Sóng dừng

$n=\dfrac{2l}\lambda=3$ bụng, $4$ nút.

$4\cdot\dfrac\lambda2=24\Rightarrow\lambda=12$ cm.

$f_n=\dfrac{nv}{2l}=20n$: $20;\ 40;\ 60$ Hz.

$A=4\cos\dfrac{2\pi}{6}=4\cos\dfrac\pi3=2$ cm.

$n=0$: $l=\dfrac\lambda4\Rightarrow\lambda=1{,}2$ m, $f=\dfrac{60}{1{,}2}=50$ Hz.