A wave on a rope has a frequency of 40 Hz and a wavelength of 0.85 m.

$v = f\lambda$: the medium sets the speed, the source sets the frequency. On a string, $v = \sqrt{\dfrac{T}{\mu}}$. Two waves in step: a path difference of whole wavelengths is loud, of odd half wavelengths is quiet. Beats: $|f_1 - f_2|$ each second.