In an acoustic horn of the type wherein a resiliently flexible membrane is stretched across an edge of a tube and vibrated by pressurized fluid forced between the tube edge and a first surface of the membrane into the tube, a positionally adjustable end cap is disposed over the membrane to permit the frequency of vibration of the membrane to be adjusted as a function of the position of the end cap relative to the membrane. The end cap position selectively limits the amount of displacement of the membrane. In addition, the end cap positional adjustability permits selectively control of the force urging the membrane against the tube edge in opposition to the pressurized fluid.