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HYDROGRAPHIC MANUAL 



poses may be disregarded for low frequen- 

 cies. Scattering losses occur when the sound 

 waves are diverted from their original direc- 

 tion of travel. This may be caused by a 

 discontinuity in the water such as turbu- 

 lence, aeration, changes in water density, 

 or by solid matter in suspension in the 

 water. 



When the sound wave reaches the bottom, 

 several things happen to it. Part of the 

 sound is reflected as an echo and returned to 

 the transducer. If the bottom is very rough 

 or if the sound wave strikes it at an angle, 

 some of the wave will be reflected away from 

 the transducer and be lost. Part of the sound 

 enters the bottom and may be absorbed or 

 reflected from deeper layers of the bottom 

 material. 



3-102 Frequency versus depth. — Deep 

 water echo sounders almost universally use 

 low frequencies or those less than 15 kilo- 

 cycles. Absorption losses are least at these 

 frequencies. The beams are wide, often as 

 much as 50 degrees (25 degrees each side of 

 center). Such wide beams are detrimental 

 to the accuracy of soundings in areas of 

 steep slopes and irregular bottom. Side 

 echoes from slopes may cover the echoes from 

 the bottom of a valley or trench. Higher 

 frequencies with resultant narrow beams are 

 desirable, but because of the high rate of 

 attenuation of such signals they are not 

 usable in great depths. Power supplied to 

 the transducer cannot be increased without 

 limit to overcome attenuation as there is a 

 limit to the driving power of a transducer. 

 When this limit is exceeded cavitation takes 

 place and decreases the power of the gen- 

 erated sound. 



Medium frequencies 15 to 50 kilocycles, 

 are used in most of the medium depth echo 

 sounders which are generally designed to 

 operate in depths less than 300 fathoms. In 

 this range the transducers are quite small, 

 the maximum dimension not exceeding 8 

 inches. However, the higher frequency re- 

 sults in a comparatively narrow beam which 

 affords more accurate definition of the 

 bottom. 



High frequency echo sounders are char- 



acterized by small transducers and narrow 

 beam widths. The short-wave length and 

 narrow beam afford excellent detailed bot- 

 tom definition, however, the high absorption 

 losses restrict use of these sounders to maxi- 

 mum depths of about 300 feet. Most of the 

 high frequency sound is reflected from the 

 top of the sea bottom and there is very little 

 reflection from lower formations. These 

 equipments are often used in conjunction 

 with dredging operations. 



3-103 The 808 transducer.— The 808 echo 

 sounder employs two identical transducers, 

 one for sending and one for receiving sound 

 waves. They are shock excited magnetostric- 

 tion transducers composed of many lamina- 

 tions of thin nickel sheets. The wire used 

 as the coupling coil between the mechanical 

 and electrical parts of the unit is wound 

 through holes in the stack. When assembled, 

 these laminations make a rectangular block 

 6I/2 X 4 X 31/2 inches. One of the large 

 faces of the block radiates sound waves or 

 receives the echoed sound. This face oscil- 

 lates at approximately 20.5 kilocycles per sec- 

 ond, when excited, or is set into oscillation 

 at this frequency by the returning signal. 

 When mounted for sounding this transducer 

 has a beam which is 80 degrees athwartship 

 and 40 degrees fore and aft. The long di- 

 mension of the transducer is in the fore and 

 aft line, where it is less sensitive to rolling 

 of the vessel. It has the disadvantage of 

 picking up unwanted side echoes. 



3-104 EDO-255 transducer.— The EDO- 

 255 echo sounder employs a Barium Titanate 

 acoustic unit and is excited from a pulsed 

 transmitter which generates 37.5 kilocycles 

 per second oscillation for about one milli- 

 second. The acoustic units are in the form 

 of thin walled cylinders, two coaxial in line, 

 which radiate sound waves radially from 

 their outer surfaces. This radial radiation 

 is horizontal until it is reflected from the 

 inner surface of a cone in which the acoustic 

 units are coaxial. The sound is then directed 

 downward towards the bottom. This entire 

 unit is housed in a plastic housing filled with 

 castor oil. One or two transducers can be 



