Convert acoustic quantities or estimate the relationship between sound pressure and sound power under ideal free-field conditions.
Sound pressure to level
Convert RMS sound pressure in air to sound pressure level using the reference pressure 20 µPa.
Sound pressure level
dB re 20 µPa
FormulaLp = 20 log10(p / p0)
Level to sound pressure
Convert a sound pressure level to RMS acoustic pressure in air.
RMS sound pressure
Pa
Formulap = p0 · 10Lp/20
Sound power to level
Convert acoustic power to sound power level using the reference power 1 pW.
Sound power level
dB re 1 pW
FormulaLW = 10 log10(W / W0)
Level to sound power
Convert a sound power level to acoustic power.
Sound power
W
FormulaW = W0 · 10LW/10
Estimate sound power level
Estimate LW from a free-field sound pressure measurement.
Estimated sound power level
dB re 1 pW
Geometric term: —
FormulaLW = Lp + 10 log10(4πr² / Q) + A
Predict sound pressure level
Estimate Lp at a distance from a source with known sound power.
Predicted sound pressure level
dB re 20 µPa
Geometric term: —
FormulaLp = LW − 10 log10(4πr² / Q) − A
Sound pressure and sound power are different quantities
Sound pressure level Lp describes the pressure fluctuation at a measurement position. It changes with distance, directivity, reflections and the acoustic environment. Sound power level LW describes the total acoustic power emitted by a source and is, in principle, independent of the measurement position.
Free-field estimation
The Lp–LW modes assume ideal geometric spreading from a compact source. The directivity factor Q accounts for radiation into a reduced solid angle. Additional attenuation A may represent known propagation losses, but room reflections and near-field effects are not included.