Two speakers send out the same note, starting together, with a wavelength of 0.5 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.