Because of their superior directional properties, shotgun microphones are preferable choices for high-quality speech and audio recording in environments with intense ambient noise. However, their low- and middle-frequency directivities are usually not sufficiently high for practical usage. Alternatively, linear microphone arrays with unequally spaced elements enable high wideband directivity with a small number of microphones. A general procedure for designing such arrays steered at the endfire direction with unequally spaced elements is proposed for high-quality audio recording from 20 Hz to 16 kHz. A simulated annealing method is used to iteratively optimize the spatial distribution for microphones that do not have matched amplitude and phase. The challenge in the optimization process arises because large bandwidth requires large matrices, which produces large accumulation error. Therefore, the optimization process needs to be carefully regularized with the error being restricted to a relatively small tolerable level. The proposed method can produce microphone arrays with higher directivity than the corresponding shotgun microphones of the same length with comparable low self-noise level.
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