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Acoustic metamaterials have recently become of interest for their ability to attenuate sound by breaking the massdensity law. In this paper, acoustic metamaterials based on Helmholtz resonators and capable of attenuating sound up to 30 dB are fabricated for sound absorption applications in the smallscale. The proposed metamaterials are subwavelength at a factor of / with respect to the lateral dimension of the units. The directional response due to the position of the acoustic source on the sound attenuation provided by the metamaterial is investigated by controlling the location of a loudspeaker with a robot arm. To enhance and broaden the absorption bands, structural modifications are added such that overtones are tuned to selected frequencies, and membranes are included at the base of the resonators. This design is made possible by innovative 3D printing techniques based on stereolithography and on the use of specific UV-curable resins. These results show that these designs could be used for sound control in small-scale electroacoustic devices and sensors.

Rayleigh surface acoustic waves (SAWs), generated on piezoelectric substrates, can interact with liquids to generate fast streaming flows. Although studied extensively, mainly phenomenologically, the effect of the SAW frequency on streaming in fluids in constrained volumes is not fully understood, resulting in sub-optimal correlations between models and experimental observations. Using microfluidic structures to reproducibly define the fluid volume, we use recent advances modeling the body force generated by SAWs to develop a deeper understanding of the effect of acoustic frequency on the magnitude of streaming flows. We implement this as a new predictive tool using a finite element model of fluid motion to establish optimized conditions for streaming. The model is corroborated experimentally over a range of different acoustic excitation frequencies enabling us to validate a design tool, linking microfluidic channel dimensions with frequencies and streaming efficiencies. We show that in typical microfluidic chambers, the length and height of the chamber are critical in determining the optimum frequency, with smaller geometries requiring higher frequencies

Piezoelectric micromachined ultrasonic transducers (PMUTs) are used to receive and transmit ultrasonic signals in industrial and biomedical applications. This type of transducer can be miniaturized and integrated with electronic systems since each element is small and the power requirements are low. The bandwidth of the PMUT may be narrow in some conventional designs; however, it is possible to apply modified structures to enhance this. This paper presents a methodology for improving the bandwidth of air-coupled PMUTs without sensitivity loss by connecting a number of resonating pipes of various lengths to a cavity. A prototype piezoelectric diaphragm ultrasonic transducer is presented to prove the theory. This novel device was fabricated by additive manufacturing (3-D printing), and consists of a polyvinylidene fluoride thin film over a stereolithography designed backplate. The backplate design is inspired by a pipe organ musical instrument, where the resonant frequency (pitch) of each pipe is mainly determined by its length. The -6-dB bandwidth of the "pipe organ" air-coupled transducer is 55.7% and 58.5% in transmitting and receiving modes, respectively, which is ~5 times wider than a custom-built standard device.

The agricultural pest, Homalodisca vitripennis , relies on vibrational communication through plants for species identification, location, and courtship. Their vibrational signal exhibits a dominant frequency between 80 and 120 Hz, with higher frequency, lower intensity harmonics occurring approximately every 100 Hz. However, previous research revealed that not all harmonics are recorded in every signal. Therefore, how the female H. vitripennis vibrational signal changes as it travels through the plant was investigated. Results confirmed that transmission was a bending wave, with decreased signal intensity for increasing distance from the source; moreover, at distances of 50 cm, higher frequencies traveled faster than lower frequencies, suggesting that dispersion of H. vitripennis signal components may enable signaling partners to encode distance. Finally, H. vitripennis generates no detectable airborne signal (pressure wave), yet their low vibrational frequency components are detectable in neighboring plants as a result of leaf-to-air-to-leaf propagation. For instance, with isolated key female signal frequencies, 100 Hz was detected at a 10 cm gap between leaves, whereas 600 Hz was detectable only with a 0.1 cm gap. Together, these results highlight the complexity of vibration propagation in plants and suggest the possibility of the animals using the harmonic content to determine distance to the signaling H. vitripennis source. Electronic supplementary material The online version of this article (10.1007/s00359-019-01366-w) contains supplementary material, which is available to authorized users.