Why do two identical sound sources only add 3 dB, not double?

Decibels are logarithmic. When you double the acoustic power (two identical sources), the result is 10 × log10(2) = 3.01 dB more. This reflects how human hearing perceives loudness — a 3 dB increase is barely noticeable, while a 10 dB increase sounds roughly twice as loud.

How does sound level change with distance?

In free field (outdoors, no reflections), sound level decreases by 6 dB for every doubling of distance from a point source. This is the inverse square law: SPL decreases by 20 × log10(d2/d1) dB. Indoors, reflections reduce the rate of decay.

Can I subtract decibel levels?

Yes, but only if the level you are subtracting is lower than the total. dB subtraction is useful for removing background noise from a measurement. The formula is 10 × log10(10^(L1/10) - 10^(L2/10)).

What is the difference between dB and dBA?

dB is a raw measurement of sound pressure level. dBA applies an A-weighting filter that approximates how the human ear perceives loudness at moderate levels — it reduces the contribution of very low and very high frequencies. Most noise regulations use dBA.

AudioCalcs

Decibel Calculator — Add, Subtract & Compare dB Levels

Add, subtract and combine decibel levels. Calculate dB change with distance using the inverse square law.

Calculation Mode

First Level

Second Level

How We Calculate This

Decibels are logarithmic units used to express ratios of power or pressure. Because they are logarithmic, you cannot simply add or subtract dB values arithmetically — you must convert to linear power values first.

Adding two dB levels

L_total = 10 × log10(10^(L1/10) + 10^(L2/10))

For example, adding 90 dB and 90 dB: 10 × log10(10^9 + 10^9) = 10 × log10(2 × 10^9) = 93.01 dB.

Multiple identical sources

L_total = L_source + 10 × log10(N)

Distance (inverse square law)

L2 = L1 - 20 × log10(d2 / d1)

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard formulas. Always verify results for critical applications.