176 Lecture 10 
Fig. 10.13. Sound distribution in vertical cross sections over a range of one to two meters 
from the projector. (a) Each scan is made with the transducer at a different depth, the top is 
at the surface, and each succeeding scan is made at an interval of depth of 0.1 in. (b) Each 
scan is made at constant depth on a line parallel to the axis of the projector, the center scan 
is on the axis, the top scan is 10cm on one side, the next-to-bottom scan is 10 cm on the 
opposite side, and the bottom scan is 30 cm from the axis. 
sentially a scanning technique which gives a complete picture on a single 
record of the sound distribution in a vertical cross section of the water either 
along the full length of the tank or across it at selected ranges. In this method, 
one of the transducers, either the transmitter orthe receiver, is mounted on the 
scanning mechanism shown in Fig. 10.14. A metal plate to which it is attached 
is driven up and down vertically between smooth guides by means of a rotating 
arm of adjustable length so that its double amplitude of motion in a vertical direc- 
tion is equal to the depth of the water. The repetition rate at which the trans- 
ducer moves between the surface and the bottom is under control, an average 
speed being approximately three times per second in each direction. The motion 
is adjusted so that the transducer point does not break the water surface nor 
strike the bottom of the tank. Experiments have shown that the result is the same 
whether the "scanning" transducer is the transmitter orthe receiver, or whether 
the scanning mechanism is mounted on the fixed platform near one end of the tank 
or on the moving platform (trolley) which travels along the rails at the side of 
the tank. A chain passing over a sprocket wheel connects the rotating arm to the 
sliding plate which supports the transducer, and the shaft of the sprocket wheel 
drives the brush contact of a potentiometer which is supplied by a 3-v battery. 
The voltage onthe brushis amplified and applied to the Y plates of the cathode ray 
