174 Lecture 10 
would be necessary to produce in this way a complete picture of the sound field 
in a vertical plane along the midline of the 20-ft steel tank. Such a large number 
of records would take a long time to produce and many disturbing changes could 
possibly occur during that time, making the resulting very complicated picture 
untrustworthy. From such considerations as these, the incentive has arisen to 
find a method which will givea complete and permanent picture of the sound field 
in the water in the shortest possible time. As a first step in this direction ex- 
periments were made to obtain a picture of the sound distribution on the bottom, 
in the hope that a satisfactory technique forthis might give a lead to its applica- 
tion in midwater. 
After trying various chemical methods using gelatine-coated plates soaked in 
dyes and adding bleach to the water in the tank, a much simpler and more effec- 
tive method was ultimately employed. In this method, and working in the small 
concrete tank, a false bottom of plate glass or metal sheet is used. This is first 
sprayed with a thin coat of "water paint" (e.g., distemper, walpamur, etc.) and 
allowed to dry, but not so long as to set or become bone-dry. The glass plate is 
then placed in the required position in the water and exposed for a few minutes, 
sometimes less, to the sound. A quartz transmitter of frequency 250 kcps is 
Fig. 10.12. Sound pattern on a hard 
sloping bottom. 
