NOW USE THE LENGTH THAT IS n times Lambda, say piston moved or frequency changed

L=1.43
v= 340.
f=3. 10^3
λ=v/f
T=λ/v
TT=1/f
k=2Pi/λ;
ω=2 Pi/T

nn=6

λλ=L/nn

λλ*nn

f=v/λλ

T6=λλ/v
1/T6
TT6=1/f
k6=2Pi/λλ;
ω6=2 Pi/T6

T6

Assume : Hard surface shift lambda half

Animate[Plot[{Sin[-k6 (L-x)+ω6 t], Sin[-k6 (L+x+λλ/2)+ω6  t], Sin[-k6 (L-x)+ω6 t] + Sin[-k6 (L+x+λλ/2)+ω6  t]}, {x, 0, L}, PlotRange->{{0, L}, {-2, 2}},PlotLabel->"λλ /2 "  λλ/2], {t, 0,T6}]

xx=L/nn

Plot[{Sin[-k6 (L-xx)+ω6 t], Sin[-k6 (L+xx+λλ/2)+ω6  t], Sin[-k6 (L-xx)+ω6 t] + Sin[-k6 (L+xx+λλ/2)+ω6  t]},  {t,0, T6},PlotRange->{-2,2}]

xx=(3/4)  L/( nn)

Plot[{Sin[-k6 (L-xx)+ω6 t], Sin[-k6 (L+xx+λλ/2)+ω6  t], Sin[-k6 (L-xx)+ω6 t] + Sin[-k6 (L+xx+λλ/2)+ω6  t]},  {t,0, T6},PlotRange->{-2,2}]

xx=0

Plot[{Sin[-k6 (L-xx)+ω6 t], Sin[-k6 (L+xx+λλ/2)+ω6 t], Sin[-k6 (L-x)+ω6 t] + Sin[-k6 (L+x+λλ/2)+ω6  t]},  {t,0, T6},PlotRange->{-2,2}]

Animate[Plot[{Sin[-k6 (L-x)+ω6 t], Sin[-k6 (L+x+λλ/2)+ω6  t], Sin[k6  x-ω6 t] +Sin[k6  x+ω6 t]}, {x, 0, L}, PlotRange->{{0, 0.001}, {-2, 2}},PlotLabel->"λλ /2 "  λλ/2], {t, 0,T6}]

{ϕ,0,2π}, AnimationRunning->False

Animate[Plot[{Sin[-k6 (L-x)+ω6 t], Sin[-k6 (L+x+λλ/2)+ω6  t], Sin[k6  x-ω6 t] +Sin[k6  x+ω6 t]}, {x, 0, L}, PlotRange->{{0+s, s+0.001}, {-2, 2}},PlotLabel->"s "  s], {t, 0,T6},{s,0,L}]