Analysis of information sources in references of the Wikipedia article "Wavelength" in English language version.
wavelength lambda light sound frequency wave speed.
The local wavelength λ of a dispersing wave is twice the distance between two successive zeros. ... the local wavelength and the local wave number k are related by k = 2π / λ.
(p. 61) ... the individual waves move more slowly than the packet and therefore pass back through the packet as it advances