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Sound Pressure Level (SPL) Calculator

Convert between sound pressure and decibels, estimate how level falls off with distance, and combine 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.

Sound pressure level (dB SPL)

Sound pressure (Pa)

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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 L_p = 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.

Distance: a point source radiating into open space follows the inverse-square law — pressure falls in proportion to distance, so 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.

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; real sources are directional; air and surfaces absorb high frequencies; and barriers block sound. Use these results to understand relationships and ballpark figures, not as a substitute for measurement with a calibrated meter.

Frequently Asked Questions

What is the 20 µPa reference?

It’s the standard reference pressure for airborne sound, p₀ = 20 micropascals, chosen to be near the threshold of human hearing at 1 kHz. A reading of 0 dB SPL means the pressure equals that reference; it doesn’t mean "no sound."

Why is 94 dB SPL equal to 1 pascal?

Because 20·log₁₀(1 Pa / 20 µPa) ≈ 93.98 dB. That’s why 94 dB SPL (1 Pa) is a common calibration point — many acoustic calibrators output exactly that level.

Why don’t two sources add up to double the dB?

Decibels are logarithmic. Independent sources add their energy, and the total level is 10·log₁₀ of the summed energy ratios. Two equal sources double the energy, which is +3 dB — not twice the number of decibels.

How much does sound drop with distance?

For an ideal point source in the open, about 6 dB for every doubling of distance (the inverse-square law). Indoors or with directional sources the real drop is usually less, because reflections and directivity change the picture.

Can this calculator measure how loud my room is?

No — it only computes formulas from numbers you enter. To measure an actual level you need a calibrated sound-level meter. Our microphone-based Decibel Meter gives a relative reading but is not calibrated SPL either.

What does "uncorrelated" mean for combining sources?

It means the sounds are independent (different machines, voices, etc.), so they add on an energy basis. Correlated sounds — the same signal from two speakers — can add coherently and behave differently (up to +6 dB or cancellation), which this simple formula doesn’t cover.

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