Two classes of waveguides are studied: acoustic waveguides conducting pressure waves generated by a constant‐velocity source; and structural waveguides excited by a constant force shaker. The high‐pass cutoff frequencies of propagating modes of both waveguide classes equal the natural frequencies of the corresponding standing‐wave modes of the waveguide cross section. In acoustic waveguides, cutoff frequencies correspond to an infinite impedance at the source. In structural waveguides, cutoff frequencies coincide with vanishing drive‐point impedance, the resonant mode short circuiting the other modes. This distinctive behavior is associated with the prescribed excitations and definition of drive‐point impedance rather than with the nature of the waveguide.