274 



MEASUREMENTS 



shown in Fig. 11.18. Curve A is recommended for measurements at the 

 lower levels and curve B for measurements around 70 db above the 

 threshold. For very loud sounds (80 to 100 db), a flat characteristic 

 should be used. 



The noise meter may be used for noise analysis in offices, factories, 

 restaurants, etc. In these measurements a large number of observations 

 should be made in various positions. The noise meter may also be used to 

 measure the transmission or attenuation by walls, floor, ceilings and doors. 

 See Sees. 11.10 and 12.2M. 



FREQUENCY IN CYCLES PER SECOND 



Fig. 11.18. Recommended characteristics for sound level meters. (American Standard 



Association.) 



The noise meter is also a useful tool in work on the quieting of machinery. 

 Since the radiation pattern of machinery noise is very complex a large num- 

 ber of measurements should be made in various directions relative to some 

 axis of the system. For these investigations a frequency analyzer of the 

 heterodyne type. See Sec. 11. 3C, is a useful adjunct for determining the 

 nature of the noise. For routine tests in manufacturing it is customary to 

 establish passable limits together with fixed geometrical configurations and 

 procedures. For routine tests it is absolutely necessary that the calibra- 

 tion remain correct within a decibel, i.e., zfc-g- decibel. 



11.8. Measurement of Reverberation Time. — The reverberation time 

 for a given frequency is the time required for the average sound energy 

 density, initially in a steady state, to decrease after the source is stopped 

 to one millionth of its initial value. The unit is the second. 



Many systems ^^ have been developed for the measurement of rever- 



2^ Olson and Massa, " Applied Acoustics," 2nd Ed., P. Blakiston's Son and Co., 

 Philadelphia, 1939. 



