Abstract: An ultrasonic transducer to which there is coupled a sonotrode or an end part with a portion functioning as a sonotrode, provided to vibrate according to a direction which can be oriented in relation to a working surface where there are plastics to be welded arranged under a free end of the sonotrode, the sonotrode internally defining a first conduit for the circulation of a cooling fluid and which exits to the exterior through the free end, and the head having a fluid supply connection. It includes a second conduit communicating the first conduit with the connection, with an inlet arranged above the sonotrode or the portion functioning as a sonotrode of the end part.