A. B. Wood 181 
0.6M (1) 4.3M 
Fig. 10.18. Longitudinal 
vertical scan and three 
transverse vertical scans 
at l-, 2-, and 3-m range. 
Frequency is 450 kcps, the 
water 2 in. deep, and the 
bottom is rubber-covered, 
(3) RANGE 2M 
(4) RANGE 3M 
When the bottom is acoustically absorbent, rubber-covered (equivalent to mud 
and sand), and the source is an omnidirectional point, the picture of the sound 
field is entirely changed. There are now fewer maxima in each scan line—perhaps 
only two or three dots (or dashes). The over-all pattern is much simpler, but for 
depth-to-wavelength ratios around 20/1 is still somewhat complicated. Examples 
of such records are shown in Fig. 10.17. The effect of the sound-absorbent or 
poorly reflecting bottom is to reduce the number of surface-to-bottom reflections 
and some of the higher modes are absent. Figure 10.18 shows a longitudinal scan 
over a rubber bottom, and three transverse scans at ranges of 1,2, and 3 m. The 
reduced number of maxima is strikingly shown in these records also. 
Picture records of sound fields such as these serve to show the weakness of 
the point-by-point method where it was impracticable to obtain more than a small 
proportion of the data required to plot the complete sound field. By the scanning 
method the complete picture can be recorded in a matter of a few minutes. 
10.4.2, Depth of Water 
As already pointed out, the effect of varying the depth of the water must al- 
ways be considered in relation to the wavelength of the sound. The nature of the 
bottom plays an important part, andthe record obtained depends also on the depth 
of the transmitter. 
Many picture records have been obtained in which these factors have been 
varied. In Figs. 10.19, 10.20, and10.21 are shown three sets of records, the water 
