Abstract: There is described a computer network in which a free space light beam is modulated to convey data to and from a network device. The light beam is generated by a signalling device which includes an optical system which collects the light beam and directs the light beam in an exit direction, and also includes a detector which detects the presence of a network device and a beam alignment system which varies the exit direction to align the light beam with the network device. The computer network has particular application in business premises such as an office. The computer network also has applications in public transport stations, for example an airport, and a public transport vehicle, for example an aeroplane.

Abstract: Apparatus for attenuating noise, particularly in an aircraft cabin, comprising a diaphragm (10) one side of which is subjected to the noise, sensing means (16) for sensing the displacement of the diaphragm in response to the noise, and deflecting means (20) responsive to the sensing means for deflecting the diaphragm in order to cancel the displacement, to attenuate noise transmitted from one side of the diaphragm to the other side of the diaphragm.