180 Lecture 10 
10.4.1. Nature of Bottom 
Reference was made in Section 10.2.3 above to the use on the model scale 
of sheet steel or plate glass as equivalent to solid rock on the full scale—sup- 
ported by observations in a model tank having a more realistic thick concrete 
bottom. Similarly it was found that a sheet of rubber 1 in. thick covering the 
bottom of the tank had an effect comparable to a layer of fine sand. In what fol- 
lows, therefore, steel, concrete, and plate glass bottoms are regarded as 
equivalent to rock and as good reflectors of the incident sound in the frequency 
range under consideration, while a rubber sheet covering these surfaces cor- 
responds to a layer of mud or sand under full-scale conditions. 
When the bottom is acoustically hard (equivalent to rock), and the source 
is a point, the average scan line of a record consists of, say, 10 or 12 small 
dots indicating sound-pressure maxima. This confirms the observations made by 
the oscillographic amplitude-recording techniques described in Section 10.2 
above. When the bottom is steel sheet (44-in.), the sound picture as a whole is 
very complex (see Fig. 10.15), particularly when the depth-to-wavelength ratio 
is large, e.g., around 20 to 1.. If, however, the bottom is plate glass carefully 
leveled parallel to the water surface, there is more evidence of regularity in the 
pattern, a characteristic V pattern being noticeable. There is considerable indica - 
tion in these hard-bottom records of the propagation of the higher modes, or of 
many reflections between surface and bottom if we regard the indications in the 
light of the ray theory. Transverse records at ranges of 1, 2, 3, and4m, in 
water 2 in. deep at a frequency of 440 kcps with a point transmitter and bottom 
of steel are shown in Fig. 10.16. These show clear evidence of a stratified pat- 
tern in the sound field with many maxima in a scan. 
Fig. 10.17. Vertical scan 
between 0.6 and 4.3 m at 
various frequencies. Water 
is 2 in. deep and the bot- 
tom rubber-covered. 
‘ a es tide | 
