Patent ID: 7409068

Claim:
A hearing instrument, comprising: a front microphone that generates a front microphone signal; a rear microphone that generates a rear microphone signal; a phase-shifting circuit that implements a frequency-dependent phase shift between the front microphone signal and the rear microphone signal to create a controlled loss in directional gain and maintain a predetermined maximum level of microphone self noise amplification (G N ) over a pre-determined frequency band, the frequency-dependent phase shift being configured to provide a maximum amount of directional gain given the predetermined maximum level of microphone self noise amplification (G N ); the frequency-dependent phase shift (θ LN ) including an inter-microphone phase shift (Δθ) in addition to a time-of-flight delay (θ C ), such that Δθ=θ LN −θ C ; and wherein the frequency-dependent phase shift (θ LN ) satisfies the following equation: θ LN = - ω ⁢ ⁢ d v - 2 ⁢ tan - 1 ⁡ [ x ⁢ ⁢ sin ⁡ ( ω ⁢ ⁢ d / v ) / ρ 1 - x ρ ⁢ cos ⁡ ( ω ⁢ ⁢ d / v ) ] , where ω is the radian frequency (2πf), d is the spacing between the front and rear microphones, v is the speed of sound, ρ = ⁢ sin ⁡ ( ω ⁢ ⁢ d / v ) ( ω ⁢ ⁢ d / v ) , and x is a function of the predetermined maximum level of microphone self noise amplification (G N ).