13-7] 



ACOUSTIC NOISE 



705 



turbojets and rockets is created by the turbulence of combustion and 

 exhaust. Even higher levels of acoustic noise are generated when after- 

 burners are used on turbojet engines. Fig. 13-1 i is an example of the sound 



155 

 150 



:§145 



i 140 

 o 



o 135 



LlI 



•-" 130 

 125 



120 



50 75 100 150 200 300 400 600 800 1200 1600 2400 3200 



100 150 200 300 400 600 800 1200 1600 2400 3200 4800 6400 



OCTAVE BANDS, cps 



Fig. 13-11 Typical Sound Spectra for Operating Jet Aircraft. "The Scream of a 



Jet in a Six-Inch Test Cell" by R. H. Jacobson, Research Engineer, Armour 



Research Foundation, Chicago, 111. 



levels that can be expected from the operation of jet aircraft. (The measure 

 of sound level is defined below.) In propeller-driven aircraft, the major 

 source of noise is usually the propeller. 



In-flight conditions do not produce the maximum levels of power-plant 

 noise. The highest levels are generated during engine run-up and acceler- 

 ation of the engine for takeoff and climb. Tests have indicated that the 

 magnitude of the sound pressure caused by the power plant will not be 

 greater than the magnitude of sound created by the aerodynamic effects 

 for an in-flight condition. 



The aerodynamic effects on the level of acoustic noise are not critical for 

 aircraft flight speeds in the subsonic range. However, high magnitudes of 

 sound pressure in the supersonic speed range are created by the air passing 

 over the external aircraft surfaces. The magnitude of sound pressure 

 generated by the aerodynamic effect is established primarily by the air- 

 speed. The effects of ambient pressure and temperature at various altitudes 

 and the actual profile shape and aircraft size are secondary effects on the 

 magnitude of sound pressure. 



Sound pressure is specified in terms of decibels. The sound pressure level 

 in db is defined as 20 X logio of the ratio of the induced sound pressure to 



