Experiments in External Noise Reduction of ... - CAFE Foundation
Experiments in External Noise Reduction of ... - CAFE Foundation
Experiments in External Noise Reduction of ... - CAFE Foundation
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.—. —=——.—--— -- —-—----—-—-----—.14 NACA TN 2079 ‘.It is worth not<strong>in</strong>g at this po<strong>in</strong>t that the graphs <strong>of</strong> the flightsshow<strong>in</strong>g sound level aga<strong>in</strong>st horizontal distance me not corrected forthe f<strong>in</strong>ite velocity <strong>of</strong> sound. In other words, the sound level shownwhen, for example, the airplane was 1000 feet away from the overheadposition is actually the sound level at the microphone at that time.S<strong>in</strong>ce that sound took some time to reach the microphone, it wasactueJly_generatedwhen the airplhe was somewhat farther away as itapproached, or nearer when it was go<strong>in</strong>g away; however, this discrepancyhas no effect on comparisons between the different configurations..Method <strong>of</strong> I?resent<strong>in</strong>gGround-Analysis Measure~ntsInformation from the ground measurements is presented <strong>in</strong>.the form<strong>of</strong> polar plot? <strong>of</strong> the over-all level and amplitude <strong>of</strong> each significantcomponent. (See fig. 27.) In tak<strong>in</strong>g the read<strong>in</strong>gs, serious fluctuations<strong>in</strong> sound level were encountered. Furthermore, on the ground the eng<strong>in</strong>eoverheated quickly and read<strong>in</strong>gs had to be taken dur<strong>in</strong>g a very shortperiod. @ each case the maximum read<strong>in</strong>g observed was recorded. The ‘propeller noise was particularly subject to fluctuation, especially <strong>in</strong>the higher harmonics, where peak read<strong>in</strong>gs varied as much as 10 decilelsover a short period. Because <strong>of</strong> this fluctuation the comparative polarplots show only eng<strong>in</strong>e and propeller fundamentals, the higher harmonicsbe<strong>in</strong>g omitted...When compar<strong>in</strong>g these plots it must be kept <strong>in</strong> m<strong>in</strong>d that the quietedconfigurations are powered by eng<strong>in</strong>es giv<strong>in</strong>g considerably more-horsepowertha.nthe standards. (See table II.)The frequencies showm on such curves are those actually measured hy‘ the analyzer. The “propeller fundamental” <strong>in</strong> each case is the lowestfrequency component observed which could be attributed to the propeller.This frequency represented <strong>in</strong> all-cases the ti~passage frequency. Forall except the stsmdard St<strong>in</strong>son, configuration.1, the lowest frequencycomponent which could le attributed to the eng<strong>in</strong>e occurred at threetimes the crankshaft rotation,frequency. This component is referredto as the “eng<strong>in</strong>e fundamental.” On the standard St<strong>in</strong>son, componentsattributable to the eng<strong>in</strong>e occurred at one and one-half and four andone~~ times crankshaft frequency...__————_.. ———.-..— .———.
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