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• 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]

• Gradient acoustic-grating metamaterial (GAGM) can realize acoustic rainbow trapping. • Detection of weak signals based on the GAGM was proposed for the first time. • Weak signals submerged by noise can be recovered more easily in the GAGM. • The GAGM helps overcome the detection limit of conventional acoustic systems. Acoustic signals detection attracts considerable attention in many areas, such as structural healthy monitoring, biomedicine, communication and earthquake prediction. However, the useful signals are always very weak and overwhelmed by background noise. In this paper, we proposed a method based on the gradient acoustic-grating metamaterial (GAGM) to detect harmonic and periodic impulse signals for the first time. The analytic analysis indicates that the incident acoustic waves with various frequencies gradually slow down and eventually stop at different positions in the GAGM. In the numerical study, we show that acoustic rainbow trapping, realized by the GAGM, can spatially separate different frequency components accompanied by the local enhancement of their amplitudes. Moreover, in the experimental investigation, we demonstrate that the GAGM is capable of amplifying pressure amplitudes about 30 times even at different measured positions. With this acoustic system, the harmonic and periodic impulse signals submerged by background noise in the GAGM can be detected much more easily. All the results indicate that the GAGM can be regarded as a functional material or an enhanced acoustic sensing device to overcome the detection limit of conventional acoustic systems. This work opens up new vistas for weak signals detection of many areas. [ABSTRACT FROM AUTHOR]