Abstract: A liquid is subjected to ultrasound within a stainless steel pipe (35) of wall thickness 2.5 mm. Three ultrasonic transducers (56) are equally spaced around the pipe, each fixed to one end of a tapered resonant coupler (46) with a nodal flange (48) by which it is supported with its wider end coaxially within a steel collar (38) welded to the outside of the pipe wall. The space around the sides and the end of the coupler is filled by a buffer liquid such as olive oil. When the transducers (56) are energised they resonate, as do the couplers (46), and ultrasound is transmitted through the buffer liquid and the wall into the liquid inside the pipe (35). Within the pipe the intensity can be sufficient to cause cavitation, and so to cause sonochemical effects.