A. B. Wood 175 
used, emitting sound energy at the rate ofabout 5 w (estimated). In a short time, 
with much less sound intensity than required by the chemical methods, a pattern 
appears on the surface of the coated glass or metal plate. The most striking 
results were noticed when one end of the plate rested on the concrete bottom of 
the tank while the other, remote edge was arranged to be fairly close to the 
water surface. In this arrangement with a hard sloping bottom, the water forms 
a wedge above the glass plate. A typical bottom picture obtained in this way is 
shown in Fig. 10.12. In addition to the primary and secondary beams from the 
transmitter, parallel interference lines can be seen at right angles to the axis 
of the primary beam. The more closely spaced equidistant interference lines 
are due to stationary waves a half-wavelength apart. Attempts made to obtain 
patterns of the longitudinal (horizontal and vertical) cross sections of the sound 
beam were not very successful. With certain illuminations of the water surface, 
however, it was observed that a stationary pattern of parallel lines was visible 
on the surface of the water. Asthe angle of the wedge formed by the bottom plate 
and the surface of the water is increased, these interference lines on the surface 
get closer together and when the angle of tilt is smali the lines are widely 
spaced. The interference lines occur at intervals corresponding to half-wave 
increases of water depth and are analogous to the case of Newton's interference 
rings inoptics. The observations ona model scale just described apply, of course, 
to the full-scale case in shallow water where the sea bed is sloping. A variable 
slope in two dimensions could result in the acoustical equivalent of an oil-film 
optical interference pattern. 
10.4, “PICTURE” RECORDS OF SOUND, DISTRIBUTION IN VERTICAL CROSS SECTIONS OF 
“OPEN” WATER — SCANNING 
In what follows a description will be given of a method of recording a "picture" 
of the sound field in the body of free undisturbed water, with sound of relatively 
low intensity. As a first step, with this aim in view, a series of cathode-ray 
oscillographic records were made, using point transducers (as described in Sec- 
tion 10.1 above) to show the distribution of pressure amplitude (a) in a vertical 
plane and (b) ina horizontal plane along the midline of the tank between the ranges 
1 and 2 m (km, full scale) from a point transmitter at a fixed depth. In the first 
series (a) the point receiver travelled on the midline of the tank on parallel 
courses displaced vertically at 0.1-in. intervals from near surface to near 
bottom. These records laid closely together are shown in Fig. 10.13. It will be 
seen that progressive changes with depth are revealed. A characteristic "V" or 
"Diamond" structure of the sound field is indicated. (This should be compared 
with later records made by the method to be described below.) In the second 
series (b) the parallel courses were in the same horizontal plane in mid- 
water, these courses being spaced horizontally 1 cm apart up and to + 10 and 
-—30 cm on each side of the midline. The resulting series of records shows no 
significant changes from one to another, the sound amplitude on any of the 
courses being the same as on any other. 
As a means of delineating a picture ofthe sound field in a cross section of the 
water, the following method has given very encouraging results and is much 
simpler and more efiective than the one just described. The new method is es- 
