Abstract: A vibration generator including an inertial body and a housing which are relatively movable and where hydraulic fluid is directed into the inertial body through a control valve mounted in the inertial body, the fluid being directed alternately into one or another working chamber which causes oscillation of the housing and any attached load. A wave form shape of pressure or flow rate of the hydraulic fluid flow is detected and used to control the frequency at which the vibration generator oscillates so that the frequency is maintained at a resonant frequency. The device has particular application in a frequency range of 20 Hertz to 1000 Hertz.

Abstract: The sound produced by the movement of the teeth of an idling circular saw or band saw blade in air is substantially reduced by exposing the blade to a radiation impedance which is effectively imaginary, thereby subjecting a preselected portion of such sound to propagation cut-off. Propagation cut-off is achieved by providing a cover including a pair of closely spaced parallel acoustically hard-wall plates positionable on both sides of the blade. Preferably the plates are spaced apart by a distance less than 21 millimeters, and extend beyond the tips of the teeth by a distance several times the space between the plates. Portions of the plates may be moveable to expose the teeth during the cutting operation.