 Wave amplitude

The wave amplitude measures the size of a disturbance relative to its reference state. For $u(x,t)=A\cos (kx-\omega t+\varphi )$ with $A\ge 0$, it is the maximum absolute deviation $A$. Its units depend on the variable: a string's displacement amplitude is a length, an acoustic wave's pressure amplitude is a pressure, and a light field can be specified by an electric field amplitude.

For a sinusoid, the peak-to-peak amplitude is $2A$ and the root mean square is $A/\sqrt{2}$. A complex amplitude stores both $A$ and a wave phase; its modulus is $A$ when the peak convention is used. In a fixed medium a progressive linear wave usually carries wave intensity proportional to $A^2$, so a small field signal and a small power signal use different scales. A finite wave packet can have an envelope whose local amplitude changes with position and time.