Level changing with distance calculator - sengpielaudio Google Page Rank
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• Damping of sound level with distance •
Changing of the sound level Δ L with the distance r
in a free field (direct field), like in anechoic chambers
Conversion: Distance values → Level changing
With sound level we usually mean a level ratio of sound pressure
These calculations are meant only for engineers and the distance from
a musicien or a loudspeaker to a microphone in a direct field -
No air damping and frequency dependance of e.g. the thunder in a distance.

Enter the three gray boxes and you get the amount of attenuation,
you can expect with a change in sound source distance, in a free field.

Reference distance r1
from source
m or ft		 Sound level L1 at
reference distance
dBSPL		 The 1/r law. There is
really no square and
no power!
New distance r2
from source
m or ft		 Sound level L2 at
new distance:
dBSPL		 Sound level difference
Δ L = L1 - L2
dB
 

The sound pressure p changes with 1/r of the distance.
Sometimes it is said, that it goes with 1/r². That is really wrong.
But the sound intensity (energy quantity) decreases with 1/r². Intensity is not pressure.
The sound pressure level shows in the free field situation a reduction of 6 dB per
doubling of distance; that means the sound pressure value is a half and not a quarter.

Sound level difference:   Distance law  or level at far distance Sound change

Δ L = L1 - L2.

The sound pressure p decreases really with 1/r from the sound source!
Distance law 01
Distance law 02
Distance law 03

In acoustics, the sound pressure of a spherical wave front radiating from a point source
decreases by a factor of 1/2 as the distance is doubled.
The behavior is not inverse-square, but is inverse-proportional:
p ~ 1 / r.

Relation of sound intensity I, sound pressure p and the distance law -
r is the distance from the sound source.

                     Intensity-distance
From this follows    sound pressure-distance
Aha!

Note: The often used term "intensity of sound pressure" is not correct.
Use "magnitude", "strength", "amplitude", or "level" instead.
"Sound intensity" is sound power per unit area, while "pressure" is a
measure of force per unit area. Intensity is not equivalent to pressure.

dB and distance ratio - sengpielaudio
Distance ratio

Conversion of sound units (levels)

For this level damping of sound with distance we have to consider the damping of air (air damping) at larger distances. See: Absorption of sound by the atmosphere

Sound pressure level and Sound pressure

Enter a value in the left or right box, then press the TAB bar or make
a mouse click at an empty space at the side, to get the solution.
The calculator works in both directions of the ↔ sign.
Sound pressure level Lp:
dB-SPL		  ↔  Sound pressure p:
Pa = N/m2
Start   Start
Standard reference sound pressure p0 = 20 μPa or 2 × 10-5 Pa (0 dB)
Inverse distance law 1/r

Inverse Distance Law
Law for Sound Field Quantities
Distance ratio Sound pressure p ∝ 1/r
1 1/1 = 1.0000
2 1/2 = 0.5000
3 1/3 = 0.3333
4 1/4 = 0.2500
5 1/5 = 0.2000
6 1/6 = 0.1667
7 1/7 = 0.1429
8 1/8 = 0.1250
9 1/9 = 0.1111
10 1/10 = 0.1000

Frequently used wrong statements in the context of sound pressure

Wrong expressions Correct version
Sound pressure decreases inversely as the square
of the distance increases with 1/r2 from the
sound source.		 Sound pressure decreases inversely as the
distance increases with 1/r from the sound source.
Sound pressure level decreases inversely as the
square of the distance increases with 1/r2 from the
sound source.		 Sound pressure level decreases by (−)6 dB per doubling
of distance from the source to 1/2 (50 %) of the sound
pressure initial value.

Sound pressure p and the inverse distance law 1/r

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