An acoustic method in air has the potential to allow for the fast and accurate characterization of objects in air. Nevertheless, it is difficult to identify acoustic signals clearly because of environmental noises and the scattering of signals on the object surface. Therefore, it is required to establish a sensing system, which enables measurements in air. In this paper, we present the acoustic imaging system using a simultaneous transmission of coded acoustic signals. We use preferred pair M-sequence signals as coded acoustic signals. The M-sequence signals enabled the improvement in signal-to-noise (SN) ratio and the stable measurement of reflected waves that cannot be detected using a conventional impulse. A preferred pair is a combination of M-sequences for which the cross-correlation only shows 3 different small values. Multiple transmissions from different sources and small aperture receiving array realize equivalent large aperture acoustic imaging system. A simultaneous transmission of M-sequence signals enables acoustic imaging with high resolution only by one transmission. Small targets could be detected in the image which was generated from the simultaneous transmission of coded signals.
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