What Is the Decibel Calculator?
The Decibel Calculator handles the four everyday jobs that trip people up because decibels are logarithmic: combining two or more sound levels, converting acoustic power in watts to dB, converting voltage to dB, and projecting a measured level out to a new distance. Pick a mode, type your numbers, and it does the log arithmetic — the same 10·log and 20·log math that a spreadsheet or a physics textbook would use, without the setup. You get the result, a supporting detail, and a plain-language note on how the change would actually sound.
Why Decibels Don't Simply Add
A decibel is a ratio on a logarithmic scale, so intuition about ordinary numbers fails. The classic example: two machines each measured at 80 dB, running side by side, produce about 83 dB together — not 160 dB. Doubling the number of equal sources adds only 3 dB because it doubles acoustic power, and 10 × log₁₀(2) is roughly 3. That single fact reshapes noise planning: silencing one of two identical sources buys you 3 dB, and getting a room 10 dB quieter — the point where it sounds about half as loud — takes cutting power to a tenth.
Power, Voltage, and Distance in One Place
The same tool covers the conversions engineers reach for constantly. Power ratios use dB = 10 × log₁₀(P₁/P₂); field quantities like voltage or sound pressure use 20 × log₁₀(V₁/V₂), because power rises with the square of voltage, which is where the factor of 20 comes from. For distance, a point source in the open loses about 6 dB every time you double how far away you stand — a 90 dB reading at 1 m becomes roughly 84 dB at 2 m and 78 dB at 4 m. Choose the matching mode and the calculator applies the right formula automatically.
Decibel Calculator
How to Use This Calculator
- Choose a Calculation Mode: Pick Add dB levels to combine sources, Power (watts) to dB for acoustic or amplifier power, Voltage to dB for field quantities, or dB at distance to project a level.
- Enter Value 1 and Value 2: In Add mode these are two levels in dB. In Power or Voltage mode they are the measured and reference quantities forming the ratio. In Distance mode, Value 1 is the known level at the reference distance.
- Set the Distance If Needed: For distance mode, enter the new distance in meters. The reference distance is the point where Value 1 was measured — typically 1 m for a rated speaker spec.
- Click Calculate and Read the Perception Note: Hit Calculate to see the combined or converted level plus its detail. Use the perception line to judge whether the change is barely audible, clearly louder, or about twice as loud.
How It Works
Decibels are a logarithmic ratio, not a plain count, so this calculator never just adds the numbers you type. It picks the right formula for the mode you choose — combining levels, converting power or voltage to dB, or projecting a level to a new distance — and does the log math for you.
The basic rule:
- Adding: dB_total = 10 × log₁₀(10^(dB₁/10) + 10^(dB₂/10))
- Power: dB = 10 × log₁₀(P₁ / P₂)
- Voltage: dB = 20 × log₁₀(V₁ / V₂)
- Distance: dB₂ = dB₁ - 20 × log₁₀(d₂ / d₁)
The distance rule assumes a point source radiating into a free field with no reflections. Indoors, walls and ceilings bounce sound back, so real rooms fall off more slowly than the formula predicts — treat outdoor-style results as a best case.
Tips & Considerations
- When two levels are within about 1 dB of each other, expect roughly +3 dB combined; when they are 10 dB or more apart, the quieter source barely moves the total.
- To cut noise, target the loudest single source first — trimming a source that is already 10 dB below the total changes almost nothing.
- Match the mode to the quantity: watts and acoustic power use the 10·log Power mode, while voltage and sound pressure use the 20·log Voltage mode.
- For distance estimates, remember the 6 dB per doubling rule only holds outdoors or in a free field; enclosed rooms fall off more slowly because of reflections.
- If you are checking against an 85 or 90 dBA workplace limit, note that this tool gives unweighted level math — a calibrated dBA meter is what compliance actually requires.
Frequently Asked Questions
Why can't I just add two decibel numbers together?
Because decibels are logarithmic, they measure a ratio of power rather than a linear quantity. Two 80 dB machines running together produce about 83 dB, not 160 dB. To combine levels you convert each back to power with 10^(dB/10), add the powers, then convert back with 10 × log₁₀ of the sum. This calculator does that for you in Add mode.
Why do two equal sources add only +3 dB?
Adding a second identical source doubles the acoustic power, and 10 × log₁₀(2) ≈ 3.01 dB. So two 70 dB fans make 73 dB, four make 76 dB, and eight make 79 dB — every doubling of sources adds 3 dB. Perceptually 3 dB is noticeable but modest, which is why halving the number of noisy machines rarely feels like half the noise.
When do I use 10·log versus 20·log?
Use 10 × log₁₀ for power or intensity quantities — watts, sound intensity, acoustic power. Use 20 × log₁₀ for field quantities like voltage or sound pressure, because power is proportional to the square of pressure or voltage. The factor of 20 is just the factor of 10 with the square pulled out of the log. Pick Power mode for watts and Voltage mode for volts and the tool applies the correct factor.
How much does sound drop as I move away from the source?
For a point source in the open, level falls about 6 dB every time you double the distance — the inverse-square law expressed as 20 × log₁₀(d₂/d₁). A speaker measuring 90 dB at 1 m reads roughly 84 dB at 2 m, 78 dB at 4 m, and 72 dB at 8 m. Line arrays, walls, and enclosed rooms change this, so use it as a free-field estimate.
How much louder is a 10 dB increase?
A 10 dB rise is generally perceived as about twice as loud, even though it represents ten times the acoustic power. By contrast a 3 dB rise doubles the power but is only a mild change to the ear, and roughly 1 dB is the smallest step most listeners can reliably detect. This is why loudness and raw power move on very different scales.
Can I subtract a known noise source from a total measurement?
Yes, decibel subtraction works the same way in reverse: convert both the total and the known source to power, subtract, then convert back. If a total reading is 85 dB and a background source is 82 dB, the remaining source is about 82 dB — not 3 dB — because the two nearly equal levels shared most of the energy. The gap between the numbers matters more than their difference.
Is the Decibel Calculator free to use?
Yes, completely free with no signup required. Everything runs in your browser, so you can add levels, convert watts or volts, and project distance as many times as you like at no cost.