Abstract: A transmission filter has a filter envelope located inside a sealed rigid container, with an intake port and a corresponding intake opening to the interior of the filter envelope located at the bottom of the container. A suction outlet port at the top of the container conveys transmission fluid passing through the filter envelope to the suction line of the transmission system. The filter envelope is formed of thermoplastic fiber felt, with two sheets or layers of the felt being ultrasonically welded to provide a continuous fluid-tight seal for the filter envelope. Ultrasonic welding of the felt is achieved by an apparatus and method that is also useful in other types of ultrasonic welding of thermoplastic fabrics. An ultrasonic horn to be vibrated at an ultrasonic frequency is provided with a knurled surface corresponding to the area to be welded.