Abstract
We present 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.
Highlights
One of the challenges in the field of sound measurements and testing is to obtain an omnidirectional, short pulse width and high-power pulsed acoustic source [1]
Noting that the laser fluence was about 5 J/cm2, which is larger than Fth, in order to guarantee the formation of Laser-induced plasma (LIP) plume
When the diameter of the resonator was reduced to 11 cm, the time constant dropped to a less than 4 to 5 ms depending on the used laser wavelength
Summary
One of the challenges in the field of sound measurements and testing is to obtain an omnidirectional, short pulse width and high-power pulsed acoustic source [1]. For example [2], the fluctuating fluid volume-mass audible sources (like the dynamic speakers, the pressurized gas nozzle injectors, the laser-induced breakdown (LIB), the electric sparks, and the explosive charges) generate a high-intensity sound with a very poor impulse response and/or bad frequency selectivity. The fluctuating fluid shear stress audible sources, where the net fluid volume nor net force equal zero (like the jet flows, the vibrating spheres and the tuning forks), have a very long pulse width in spite of their good frequencyselectivity. We have combined the fluctuating fluid volume-mass and the fluctuating fluid shear stress audible sources into a new hybrid category The idea behind this combination, is to make use of the high power and the short pulse-width LIB, to induce a Helmholtz resonance inside a hollow metallic spheres. To maximize the acoustic yield of the resonator, the pulsed laser beam is focused through a fine hole, on the internal wall of the spherical cavity (as shown in figure 1a), resulting in ablating the thin metallic layer and the generating a high pressure plasma plume (as shown in figure 1b)
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