2. Description of the Apparatus
The apparatus is composed of two parts: a sound generator and a parabolic reflector. The external appearance of the part is shown in Fig. 1a. An explanation of the operation of the sound generator will be made with the help of the schematic diagram, Fig. 2a. It consists of a combustion chamber and a resonant tube coupled together, plus a water jacket which surrounds the entire unit. The combustion chamber is shown at the left end of Fig. 2a. Oxygen at a pressure of 6 atmospheres is introduced into the combustion chamber through an axially located tube having a 6 mm orifice. Methane is introduced at the same pressure through a second tube located near the side of the chamber. This gas is made to pass through a set of vanes which causes it to assume a whirling motion before it reaches the combustion region near the orifice of the oxygen tube. The arrangement of the vanes is illustrated in Figure 2b.
The rate of combustion is made to vary by the use of a quarter wave-length resonant tube acoustically coupled to the combustion chamber. The resonant tube has a small conical flare. The exact rate of flare has no significant effect upon the action of the device as long as its valve is small. The tube acts as if the right end is free and the left end blocked so that the fundamental resonance of the pipe is such that its length is equal to a quarter of a wave-length. A standing wave within the pipe produces fluctuations in pressure within the combustion chamber, which fluctuations will cause the rate of combustion to vary at the same frequency. The periodic release of thermal energy serves to maintain the system in oscillation. In order to increase the thermo-dynamical efficiency of the unit, both the combustion chamber and the resonant tube are surrounded by a pocket through which water is circulated, thus cooling the combustion products as they pass through the combustion chamber. An approximate equal-pressure contour of the sound field produced by the generator above is shown in Figure 3. It is obvious that most of the sound is propagated radially from the sound generator, that is to say, a “disk” of sound is produced. The absence of propagation in the axial direction is caused by the well-known phenomenon of the scattering of sound by hot columns of gases. The purpose of the original project was to produce a beam of sound—not a “disk” of sound. In order to produce a beam, the sound generator is mounted along the axis of a parabolic mirror with the open end of the resonant tube located in the focal plane. When so mounted, the contour of the sound field is changed from the disk of Figure 3 to the conical beam of Figure 4. This beam is neither narrow (as shown by the 30o spread between the first minima) nor highly directive (as shown by the 3 to 1 ratio of amplitudes between the main beam and the first side lobe). Several models using the above principles were constructed which varied in operating frequency from 400 to 1000 cps. The most powerful unit built operated at 800 cps and used the larger of the parabolic mirrors shown in Figure 1b, the diameter of which was 320 cm. This unit was able to produce a pressure of 1000 microbars on the axis of the main beam 60 meters from the generator. The development thus culminated in a powerful source of sound radiating over 100 watts of acoustical energy.
Remarks
The device failed in its original objective, as might have been expected, for it was concluded that it would be of no military value at ranges greater than 100 meters. The project is of importance because it furnishes concrete evidence of the failure of an idea after proposed as a military weapon. Furthermore, this method of producing intense audio frequency sounds in air may be of some scientific interest.
Prepared by: H. M. TRENT, Technician.
