SmE REMARKS ON THE ACCELERATION RECORD 



The output of the SPLASHNIK is recorded as a filtered variable d-c 

 voltage proportional to the acceleration experienced by the system. 

 Several aspects of the SPLASHNIK output must be discussed before one can 

 safely proceed to computation of the end product, the wave height spectrum. 



The low-pass filter has already been mentioned. High-frequency 

 wave information above 1 cps cannot be recorded accurately because it is 

 distorted by the frequency response of the 3-foot float. In addition, 

 wave frequencies above 0.5 cps are usually of little concern to ship 

 motion studies but do contribute rather large accelerations. If the 

 sensitivity of a recording channel is adjusted to accept the highest 

 signal, then the contributions in the important lower frequency range 

 will be considerably smaller and may even be hardly discernible. Elimin- 

 ation of the higher frequency content serves to emphasize the important 

 wave components. The adjustable frequency cutoff in the low-pass filter 

 provides a choice for elimination of undesired information. 



Dorrestein^ points out that an error in the acceleration signal 

 results from the tilt of the raft on the side of a wave. He concludes 

 that the error is small, but being proportional to the square of the 

 slope of the raft, it has a d-c component which must be removed before 

 double integration. The SPLASHNIK is, of course, subject to the same 

 error. Even if the accelerometer were satisfactorily stabilized, the 

 low quality electronics (designed to keep cost down) still produces a d-c 

 drift in the acceleration record. However, our method of analysis requires 

 computation of the acceleration spectrum and algebraic operation on this 

 function to obtain the wave spectrum. Consequently, double integration 

 is not necessary and the need for a high- pass filter is eliminated. The 

 result of this is an acceleration spectrum showing energy out to zero 

 frequency, which is known not to exist. A "human filter" is applied at 

 this stage by arbitrarily cutting off the acceleration spectrum where it 



