110 



USRL TEST STATIONS 



except for its staining and slightly poisonous nature. 

 However, a commercial product, "Nalco 2 1M," manu- 

 factured bv The National Aluminate Corporation, 

 was finally obtained and used quite successfully. This 

 substance comes in small briquettes which are placed 

 in a holder in the inlet water line and requires no at- 

 tention except replenishing. Its only disadvantage is 

 that it makes the water slightly milky, as do most 

 products of this nature. Its poisonous qualities are 

 negligible as long as the concentration is kept at 3 

 to 5 parts per million. 



The positioning equipment of the transducer (Fig- 

 ure 43) consists of two heavy carriages which can be 

 moved the length of the tank on two steel rails, 

 mounted lengthwise on top. Each carriage provides 

 for positioning the transducer by one screw parallel 

 to the long dimension of the tank and a second at 

 right angles. The vertical adjustment is made by four 

 screws driven at the same rate by a sprocket chain. 

 Rotation may be made about a vertical axis for a 

 full circle. 



In addition to these degrees of freedom, one car- 

 riage is provided with two more adjustments of the 

 transducer suspension. One permits the vertical angle 

 of the transducer to be changed and the other allows 

 it to be displaced with respect to the axis of rotation. 

 These arrangements are necessary to permit adjust- 

 ment of the direction of the acoustic axis of the trans- 

 ducer in the vertical plane lengthwise of the tank and 

 to permit rotation of the transducer about an axis 

 through its acoustic center. This same carriage has 

 motor-driven rotation about the vertical axis to fa- 

 cilitate the taking of directivity patterns. 



The transducer holders are designed to include 

 the cylindrical case of the preamplifier. Adapters are 

 provided for suspending the nonstandard transduc- 

 ers. The positioning equipment is very rugged in con- 

 struction in order to reduce vibration and other 

 extraneous motion to a minimum. This is necessary 

 because of the small wave lengths and extremely 

 sharp directivity patterns involved. In addition to 

 the two carriages on the guide rails, platforms are 

 available for suspending other equipment for more 

 complex measurements. 



For tests in the outdoor areas, two extra T rails are 

 available that can be suspended from the guide rails. 

 When these rails are fastened in position, the instru- 

 ment platforms can be mounted on them in the same 

 manner as on the top of the tank. Simpler rigging, 

 consisting of a plate with several cylinders attached, 



21 ABSORBERS 



Figure 44. Schematic arrangement of absorbers and trans- 

 ducers in high-frequency tank. 



is available for special tests. The cylinders fit the 

 standard transducers and the plate is of a size to fit 

 across the guides in the test area. 



Reverberation Control. There are a number of fac- 

 tors which influence the reverberation in a tank. 

 Among these are the size and nature of the walls and 

 the directivity pattern (or beam width) of the projec- 

 tor. It is fairly obvious that a narrow beam is much 

 easier to control than is a broad beam that spreads 

 out and strikes the walls of the tank close to the pro- 

 jector. 



Since at these high frequencies the beam widths are 

 relatively narrow, it was decided to place absorbing 

 units around and behind the hydrophone only, in 

 such a manner that the portion of the sound beam 

 that passes the instrument will strike the absorbers 

 and be reduced to a negligible intensity. The particu- 

 lar arrangement used is a cluster of 21 cylindrical 

 absorbers grouped as shown in Figure 44. The pro- 

 jector is beamed at the hydrophone from the opposite 

 end of the tank. The tank was made elliptical with 

 the thought that with the projector at one focal point, 

 any stray radiation would strike the walls and pass 

 through the other focal point. If the hydrophone 

 were then placed about 4 inches in front of the focus, 

 the stray radiation would have to pass through the 

 absorbers before striking it. However, it was found 

 in practice that this action contributed little to the 

 reduction of reverberation and that the projector 



