114 



One problem in which we have a common interest is the ambient noise of 

 the ocean: its directional properties, its spectrum, and its relation to measur- 

 able properties such as wind force or wave height. Since animal sounds are 

 sometimes a predominant factor in the noise background, it is important to know 

 what animals are responsible for what sounds, as well as the seasonal and geo- 

 graphic distributions of these noise sources. I agree wholeheartedly with Dr. 

 Mersey's feelings concerning the value of making magnetic tape recordings dur- 

 ing the conduct of passive listening experiments. The possibility of obtaining 

 an objective classification of animal sounds through the use of sound spectro- 

 graph techniques is extremely interesting and if successful will remove much 

 speculation from this field. 



Dr. Hersey noted that past measurements of ambient noise for the most 

 part have been made with non-directional hydrophones and that the state of 

 knowledge in this field can now be most readily advanced by the use of direction- 

 al systems. This problem has been examined from a theoretical standpoint by 

 R.J. Urick (1951) under the assumption that deep-water noise is of surface ori- 

 gin and that the radiation from an element of surface area is distributed in angle 

 according to some power, n, of the cosine of the angle with the vertical. Deter- 

 mination of n, and its dependence on frequency and environmental factors is of 

 considerable interest. 



At the Underwater Sound Laboratory we have undertaken to build an 

 electrically steered array to study this problem. The array consisting of 

 thirty-six (36) individual hydrophone elements is shown in Figure 1. The ele- 

 ment spacing is slightly less than one-half wavelength at a frequency of 8,000 

 cps, giving a total length of about ten (10) feet. Each element of the hydrophone 

 is brought through its own pair of leads to a pre-amplifier and compensator 

 which permits the array to be steered electrically from end-fire toward the sur- 

 face, through the horizontal to end-fire toward the bottom. Compensation or 

 steering of the array is accomplished through the insertion of time delay net- 

 works between the adjacent elements of the array so that only signals from a 

 particular direction will add in phase. When no time delay is inserted, the ar- 

 ray has its maximum response for signals (or noises) originating in a plane per- 

 pendicular to the axis of the line of elements. When a time delay is inserted, 

 equivalent to that required for the acoustic wave to travel in the water a distance 

 equal to the element spacing, the array will have its maximum response along 

 the line of the array. For time delays smaller than this, the line of maximum 

 response generates a cone about the axis of the line array and makes an angle 

 9 with that axis, 



where cos 6 = f" c. 



c s velocity of sound in water 

 d = element spacing 

 > z time delay 



The method of continuous compensation or steering employed with this 

 array has been described by Holt (1947) and will not be repeated here. 



At the half wavelength frequency this array has a directivity index which 

 is independent of the angle to which it is steered. At frequencies lower than 

 that required for half wavelength spacing, the directivity index in the "end-fire" 

 compensated position is approximately 3 db higher than it is for the "broadside" 

 position. 



