Sound - Attenuation and the Directivity Coefficient
For a continuing sound source the sound level in a room is the sum of direct and reverberant sound. The sound pressure for a receiver can be expressed as
Lp = LN + log (D / (4 π r2) + 4 / R) (1)
where
Lp = received sound pressure level (dB)
LN = sound power level from source (dB)
D = directivity coefficient
R = room constant (m2 Sabine)
π = 3.14 .....
r = distance from source (m)
Directivity coefficient - D
The figure below can be used to approximate the Directivity coefficient - D - for typical locations of the receiver and the sound source
(1) can be transformed to express the difference between the received sound pressure level and emitted sound power level - the attenuation - as:
Lp - LN = 10 log (D / (4 π r2) + 4 / R) (2)
Room Sound Attenuation Calculator
D - directivity coefficient
R - room constant - (m2 Sabine)
r - distance from source (m)
Room Sound Attenuation (Lp - LN) - (dB) :
The attenuation (Lp - LN) can also be estimated from the diagram below by modifying (2) to be expressed as the ratio between "distance r between source and receiver" and "square of the directivity coefficient D" - and the total room sound absorption in m2 Sabine.
Example - Sound Attenuation in a Room
For a room the acoustic property total room sound absorption is 1000 m2 Sabine. For a listener in the middle of the room with a directivity constant 1 and at distance 1 m from the sound source - the ratio
r / D1/2 = 1 / (1 m)1/2
= 1
From the diagram above the attenuation can estimated to approximately
10 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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