Abstract: An acoustic horn imparts energy at a selected wavelength, frequency, and amplitude. The horn has at least one nodal plane and a natural frequency of vibration. The horn has an outer surface and at least one cutout located in the outer surface. The cutout is located at a longitudinal location on the surface that does not contact the nodal plane. The horn length is a function of the shape, size, number, and location of the cutouts, and is less than the length of a solid horn having the same natural frequency of vibration. The horn can vibrate at a natural frequency and the length of the horn can be less than one-half wavelength of vibration.