T 



Reduction of Airplane Noise and Vibration* 



By C. J. SPAIN, D. P. LOYE and E. W. TEMPLINf 

 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 charac- 

 teristics 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 identi- 

 fied and measured. 



It is also desirable to be able to explore surfaces as to the extent to 

 which they radiate noise. A microphone attachment has therefore 

 been developed which quickly measures the characteristics of various 

 interior surfaces. As a result, it has been found possible to improve 

 the efficiency of distribution of the sound absorbing material, increasing 

 its weight in certain locations and reducing it in others, thereby both 

 lowering the noise level in the cabin and decreasing the total weight of 

 acoustic treatment. 



In order to measure the noise reduction provided by the cabin walls, 

 another device known as a high-speed automatic level recorder has 



* Digest of a paper presented before the Los Angeles (California) Section of the 

 American Society of Mechanical Engineers, May 27, 1936. Published in full in 

 Transactions A.S.M.E., Vol. 58, No. 6, pp. 423-431, August 1936. 



t The authors of this paper are engaged in technical work with the Electrical 

 Research Products, Inc., a division of the Bell Telephone System. 



626 



