Distance Attenuation Calculator

Enter your initial sound pressure level (SPL₁) and initial distance (R₁), then set a new distance (R₂) to find the attenuated sound level (SPL₂) at that point. The Distance Attenuation Calculator applies the inverse square law formula — SPL₂ = SPL₁ − 20·log(R₂/R₁) — to show you exactly how many decibels are lost as sound travels further from its source.

dB

The known sound level at the reference distance.

m

The distance from the sound source where SPL₁ was measured.

m

The new distance at which you want to know the sound level.

Results

Sound Level at Point 2 (SPL₂)

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Attenuation (Level Drop)

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Distance Ratio (R₂ / R₁)

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Sound Level Category

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SPL at Reference vs New Distance

Results Table

Frequently Asked Questions

What is Sound Pressure Level (SPL)?

Sound Pressure Level (SPL) is a logarithmic measure of the effective pressure of a sound relative to a reference value (typically 20 µPa, the threshold of human hearing). It is expressed in decibels (dB). A whisper is around 30 dB, normal conversation around 60 dB, and a jet engine can exceed 140 dB.

How do I calculate the sound pressure level change with distance?

Use the formula SPL₂ = SPL₁ − 20·log(R₂/R₁), where SPL₁ is the known level at distance R₁ and SPL₂ is the level at the new distance R₂. This formula assumes a free-field (anechoic) environment with a point source radiating in all directions.

What is the inverse square law for sound?

The inverse square law states that sound intensity decreases proportionally to the square of the distance from the source. In practical terms, every time you double the distance from a sound source in a free field, the sound level drops by approximately 6 dB.

What is the 6 dB rule?

The 6 dB rule states that for every doubling of distance from a point sound source in a free field, the sound pressure level decreases by approximately 6 dB. For example, if a speaker measures 90 dB at 1 m, it will measure about 84 dB at 2 m and 78 dB at 4 m.

What is the 3 dB rule?

The 3 dB rule applies to line sources (such as traffic along a road or a long pipe). For these sources, SPL decreases by roughly 3 dB for every doubling of distance, compared to the 6 dB drop for point sources. This calculator uses the point source (6 dB per doubling) model.

How much louder is 40 dB than 20 dB?

A 20 dB difference corresponds to a 10× increase in sound pressure and a 100× increase in sound intensity. So a 40 dB sound has 10 times higher pressure than a 20 dB sound. Perceptually, listeners typically perceive it as roughly 4 times louder due to the logarithmic nature of human hearing.

Does this calculator account for air absorption or reflections?

No — this calculator uses the basic inverse square law for a point source in a free field (like an anechoic chamber or outdoor open space). It does not account for air absorption at high frequencies, reflections off walls, ground effect, or other environmental factors. For precise acoustic engineering, additional corrections should be applied.

What units does this calculator use for distance?

Distance inputs are in metres (m). You can use any consistent unit as long as both R₁ and R₂ use the same unit, since the formula depends only on their ratio. For example, entering distances in feet will give the same result as metres for the same ratio.

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