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

14.13 Gas Jet Noise Control 395Figure 14.15. Air shroud silencer. The micrometer shown here adjusts the amount ofair supply bypassing the nozzle instead of going through it. (Courtesy of ITW Vortec,Cincinnati, Ohio.).Impingement NoiseImpingement noise occurs when a gas jet is brought close and impinges upon asolid surface or object. A sharp increase in the noise level occurs, particularly inthe range of higher frequencies. In addition, the impingement of the gas jet onthe surface can bring about unsteady forces in the form of aerodynamic dipoles,which can be described as a pair of point sources of equal magnitudes separated bya small distance and oscillating with an angular phase difference of 180 ◦ . This is tosay that these two point sources are out of phase, i.e., when one of the point sourcesis positive, the other is negative. These dipoles constitute the basic mathematicalmodels used to describe the noise and radiation of many common noise sourcesincluding propellers, valves, loudspeakers, air duct diffusers, a number of musicalinstruments, and so on. There are also some directivity patterns present when thenoise evolves from dipole sources.Impingement noise, as with jet noise, is difficult to predict in the way of itsamplitude and spectral characteristics of the noise-generating mechanism. Fromboth analytical and experimental considerations, the radiated sound power forimpinging subsonic jets depends in the first order on the fifth or sixth powerof the flow velocity. Again, as with the free jet flow, even a slight reduction inthe flow velocity can bring about appreciable reductions in impingement noise.If a jet flows over a sharp edge or discontinuity, even more noise is likely tobe generated. Whistle-like edge tones will also occur. Cutting down on the flowturbulence created by jet flows over a cavity or an obstruction can lessen theperiodic components and the impingement noise. In these cases, the impingementnoise can be lessened by avoiding or eliminating the presence of cavities and byredirecting the jet away from the edge.