The everyday decibel conversions in one place: voltage to dBu and dBV, combining multiple SPL sources, and the change in level over distance.
The everyday conversions.
dBu = 20·log₁₀(V/0.775) · dBV = 20·log₁₀(V/1). Combine: 10·log₁₀(Σ10^(Lₙ/10)). Distance: ΔSPL = −20·log₁₀(d₂/d₁) (−6 dB per doubling).
The everyday decibel conversions in one place: voltage to dBu and dBV, combining multiple SPL sources, and the change in level over distance.
You cannot just add the decibel numbers. Convert each level to power with 10 to the power of (level over 10), add those, then convert back with 10·log10 of the sum. Two equal sources add up to about 3 dB louder, not double the number, because decibels are logarithmic.
Both express a voltage as a decibel level but against different references. dBu is referenced to 0.775 volts (the voltage that delivers 1 mW into 600 ohms), while dBV is referenced to 1 volt. The two differ by a fixed 2.2 dB, so 0 dBV equals about +2.2 dBu.
In a free field, level follows the inverse-square law: it falls by 20·log10 of the distance ratio, which is 6 dB for every doubling of distance. So moving from 1 to 2 metres loses 6 dB, and 2 to 4 metres loses another 6 dB.
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