  Relativistic bound state problem in hadron physics is studied. Mesons and
their resonance excitations in the framework of Relativistic Quantum Mechanics
(RQM) are investigated. Two-particle wave equation for the Lorentz scalar QCD
inspired funnel-type potential with the coordinate dependent strong coupling
$\alpha_\S(r)$ is derived. The concept of distance dependent particle mass is
developed. Two exact asymptotic expressions for the system's squared mass are
obtained and used to derive the meson interpolating complex-mass formula. Free
particle hypothesis for the bound state is developed: quark and antiquark move
as free particles in of the bound system. Practical applications of the model
are given.
