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The detection and localization of weak sound sources are a topic of great interest in a wide range of applications, but generally require a pressure amplification and directivity acoustic system. Although acoustic technologies have been advanced in recent years, the realization of pressure amplification and directivity acoustic systems remains a slippery subject. Here, we propose a gradient acoustic metamaterial (GAM) coupled with space-coiling structure (GAM-SCS) for the detection and localization of weak sound sources. The GAM-SCS is evaluated both numerically and experimentally. Compared with the GAM without space-coiling structure, the GAM-SCS has a better pressure amplification performance which enables the sound signals to be pre-amplified. Meanwhile, the induced space-coiling structure, making use of subwavelength features, also enables the GAM-SCS to work in a lower frequency. Furthermore, the directivity of the GAM-SCS also shines lights on directional acoustic sensing to resist noise interference. We believe that this work opens up a new vista to realize the detection and localization of weak sound sources based on acoustic metamaterials. [ABSTRACT FROM AUTHOR]

• Phononic crystal (PC) point defects coupled with Helmholtz resonators is proposed. • Coupled resonator (CPCR) can realize acoustic enhancement and directional sensing. • The CPCR helps overcome the detection limit of conventional acoustic systems. • The CPCR can be used for sound source localization. The detection and localization of acoustic signals play a vital role in many areas, such as structural health monitoring, gas pipeline leakage detection and underwater acoustic communication. However, the realization of both high-sensitivity and high-directivity acoustic systems at the same time remains a slippery subject. Besides, the detection limit, namely the detection capability of minimal detectable pressure, of sensitivity and directivity still hinders the performance of ordinary acoustic sensors where weak signals or low signal to noise ratios (SNRs) exist. Here, we propose a structure that couples phononic crystal (PC) point defects with four-sided Helmholtz resonators (HRs) to realize acoustic enhancement and directional sensing simultaneously in a high energy density cavity. The proposed coupled PC resonator (CPCR) shows a much better acoustic enhancement performance than PC point defects or four-sided HRs individually. Meanwhile, the remarkable directional response of the CPCR ensures the success in directional sensing. Furthermore, both in numerical and experimental studies, we demonstrate that the CPCR can be regarded as an acoustic device or sensor to detect harmonic signals and Gaussian pulse signals effectively. This work breaks through the detection limit of conventional acoustic sensing systems and provides a new routine for the development of coupled acoustic sensing devices that are highly desirable in practical engineering applications. [ABSTRACT FROM AUTHOR]

• Graded reflection acoustic grating metamaterial(GRAGM) broaden the working bandwidth. • The GRAGM enables to realize a deep subwavelength acoustic sensing. • The strong directional response of the GRAGM is able to localize sound sources. • The GRAGM can be regarded as an acoustic antenna. Acoustic grating metamaterials (AGMs) with exotic enhancement property and remarkable directional response are promising candidates to advance the metamaterial-based acoustic sensing. In this paper, a graded reflection AGM (GRAGM) is proposed to broaden the working bandwidth and reduce the device volume. In contrast with conventional periodic AGM, GRAGM with graded slit width can be viewed as a series of combined Fabry-Perot-like (FP-like) resonators. As a result, the working band is broadened twice by combining multiple resonant bands. The added reflection plate is capable of reducing the working frequency of GRAGM by manipulating the reflection phase at the boundaries with the maximum variation over than 280 Hz, which enables the GRAGM to work in a deep subwavelength scale. Moreover, the strong directional response of the GRAGM is able to localize sound sources. These exotic properties promise a number of potential applications, such as sound source localization, condition monitoring, and remote whispering. [ABSTRACT FROM AUTHOR]