The FAB hydrophone is sufficient to its task 
and showed no change in sensitivity with pres- 
sure to 9000 psi and has since performed effec - 
tively over the last 9 months. However, its 
sensitivity was not all one would wish and its 
upper frequency limit was too low for many 
applications. We therefore embarked ona 
hydrophone development program utilizing the 
same basic equalization system but seeking 
ways to increase the sensitivity and to drive 
down the Helmholtz Resonance and to increase 
the basic cylinder resonances. The design of 
one of the resulting hydrophones is shown in 
slide 3 and whose response is shown in slide 4. 
As can be seen, this hydrophone is quite an 
improvement on the FAB unit. The use ofa 
very small orifice and a smaller cavity reduced 
the frequency of the Helmholtz Resonance. The 
sensitivity of the unit was increased while the 
cylinder size was reduced by decreasing the 
wall thickness, 
by use of a shorter cylinder and aluminum end 
caps. We also feel we get some increase in 
sensitivity by the end caps acoustically coupling 
to the cylinder ends. 
PROJECTOR DESIGNS 
The projectors I shall discuss are of the 
pressure equalized segmented cylinder type. 
The MBP~-1 represents the first real experience 
with the design of an acoustic projector for a 
very high pressure ambient. Projector design 
is in general a more difficult problem than 
hydrophone design because of the necessity for 
good efficiency. This is especially true in the 
case of deep transducer design where the back 
radiation is a problem. In the case of the 
MBP -1 our approach was to take a transducer 
mechanism which is basically efficient (namely 
the large cylinder made up of staves polarized 
and energized in the circumferential direction) 
and accept the losses which the lack of pres- 
sure release on the inside of the cylinder 
causes. This was done because of the need 
for this transducer to series of experiments 
we carried out with the Naval Research Labora- 
tory under the direction of Mr. Benhannan. 
The cylinder was 20 in. OD, 9 in. high witha 
1 in. wall thickness; the inside was filled with 
castor oil, the top and bottom plates of the 
cavity were 1/4 in. steel. The unit is shown 
in slides 5 and 6. The response of this unit 
is shown in slide 7. The efficiency of this 
unit, about 60%, indicates that the back wave 
and the lack of pressure release causes us no 
significant problem. The calibration of this 
unit at 1000 psi shows no change from shallow 
water measurements and, at sea measure- 
ments at depths up to 12,000 ft showed no 
change in response within the accuracy of 
measurement. One of the MBP-1 units has 
been operated for more than 150 hours at 
depths exceeding 10,000 ft. At the end of this 
series of tests the unit was recalibrated and 
shows no sign of deterioration either acoustical 
or mechanical. We feel that the attainment of 
The upper resonance was raised 
27 
such high efficiency in a unit of such simple and 
reliable design whose performance is independent 
of depth is a significant step forward in trans- 
ducer design. 
The MBP -2 is an improved version of the 
proven techniques of the MBP-1 and is shown 
in slides 8 and 9. It was designed and built by 
The Martin Company under contract to the 
Bell Telephone Laboratories. As can be seen 
from the slide, it uses two rather short rings 
4 in. high 0.4 in. thick and 20 in. in diameter. 
These rings are contained in separate water- 
tight housings. The thinness of the rings has 
two main effects on the performance. It lowers 
the mechanical Q and decreases the resonance 
frequency since the mass loading effect of the 
medium is a more significant part of the 
vibrating system mass. These effects can be 
seen in slide 10. Even though the circumference 
is the same as that of the MBP-1 the frequency 
has dropped to 1900 cps and the mechanical Q 
is only two. The efficiency has dropped to 35% 
because of the power loading at the lower 
frequency. Stacking more than two rings should 
improve the loading and therefore, the efficiency 
appreciably. A single ring by itself has an 
efficiency of only 20%. Slide 11 shows the 
efficiency of the MBP~-2 as a function of fre- 
quency. The smooth response and low Q shown 
in slide 10 lead to a transducer which has a 
useful efficiency over a broad frequency band 
making it of real value as a general purpose 
sound projector. This is especially true when 
one considers that they can be stacked in the 
vertical to get higher efficiency, better power 
handling and some directivity. The calibration 
of this unit has been checked at 1000 psi with 
no change in characteristics and it is expected 
that it will be as insensitive to depth as was the 
MBP-1. 
CONCLUSIONS 
We have reviewed the design of a number 
of transducers which are suitable for operation 
at great depth. Two of these have operated at 
depth for appreciable time. The design of 
transducers for this environment presents 
problems new to the transducer designer none 
of these problems,however, are insuperable. 
By the application of known transducer theory 
and by careful testing under pressure, we feel 
that the problem deep transducer design can 
be placed on as firm a foundation as present 
day shallow transducer design. We have con- 
fined our remarks to a cylindrical ceramic type 
but there are several other configurations under 
development which fit other needs and appli- 
cations. 
