Well I had never heard of a Diaphone before! Why didn’t they call it a foghorn, which is what it is in form and function? Anyway, the plans revealed a ‘Diaphone Turret’ on the roof of the superstructure, just in front of the lantern mast. The structure is about 10ft tall overall and 5ft diameter. As it is called a turret, I assume that it rotates, although I am not sure why. It would explain the ‘MOTOR DRIVEN TURNING GEAR’ label inside the turret – assuming I have interpreted the rather indistinct third word correctly.
The Diaphone can be seen (just) in the 1950s photo of the Lady Dixon on station near Carrickfergus.
I found a photo of an up-to-date version, which has the ‘deflector’ at the business end, although the noise-generating end has a 90° bend.
The diaphone horn was based directly on the organ stop of the same name invented by Robert Hope-Jones creator of the Wurlitzer organ. Hope-Jones' design was based on a piston which was closed only at its bottom end and which had slots, perpendicular to its axis, cut through its sides; the slotted piston moved within a similarly slotted cylinder.
Outside of the cylinder was a reservoir of high-pressure air. Initially, high-pressure air would be admitted behind the piston, pushing it forwards. When the slots of the piston aligned with those of the cylinder, air passed into the piston, making a sound and pushing the piston backwards to its starting position, whence the cycle would repeat. A modification of Hope-Jones' design was patented by John Pell Northey, who added a secondary compressed air supply to the piston in order to power it during both its forward and reverse strokes and thus create an even more powerful sound, which carried for miles. The entire horn apparatus was driven by a compressor.
There is unfortunately nothing remaining of this wonderful sea organ, but that is probably a good thing as the temptation to try it out would be overwhelming.