💡 Physics

Acoustics & Lighting Units

Convert sound pressure levels and lighting illuminance. Choose the category, enter a value and units, then press Convert.

Written and reviewed by SM Arijul Islam · Last reviewed 28 October 2026 · Editorial standards · Report an error

About the Acoustics & Lighting Units

Sound pressure level is logarithmic: dB SPL = 20 log₁₀(p ÷ 20 µPa). That reference is the threshold of hearing, so 0 dB SPL is not silence in a physical sense but the bottom of the human scale. A conversation at 60 dB is 1,000 times the pressure of 20 dB, not three times.

Lighting units are easier but still often confused. Lux is lumens per square metre; a foot-candle is lumens per square foot, so 1 fc = 10.764 lux. A phot is 10,000 lux. Lumens measure the total output of a lamp; lux measures how much of it lands on a surface, which is why the same bulb gives different lux at different distances.

How the calculation works

dB SPL = 20 × log₁₀(pressure ÷ 20 µPa).
Pressure = 20 µPa × 10^(dB ÷ 20).
1 foot-candle = 10.7639 lux.
1 phot = 10,000 lux.

Reference values

Sound pressure levels and equivalents
Sound source	dB SPL	Pressure
Threshold of hearing	0 dB	20 µPa
Whisper	30 dB	0.63 mPa
Conversation	60 dB	20 mPa
Busy traffic	80 dB	0.2 Pa
Concert	100 dB	2 Pa
Pain threshold	120 dB	20 Pa

Worked example: decibels and lux

A sound pressure of 0.2 Pa is 20 log₁₀(0.2 ÷ 0.00002) = 80 dB SPL. A 10 dB increase is ten times the sound power but roughly twice the perceived loudness. For lighting, a 500 lux office is 46.5 foot-candles; a 100 fc workshop is 1,076 lux.

The logarithmic scale is why a “quiet” 40 dB room and a 70 dB vacuum cleaner differ by a factor of about 31 in pressure, not 1.75. It is also why halving the distance to a point source raises the level by about 6 dB, not 2. Use the scale for comparisons, and use a meter when a specification matters.

Weighting, distance and measurement

Sound level meters apply a frequency weighting because the ear is not equally sensitive across the spectrum. A-weighting approximates that response at moderate levels and is the basis of most environmental and workplace limits; C-weighting is flatter and used for peak measurements. The same noise can read several decibels differently under the two, so the weighting must be quoted with the number.

Distance matters just as much. In a free field, sound pressure falls about 6 dB per doubling of distance, but indoors reflections change that, and a measurement at 1 m from a machine is not comparable with one at the operator's ear. Lighting has the same issue: lux is measured at the surface, so a lamp's lumen output says nothing about the light on your desk until distance and beam angle are known.

Frequently asked questions

Is dB a unit of power?

It is a logarithmic ratio, not an absolute unit. dB SPL uses 20 µPa as its reference; dBm uses 1 mW; dB FS uses full scale.

Does doubling the power double the dB?

No. Doubling power adds about 3 dB; doubling perceived loudness is roughly 10 dB.

What is the difference between lux and lumens?

Lumens are the total light emitted; lux is lumens per square metre at a surface. A 1,000-lumen lamp lighting 1 m² evenly gives 1,000 lux.

Does doubling the distance halve the sound level?

In a free field, doubling distance reduces sound pressure level by about 6 dB. That is a factor of four in pressure but only about 1.5× in perceived loudness.

Sources and standards

The decibel is a logarithmic ratio rather than an absolute unit, and the lux is a photometric measure. The SI Brochure defines the candela and lumen behind the lighting figures, and the page states the reference pressure and weighting assumed for sound levels.