Yönak Serdar H, Dowling David R
Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2133, USA.
J Acoust Soc Am. 2003 Dec;114(6 Pt 1):3167-78. doi: 10.1121/1.1628250.
The photoacoustic effect is commonly exploited for molecular spectroscopy, nondestructive evaluation, and trace gas detection. Photoacoustic sound is produced when a photoactive material absorbs electromagnetic radiation and converts it to acoustic waves. This article focuses on the generation of photoacoustic sound from thermal expansion of photoactive gases due to unsteady heating from a laser light source, and extends the work of prior studies on photoacoustic sound generation in an open environment. Starting with the forced free-space wave equation, a simple model is constructed for photoacoustic sounds produced by both acoustically distributed and compact gas clouds. The model accounts for laser absorption through the Lambert-Beer law and includes the effects of photoactive gas cloud characteristics (shape, size, and concentration distribution), but does not include molecular diffusion, thermal conduction, convection, or the effects of acoustic propagation through sound-absorbing inhomogeneous media. This model is compared to experimentally measured photoacoustic sounds generated by scanning a 10.6-micron carbon dioxide (CO2) laser beam through small clouds of a photoactive gas, sulfur hexafluoride (SF6). For the current investigation, the photoactive gas clouds are formed either by low flow-rate calibrated leak sources or by a laminar jet emerging from a 1.6-mm-diam tube. Model-measurement comparisons are presented over a 3- to 160-kHz bandwidth. Signal pulse shapes from simple gas cloud geometries are found to match calculated results when unmeasured gas cloud characteristics within the model are adjusted.
光声效应通常用于分子光谱学、无损评估和痕量气体检测。当光活性材料吸收电磁辐射并将其转换为声波时，就会产生光声。本文重点研究由于激光光源的不稳定加热导致光活性气体热膨胀而产生光声的过程，并扩展了先前在开放环境中光声产生的研究工作。从强迫自由空间波动方程出发，构建了一个简单的模型，用于描述由声学分布和紧凑气体云产生的光声。该模型通过朗伯-比尔定律考虑激光吸收，并包括光活性气体云特性（形状、大小和浓度分布）的影响，但不包括分子扩散、热传导、对流或声音在吸声非均匀介质中传播的影响。将该模型与通过扫描10.6微米二氧化碳（CO₂）激光束穿过光活性气体六氟化硫（SF₆）的小云团产生的光声实验测量结果进行比较。在当前的研究中，光活性气体云团由低流量校准泄漏源或从直径1.6毫米的管子中喷出的层流射流形成。在3至160千赫兹的带宽范围内给出了模型与测量结果的比较。当调整模型中未测量的气体云特性时，发现简单气体云几何形状的信号脉冲形状与计算结果相匹配。
