Decibels don’t add the way regular numbers do. Two sound sources each producing 80 dB don’t combine to make 160 dB — they make 83 dB. This calculator handles the logarithmic math for you, whether you’re adding sound levels, converting pressure to dB, or calculating signal gain.

What This Calculator Does

The calculator covers four common decibel operations:

Add two dB levels. Enter any two dB values and get the combined level. Useful for understanding how multiple noise sources interact in a room, or how much louder two speakers are than one.

Subtract dB levels. Calculate the residual noise level when one source is removed, or find the difference between two measurements.

Convert sound pressure to dB. Enter a pressure value in pascals (Pa) or micropascals (μPa) and get the equivalent dBSPL reading referenced to 20 μPa.

Calculate dB gain or loss. Enter an input level and output level to find the gain in dB — useful for amplifiers, mixers, and signal chains. Or enter a dB gain value to find the equivalent linear ratio.

For an explanation of why these calculations require logarithmic math rather than simple addition, the guide to why the decibel scale is logarithmic covers the underlying reason in detail.

How to Use the Calculator

Step 1 — Choose your calculation type. Select from the dropdown: Add Levels, Subtract Levels, Pressure to dB, or dB Gain.

Step 2 — Enter your values. Type your dB readings or pressure values into the input fields. For pressure conversions, make sure you’re using the correct unit (Pa or μPa).

Step 3 — Read your result. The output appears instantly. No submit button needed — the calculator updates in real time as you type.

Common Calculations Explained

Adding Two Sound Sources

When two identical sound sources combine, the rule is:

• Two sources at the same level → combined level = original + 3 dB
• 70 dB + 70 dB = 73 dB (not 140 dB)
• 80 dB + 80 dB = 83 dB
• 80 dB + 70 dB = 80.4 dB (the louder source dominates)

The formula: Combined dB = 10 × log₁₀(10^(L1/10) + 10^(L2/10))

This matters practically in acoustics. Adding a second identical machine to a factory floor doesn’t double the noise level — it adds 3 dB. Adding a third adds another 1.8 dB on top of that. The returns diminish logarithmically.

Converting Pressure to dB

The standard conversion formula is:

dBSPL = 20 × log₁₀(P / 20μPa)

Where P is the measured pressure in micropascals (μPa). For context: normal conversation produces about 20,000 μPa, which converts to approximately 60 dBSPL. The sound pressure level guide explains what the 20 μPa reference represents and why it’s used.

Calculating dB Gain

Gain in dB expresses how much a signal is amplified or attenuated:

• dB gain = 20 × log₁₀(Output voltage / Input voltage)
• dB gain = 10 × log₁₀(Output power / Input power)

Note the difference: voltage and pressure use a factor of 20; power and intensity use a factor of 10. This trips up a lot of people — make sure you’re using the right formula for your signal type.

A gain of 0 dB means no change. Positive dB = amplification. Negative dB = attenuation.

The 3 dB Rule

A 3 dB increase represents a doubling of acoustic power. A 6 dB increase represents a doubling of sound pressure. A 10 dB increase sounds approximately twice as loud to the human ear.

These relationships come directly from the log math and are worth keeping in mind when interpreting your results.

Practical Use Cases

Workplace noise assessment. If multiple machines are running simultaneously, calculate the combined noise level to check against the NIOSH noise exposure limits. The combined level, not individual machine levels, determines your exposure risk.

Acoustic treatment. If your room measures 65 dB background noise and your target is 40 dB, you need 25 dB of reduction. The calculator helps you model the impact of adding absorption panels, sealing gaps, or installing acoustic doors.

Audio equipment. Calculate the gain staging in your signal chain, find the output level from a known amplifier gain, or verify that your headphone amplifier is delivering sufficient power for your headphones.

Speaker placement. Sound level drops approximately 6 dB every time you double the distance from the source (the inverse square law in free space). Use the calculator to estimate how levels change as you move further from or closer to a speaker.

Hearing protection planning. If your environment measures 100 dB and your earplugs have an NRR of 30, your effective protected level is approximately 100 − (30/2) = 85 dB. Check this result against the safe exposure levels in the hearing damage decibel chart.

FAQ

Why can’t I just add decibels directly?

Decibels are a logarithmic scale, not a linear one. Doubling the number of identical sources adds exactly 3 dB to the combined level — not another equal amount. The calculator applies the correct logarithmic formula automatically.

What is the reference level for dBSPL?

20 micropascals (μPa) — the internationally agreed threshold of human hearing, chosen as the softest sound a young adult with healthy hearing can detect under ideal conditions.

What’s the difference between dB (power) and dB (voltage)?

For power and intensity, the formula uses a factor of 10: dB = 10 × log₁₀(P₂/P₁). For voltage and pressure, it uses a factor of 20: dB = 20 × log₁₀(V₂/V₁). This difference exists because power is proportional to the square of voltage, and squaring inside the log is equivalent to doubling the coefficient outside it.

How many dB is twice as loud?

Perceived loudness roughly doubles every 10 dB, based on the equal-loudness research underlying the phon scale. So a sound at 70 dB sounds about twice as loud as one at 60 dB. In terms of acoustic intensity (energy), a doubling occurs every 3 dB. These are different things — the calculator works with acoustic intensity and pressure, not perceived loudness.

Can I use this calculator for headphone sensitivity?

Yes. Headphone sensitivity is typically rated in dB/mW or dB/V. If you know your amplifier’s output voltage and your headphone’s sensitivity rating, the gain calculator can tell you how loud your headphones will play at a given output level.

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