miu0 = 4*pi*10^(-7);
miur = 2000;
S0 = pi*(0.25*10^(-3))^2/4;
n = 1000;
I0 = 1;
deta0 = 0.01;
L = 0.642;
rho = 0.385;
T = 162;
solve = NDSolve[{D[u[t, x], t, t] - T*D[u[t, x], x, x]/rho ==
miu0*(miur)^2*S0*(n*I0)^2*
exp (-(x - L/2)^2/0.001)*(cos (2*pi*329*t))^2/(rho*8*deta0^2),
u[t, 0] == 0, u[t, L] == 0, u[0, x] == 0,
Derivative[1, 0][u][0, x] == 0}, u[t, x], {t, 0, 20}, {x, 0, L}]
报错::
NDSolve::nlnum: The function value {0.,0.,0.,0.,0.,0.,0.,0.,0.,<<33>>,0. -2.58132*10^7 cos^2 exp pi^4,0. -3.7171*10^7 cos^2 exp pi^4,0. -5.05938*10^7 cos^2 exp pi^4,0. -6.60817*10^7 cos^2 exp pi^4,0. -8.36346*10^7 cos^2 exp pi^4,0. -1.03253*10^8 cos^2 exp pi^4,0. -1.24936*10^8 cos^2 exp pi^4,0.} is not a list of numbers with dimensions {50} at {t,u[t,x],(u^(1,0))[t,x]} = {0.202631,{0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.},{0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.,0.}}.
画个图
{{u[t, x] ->
InterpolatingFunction[{{0., 0.}, {0., 0.642}}, {
5, 5, 2, {1, 25}, {4, 6}, 0, 0, 0, 0, Automatic, {}, {},
False}, {{0.}, {0., 0.02675, 0.0535, 0.08025, 0.107, 0.13375,
0.1605, 0.18725, 0.214, 0.24075000000000002`, 0.2675, 0.29425,
0.321, 0.34775, 0.3745, 0.40125000000000005`, 0.428, 0.45475,
0.48
15000000000
0004`, 0.50825, 0.535, 0.5617500000000001,
0.5885, 0.61525, 0.642}}, {
Developer`PackedArrayForm, {0, 4, 8, 12, 16, 20, 24, 28, 32, 36,
40, 44, 48, 52, 56, 60, 64, 68, 72, 76, 80, 84, 88, 92, 96,
100}, CompressedData["
1:eJxTTMoPSmVkYGBIYcAFHJxxSo3Kj8qPIHkALbkO2g==
"]}, {Automatic, Automatic}][t, x]}}
Plot3D[Evaluate[u[x, t] /. First[%]], {x, 0, L}, {t, 0, 20},
PlotPoints -> 10]
结果什么也画不出来

哦, 我似乎是调用错了, 调用常数或者函数用大写

另外有有个小问题, 求出来solve之后想画一个x=0.321, t:0~1 的二维曲线, 这个应该是怎么去画??

ClearAll["Global`*"]
miu0 = 4*\[Pi]*10^(-7);
miur = 2000;
S0 = \[Pi]*(0.25*10^(-3))^2/4;
n = 1000;
I0 = 1;
deta0 = 0.01;
L = 0.642;
rho = 0.385;
T = 162;
solve = NDSolve[{D[u[t, x], t, t] - T*D[u[t, x], x, x]/rho ==
miu0*(miur)^2*S0*(n*I0)^2*
Exp [-(x - L/2)^2/0.001]*(Cos [2*\[Pi]*329*t])^2/(rho*8*deta0^2),
u[t, 0] == 0,
u[t, L] == 0,
u[0, x] == 0,
Derivative[1, 0][u][0, x] == 0
}, u, {t, 0, 20}, {x, 0, L}, MaxStepSize -> 0.01];
Plot3D[Evaluate[u[x, t] /. First@solve], {x, 0, L}, {t, 0, 20},
PlotPoints -> 10]