Physics
Sound pressure level
Twenty rather than ten because pressure squared is proportional to intensity, and the logarithm of a square doubles.
The Formula
dB = 20 x log(p / p0)
Twenty rather than ten because pressure squared is proportional to intensity, and the logarithm of a square doubles.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| L | Sound pressure level | dB | Level in decibels relative to the reference. |
| p | Sound pressure | Pa | Measured root mean square pressure. |
| p0 | Reference pressure | Pa | Twenty micropascals for sound in air. |
What Each Symbol Means
LdB
Sound pressure level
Level in decibels relative to the reference.
pPa
Sound pressure
Measured root mean square pressure.
p0Pa
Reference pressure
Twenty micropascals for sound in air.
See It Worked With Real Numbers
A formula on its own does not tell you which number goes where. The how to calculate decibels page applies this equation to a concrete case, shows every intermediate step, and names the mistakes that produce a wrong answer even when the algebra is right.
Where This Formula Appears
This equation is the working core of How To Calculate Decibels, filed under Science. Every number the guide produces comes from substituting into this one line.