178 Lecture 10 
With this new technique numerous records have been made in a study of 
sound propagation in shallow water. With either the mode or ray theories of 
propagation, where interference between direct and reflected sound is the main 
feature, the important influences which must be studied are the depth of the water, 
the wavelength of the sound, and the ratio of these two quantities; the depth of the 
source of sound; and the nature of the bottom. Other factors which have already 
been mentioned, such as temperature effects and state of sea surface and bot- 
tom, must of course also be considered, but for the present it will be suffi- 
cient to deal with these main considerations. In the earlier experiments which 
were described at a meeting of the Acoustical Society of America at Chicago in 
November 1958 [9], records were made using the sheet steel bottom of the model 
tank as representative of a flat, acoustically hard, reflecting bottom, and when 
covered with sheet rubber, as an acoustically absorbent or relatively poor re- 
flecting bottom. The records obtained, particularly those on the hard steel bot- 
tom, were very complicated and it was thought at the time that this was partly 
due to the fact that the steel bottom was not sufficiently flat, irregularities in 
some parts amounting to as much as 2or 3 mm, i.e., comparable with the wave- 
lengths of the sound. Since then the bottom has been covered with plate glass 
", in. thick of good quality, carefully leveled by ebonite wedges around the 
edges and made parallel to the water surface by the use of a number of metal 
cones accurately turned to have heights equal to the water depths required (viz., 
2, 1, °4, and '/ in.), the tip of each cone just touching the underside of the water 
surface. Care was taken to ensure that the space between the plate glass and the 
steel bottom was water-filled and free from air bubbles. Before making records 
the water was well stirred, by dragging a long "comb" through it, to ensure 
isothermal conditions. 
Scan records have been made covering a wide variety of conditions affecting 
the propagation of sound in shallow water. As in the point-by-point technique, 
it was found in the early stages ofthe investigation that there was a very marked 
contrast in the sound fields according to the nature of the bottom (i.e., acoustical- 
ly hard and a good reflector like steel or rock, or a poor reflector like rubber or 
mud and sand). Consequently, when other factors influencing propagation have 
been examined, e.g., depth of water, wavelength (frequency) of sound, directional 
properties of transmitter, depth of transmitter, and so on, these have been con- 
sidered in relation to the nature of the bottom, reflecting or absorbent. The "pic- 
ture" records of sound fields to which reference is made later therefore generally 
relate to the variable factor considered as it applies to the two bottom types 
just mentioned. It will be necessary therefore to refer to the same records 
in different connections. 
Most of the illustrations of picture records of sound fields were made with 
a point (omidirectional) transmitter, but a few are reproduced to show the 
markedly different characteristics when a directional transmitter is used. The 
scanning receiver is a point in all cases. Records have been made showing 
longitudinal vertical cross sections of the sound fields extending to the full length 
of the model tank, and of transverse vertical sections at a series of ranges 
from the transmitter. A simple method has been found useful for the intensity 
calibration of records by the introduction of a series of known db steps in the 
transmission voltage or in the circuit of the receiver. 
