here is a valley in a directional distribution where the energy density is 

 less than half that at the lesser of the two adjacent distribution peaks. 

 This definition is subjective, and analysis results will change if the defini- 

 tion is changed. For all 1,046 cases in the present data set, the parameter 

 K was scanned and the number of unimodal (K = 1) , bimodal (K = 2) , trimodal 

 (K = 3), etc., cases were counted for each frequency. These numbers were then 

 converted to percentages of total cases. 



205. Results of this computation are shown in Figure 23. Along the 

 frequency axis the center of each bar is at the representative frequency f„ , 

 and the bar width is the resolution bandwidth df . For a given frequency, 

 the length of each shading pattern represents the percent of cases having the 

 number of modes associated with the shading pattern. 



206. Figure 23 indicates that virtually all of the directional dis- 

 tributions at 0.09 to 0.10 Hz (the tallest, black bar) are unimodal. The 

 percent of unimodal cases drops off on either side of this. At the highest 



Figure 23. Frequency-by- frequency distributions of 

 numbers of modes 



84 



