About this tool
Decibels describe ratios on a logarithmic scale. The dB calculator converts both ways between dB and ratio – using 20·log10 for amplitude quantities such as voltage, current and sound pressure, and 10·log10 for power quantities. That is why +6.02 dB doubles the voltage, but +3.01 dB already doubles the power.
It also adds the levels of incoherent sources (90 dB + 90 dB = 93.01 dB), calculates the level of several equal sources, subtracts background noise from a measurement, converts between dBu, dBV, dBFS and volts (0 dBu = 0.7746 V, 0 dBV = 1 V, 0 dBFS configurable) and uses the inverse square law to find the level at another distance. To generate a test tone at an adjustable level, use the Tone Generator.
How it works
- Choose the mode at the top: dB ↔ ratio, add levels, dBu / dBV / dBFS or distance.
- Enter your values – for ratios you can edit either the dB field or the ratio field.
- Read and copy the result; the link in the address bar keeps your inputs.
Frequently asked questions
Why is 90 dB + 90 dB not 180 dB?
Decibels are logarithmic. Two equally loud, independent sources double the sound power, and doubling corresponds to 10·log10(2) = 3.01 dB. So 90 dB + 90 dB = 93.01 dB. Ten equal sources add 10 dB. A source that is 10 dB quieter than another raises the total by only about 0.41 dB.
When do I use 20·log10 and when 10·log10?
Power quantities such as power in watts or sound intensity use 10·log10. Field quantities such as voltage, current or sound pressure use 20·log10, because power rises with their square. That is why +6.02 dB is double the voltage but four times the power.
What is the difference between dBu, dBV and dBFS?
dBu refers to 0.7746 V (the voltage that produces 1 mW in 600 Ω), dBV to 1 V. Professional gear runs at +4 dBu (1.228 V), consumer gear at −10 dBV (0.316 V). dBFS is digital and refers to full scale; which analogue voltage 0 dBFS corresponds to depends on the converter – EBU R68 specifies +18 dBu, +24 dBu is common in the US.
How much does the sound level drop with distance?
For a point source in the free field the level falls by 20·log10(r2/r1) according to the inverse square law: 6.02 dB per doubling of the distance and 20 dB per tenfold distance. In rooms with reflections and for large sources such as line arrays the drop is smaller.
What is subtracting background noise good for?
When you measure a machine while background noise is present, the reading contains both sources. Subtracting the separately measured background gives the level of the machine alone: 85 dB total with 80 dB background leaves 83.35 dB. If the background is less than about 3 dB below the total, the result becomes unreliable.