A rotary acoustic horn. The rotary acoustic horn includes a shaft having an axial input end and an axial output end; and at least one welding portion; wherein the welding portion comprises an outer weld face that expands and contracts with the application of acoustic energy; wherein the welding portion comprises a first opposing end portion and a second opposing end portion; wherein the welding portion comprises through holes extending substantially along an axial direction of the welding portion from the first opposing end portion to the second opposing end portion; and wherein the through holes are located between the weld face and the shaft.