Abstract: An actuator having a linear current force characteristic comprises a moving coil transducer and a hydraulic lever. The moving coil is capable of producing only a relatively low force but with a relatively large displacement. The coil is formed on a piston which acts as an input piston to a hydraulic level. An output piston has a much larger area than the input piston. Thus, the long throw, low force input to the hydraulic lever is converted into a short throw, high force output at the second piston.

Abstract: A system for reducing periodic noise, which includes a plurality of harmonically related noise signals, comprises an actuator for producing a canceling acoustic signal, a sensor for detecting a residual noise signal, a synchronizing signal generator and processing circuitry. The processing circuitry comprises a plurality of tunable harmonically related band pass filters, a tuning signal generator and a summer which sums the outputs of the filters. The tuning signal generator receives the synchronizing signal from the synchronizing signal generator and outputs the tuning signals to the band pass filters. As the frequency of the synchronizing signal changes, the tuning signal generator causes the tunable filters to track harmonics of the noise to be canceled. After summing by the summer and suitable amplification, the outputs from the filters are used to drive the actuator so as to reduce the residual noise detected by the sensor.

Abstract: A muffler for muffling noise in a fluid flow, such as exhaust gases from an internal combustion engine comprises a chamber, loudspeakers for injecting anti-noise into the chamber under the control of processing circuitry in dependence upon residual noise detected by a microphone. The gas flows from an inflow pipe to an outflow pipe past a radial aperture that produces an acoustic coupling between the chamber and the gas flow, to allow the anti-noise to mix with noise in the gas flow and produce noise cancellation. The coupling is so arranged that the chamber becomes pressurized by the gas flow, in use, so as to restrict hot gas flow through the chamber and thereby provide an essentially benign environment for the loudspeaker.

Abstract: A headphone assembly for an active noise cancellation system has a rigid moulded plastics shell having a rigid baffle dividing the shell interior into a front volume and a closed rear volume, both of which may be filled with an acoustic foam. The baffle has a central opening in which is mounted a headphone transducer, and a seal cushion is disposed around the mouth of the shell, to effect an air-tight seal against the user's head. The headphone transducer has two effective sound radiating surfaces of different sizes, and may be constructed as a form of orthodynamic drive unit. In the latter case, the orthodynamic drive unit diaphragm may have two separate coils of different radial extents.