  The direct proportionality relation between the boson peak maximum in
glasses, $\omega_b$, and the Ioffe-Regel crossover frequency for phonons,
$\omega_d$, is established. For several investigated materials $\omega_b =
(1.5\pm 0.1)\omega_d$. At the frequency $\omega_d$ the mean free path of the
phonons $l$ becomes equal to their wavelength because of strong resonant
scattering on quasilocal harmonic oscillators. Above this frequency phonons
cease to exist. We prove that the established correlation between $\omega_b$
and $\omega_d$ holds in the general case and is a direct consequence of
bilinear coupling of quasilocal oscillators with the strain field.
