under a particular calibration curve, purporting to represent a 

 5-cps filter, to be 137 units . The height of the calibration 

 curve is measured as 15.2 units and a frequency band of 10 cps is 

 equivalent to 16 scale divisions on the analysis frequency scale. 

 Equation [6] shows that the effective filter bandwidth is 5.63 cps 

 and states that the effective filter bandwidth is equivalent to the 

 width of a rectangle whose height is the height of the calibration 

 curve and whose area is the area under the calibration curve. In 

 principle, the effective bandwidth of each filter should be constant 

 but variations of several percent indicate instability of the crystalline 

 structure of the filter which, it is hoped, will be corrected with air 

 conditioning of the space in which the system is installed. 



To complete the universalized ordinate scale, in terms of the 

 seakeeping event being studied, it is necessary to introduce the 

 instrument calibration. The signal being analyzed is a fluctuating 

 voltage which may represent heave, pitch, etc. Before each run, 

 a calibration should be applied to the tape which relates particular voltage 

 settings to particular transducer signals. The value of the calibration 

 squared (C~, ) is all that is required to complete the ordinate scale. 



If the squared calibration is taken together with Equations [5] 



and [6] , the resultant spectral density ordinate $ (cD ) associated with 



the peak of the calibration curve is 



C^ V^ 2C^ V L 

 ^ , . T p T ms r-T 



*K> = Tf = -Ak ^^^ 



a 

 which has the dimensions of seakeeping units (degrees, feet, etc.) 



22 



