Abstract: A closed-loop ultrasonic welding apparatus for sealing or welding thermoplastic material on a continuous basis wherein the material to be welded is fed through a weld station comprising a horn and anvil. The apparatus includes means for providing a first signal commensurate with a desired ultrasonic energy density in the material when welded, a second signal responsive to the feed speed of the material through the weld station, and a third signal commensurate with the power coupled by a power supply via a transducer and the horn to the material at the weld station. Control means receive the first, second and third signals and produce an error signal, which is responsive to the difference between the desired energy density and the actual energy density coupled to the material. After being subjected to further processing the error signal is converted to a control signal for controlling the output from said power supply and/or controlling the engagement force between the horn and material.