With both ends of the cord fixed one obtains the
fourth harmonic
when, besides the
nodes
at the end of the cord,
appear three more nodes in the cord.
This frequency must be such that the space between the two ends of the cord,
 ,
corresponds to twice the wavelength,
=2λ.
Since the speed of waves in the cord, v,
is supposed to be constant, one has
v = f × λ
→
f = 2v /
= 4v / 2
Remember that,
although you do not observe waves moving,
the standing wave is the result of the constructive interference
of many waves which all the time
reflect at the ends of the cord.
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