Experiments in External Noise Reduction of ... - CAFE Foundation

10 NACA TN 207’9●.To get the microphone away from reflections caused by the instrumentcases and from internal noises of the Graphic Level Recorder, a ~>foot .extension cable was used. The microphone was mounted in a stand whichheld it 8 inches above the ground md was enclosed by two cloth windscreens, one inside the other; one of these formed a l~inch cube, theother, a 12—inch cube. This cut down wind noises so’that ,thebackgroundnoise (with flat weighting) was about 60 decibels in a 15-mil*an-hourwind. The wind screens served the further purpose of keeping the sunoff the microphone. With the extension cable used, the readings of theSound Level Meter had to be corrected for cable losses. The normal .correction was about 3 decibels, but when the microphone temperaturewent above 800 F this correction became greater, and it was thereforenecessary to know the microphone temperature for work on sunny, warmFor exsmple, on a hot June day with the air temperature about~~s~, the microphone temperature rose to 105° F, giving a cable correctionof 6 decibels, The microphone temperature was determined byholding a thermometer against its metal case.Aside from equipment calibration and maintenance, the only problemencountered was how to place some reference mark on the record of theGraphic Level Re’corder. A momentary shorting switch was developed whichwould “short” the input to the recorder for an instant when a handswitch was pressed. In this way the record could be marked just as theairplane passed overhead. It was felt that estimates by eye, of theoverhead position, were sufficiently accurate for the purposes of-this ‘work. The take-offs were similarly marked as the airplane passed themicrophone.1!.When the Graphic Level Recorder was used, a continuous record wasmade of the sound level of each flight from the time the airplane wasfirst audible until its noise faded into the background. Concurrentlythe pesk reading of the Sound Level Meter was observed for all flightmeasurements. For the tak=ff measurements, the Graphic Level Recorderwas started with the tsk~ff and kept running until the airplane noisefaded into the background noise. Also concurrently the peak reading ofthe Sound Level Meter was recorded.Grounddistance ofside of thetests included ove~all sound-level measurements from a50 feet at singlesof 0°, 30°, 600, 90°, 120°, and 150° on eachairplane measured from the dead-ahead position. The analyzer..