Photoacoustic Signal Generation in a Long Open Tube

Abstract

The well known theories of photoacoustic signal generation seem to be insufficient for explaining the results of some recent PA experiments. For investigating the PA signal generation, the simplest possible device, a long open tube was chosen as PA cell. The PA signal generated by the absorption of the light of a CO₂ laser in air was investigated as a function of the chopping frequency. The whole setup was placed into an anechoic room in order to reduce the effect of the outside noises. The acoustic properties of the open tube were measured by using the external white noise source, then the PA signal and the background noise were measured by opening and closing the beam shutter of the laser. According to the present theory of PA signal generation only the odd longitudinal eigenmodes may be excited photoacoustically, because of the perfect symmetry of the tube. The results of the experiment clearly show, that the even modes will be excited, too, and the effectiveness of the excitation is quite the same for odd as for even modes. The experiment may be modelled by assuming simultaneous pressure and velocity generation.