Calculation
L2 = L1 − 20·log10(r2/r1)
For a point source in free field, sound level drops by 6 dB for every doubling of distance.
Dubbel Meßtechnik W 2.9.2: Lx = 20·lg(X/X0), „bei Abstandsverdoppelung fällt der Schallpegel um ca. 6 dB" (Punktquelle, Freifeld)
Sound level decrease with distance
For a point source in free field, sound level drops by 6 dB for every doubling of distance.
MINTSI
02
Result
Select a target and calculate.
Understand the inputs
- Sound level at r2 L2 in dB — Sound level at the new distance r2 from the source.
- Sound level at r1 L1 in dB — Known sound level at reference distance r1.
- Reference distance r1 — Distance at which L1 was measured or specified.
- New distance r2 — Distance for which the sound level is sought.
Example
L1=90 dB at r1=1 m gives a level L2=90−20·log10(10)=70 dB at r2=10 m.
Assumptions and limits
Point sound source in unobstructed free field with no reflections, ground attenuation or air absorption; real rooms and obstacles change the behaviour.
Understand Sound level decrease with distance
This calculator determines the sound level at a new distance from a point sound source in a free field, from a known reference level.
What does this quantity describe?
In a free field, sound pressure falls inversely with distance, and sound level accordingly falls logarithmically: L2=L1−20·log10(r2/r1).
Formula and variables
L2 = L1 − 20·log10(r2/r1)
L2 = L1 − 20·log10(r2/r1)
| Symbol / input | Meaning |
|---|---|
| Sound level at r2 L2 in dB | Sound level at the new distance r2 from the source. |
| Sound level at r1 L1 in dB | Known sound level at reference distance r1. |
| Reference distance r1 | Distance at which L1 was measured or specified. |
| New distance r2 | Distance for which the sound level is sought. |
Choose the inputs correctly
L1 is the known sound level at reference distance r1, r2 the new distance for which the level is sought.
How to use the calculator
Take L1 and r1 from a measurement or datasheet; use r2 as the actual distance to the location of interest.
Worked example
L1=90 dB at r1=1 m gives a level L2=70 dB at r2=10 m.
Understand the result and units
Every doubling of distance lowers the level by about 6 dB, so the decrease is markedly faster than linear with distance.
L1 and L2 are sound levels in decibels, r1 and r2 lengths.
Typical applications
Estimating noise level at a given distance from machines, plant or loudspeakers, e.g. for noise-impact assessments.
Assumptions, limits and common mistakes
Point source in unobstructed free field with no reflections, ground attenuation, air absorption or directivity; indoor spaces and extended sources follow a different behaviour.
Common mistake: Do not confuse the factor 20 (for sound pressure level) with the factor 10 (for sound power or intensity level under area spreading).
Frequently asked questions
What is “Sound level decrease with distance” used for?
Estimating noise level at a given distance from machines, plant or loudspeakers, e.g. for noise-impact assessments.
Where do the input values come from?
L1 is the known sound level at reference distance r1, r2 the new distance for which the level is sought.
What does the result not cover?
Point source in unobstructed free field with no reflections, ground attenuation, air absorption or directivity; indoor spaces and extended sources follow a different behaviour.