Generation of Custom Acoustic Harmonic Bursts from Spherical Helmholtz Resonators Using Q-Switched Nd:YAG Laser Induced Plasma

We reported a new method for generating a 4 to 25 ms high power acoustic harmonic bursts, reaching more than 110 dB Sound Pressure Level (SPL), from the spherical Helmholtz resonators. The method uses Q-switched Nd:YAG laser pulses (wavelength = 1064 nm, pulse width = 6 ns, and energy = 450 mJ) to induce plasma shocks inside an AISI 316L stainless steel cavity. The confined plasma shock produces an acoustic burst of temporal standing waves which are characterized by a wide harmonic bandwidth. The frequency response of the system depends on the geometry of the used Helmholtz resonator as well as the laser wavelength (with constant laser pulse duration and fluence). The experiments reveal the dependence of the odd/even harmonic on laser wavelength. This method is a prospective alternative for the dodecahedron loudspeakers, other sources in ISO and standard audio tests.

Scatter-free acceleration of particles by interaction with plasma shock waves in the interstellar medium

ABSTRACT Scatter-free acceleration is investigated for a test particle thrusted by a moving magnetized cloud in the presence of the uniform interstellar magnetic field. It is found that depending on the orientation of the background magnetic field, three different scenarios occur for the interacting particle. In some cases, the particle reflects into space with a negligible increase in energy. Otherwise, the particle is either trapped at the wavefront or is injected inside the cloud. The trapped particle moves with the cloud and gains energy through the magnetic trapping acceleration mechanism, which is already investigated in previous reports. The injected particle accelerates through a different mechanism, which is introduced in this paper as the spiral acceleration. In this mechanism, the particle moves in a spiral path and gains energy by the convective electric field of the cloud. The radius of the spiral increases as the particle gains more energy and the process continues until the particle is located inside the cloud. Since in most cases the trapping condition is not satisfied, the spiral acceleration mechanism is of great importance.

Soft Mango Firmness Assessment Based on Rayleigh Waves Generated by a Laser-Induced Plasma Shock Wave Technique

Many methods based on acoustic vibration characteristics have been studied to indirectly assess fruit ripeness via fruit firmness. Among these, the frequency of the 0S2 vibration mode measured on the equator has been examined, but soft-flesh fruit do not show the 0S2 vibration mode. In this study, a Rayleigh wave is generated on a soft mango fruit using the impulse excitation force generated by a laser-induced plasma shock wave technique. Then, the flesh firmness of mangoes is assessed in a non-contact and non-destructive manner by observing the Rayleigh wave propagation velocity because it is correlated with the firmness (shear elasticity), density, and Poisson’s ratio of an object. If the changes in the density and Poisson’s ratio are small enough to be ignored during storage, then the Rayleigh wave propagation velocity is strongly correlated to fruit firmness. Here, we measure the Rayleigh wave propagation velocity and investigate the effect of storage time. Specifically, we investigate the changes in firmness caused by ripening. The Rayleigh wave propagation velocity on the equator of Kent mangoes tended to decrease by over 4% in 96 h. The Rayleigh wave measured on two different lines propagated independent distance and showed a different change rate of propagation velocity during 96-h storage. Furthermore, we consider the reliability of our method by investigating the interaction of a mango seed on the Rayleigh wave propagation velocity.