance  r  from a linear power source doubles, the sound pressure level decreases with 3 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. 
Noise and Attenuation
Noise is usually defined as unwanted sound  noise, noise generation, silencers and attenuation in HVAC systems.
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