In a free field - a doubling of the distance from a noise source reduces the sound pressure level with 6 decibel.
This - the Inverse Square Law - can be expressed in a diagram like
dL = Lp2 - Lp1
= 10 log (R2 / R1)2
= 20 log (R2 / R1) (1)
dL = difference in sound pressure level (dB)
Lp1 = sound pressure level at location 1 (dB)
Lp2 = sound pressure level at location 2 (dB)
R1 = distance from source to location 1 (ft, m)
R2 = distance from source to location 2 (ft, m)
A "free field" is defined as a flat surface without obstructions.
Example - Rifle Shot and Sound Pressure at Distance
If the sound pressure from a rifle shot is measured to 134 dB at 1.25 feet - the reduction in sound pressure level at distance 80 feet can be calculated as
dL = 20 log ((80 ft) / (1.25 ft))
= 36 dB
The sound pressure level at distance 80 ft can be calculated as
Lp2 = (134 dB) - (36 dB)
= 98 dB
Inverse Square Law Calculator
Use the calculator below to calculate the sound pressure level at distance.
Lp1 - sound pressure level at location 1 (dB)
R1 - distance from source to location 1 (m, ft)
R2 - distance from source to location 2 (m, ft)
Example - Noise from a Machine
The noise from a machine in distance 1 m is measured to 110 dB. The noise reduction due to the inverse square law to a working area at distance 5 m can be calculated as
dL = 20 log ((5 m) / (1 m))
= 14 dB
The sound pressure level in the working area can be calculated as
Lp2 = (110 dB) - (14 dB)
= 96 dB
This noise level is only permitted for a limited amount of time and some action with partial barriers or enclosure of the machine should be done.
Room acoustics and acoustic properties, decibel A, B and C, Noise Rating (NR) curves, sound transmission, sound pressure, sound intensity and sound attenuation.
The rules of logarithms - log10 and loge for numbers ranging 1 to 1000.
The noise level should not exceed certain values to protect public health and welfare.
Barriers and enclosures to reduce noise sound pressure levels from machines.
Geometric dilution of light vs. distance.
Distance and perception of voice.
The disruption of the sound pressure wave and the reduction of noise is called attenuation - Sound Pressure Level vs. distance calculator.
Sound Pressure is the force of sound on a surface perpendicular to the propagation of sound.