  In a linear magnetoelectric the lattice is coupled to electric and magnetic
fields: both affect the longitudinal-transverse splitting of zone-center
optical phonons on equal footing. A response matrix relates the macroscopic
fields (D,B) to (E,H) at infrared frequencies. It is shown that the response
matrices at frequencies 0 and \infty fulfill a generalized Lyddane-Sachs-Teller
relationship. The rhs member of such relationship is expressed in terms of
weighted averages over the longitudinal and transverse excitations of the
medium, and assumes a simple form for an harmonic crystal.
