Why can't I just add decibel values together?

Decibels are a logarithmic scale representing acoustic power ratios. Adding two 80 dB sources does not yield 160 dB — it yields approximately 83 dB. You must convert each level back to a linear power ratio, sum those ratios, then convert back to decibels.

What is the formula used here?

The combined level is L_total = 10 × log₁₀(10^(L1/10) + 10^(L2/10) + … + 10^(Ln/10)). This is the standard ISO/IEC acoustic power-summation formula valid for incoherent (uncorrelated) sources such as separate machines or speakers.

What does "two identical sources add 3 dB" mean?

If both sources are at the same level, 10^(L/10) + 10^(L/10) = 2 × 10^(L/10), and log₁₀(2) ≈ 0.301, so 10 × 0.301 ≈ 3 dB is added. This is a useful rule of thumb: doubling incoherent power sources raises the combined level by about 3 dB.

Is this formula valid for correlated (coherent) sources?

No. For coherent sources — such as two speakers playing the same signal in phase — the combination is amplitude-based (voltage or pressure addition), which can add up to 6 dB. This tool assumes incoherent sources, which is standard for noise and environmental acoustics.

What is the valid input range?

The tool accepts values from 0 dB to 200 dB. Typical environmental noise measurements fall between 30 dB (quiet room) and 130 dB (near a jet engine). Values outside 0–200 dB will show an inline validation error.

How do I remove a source?

Click the trash icon to the right of any source row to remove it. You must have at least one source in the list.

Combine Decibels Calculator

Add multiple sound sources together and calculate the combined decibel level using the logarithmic power-summation formula.

Sound Sources

Quick Presets

Combined Sound Level

— dB

Formula: 10 × log₁₀(Σ 10^(L_n/10))

Power Contribution Breakdown

Percentage of total acoustic power each source contributes.

The +3 dB Rule

Two identical incoherent sound sources always combine to exactly 3 dB above each individual source — regardless of their level. Four identical sources add 6 dB; ten add 10 dB.

Why Louder Source Dominates

A source 10 dB quieter contributes only 10% of the acoustic power of the dominant source. A source 20 dB quieter contributes just 1%. The loudest source almost always sets the combined level.

Summary

When two or more sound sources play simultaneously, you cannot simply add their decibel values — decibels are logarithmic. This calculator uses the correct acoustic power-summation formula (10 × log₁₀ of the sum of 10^(dB/10) for each source) to find the true combined SPL. Enter as many sources as you need, get the result instantly, and see each source's contribution to the total.

How it works

Enter the sound level (in dB or dBSPL) of each source in the input fields.
Click "Add Source" to include more sound sources — up to 20 are supported.
The combined level updates in real time using logarithmic power summation.
A contribution bar shows what percentage of acoustic power each source adds.
Use the preset buttons to load classic examples like two identical sources (+3 dB rule).
Click the trash icon next to any source to remove it from the calculation.

Use cases

Calculate the combined noise level of multiple HVAC units running simultaneously.
Determine total SPL when stacking identical PA speakers on a live sound rig.
Verify that ambient noise sources in a room sum to an acceptable level.
Check how much a second machine adds to an existing factory noise measurement.
Understand the +3 dB rule: two equal sources combine to exactly 3 dB louder.
Plan hearing protection requirements when multiple noise sources are present.
Estimate combined traffic, construction, and HVAC noise for an acoustic report.
Teach students the difference between linear and logarithmic addition.

Frequently Asked Questions

Last updated: 2026-05-29 · Reviewed by Nham Vu