Ci. Malme, DEVELOPMENT OF A HIGH TARGET STRENGTH PASSIVE ACOUSTIC REFLECTOR FOR LOW-FREQUENCY SOLAR APPLICATIONS, IEEE journal of oceanic engineering, 19(3), 1994, pp. 438-448
The objective of this study was development of broadband, high target
strength, passive acoustic reflectors. These reflectors would provide
convenient, low-cost targets for low-frequency sonar trials and fleet
exercises. The primary development goals included controlled, stable m
onostatic and bistatic reflectivity, and adjustable deployment depth d
own to 90 m (300 ft). The development process used both computer model
s and scaled physical models to find the most effective configuration
capable of meeting the project goals. Review of acoustic scattering th
eory showed that acoustically soft reflectors (bubbly liquids or air-f
illed spheres and cylinders) provide higher target strength values tha
n similarly sized hard reflectors (metal spheres or corner reflectors)
. Air-filled cylinders were found to provide the highest target streng
th values and widest useful bandwidth for a given reflector volume. Th
is is a result of the tube resonance that occurs at ka = 0.02 at the d
epth range of interest where k is the acoustic wavenumber and a is the
tube radius. Air-filled cylinder target strength at resonance is abou
t equal to its target strength at ka = 1.0 with a deviation less than
+/-4 dB between these values. Guided by these study results, a prototy
pe reflector was assembled using a 16-m length of air-filled gum rubbe
r tubing, 5.7 cm in diameter (3.2-cm ID). A SCUBA regulator attached t
o a small air tank maintained tubing inflation. Te-st results at a dep
th of 90 m showed an effective target strength of 12 dB at 250 Hz with
a deviation less than +/-2 dB from 200 to 400 Hz. This is equivalent
to the target strength of a 16-m-diameter perfectly reflecting sphere-
an object with a volume about 52000 times that of the prototype cylind
er reflector.