Sound Pressure Level (SPL) Calculator
This sound pressure level (SPL) calculator converts between sound pressure and decibels, estimates how level falls off with distance, and combines multiple sound sources — using the standard acoustics formulas, all in your browser.
These are idealized textbook formulas (a point source in a free field, and uncorrelated sources). Real rooms add reflections, absorption and directivity, so treat the results as estimates. This is a calculator — it does not measure anything.
Sound pressure ↔ level
Edit either field. dB SPL is referenced to 20 µPa (the standard threshold-of-hearing reference). 94 dB SPL = 1 Pa.
Level change with distance
Inverse-square law for a point source in free field: doubling the distance drops the level by 6 dB. L₂ = L₁ − 20·log₁₀(d₂ / d₁).
Combine sound sources
Adds uncorrelated sources on an energy basis: L = 10·log₁₀(Σ 10^(Lᵢ/10)). Two equal sources add 3 dB, not double. Enter levels in dB separated by commas.
Common sound levels (approximate dB SPL)
Rough, typical figures for orientation — real levels vary with distance and source. Prolonged exposure above ~85 dB can damage hearing; 120 dB+ is painful.
How SPL Math Works
Sound pressure level (SPL) expresses a sound pressure on a logarithmic decibel scale relative to a reference. The reference is p₀ = 20 µPa (20 millionths of a pascal), roughly the quietest sound a healthy young ear can detect. The formula is Lp = 20·log₁₀(p / p₀), so each time the pressure ratio multiplies by 10, the level rises by 20 dB; a doubling of pressure is about +6 dB. To convert pascals to dB SPL, plug the measured pressure into that formula — the Pressure tab does it instantly for any value you enter. Unweighted dB SPL treats all frequencies equally; in practice, noise regulations quote A-weighted levels (dBA), which apply a frequency curve matching human hearing sensitivity, reducing the contribution of bass and very high frequencies. Our amplitude-to-dB converter handles the related voltage/amplitude side of the same logarithmic math.
Distance: a point source radiating into open space follows the inverse-square law — acoustic intensity falls with the square of distance, so pressure falls in proportion to distance and the level drops 6 dB per doubling of distance. Combining sources: independent (uncorrelated) sounds add by energy, not by level, so two equally loud sources give +3 dB and ten give +10 dB — never simple addition. When you need to combine decibels from multiple machines or speakers, or simply add sound levels together, use the Combine Sources tab to get the correct energy-sum result. If you need to add or subtract levels numerically in other contexts, the dB addition calculator streamlines those same energy-sum steps.
Why these are estimates
The free-field, point-source assumptions rarely hold exactly. Indoors, reflections build up a reverberant field that makes distance matter less — the level eventually flattens to a diffuse-field average determined by the room's total absorption and the source power. The distance at which reverberant energy dominates free-field energy is called the critical distance. Real sources are also directional; air and surfaces absorb high frequencies; and barriers block sound. For room-specific estimates, a reverb-time calculator accounts for absorption that this SPL calculator cannot. Use these results to understand relationships and ballpark figures, not as a substitute for measurement with a calibrated meter.