EVOLUTION OF THE DESIGN CONCEPT 
The Nonresonant Acoustic Projector (NRAP) Project got its start with this author’s 
suggestion for a system called Friendly Acoustic Sources. On finding that there were no suit- 
able, inexpensive sources in the 5 to 50 Hz spectral region available for the Friendly Acoustic 
Sources concept, he was Jed to investigate first resonant then nonresonant sources of the me- 
chanical type. Resonant sources were abandoned because they operate on the principle of 
the resonating gas bubble wherein the resonant frequency of oscillation is given by the follow- 
ing relationship: * 
1 kYP’ Hes 
(Riess Re ; (1) 
27 RON Gp 
where 
f = resonant frequency in Hz 
R = linear dimension of bubble in cm 
k = number of order ] dependent on the configuration of the bubble 
(for example, k = 3 for a spherical bubble with R denoting the radius) 
y = ratio of specific heats of the gas at constant pressure and volume 
y . : fe) 
P = ambient ocean pressure in dynes/cm= 
p = density of gas in g/cm3. 
Equation (1) shows that a resonant source’s linear dimension, R, must be large — thus the 
source must be relatively expensive — when the design frequency is low and the depth of 
operation is great. Also, small excursions of the source in depth change the pressure, P’, 
as well as the source operating frequency. f. (Or if fis forced to remain constant, changes 
in P’ generally cause the source to operate off resonance, with corresponding large changes 
in the source’s radiated power level.) 
All these problems can be avoided by going to a nonresonant type of source. 
Therefore, the nonresonant acoustic projector design concept shown in figure | was for- 
mulated, and the NRAP project was sponsored by NOSC management.** 
During the initial phase of the design effort the author concluded that the approach 
shown in figure 1 would entail overly large aerodynamic windage losses. A suggestion by 
FR Abbott concerning the MK VI minesweeper source then led to a cam-type design (fig 
2). Further work showed, however, that this system would require an unduly expensive 
cam plus large, expensive cam followers. GO Pickens and JM Walton had independently 
suggested that a flat linkage system driven by an eccentric might do the job. However, all 
such systems apparently needed a sliding element, which would be energetically lossy as well 
“as somewhat unreliable in operation. LE McKinley then suggested that the Walschaert 
valve gear (for old-time steam locomotives) be looked into, and this quest resurrected the 
Baker modification of the Walschaert system. Finally, a modification of the Baker system 
in turn vecame the basis for the design that was used successfully in the project. 
*See, for example, NAVMAT P-9675, Physics of Sound in the Sea, p 462. 
**Other low-frequency nonresonant sources exist, but none combines all the advantages of this one so 
far as the author is aware. 
aE ACES Re 
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