Diaphone Foghorn, Point Lonsdale Lighthouse, Victoria (1928 - c1978)

'The Rip', the 3 km wide entrance to Port Phillip Bay from Bass Strait, has a long reputation as a very dangerous zone for shipping, with its narrow navigable channels, strong currents and often high winds. The lighthouses at Queenscliff (Shortland Bluff / Black (High) lighthouse 1843, 1862; White (Low) lighthouse 1853, 1862), and at Point Lonsdale (1863, 1902)), were built to aid ships navigating and traversing 'The Rip'.

In 1928, the Melbourne Port Authority installed an audible warning system in the form of a diaphone foghorn, at Point Lonsdale lighthouse, to provide an alert for ships approaching the entrance in foggy weather, and it remained in-service for about fifty years. The plant is contained in the weatherboard shed with a galvanised iron gable roof, located immediately to the east of the lighthouse. The low frequency sound generated, at around 180 Hz, is as loud as a jet engine (138 decibels) and can carry 15 km or more out to sea in foggy conditions. After it ceased operating, many locals missed the old horn’s mournful warning on foggy winter nights, although more than one or two visitors to Point Lonsdale, who wandered too close to the lighthouse as the old horn sounded, have been thrown sideways by its assault on their ear drums. The Port Authority replaced it with the current smaller fog warning system, that uses a diaphragm horn, installed in a nearby former defence bunker.

The 'heart' of the diaphone foghorn is the diaphone horn, a device originally developed to produce the low notes of pipe organs, invented by Robert Hope-Jones considered also to be the inventor of the Wurlitzer theatre organ. The diaphone consists of a hollow bronze cylinder, open at its outward end, with close-spaced narrow circumferential slits. The piston fits snugly inside a similarly slotted cylinder. At the inner flanged end of the piston, 'operating air', when admitted to it, causes the piston to oscillate about 90 times per second. As a result, the piston and cylinder slits align and then misalign at twice this frequency. Each time the slits align, a large volume of separately valved 'sounding air' passes into the hollow piston and escapes to free air. In the case of the Point Lonsdale Foghorn, the successive compression and rarefactions of the air issuing from the diaphone, are reinforced by the resonator trumpet protruding from near the top of the roof gable.

The foghorn at Point Lonsdale was manufactured by Chance Brothers, of Birmingham (UK), lighthouse equipment manufacturers, and is their 'Type F' diaphone. It follows a modification of the Hope-Jones' design where a compressed air supply drives the piston. It has been restored to full working order by a group of local volunteers, and is operated on special occasions. It operates with compressed air supplied by a Gardiner two-cylinder kerosene engine coupled to a Reavell 4-cylinder radial air compressor. The air compressor feeds into two large air receivers and the diaphone is supplied with air at up to 35 psi (240 kPa) from these receivers, via pneumatically operated valves. In order to produce a pre-determined sequence of the horn blasts there is a mechanically driven timing unit providing pneumatic signals to the 'operating' and 'sounding' air valves. This timer was originally powered by a belt drive from the Gardiner engine but is now operated by a small electric motor. It originally had two Gardiner engine and Reavell air compressor sets. The second set was replaced by an electric motor driven, vertical reciprocating air compressor when mains electricity supply became available to the lighthouse. (The second engine-compressor unit is in the possession of the Queenscliffe Maritime Museum).

Operative diaphone foghorns are now quite rare in the world, however, across Bass Strait, the Low Head light station on the eastern side of the Tamar River mouth has a similar foghorn, but with the larger Chance Brothers 'Type G' diaphone. It has also been restored to working order by a group of Tasmanian volunteers.