Figure 8 gives results similar to those of figure 7 except that the air overpressure in 
the NRAP was maintained at about 5 psi (versus about 2 psi in the first test). No significant 
changes in performance Were detectable from this change of internal pressure. 
Figure 9 shows the spectral output when the NRAP was set for (a nominal) zero 
amplitude of piston motion The fundamental was down about 45 dB relative to that shown 
in figure 7. More accurate zeroing can readily be achieved by closed-loop methods, if desired. 
Figure 10 shows the output spectra when the NRAP is operated at frequencies of 12, 
1S,and 16 Hz. The maximum amplitudes vary precisely with the fourth power of frequency, 
as predicted by equation (5). 
LAKE PEND OREILLE TESTS 
DL Carson, Code 712, expressed interest in and provided funding for testing the 
NRAP ata depth of 500 feet at Lake Pend Oreille, Idaho. Therefore, TE Stixrud trucked 
the NRAP to that facility and conducted a series of tests there. The tests all went without 
difficulty. Stixrud’s 1-port of the tests is attached as appendix A, and details of the sound 
pressure level calculations are included as appendix B. 
PROJECTOR EFFICIENCIES 
While no great attempt has been made te measure or improve the NRAP efficiency, 
general observation of the voltage and current levels required to operate the rotor drive 
motor under various conditions shows that its efficiency is about | to 1.5 percent. 
EVALUATION AS A STANDARD SOURCE 
On 11 September 1980 the NRAP projector was calibrated at the Navy’s Acoustic 
Calibration Facility (headquartered at NRL/USRD, Orlando, Florida) to evaluate the NRAP 
as a standard source. The calculated NRAP source level was 170.61 dB re 1 wPa at 1 m under 
the test conditions pertaining, while the measured output level was 169.9 + 1.0 dB: hence 
the NRAP capability as a standard source was validated. 
THE FUTURE 
Since the NRAP design was originally conceived, modifications to the linkage system 
(appendix C) have made it possible to contro! the device as follows at suitable points in each 
cycle when the linkage system velocities momentarily come to zero: 
Turn the NRAP from zero to full-on and back again 
Reverse the phase of the output signal 
Switch the frequency from the fundamental to twice the fundamental and back 
again. 
The switching operations can be accomplished at the zero-velocity points without changing 
the flywheel velocity or jarring the other parts of the NRAP machinery. These features will 
make the NRAP suitable for such Navy missions as measuring sound speeds in the ocean, 
communicating acoustically between distant locations, and obtaining Doppler and other 
important information about the acoustic channels in the ocean. 
The cost of an NRAP unit, in production lots of 100 units, was estimated 
independently by JM Walton, GO Pickens, TE Stixrud, and the author. The cost estimates 
lay in the range from $5,000 to $10,QU0. i Sheen ianp cata 
15 
