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Borgman (1969) has developed a technique to estimate higher 

 order directional Fourier coefficients which does not require the 

 subtraction of signals from adjacent pressure gauges. If the 

 distance and angle between a pair of pressure gauges in an array is D 

 and A , respectively, then the cross-spectral density, normalized by 

 the unidirectional spectral density S (6) can be expressed as: 



c'+iq' = J (kD) + 2 Jl (a cos ng + b sin nB)iT(i)"j (kD) (F.l) 

 n=l 



for some frequency G . J^ (argument) and J^ (argument) are Bessel 

 functions. In terms of real and imaginary parts, Eq. F.l can be 

 written: 



