120 



D, and an adjustment in the equivalent SPSL N(f) would be 

 made. (It is assumed that while the random-noise level may 

 change somewhat, the spectrum shape of the random noise 

 signal does not. ) Ambient-noise spectrum levels are then 

 determined by a direct comparison with the random -noise 

 calibrate signal at each frequency. 



The use of this tone and random-noise calibration 

 method eliminates the need for time-consuming calibration 

 using many different frequencies to measure the frequency 

 response of the system, although such calibrations can be 

 made from time to time for comparison, using the external 

 calibrate signal input. 



i 1 1 — i — i — r~r 



110 



u 



z 



>- 



Q 



o 

 o 

 o 



100 - 



LU 



a. 90 



_i 

 </) 



D. 



z 80 



< 

 > 



° 70 



60 



1 1 1 1 — i — i — r~r 



-SPSL OF TONE 



1/3-OCTAVE BAND LEVEL 

 / AT 1000 HZ 



MOD 2 



'MOD 1 



J I I I I L_L 



J I I I I I L 



J I I I I I I ' 



10 



FREQUENCY (HZ) 



Figure 10. Equivalent sound-pressure spectrum level of random-noise 

 calibrate signal. 



16 



