Abstract: In the present paper general formulas are derived for the resonant frequency, the amplication factor M_P, the form coefficient φ, and the input impedance of a composite concentrator horn. On the bases of these general formulas, formulas for computation are given for the step type concentrator with either conical, or exponential, or catenoidal transition section at the step; for the conical concentrator with a cylinder at its small end; and for the catenoidal concentrator with a cylinder at its large end. Values of M_P and φ as well as the dependence of the input impedance on kl₂ in the neighborhood of the resonant frequency are actually computed for the above mentioned composite concentrators. Design curves are given. Check by experimentation shows that the theoretical values basically agree with the experimental results.