Figure Al2 compares the acoustic intensity at three different frequencies. The 
frequencies, as read on the hydrophone monitor, were set by adjusting the voltage to the 
drive motor while the piston throw remained at maximum. 
Figure Al3 is the usual spectrum at 15 Hz with two calibrate signals superimposed. 
The calibrate signals are intended to represent the monitor hydrophone output when it is 
exposed to a sound pressure level of 174 dB re | wPaat 1 m. It can be seen that the 15 Hz 
calibrate level is | dB lower than the 19.8 Hz level. The crew explained that this was due to 
the diminishing response of the measurement equipment at lower frequencies. The crew 
established the source level of the HAW-15 projector at an apparent value of 167.5 dB re 
] wPa at 1 m- This was based on a digital readout in dB of the amplified monitor hydro- 
phone output compared to the same readout when the 15 Hz calibrate voltage replaced the 
hydrophone voltage. The crew stated that they had used a hydrophone sensitivity of -203.5 
dB re | Vat | wPa in their calculations. This sensitivity was based upon a reciprocity calibra- 
tion at | kHz. The same calibrations gave a sensitivity of -206 dB re 1 Vat 1 wPa at 10 kHz. 
The crew further stated that they had no way to verify the monitor hydrophone sensitivity 
at 15 Hz. 
SOURCE LEVEL CONCLUSIONS. The HAW-15 transducer radiated a constant 
source level at all test depths and for an ]8-hour period at 500 ft during tests at Lake Pend 
Oreille. After the tests the full piston-throw amplitude was measured and found to be the 
same as before the Transdec tests in April. The calculated sound pressure level expected 
to be radiated at 15 Hz and at the measured displacement was 176.88 dB re 1 Pa at I m.a 
little more than | acoustic watt of radiated power. The details of the calculations are in- 
cluded as appendix B. 
<< 
“Windage”’ Loss 
MEASUREMENTS. One of the goals of the test program was to gain insight into 
the power loss due to the drag of the interior gas (in this case nitrogen) on the various com- 
ponents. Moving components include the motor armature, the flywheel, the linkages lead- 
ing to the piston faces, and perhaps the pistons themselves. Stationary components which 
have their effects by retarding the flow of gas caused by the moving parts include the motor 
stator and housing, the flywheel housing, and to a lesser degree the projector case and 
structural parts interfacing the gas. 
Figure Al7 shows the rotor drive motor input power vs depth curve resulting from 
the following four point measurements: 
A — 104.5 W at 25 ft 
B — 114.0 W at 100 ft 
C — 129.3 W at 250 ft 
D — 148.2 W at S00 ft. 
WINDAGE LOSS CONCLUSIONS. In figure A17 the input power vs depth was found 
to be approximately a straight line. This is to be expected since windage loss is proportional 
to gas density, which is, in turn, proportional to pressure, hence depth. (There is an addi- 
tional small increase in density with depth due to the cooler water at greater depths.) In 
order to separate the windage loss power from the other losses (bearing loss, motor losses 
and acoustically radiated power), the measured (gross) power loss curve was extrapolated 
from point A to the -45.5 ft point labeled E. This hypothetical point, calculated by 
37 
3 
SS. rearrewewwcmboiaE SaeTs 
4 
