3. EQUIPMENT AND INSTRUMENTS 



85 



used extensively to generate and detect un- 

 derwater sound. 



There are two principal methods of excit- 

 ing a transducer to provide a sound produc- 

 ing power. They are shock excitations and 

 excitation from a pulsed transmitter. A 

 transducer may be shock excited by dis- 

 charging into it electrical energy stored in a 

 capaciter or condenser. The transducer rings 

 and sends out a train of decaying acoustic 

 waves into the water. There are two prin- 

 cipal advantages to shock excitation, and, 

 when two transducers are used it is possible 

 to obtain soundings in very shoal water. 



A pulsed transmitter will excite a trans- 

 ducer by producing oscillation at the designed 

 frequency for a specific period of time. The 

 advantages of this type of excitation are 

 that there is control of the length of the 

 transmitted sound pulse and more power can 

 be delivered to the transducer. 



3-100 Transducer beam width. — The 



transducer is placed in the bottom of the 

 vessel with the radiating face toward the sea 

 bottom. The sound energy is directed toward 

 the bottom in the form of a beam which 

 may have a width varying from two degrees 

 to more than fifty degrees. The beam may 

 be symmetrical about its main axis or it may 

 be wider in one direction than another. The 

 beam width has an inverse relationship to 

 the area of the transducer face or diaphragm. 

 At a fixed frequency of operation the beam 

 becomes narrower, or sharper, as the size of 

 the transducer face increases ; or for a fixed 

 transducer size, the beam sharpens as the 

 frequency is increased. 



The width of the transducer beam has an 

 important effect on the accuracy and ap- 

 pearance of the recorded echo. When the 

 same power is used, the energy directed to- 

 wards the bottom will increase as the width 

 of beam is decreased, and, at the same fre- 

 quency, the energy in the echo will be in- 

 creased. The smaller beam also reduces the 

 adverse effects of noises arriving from direc- 

 tions other than from the direction of the 

 bottom. In order to obtain the greatest ac- 

 curacy in echo sounding the beam should be 

 extremely narrow. Then the echo comes from 



a very small area on the bottom and side 

 echoes from slopes and other irregularities 

 are reduced to a minimum. However, there 

 are certain practical aspects of the operation 

 which govern the selection of beam width 

 to be used. 



For example, it is necessary to use low 

 frequency sound waves in deep water, but 

 it is not practical to install in the hull a 

 transducer large enough to produce a narrow 

 beam at low frequencies because of the 

 limited space between the frames of a ship 

 which, on Coast and Geodetic Survey ships, 

 varies from about 20 inches to 27 inches. A 

 special narrow beam transducer with an 

 intermediate frequency is being used experi- 

 mentally for deep sea sounding. The trans- 

 ducer is mounted in a blister on the outside 

 of the hull and at the end of a shaft extended 

 through a universal joint in the hull. The 

 unit is stabilized in a vertical position by a 

 hydraulic system so that the beam is always 

 directed toward the bottom regardless of the 

 ship's motion. More accurate delineation of 

 bottom slopes is possible through partial 

 elimination of hyperbolic curves recorded on 

 the fathogram by side echoes when the wide 

 beam transducer is used. 



Unless a stabilizing system is used nar- 

 row beaming can be troublesome when 

 sounding in deep water and rough seas. As 

 the vessel rolls the sound is beamed in a 

 slanted line, and while the echo may not be 

 lost, it will not be a true depth. 



3-101 Attenuation of acoustic signals. — 



After the sound wave leaves the transducer it 

 is continually subjected to losses in strength 

 and is often quite weak when the echo re- 

 turns to the transducer. The sound wave 

 spreads out or radiates as it travels to the 

 bottom and again as it returns as an echo. 

 This causes a gradual and continuous loss 

 of energy. Some of the energy is absorbed 

 in the water by conversion to heat. This is 

 similar to friction losses in mechanical sys- 

 tems. Absorption losses increase as the fre- 

 quency of the signals is increased. These 

 losses are important when considering the 

 design of echo sounders intended to operate 

 at a high frequency, but for practical pur- 



