172 Lecture 10 
change of frequency (wavelength) of transmission. This has been confirmed by 
making a series of bottom-to-surface records at a mean frequency of 410 kcps 
and varying by about 1.6 kcps on either side of the mean. The small but definite 
variation in appearance of the records is equivalent to that which is produced by 
a total isothermal temperature variation of 3.5°C. This has been verified by 
experiments in the small concrete tank. In case (b), when temperature gradients 
are established in the water layer, the first problem is to produce the required 
temperature gradient. This is simply achieved by inserting an electric immersion 
heater in the water for the appropriate time, quick stirring, then allowing the 
water to stratify for a few minutes; the warm water rises to the surface and 
leaves the colder water in contact with the concrete bottom of the tank. A cali- 
brated point thermistor mounted near the point transducer is raised from bot- 
tom to surface with the transducer, and a two-beam cathode ray oscillograph 
records bottom-to-surface sound amplitude with the corresponding temperature 
variation (and its calibration marks). Many such records (see Fig. 10.10) have 
shown that, although changes in the sound distribution occur due to small tem- 
perature gradients, the records are still much the same in general character as 
those due to correspondingly small isothermal temperature changes. 
10.2.6. Depth of Water 
Reference has been made earlier to constancy tests of bottom-to-surface 
records using point transducers, when it was shown that changes could occur 
overnight when the experimental setup was fixed. After suspecting temperature 
efiects, small frequency changes, etc., it was eventually demonstrated that a 
small change of the order of a fraction of a millimeter in a total water depth of 
about 50 mm could sometimes result in a change of about 30 db in the received 
signal, both transmitter and receiver having been kept at a constant depth and 
distance apart, and the wavelength of the sound constant at near 3.5 mm. In 
pursuance of such observations, a series of records was made in the small con- 
crete tank to discover the effect of varying the water depth over a much wider 
range, e.g., from 0 to 5cm maximum depth. In these experiments the bottom was 
varied from "soft" to "hard" and "absorbent." These three cases were repre- 
sented by "air" (in the form of rubber mousse cemented to the upper side of a 
metal-plate sinker), steel, and rubber sheet. In all cases two point transducers 
are placed a fixed distance apart on the bottom of the tank, and water is allowed 
to fill the tank slowly (without making waves or ripples) while a logarithmic 
record of received sound intensity as a function of water depth is being made. 
Three typical records are shown in Fig. 10.11. Record (a), where the bottom is 
rubber-covered, indicates a fairly regular series of maxima and minima differ- 
ing by 20 or 30 db in intensity. It will be noted that at certain critical depths the 
sound intensity changes very rapidly. Records of this type, the bottom being 
rubber (sound-absorbent and equivalent to mud and sand) have been made at 
various frequencies, the spacing of the intensity "crevasses" becoming progres - 
sively smaller as the sound frequency increases. Records (b) and (c) in Fig. 10.11 
were made ina similar manner whenthe bottom was soft or hard. Deep crevasses 
in intensity level are again observed, occurring muchmore frequently than when 
the bottom is rubber-covered and sound-absorbent* Such rapid variations of 
*Note: the frequency is 430 kcps in records (a), (b), and (c). 
