A guitar string, a skipping rope, a drumhead: small vibrations of a stretched string obey the wave equation. It differs from the heat equation by one derivative in time, and that changes decay into oscillation.
Idea clave
$u_{tt} = c^{2}u_{xx}$ on $0 < x < L$, fixed ends $u(0, t) = u(L, t) = 0$, initial shape $u(x, 0) = f(x)$ and velocity $u_t(x, 0) = g(x)$: $$u = \sum_{n=1}^{\infty}\left(A_n\cos\frac{n\pi ct}{L} + B_n\sin\frac{n\pi ct}{L}\right)\sin\frac{n\pi x}{L},$$ where $A_n$ are the sine coefficients of $f$ and $B_n\cdot\frac{n\pi c}{L}$ are the sine coefficients of $g$. Mode $n$ has angular frequency $\frac{n\pi c}{L}$ and period $\frac{2L}{nc}$.
Por qué funciona
A short piece of string with density $\rho$ and tension $\tau$ feels a net vertical force from the tension that is proportional to the string's curvature, $\tau u_{xx}\,\Delta x$; mass times acceleration is $\rho\,\Delta x\,u_{tt}$. So $u_{tt} = c^{2}u_{xx}$ with $c = \sqrt{\tau/\rho}$, the wave speed. Separating, $X'' + \lambda X = 0$ with fixed ends gives $\sin\frac{n\pi x}{L}$ as before, but now $T'' + c^{2}\lambda T = 0$ is a simple harmonic oscillator, so each mode vibrates forever with no damping. Two initial conditions are needed, since the time equation is second order: $f$ fixes the $A_n$, and differentiating at $t = 0$ shows $g$ fixes $\frac{n\pi c}{L}B_n$.
Ejemplo resuelto: Shape and velocity
Solve $u_{tt} = u_{xx}$ on $(0, \pi)$ with fixed ends, $u(x, 0) = -4\sin 3x$, $u_t(x, 0) = -4\sin 4x$.
- $c = 1$, $L = \pi$: modes $\sin nx\cos nt$ and $\sin nx\sin nt$.
- Shape: $A_3 = -4$. Velocity: $4B_4 = -4$, so $B_4 = -1$.
Respuesta: $u$ $= -4\sin 3x\cos 3t$ $- \sin 4x\sin 4t$
Ejemplo resuelto: Struck from rest position
Solve $u_{tt} = 4u_{xx}$ on $(0, \pi)$, fixed ends, $u(x, 0) = 0$, $u_t(x, 0) = -6\sin x$.
- $c = 2$: the mode $n = 1$ has frequency $nc = 2$.
- $A_n = 0$; $2B_1 = -6$, so $B_1 = -3$.
Respuesta: $u = -3\sin x\sin 2t$
Ejemplo resuelto: A guitar string
A string 0.65 m long has wave speed $c = 286$ m/s. What is its lowest frequency in hertz?
- The fundamental has period $\frac{2L}{c} = \frac{1.3}{286}$ s.
- Frequency $= \frac{c}{2L} = \frac{286}{1.3}$.
Respuesta: 220 Hz (the note A); the overtones are 440, 660, 880, … Hz
With $g = 0$ the solution can also be written as $u$ $= \frac{1}{2}\left(F(x - ct) + F(x + ct)\right)$, where $F$ is the odd periodic extension of $f$ (d'Alembert's formula). It follows from $\sin a\cos b$ $= \frac{1}{2}\left(\sin(a - b) + \sin(a + b)\right)$ and shows the initial shape splitting into two halves that travel in opposite directions at speed $c$.