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

14.13 Gas Jet Noise Control 393SolutionApplying Equation (14.46)0.15 × 344 m/sf peak = = 1032 Hz0.05 mThus, the peak frequency should be expected to occur in the 1 kHz octaveband.On either side of the peak frequency, the octave band distribution of the acousticalpower falls off. Experimental data indicated that the spectrum falls off at theaverage rate of –6 dB per octave above the peak frequency and −7 dB per octavebelow the peak frequency. The upshot is that the magnitude and the spectralcharacter of gas jet noise can be only estimated roughly.14.13 Gas Jet Noise ControlThe major challenge in dealing with jet engine noise is to reduce the high noiselevels with minimal impact on the thrust. The greatest progress came about byreducing the jet velocity while keeping the thrust constant because the soundpower is proportion to (thrust) × U 6 /c 0 5 , according to the Lighthill equation(14.41). Early efforts were centered on modifying the nozzle shape to variationsof the “cookie-cutter” forms or using multiple smaller nozzles. Some of thesedesigns produced up to 10 dB noise reductions with a rather small loss of thrust.Newer by-pass and fan-jet engine designs entail much lower jet velocities, largestreamlined enter bodies, and annular jets that may be subdivided in any one ofseveral ways.In the case of simple high-velocity air jets in industrial environments, such asthose used to power air tools, provide cooling or venting, parts ejection, and so on,a number of straightforward noise reduction measures can be applied. The basicsteps include the following:1. Reduction of the required air velocity by moving the nozzle closer to a partbeing ejected, while maintaining the same value of thrust.2. Adding additional nozzles, reducing the required velocity but again sustainingthe same thrust magnitude.3. Installation of newer models of quieter diffusers and air shroud nozzles.4. Interruption of airflow in sequence with ejection or blow-off timing.Methods 1 and 2 above result in noise reduction from cutting down on the jetvelocity. Reduction of the airstream velocity should be the first consideration ofany noise reduction program. Usually, the only constraint is the preservation ofthe air-jet thrust. The thrust T j of a jet is given byT j = m V (N) (14.47)wherem = mass flow rate, kg/s