Estimating radiation modes using wave-number filtering

Abstract

Acoustic radiation modes have been investigated in recent years as a means of estimating the far-field radiated acoustic power from a structure. The authors have been investigating two possible methods of estimating the amplitudes of the radiation modes associated with radiation from a baffled beam. The first method, based on using distributed sensors shaped according to the radiation modes, has been reported previously [S. D. Sommerfeldt and D. Lin, J. Acoust. Soc. Am. 105, 1088 (1999)]. This paper will report a second method, based on using distributed sensors designed to act as low-pass wave-number filters. The spatial filtering provided by the sensors allows the array to estimate the radiation mode amplitudes with only a small number of sensors, while still avoiding spatial aliasing problems. Numerical analysis indicates that the method is capable of providing good estimates of the radiated acoustic power for a normalized frequency range extending up to kl=10, using an array of only six sensors. Although the radiation mode shapes are frequency dependent, the method has demonstrated the ability to provide good modal estimates throughout this entire frequency range. Broadband estimates of the acoustic radiation are typically within 1 dB of the actual value.