26 Lecture 1 
DR. SCHOFIELD: No investigations into this possibility have been made. 
PROFESSOR R.E.H. RASMUSSEN referred to a formula quoted by Dr. 
Schofield giving the maximum intensity of radiation (/, in w per square centi- 
meter) from a barium titanate transducer interms of the depth (H) of immersion 
in feet, when cavitation is the limiting restriction as !=0.3 (H/32 + 1.8)?. 
DR. SCHOFIELD: This formula should only be usedasa "rule-of-thumb" indi- 
cation of the cavitation level. The presence of dissolved gases and impurity 
nuclei could cause large deviations from the levels predicted by this formula. 
MR. J.S.M. RUSBY commented that the pattern of cavitation observed on the 
radiating face of the 10-kcps longitudinal projector mentioned in the lecture 
could result from the regional lines of high displacement arising from flexural 
vibrations, and so lowering the cavitation threshold as was observed. He also 
enlarged upon the suggestion concerning planar arrays of projectors in which it 
was stated that a method of overcoming the pressure-release problem in deep 
water was to make the rear ends of the individual elements inefficient as radia- 
tors compared with their front faces. Mr. Rusby said that this could be achieved 
by reducing the cross-sectional areas of the rear ends and separating them; in 
this way, the value of the total radiation impedance as seen by the rear of the 
array would be lowered. 
DR. SCHOFIELD: It is agreed that the cavitation erosion marks could be 
caused by "flapping" of the radiation heads. NRE have been considering methods 
of mismatching the tails of elements to water as a means of reducing back 
radiation. 
DR. H. A.J. RYNJA following upon a point raised in the discussion of low- 
frequency transducers on the use of magnetostrictive devices, spoke of his own 
experience with a scroll of 1m diameter of a nickel—four-percent cobalt ma- 
terial forming a resonator of 1.5 kcps with a tuned bandwidth of one octave, 
With two such rings mounted coaxially and driven with 90° phase difference and 
with pressure-release material on the outside surface of the rings, he had suc- 
ceeded in obtaining quite sufficient power in unidirectional radiation. Dr. Rynja 
agreed, however, with Dr. Schofield on the difficulties of manufacturing the 
scrolls. 
REFERENCES 
1. F.A. Fischer, Fundamentals of Electroacoustics (Interscience Publishers Inc., New York, 1955). 
2. W. P. Mason, Electromechanical Transducers and Wave Filters (D. Van NostrandCo. Ltd., 1942, 1948). 
3. D.W. Dye, "The Piezoelectric Quartz Resonator and Its Equivalent Electrical Circuit,” Proc. Phys. 
Soc., Vol. 38, 339-457 (1926), 
4, W. P. Mason, "An Electromechanical Representation of a Piezoelectric Crystal Used as a Transducer," 
Proc, Inst. Radio Eng., Vol. 23, 1252-1263 (i935). 
5. W. P. Mason, Piezoelectric Crystals and Their Applications to Ultrasonics (D. Van Nostrand Co. Ltd., 
1950). 
6. P. Vigoureux and C. F. Booth, Quartz Vibrators (H M. Stationery Office, 1950). 
7. H. Hecht, Die Elektroakustischen Wandler (Johann Ambrosius Barth, Leipzig, 1954), 
8. F. V. Hunt, Electroacoustics (Harvard Univ. Press, 1954). 
