Low frequency, log-periodic acoustic array

Claim 1

A method of determining the sequential dipole spacing of a plurality of hydrophone dipole pairs about an array acoustic axis so as to form a low frequency, log-periodic acoustic line array wherein the acoustic beam pattern formed by each independent dipole pair has a quasi-constant directionality over a frequency band and the assembled array covers at least a decade range of frequencies through juxtaposition of contiguous frequency bands, said method comprising the steps of: selecting an operational frequency range; determining the spacing (2d n ) of the innermost dipole pair using the following relationship 2 d.sub.n =(c/f) (A) wherein d n is the distance of each dipole element from the array acoustic axis, c is the speed of sound in seawater at the operational frequency, temperature and pressure the array will experience and f is the maximum desired operational frequency; selecting a minimum null depth commensurate with desired directionality descrimination; selecting a (d/λ) max for the ideal null from a graph wherein d equals 2d n and represents the distance between dipole pairs and λ is the wavelength of the associated acoustic frequency; selecting a (d/λ) min from said graph for said minimum null depth; solving the following relationship for τ ##EQU8## wherein τ is an arbitrary constant associated with the log-period; solving the following relationship for d n-1 ##EQU9## using τ from (B) supra; repeating (C) supra iteratively substituting the previous d n-1 result for d n in (C) and solving for new d n-1's until 2d n , 2d n-1 , etc. spacing for a frequency decade has been described; and arranging said hydrophones in accordance with said 2d n , 2d n-1 , etc. spacing.

Claim 2

A method of determining the sequential dipole spacing of a plurality of hydrophone dipole pairs about an array acoustic axis so as to form a low frequency, log-periodic acoustic line array wherein the acoustic beam pattern formed by each independent dipole pair has a quasi-constant directionality over a frequency band and the assembled array covers at least a decade range of frequencies through juxtaposition of contiguous frequency bands, said method comprising the steps of: selecting an operational frequency range; determining the spacing (2d n ) of the innermost dipole pair using the following relationship 2 d.sub.n =(c/f) (A) wherein d n is the distance of each dipole element from the array acoustic axis, c is the speed of sound in seawater at the operational frequency, temperature and pressure the array will experience and f is the maximum desired operational frequency; selecting a minimum null depth commensurate with desired directionality descrimination; selecting a (d/λ) max for the ideal null from a graph wherein d equals 2d n and represents the distance between dipole pairs and λ is the wavelength of the associated acoustic frequency; selecting a (d/λ) min from said graph for said minimum null depth; solving the following relationship for τ ##EQU8## wherein τ is an arbitrary constant associated with the log-period; solving the following relationship for d n-1 ##EQU9## using τ from (B) supra; repeating (C) supra iteratively substituting the previous d n-1 result for d n in (C) and solving for new d n-1's until 2d n , 2d n-1 , etc. spacing for a frequency decade has been described; and arranging said hydrophones in accordance with said 2d n , 2d n-1 , etc. spacing.