Technical Digests---Reduction of Airplane Noise and Vibration

HE three principal sources of airplane noise are the engine, the propeller, and air turbulence. Because of the impossibility of generating each kind of noise separately from the others, it has been necessary to develop what are in effect means for separating them and studying each one independently as they vary with speed of ship, speed of engine, and horsepower. In brief, the method that was used employs a series of tests under various flight conditions, the resulting data making it possible to solve a set of simultaneous equations. The paper gives numerous curves showing the variation with engine speed of the noise from these three sources. Fundamental to any consideration of airplane noise are the characteristics of the ear itself. For the most part, physiology does not cooperate with the acoustical engineer when he sets out to increase the comfort of air travel. In fact, it has been necessary to develop several specialized measuring devices in addition to the familiar type of noise meter. Among these may be mentioned particularly a frequency analyzer which permits of selecting either a 20-cycle or a 200-cycle band out of any portion of the noise spectrum from 40 to 11,000 cycles per second. With the 200-cycle band filter the general shape of the noise characteristic is measured, while with the 20-cycle filter the frequency components of engine, propeller and other noise are identified and measured. It is also desirable to be able to explore surfaces as to the extent to which they radiate noise.