Propagation of Sound Outdoors - Attenuation vs. Distance
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
Lp = LN + 10 log [(Q / 4 π) (1 / r12+ 1 / r22+ ...+1 / rn2) + 4 / R] (1)
or modified for a single source:
Lp = LN + 10 log (Q / (4 π r2) + 4 / R) (1b)
where
Lp = sound pressure level (dB)
LN = 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 (m2)
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
Lp = LN + 10 log (1 / (4 π r2)
= LN - 10 log (4 π r2) (2)
since
Q = 1
R ≈ ∞
(2) can also be expressed as:
Lp = LN - 20 log (r) + K' (2b)
where
K' = -11 (single sound source and spherical radiation)
Single Sound Source - Hemi-Spherical Propagation
When the sound source propagates hemi-spherically 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:
Lp = LN - 20 log r + K' + DI - Aa - Ab (3)
where
DI = directivity index
Aa = attenuation due to atmospheric conditions
Ab = attenuation due to barriers
Linear Sound Source
With a linear sound source, like a road or high-way with heavy traffic, (1) can be summarized (integrated) to express the sound pressure as
Lp = LN - 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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