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 . hence , the welding apparatus causes a predetermined energy density in the welded material irrespective of disturbances or changes in feed speed , engagement force between the horn and material , or the material itself .