A. B. Wood 187 
10.4.3. Wavelength of Sound 
Assuming the velocity of sound inthe water is constant, variation of frequency 
will result in a corresponding variation of wavelength of the sound. Many of the 
earliest picture scan records [9] showed the effect of varying frequency, e.g. 
in steps of 100 kcps over a range 640 to 400 kcps. Such records, particularly 
those made over a rubber (sound-absorbent) bottom, showed a marked similarity 
in general appearance, but it was difficult to trace the transition from one 
frequency to the next in the series (see Fig. 10.17). When the frequency steps are 
smaller however, e.g., 400, 425, 450 kcps as in records shown in Fig. 10.26, 
the change in position on the record of certain clearly marked features can 
easily be observed. These changes are comparable to those of Section 10.4.2 
where the results of small depth changes were discussed. 
10.4.4. Temperature of Water 
The question of the temperature of the water, isothermal and with gradients, 
was discussed briefly in Section 10.2.5. Many bottom-to-surface records were 
made by the point-by-point oscillographic technique which showed that small 
temperature changes had the same effect as small changes of frequency. Thus, 
around 400 kcps a change in temperature of 3.5°C could be expected to produce 
the same effect as a change of frequency of 3.2 kcps (i.e., 1°C is approximately 
equivalent to a 1-kcps change of frequency at 400 kcps). Variation of tempera- 
ture in the water results in a change of velocity of propagation of the sound and, 
the frequency being assumed constant, is just another way of varying the wave- 
length. The frequency (wavelength) effects shown in Fig. 10.26 should therefore 
be reproducible by a sufficient change of temperature of the water. 
10.4.5. Directional Transmission 
In all the scan records mentioned hitherto, the transmitter has been of the 
omnidirectional type permitting the propagation of modes or rays in almost all 
directions. When the transmitter is directional, however, relatively few modes 
are propagated and the scan picture of the sound field along the model tank be- 
comes much simpler. A series of four such records, using a 2-cm-diameter 
transmitter at a frequency of 560 kcps (semiangle of primary beam, 9.3°) in 
water 5 cm deep, is shown in Fig. 10.27. In record 1 the bottom is plate glass 
and the directional transmitter is in midwater. It will be seen that this record 
resembles that of a point source transmitting over a rubber (mud) bottom (see 
for example Fig. 10.19) rather than that over plate glass (rock). In both cases, 
of course, the higher modes are suppressed, but in different ways. In records 
2, 3, and 4 the plate-glass bottom is rubber-covered, the directional transmitter 
being in midwater, near bottom and near surface, respectively. Changes in 
pattern due to these changes in depth of transmitter are apparent, but the gen- 
eral character of the record is much the same with all three cases, and indeed 
with that of record 1 when the bottom was plate glass. 
In Fig. 10.28 the directional characteristics of a transmitter are shown at 
1-m range (a) by rotating the transmitter, the point receiver scanning vertically 
at 1-m range, and (b) by rotating the transmitter, the receiver at 1-m being 
fixed in midwater. Method (b) is of course the conventional method of recording 
beam characteristics. The record in (b) represents the intensity variations 
