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| Jun 11, 2010 4:16 pm |
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What is a Large Phased Array and What are its Advantages? |
Richard Weisenberger
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A large phased array is a method of increasing the output of a radiating system, either RF or acoustical, by virtue of having multiple sources radiate additively in phase. In the case where the array is as large as ½ wavelength, additional gain can be realized, as it increases the directivity index as well. Broadcast antennas make use of a phased array to increase their effective radiated power by limiting their vertical coverage. This is how it’s done in RF applications. The same principles apply to acoustical applications as well.
The new generation of electronic warning sirens, such as the Federal Signal’s Modulator series or Whelen’s WPS series use multiple stacked 360 degree radial horns to increase their dB rating over a 360 degree horizontal plane. By limiting the vertical dispersion, higher SPLs can be obtained for a given amount of acoustical power by increasing the directivity index. An actual increase in in acoustical power is achieved by using multiple drivers on each of the horns of the array. In the case of electronic sirens the actual acoustical power is limited to about 1 kW.
Phased arrays are not limited to electronic systems. Sirens have made use of this principle for decades, as the multiple ports you find in most sirens all open and close at the same time and thus form a phased array with the output of each port adding to the siren’s total output. Sentry Siren has even gone as far as vertically stacking two identical arrays above each other to not only add to the output, but further increase it by increasing the directivity index on their mechanical sirens. As far as I’m aware, Sentry is the only manufacturer of mechanical sirens to do this.
Prior to the toroidal whistle, whistles were basically nothing more than cylindrical cavity resonators with a relatively small radiating mouth. The output was determined by the size and scale of the whistle, as it was basically an omnidirectional point source. The toroidal whistle is not a cavity resonator, but a ring source that resembles a large number of nearly 30 such whistles arranged within a single ring or toroid. As the entire periphery radiates in phase, it is in fact a large virtual phased array having the output of about 30 conventional steam whistles all of the same pitch. As the total slit or driving area is far less than the slit area of the equivalent number of typical cylindrical whistles, not only is the output increased, but also the efficiency. The efficiency of the whistle is further increased by making use of a slit width that will provide the required flow rate at the least possible pressure.
The efficiency of a toroidal whistle can be increased by a further 25% by inverting the toroid and blowing it from the inner edge as opposed to the outer edge. Horn loading the virtual array increases the directivity index by 13 dB, thus further increasing the unit’s output by 10 dB above the noninverted version with a directivity index of only 3 dB, all while using 25% less drive power.
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