Chapter 13 

 EXPERIMENTAL PROCEDURES 



THIS CHAPTER describes the principal methods 

 which have been used in the gathering and 

 analyzing of reverberation intensities. It will be seen 

 that the techniques for the study of reverberation 

 have been greatly improved since the first studies. A 

 great many systems have been conceived for such 

 studies; but only those which have actually been put 

 into operation and used extensively in the gathering 

 and processing of data will be discussed here. Refer- 

 ences to sources which give more detailed informa- 

 tion are included in the body of the chapter. 



The experimental determination of the frequency 

 characteristics of reverberation will be discussed in 

 Chapter 16. 



13.1 EQUIPMENT AND FIELD 



PROCEDURES 



Reverberation measurements have been made 

 under a wide variety of oceanographic conditions, 

 over many different types of bottoms, and at water 

 depths between 10 and 2,500 fathoms. The most 

 common projector depth has been 16 ft, but oc- 

 casionally various other projector depths have been 

 used. Most of these reverberation studies have been 

 made by UCDWR; quite recently, however, WHOI 

 has undertaken a reverberation program of its own. 

 Although differing in details, the field procedures 

 have in all cases been similar in broad outline. 



In the early measurements off San Diego, made 

 aboard the USS Jasper (PYcl3), this procedure was 

 followed. Upon arrival at the chosen location, the 

 main engines of the vessel were shut down, and the 

 Jasper was permitted to drift freely. The rate of 

 drift during the working day varied between 0.5 

 knot and 2 knots, depending on the wind velocity 

 and ocean currents. The transducer units with their 

 supporting frames were hoisted by means of an 

 electrically driven winch and boom, given the de- 

 sired orientations, swung over the rail, and lowered 

 into the water to the working depth. With the sound 



gear overside, the projector and hydrophone cables 

 were led through a doorway into the wardroom, and 

 attached to the respective pieces of equipment. 



In later studies, the depth to which the transducer 

 was to be lowered and the angle the transducer was 

 to make with the vertical were "built in" to the 

 equipment at the time of installation. Hoist train 

 mechanisms were provided, which lowered the trans- 

 ducer to the working depth from sea chests recessed 

 in the keel. The bearing of the transducer in the 

 horizontal plane was adjusted by means of a remote 

 control training system. In these later modifications, 

 the transducers were permanently wired to terminal 

 boards, from which they could be connected to the 

 regular electronic equipment or "sound stack," or to 

 specially constructed research stacks. 



After all connections are made, a sound pulse of 

 controllable duration is sent into the water by means 

 of a keying arrangement. As a result of this pulse, 

 scattered sound returns to the transducer and gener- 

 ates a voltage in the receiving circuit. This voltage, 

 after amplification, is recorded as reverberation. 

 Somewhat different methods are used by UCDWR 

 and WHOI for recording the reverberation. Systems 

 for measuring reverberation intensities are discussed 

 in detail below. 



13.1.1 Transducers and Electronic 

 Equipment 



Most reverberation measurements have been taken 

 at a frequency of 24 kc, which is the prescribed 

 frequency for most Navy echo-ranging gear. Several 

 types of transducer units have been employed at San 

 Diego. Most of the early data ' were obtained with a 

 pair of similar magnetostrictive units (QCH-3), one 

 used as a projector and the other as a receiver; some 

 of the data reported there were obtained with a 

 crystal transducer, the QB. The QB crystal unit 

 proved to be superior to the QCH-3 units for rever- 

 beration studies because of its higher power output 



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