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

394 14. Machinery Noise Control(a)(b)Figure 14.14. (a) Multi-jet diffuser and (b) restrictive diffuser nozzle.V = average jet velocityEquation (14.47) indicates that thrust can be preserved if the mass flow is increasedwhile reducing the jet velocity. Increasing the nozzle exit area and moving the nozzlecloser to the ejection target will also provide considerable velocity reduction.Adding on two or more nozzles will also permit reduction in air-jet velocity and acorresponding reduction in noise level.Multiple-Jet and Restrictive Flow Silencer NozzleFigure 14.14(a) shows an example of a multi-jet diffuser. The noise reductionaccrues mainly from the reduction in jet air core size, which also lessens theturbulence in the mixing regions. Also, the smaller inner jets flow along with theouter layer of high velocity air, thereby reducing the shearing action with the staticambient air. In the restrictive flow nozzle of Figure 14.14(b), the high-velocity coreis minimized by the sintered metal restrictor, typically mounted into the nozzleexit. The flow velocity is reduced somewhat, with a corresponding drop in theamount of noise radiation. In both of these nozzle types, there will be some loss inthe jet thrust, so additional nozzles may be required to keep up the same amountof the jet thrust.Air Shroud Silencer NozzlesIn Figure 14.15, an air shroud silencer nozzle is shown with its airflow pattern.Here the noise reduction is achieved through bypassing some of the primary airflowaround and over the nozzle. The bypassed air lowers the velocity gradients betweenthe primary jet and the ambient air, thus cutting down on the shearing action andthe resultant radiation of noise. There is usually little change in the mass flow forthis type of silencer and the jet thrust is generally preserved. A micrometer dialprovides control of amount of bypassed air. The typical noise reduction throughthe use of air-shroud silencer nozzles is 10–20 dB in the critical high-frequencyrange of 2–8 kHz for small high-velocity jets.