DEEP TRANSDUCER DESIGN 
by R. P, DELANEY 
The Martin Company 
Baltimore, Maryland 
ABSTRACT 
The problem of design of deep sea (10, 000 ft) 
transducers is a severe one particularly from 
the point of view of maintaining good efficiency 
without conventional pressure release materials. 
The design of the MBP-1 Transducer met 
this problem by pressure equalization. This 
projector has been operated at 10,000 ft deep 
with 50% efficiency at 2700 cps. Similar trans- 
ducers designed for lower frequencies have 
displayed similar performance. Design data 
and performance of these transducers are 
discussed. 
INTRODUCTION 
The title of my paper is somewhat mis- 
leading in that I intend to confine my remarks 
to a particular type of transducer, namely, 
configurations based on ceramic cylinders. 
Since 1958 The Martin Company has designed, 
built, and tested a number of projectors and 
hydrophones suitable for employment at great 
depth (below 5000 ft). I have selected 4 of the 
more successful configurations to describe 
today and I shall try to point out how and why 
they work in order to assist others in avoiding 
pitfalls attending this field. 
When faced with a new environment and new 
design problem, it is a common approach to 
take a familiar design suitable for another 
environment and seek to adapt or modify it for 
the new environment. This tendency probably 
accounts for our use of ceramic cylinders when 
faced with the problem of designing hydrophones 
and projectors for deep use, because for near 
surface applied ceramic cylinders are a very 
common transducer type. Operated in the 
fundamental circumferential mode as pro- 
jectors, the outside surface of the cylinder is 
commonly exposed to the medium by means of 
a rubber boot while the inside wall is pressure 
relieved with air or one of the common pressure 
release materials, celltite rubber or corprene. 
They are a simple design acoustically and 
mechanically and a very effective one. Hydro- 
phones of this design exhibit desirable broad 
band characteristics below resonance and 
projectors when using the stave construction 
methods of Mr. Green of NEL show excellent 
efficiencies. However, this rather pleasant 
situation is upset when the static pressure of 
the deep ocean environment is introduced. On 
a theoretical basis the wall thickness of 
ceramic cylinders can be increased to the part 
where an air backed hydrophone becomes 
25 
possible. We have never been able to build an 
air backed unit which survives long periods under 
pressure. We have had units last 6 weeks at 
9000 psi only to implode. We therefore turned 
our attention toward the development of con- 
figurations which are pressure equalized using 
oil or water at ambient pressure on the back or 
inside of the cylinder to reduce the stress levels 
in the ceramic, and using one means or another 
to prevent the loss of sensitivity (about 15 db) 
which occurs when the inside is acoustically 
short circuited. 
HYDROPHONE DESIGNS 
I should now like to describe two hydrophone 
configurations which we have successfully tested 
and used. I would first like to acknowledge a 
debt of gratitude we owe to the Hudson Labora- 
tories who first developed the equalization method 
described below and who were most generous in 
their advice and assistance. Mr. Oberlin has 
discussed the Fixed Acoustic Buoy installed and 
the first hydrophone I shall describe was designed 
for the FAB. The active element is a PZT-5 
cylinder 2 in. tall by 1.5 in. OD and 0, 25 in. 
wall thickness. The inside of the cylinder is 
pressure equalized with castor oil which is 
connected to a reservoir outside the cylinder via 
a small orifice in the rigid end caps. Mr. Ted 
Madison of General Electric gave a paper in the 
Fall of 1960 at the Under Water Acoustic Sympo- 
sium on the theory of operation of such a device. 
It is based on Helmholtz Resonator theory where 
the orifice is transparent at frequencies below 
the resonance frequency of the orifice --chamber 
combination, so that the inside and outside are 
in static equilibrium; while at frequencies above 
resonance the orifice is opaque and the acoustic 
pressure will not be transmitted. In sucha 
transducer the main design problems are to make 
the cylinder large enough for the desired sen- 
sitively, design the orifice and chamber tc push 
the Helmholtz Resonance below the band of 
interest and yet have the cylinder small enough 
so that its lowest length resonance considering 
the mass loading of the end cap is above the 
upper end of the desired frequency band. I 
mention the length resonance because the mass 
loading effect of the end caps will often reduce 
the length resonance below the fundamental 
circumferential mode resonance. Slide 1 gives 
an idea of the FAB hydrophone configuration. 
Slide 2 is the receiving response of the FAB 
hydrophone. 
