Speaker SPL at Distance Calculator
Predict how loud a speaker is at any listening distance from its 1 W / 1 m sensitivity and the power you feed it, using the inverse-square law.
How it works
SPL(d) = sensitivity + 10·log₁₀(P) − 20·log₁₀(d)
A speaker’s sensitivity rating tells you its output at 1 metre with 1 watt of input. Adding power raises the level by 10·log₁₀ of the power ratio, so ten times the power adds 10 dB. Distance takes it away by the inverse-square law: sound spreads over a sphere, so every doubling of distance drops the level by 6 dB. The two effects combine linearly in decibels.
Worked example
An 88 dB speaker fed 100 W gains 10·log₁₀(100) = 20 dB, giving 108 dB at 1 m. At 4 m the distance loss is 20·log₁₀(4) = 12 dB, so the listener hears about 96 dB.
Frequently asked questions
Why does doubling the distance cost 6 dB, not 3?
Sound power spreads over the surface of a sphere, which grows with the square of the radius. Doubling the distance quadruples the area, and 20·log₁₀(2) = 6 dB. The 3 dB figure applies to power, not distance.
Does this account for room reflections?
No. This is the free-field (anechoic) prediction. In a real room, reflections raise the level closer to the walls and the reverberant field can add several decibels, so measured SPL is usually a bit higher than the free-field estimate at longer distances.
How much power do I need to get 3 dB louder?
You must double the power for every 3 dB. Going from 100 W to 200 W adds 3 dB; reaching 6 dB louder needs 400 W. This is why amplifier headroom matters far more than the last few watts of continuous power.
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This calculator is for educational purposes. Double-check important results before acting on them.