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The optimum gains of a delay-and-sum microphone array for the near sound field, where the sound waves are spherical, have been investigated. The optimum gains give the highest signal-to-noise ratio in a sound field with background noise, and they give constant sensitivity to the sound source at every focal point. The optimum gains were theoretically derived as gi=C/ri, where ri represents the distance between the array focal point and the ith microphone element and C represents the normalizing parameter that keeps the sound source sensitivity constant, independent of the focal point. Parameter C is a function of ri and rc, where rc is the critical distance of the reverberant room. The results of computer simulation and indoor experiments (room volume: 83 m3, reverberation time: 0.2 s) showed a good array sensitivity distribution and power distribution estimation when each gain was optimized.
Nomura et al. (Tue,) studied this question.