Noise Propagation

What is a Point Source?

If a sound source is so small in all dimensions, when compared to the distance a receiver is located, this can be considered a point source. A true point source radiates sound equally in all directions. Examples of point sources could be a condenser or a generator (where the receiver is a significant distance away). For every doubling of distance, the sound level from a point source decreases by 6 dB.

How to Calculate Sound Attenuation from a Point Source?

This can be estimated using the Inverse Square Law, the formula of which is:

L_P(receiver) = L_P(source) – 20 x log₁₀ (R₂/R₁)

Where:

• L_P(receiver) = the calculated Sound Pressure Level at the receiver
• L_P(source) = the Sound Pressure Level of the source (e.g. manufacturer noise data or measured levels)
• R₁ = distance in metres from the sound source to the location of L_P(source) (e.g. the manufacturer noise data may have been measured at 1 metre)
• R₂ = distance in metres from L_P(source) to the location of L_P(receiver)

What is a Line Source?

For a line source, the sound is spread out cylindrically in a line in one dimension. The distance the sound is spread out in a line is large when compared to the distance a receiver is located. Examples of line sources could be busy roads and rail lines or duct work. For every doubling of distance, the sound level from a line source decreases by 3 dB.

How to Calculate Sound Attenuation from a Line Source?

Line Source attenuation can be calculated using the following formula:

L_P(receiver) = L_P(source) – 10 x log₁₀ (R₂/R₁)

Where:

• L_P(receiver) = the calculated Sound Pressure Level at the receiver
• L_P(source) = the Sound Pressure Level of the source (e.g. measured levels)
• R₁ = distance in metres from the sound source to the location of L_P(source) (e.g. the road may have been measured at 10 metres)
• R₂ = distance in metres from L_P(source) to the location of L_P(receiver)

What is a Plane Source?

If the sound source is large in both horizontal and vertical directions, when compared to the distance to a receiver, it can be thought of as a plane source. Examples of plane sources could be building walls or louvres.

The sound attenuates at different rates, depending on the distance from the source to the receiver:

• Near Field: no attenuation
• Medium Field: line source attenuation is exhibited
• Far Field: point source attenuation is exhibited

How to Calculate Sound Attenuation from a Plane Source?

Plane Source attenuation can be calculated as follows:

• Near field attenuation: no attenuation from 1 metre to a/π
• Medium field attenuation: line source attenuation from a/π to b/π
• Far field attenuation: point source attenuation from b/π to R

Where:

• a = smallest dimension of sound source in metres
• b = largest dimension of sound source in metres
• R = distance from sound source to receiver