THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

Belt Drivesf CL = n 1N 1 2400 × 12= = 24 Hz30N L 30 × 40f TE/CA = n 1N 130=2400 × 123014.12 Gas-Jet Noise 385= 960 HzBelt drives are often used instead of gears or chains in order to save on costs andprovide some degree of noise control. The elasticity of the belt prevents shock loadsat a driven machine from being transmitted back to the driver. Solid-borne noise andattendant vibration are reduced. Flat belts and V-belts mounted on smooth pulleysdepend on friction to transmit power, so there must be adequate belt tension toprevent slipping as well as to oppose centrifugal effects. When suddenly loaded,these belts can slip, causing a squealing noise. This sometimes can be fixed byincreasing the tension; but excessive tension can shorten the life of a belt and induceexcessive bending moments in connected shafts. When precise speed ratios mustbe sustained, such as the camshaft of an automotive engine, toothed belts mountedon toothed pulleys can be utilized. Toothed belts can maintain timing and phaserelationships just as well as meshing gears, but these belts isolate vibration andshock forces rather than transmitting them between the driving and driven elements.Universal JointsUniversal joints are used when the relative position of a driving element changeswith respect to a driven element, such as with an automobile transmission thatis connected through a drive shaft to a rear axle. Flexible couplings and flexibleshafts can also be used, but the former can accommodate only relatively slightmisalignments, and the latter can handle large misalignments but cannot handlelarge amounts of torque. In general, for a given rotational speed n, the frequencyassociated with noise and vibration is given byf = n 30 Hz14.12 Gas-Jet NoiseA most common and also worrisome noise source is the gas jet. This is alsoreferred to as aerodynamic noise, and examples include blowdown nozzles, gas oroil burners, steam valves, pneumatic control discharge vents, aviation jet engines,and so on. An acoustically unmitigated steam valve of a large cooker in a major foodprocessing plant can measure as much as 120 dB. Research on aerodynamicallygenerated noise began in earnest in the early 1950s as the result of the appearanceof the commercial jet engine, when it became obvious that its mechanism of soundgeneration had to be understood better in order to effect noise control (Lighthill,1952, 1978; Hubbard, 1995).