An acoustic method that can be used in air has the potential to allow for a fast and accurate characterization of objects in air. Nevertheless, it is difficult to identify acoustic signals from small objects clearly because of environmental noise and the scattering of sound on the object surface. Therefore, a sensing system that enables the measurement of small objects in air must be developed. In this study, we performed the localization of small objects of size comparable to the sound wavelength using an M-sequence signal and the phase information of received signals in a noisy indoor environment. Using the M-sequence signal, we are able to improve the SN ratio and to measure in a stable manner the reflected waves that cannot be detected using a conventional impulse. The arrival direction information was used to extract signals reflected by targets from unwanted signals of the floor or ceiling. Using an M-sequence signal and the arrival direction information, the position detection of small objects in the indoor environment was enabled.
Acoustic sensing in the air might serve as an effective technique for acquiring information about an object, such as its distance, shape, and surface configuration. Nevertheless, it is difficult to identify acoustic waves clearly because many mirror reflections or environmental noises exist in the air. In addition, acoustic measurement requires a longer time than measurements using lasers or electromagnetic waves because sound propagates much more slowly than light. For this study, we attempted highly precise position measurement using an M-sequence signal in an indoor environment. The degrees of M-sequence were 7–14; the transmitting frequency was 25 kHz. A signal reflected from various angles was received by moving the receiving unit, which contained three microphones. Measurement results with the M-sequence signal showed that the signal-to-noise ratio (S/N) improved 20–40 dB compared with a burst wave. Furthermore, the distance resolution improved when the M-sequence degree was changed. The distance and the angle of multiple objects installed indoors or in walls were detectable as a result of construction of a two-dimensional plane based on received signals.
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