RESUMEN
Thermally induced optical reflection of sound (THORS) provides a means to manipulate sound waves without the need for traditional acoustically engineered structures. By photothermally exciting a medium, with infrared light, a barrier can be formed due to abrupt changes in compressibility of the excited medium. Discovery and initial characterization of the THORS phenomenon utilized air saturated with ethanol vapor as the absorbing medium and a CO2 laser, operating at 9.6 µm, as the excitation source to achieve acoustic reflection efficiencies of 25-30% of the incident wave. In this work, we demonstrate for the first time, the ability to generate THORS barriers in ambient air (i.e., without the need for ethanol vapor). Employing atmospheric water vapor as the absorbing medium and a modulated, multiline carbon monoxide laser, operating at 5.5 ± 0.25 µm, THORS barriers capable of acoustic and ultrasonic reflection-suppression efficiencies greater than 70% are readily generated. To achieve these significant reflection-suppression efficiencies, the temporal dynamics of THORS barriers in ambient air were characterized using 300 kHz ultrasonic pulses incident on the barriers, revealing three different operational regimes. In the first regime, a single laser pulse generates a transient THORS barrier that lasts tens of milliseconds and exhibits minimal acoustic reflectivity. In the second regime, multiple laser pulses interact with the water vapor prior to complete relaxation of the THORS barrier from the previous excitation pulse, resulting in an additive response and reflectivity/suppression efficiencies as great as 72%. Finally, in the third regime, non-modulated continuous wave (CW) excitation of the water vapor occurs resulting in no measurable acoustic reflectivity/suppression from the THORS barrier. This work characterizes these different regimes and the optimal modulation timing to generate efficient continuous acoustic barriers using THORS.
RESUMEN
Using the recently discovered THermally induced Optical Reflection of Sound (THORS) phenomenon, it is possible to generate optically induced, local density barriers in air by the absorption of intense, modulated laser light (the THORS phenomenon), which results in abrupt differences in compressibility of the air at these barriers that can efficiently reflect incident acoustic waves. In this note, we demonstrate the ability to optically manipulate and reflect acoustic waves in air as well as optimize the functional parameters (optical modulation and acoustic frequency) and characterize the effects of common physical parameters, including localized thermal gradients and incident angle of reflection on the efficiency of the resulting acoustic reflection. Finally, the ability to efficiently steer acoustic waves around a physical obstruction using THORS is also demonstrated.
