into a position somewhere between are H and are G shown in the figure, and the amplitude 
of motion of the rocking cross arm and pistons increases to something less than what it was 
in figure 3. 
Note that both the frequency and the amplitude of piston motion can be adjusted 
independently or conjointly while the source is in full operation. Changing one without 
changing the other generally causes the output power of the projector to change — see equa- 
tion (5). 
When the piston amplitude is set to zero, the rotor drive motor can be started for 
any depth of operation of the projector, since the motor then has to overcome only the 
friction of the internal bearings. Once the rotor is turning, the piston amplitude can be ad- 
justed as desired, without difficulty. 
When the amplitude adjustment link has been set to a definite location, the pistons 
are coupled tightly to the rotor. Therefore, the frequency of the piston motion is determined 
only by the rotor turning rate. Also, the link amplitudes of motion are determined only by 
the rotor-linkage geome2try. Asa resuJt, both frequency and amplitude of the piston motion 
are independent of the pressure of the air inside the housing and the pressure of the ocean 
outside the housing. Therefore, changes in the operating depth of the projector do not 
change either the frequency or amplitude of piston motion. 
Consequently. the system can be designed to operate in a pressure compensated 
housing. A nominal overpressure such as 5 psi can be maintained within the housing, thus 
eliminating the need for heavy (hence expensive) housing walls. If a leak develops in the 
housing, t! 2verpressure inside prevents intrusion of ocean water into the interior. The 
overpress!ue also prevents bearing play or “backlash,” which would produce distortion of 
the output wave form and energy loss in the piston drive train. Additionally, the overpres- 
sure prevents reversal of the rolling seals during operation. 
Since the amplitude of piston motion is determined by the location of the amplitude 
adjustment link and the frequency of motion is determined by the rotor speed, the NRA 
projector becomes in effect a “‘standard source” whose acoustic output power level is con- 
trolled only by the projector’s operating parameters together with the intrinsic parameters 
(density and sound velocity) of the ocean medium. Since small excursions in the operating 
depth of the source de not influence the latter quantities to any great degree, the NRAP 
does not require complex or precise calibration with exterior devices cuch as calibrated 
hydrophones located at well-measured distances from the source, etc. 
Moreover, the acoustic intensities radiated at the fundamental as well as the higher 
harmonic frequencies can be calculated directly from the values of the linkage parameters 
used in the projector. 
Important in the concept of the NRAP is the flywheel. This unit is rigidly connected 
to the rotor and hence tc the pistons. Therefore, the high reactive power tiow into the 
ocean during half of each piston cycle is recovered by way of an equal reactive power flow 
back into the flywheel during the next half of each cycle. As a result, this source has higher 
efficiency than many types of low-frequency projectors. 
Because large peak forces exist in the linkage system under most conditions of 
operation, the bearing frictions are correspondingly large and all linkage elements must be 
sized to withstand the peak forces. 
For the purposes of this “‘proof of principle” project .he NRAP was designed to 
provide an output sound pressure level of 171 dB re 1 uPa at 1 m — about | W of acoustic 
power — at a frequency of 15 Hz. The diameters of the pistons were chosen to be 11.25 
