09-09-2017, 11:26 AM
The photo-acoustic effect is commonly exploited for molecular spectroscopy, non-destructive evaluation and trace gas detection. Photo-acoustic sound occurs when a photo-active material absorbs electromagnetic radiation and converts it into acoustic waves. This article focuses on the photo-acoustic sound generation of the thermal expansion of photo-active gases due to the unstable heating of a laser light source and extends the work of previous studies on the generation of photo-acoustic sounds in an open environment. From the forced free space wave equation, a simple model is constructed for photo-acoustic sounds produced by acoustically distributed and compact gas clouds. The model explains the absorption of the laser through the Lambert-Beer law and includes the effects of photo-active gas cloud characteristics (shape, size and concentration distribution), but does not include molecular diffusion, thermal conduction, convection or effects of acoustic propagation through inhomogeneous means that absorb the sound. This model compares with the experimentally measured photo-acoustic sounds generated by scanning a 10.6 micron carbon dioxide (CO2) laser beam through small clouds of a photo-active gas, sulfur hexafluoride (SF6).
The photo-acoustic effect is commonly exploited for molecular spectroscopy, non-destructive evaluation and trace gas detection. Photo-acoustic sound occurs when a photo-active material absorbs electromagnetic radiation and converts it into acoustic waves. This article focuses on the photo-acoustic sound generation of the thermal expansion of photo-active gases due to the unstable heating of a laser light source and extends the work of previous studies on the generation of photo-acoustic sounds in an open environment. From the forced free space wave equation, a simple model is constructed for photo-acoustic sounds produced by acoustically distributed and compact gas clouds. The model explains the absorption of the laser through the Lambert-Beer law and includes the effects of photo-active gas cloud characteristics (shape, size and concentration distribution), but does not include molecular diffusion, thermal conduction, convection or effects of acoustic propagation through inhomogeneous means that absorb the sound. This model is compared to the experimentally measured photo-acoustic sounds generated by scanning a 10.6 μ carbon dioxide (CO2) laser beam through small clouds of a photo-active gas, sulfur hexafluoride (SF6). For current research, photo-active gas clouds are formed by low flow calibrated leak sources or by a laminar jet emerging from a 1.6 mm diameter tube. Comparisons of the model measurement are presented in a bandwidth of 3 to 160 kHz. The signal pulse forms of the simple gas cloud geometries coincide with the calculated results when adjusting the characteristics of the gas clouds not measured in the model.
The photo-acoustic effect is commonly exploited for molecular spectroscopy, non-destructive evaluation and trace gas detection. Photo-acoustic sound occurs when a photo-active material absorbs electromagnetic radiation and converts it into acoustic waves. This article focuses on the photo-acoustic sound generation of the thermal expansion of photo-active gases due to the unstable heating of a laser light source and extends the work of previous studies on the generation of photo-acoustic sounds in an open environment. From the forced free space wave equation, a simple model is constructed for photo-acoustic sounds produced by acoustically distributed and compact gas clouds. The model explains the absorption of the laser through the Lambert-Beer law and includes the effects of photo-active gas cloud characteristics (shape, size and concentration distribution), but does not include molecular diffusion, thermal conduction, convection or effects of acoustic propagation through inhomogeneous means that absorb the sound. This model is compared to the experimentally measured photo-acoustic sounds generated by scanning a 10.6 μ carbon dioxide (CO2) laser beam through small clouds of a photo-active gas, sulfur hexafluoride (SF6). For current research, photo-active gas clouds are formed by low flow calibrated leak sources or by a laminar jet emerging from a 1.6 mm diameter tube. Comparisons of the model measurement are presented in a bandwidth of 3 to 160 kHz. The signal pulse forms of the simple gas cloud geometries coincide with the calculated results when adjusting the characteristics of the gas clouds not measured in the model.