REDUCTION OF AIRPLANE NOISE AND VIBRATION 627 



been developed. It is in effect a rectifying and recording oscillograph 

 whose stylus is capable of traveling at various speeds, the highest being 

 such as to indicate in one second a difference of level of 840 db. 



To reduce airplane noise within the passengers' and pilot's compart- 

 ments, it is necessary to provide sound absorbing material as well as 

 sound insulation. If there were no absorption within the cabin, the 

 sound reduction would be zero, no matter how efficient the insulation, 

 as the insulation would in this case only serve to delay the building up 

 of the sound inside to the same intensity as outside. Laboratory 

 equipment suitable for the study of absorbing materials and the 

 measurement of their coefficients is therefore a very necessary adjunct. 



Finally, mechanical vibration of audible rates to which various parts 

 of a ship respond must be carefully studied. For this purpose, a so- 

 called vibrometer has been perfected. With it, data are obtainable 

 indicating the extent to which noise is transmitted into the cabin 

 through the fuselage structure as compared to that coming through 

 the air. 



