A. B. Wood 163 
second) between the bottom and surface of the water. Transducers could also be 
rotated at a constant angular speed if required for the purpose of recording 
their directional characteristics. In some of the earlier experiments, use was 
made of a small concrete tank as a more realistic approach to solid rock than 
is the case with the bottom of the steel tank. The dimensions of this tank were 
38 by 33 by 4 in., the solid concrete bottom being 24 in. thick. It was used main- 
ly in studies of temperature gradients and of variation of water depth, and also 
served in a comparison with the steel bottom of the large tank. 
As had been anticipated, the sides and ends of the steel tank were found to be 
good sound reflectors. Using directional pulse transmissions at 1 Mcps, a regular 
succession of end-to-end reflections was observed. Various methods were tried 
to prevent these reflections, and the one ultimately used was very effective. This 
method employed sheets of thin rubber covered with "spikes" about %% in. high 
and spaced about 0.3 in. apart in an equilateral triangle arrangement. The ma- 
terial, shown in Fig. 10.3, is a commercial product used in the manufacture of 
batting gloves, soap mats, etc. These spiked rubber mats, 15 in. long and 4\/, in. 
wide, were used with the spikes vertical in piles equal to the depth of water so as 
to screen the sides and ends of the tank. In this way a reduction of echo strength 
of about 25 to 30 dbwas obtained. The mats are easily removable for cleaning and 
remain constant in acoustic properties for long periods. 
Both directional and omnidirectional (point) transducers have been used with 
quartz, barium titanate, or PZT as the piezoelectric material. The directional 
transducers, varying in diameter from 1.0 to 2.4 cm and in thickness according 
to the frequency required, produced primary beams of semiangles ranging be- 
tween 5° and 30°, approximately, according to diameter and frequency. These 
directional transducers, although having large diameters as compared with the 
scale we are considering, served a useful purpose in providing known directional 
characteristics of transmission in certain propagation tests which will be men- 
tioned later. An approximation to an omnidirectional transmitter required that 
the diameter ofthe active face should not appreciably exceed one wavelength of the 
transmitted sound, i.e., 1.5 mm at 1 Mcps or 3.0 mm at 0.5 Mcps. This was 
ultimately achieved to a sufficient approximation in the following way. A short 
length (say, about 4 in.) of a metal wire having a flat conical end, as shown in 
Fig. 10.3. Pile of spiked rubber mats used to suppress reflection from sides of steel tank. 
