METHOD AND APPARATUS FOR THE TRANSMISSION OF AN ACOUSTIC SIGNAL IN A PHOTOACOUSTIC CELL

PCT No. PCT/DK91/00061 Sec. 371 Date Sep. 1, 1992 Sec. 102(e) Date Sep. 1, 1992 PCT Filed Mar. 4, 1991 PCT Pub. No. WO91/14176 PCT Pub. Date Sep. 19, 1991.The microphone in a photoacoustic cell that is part of an apparatus for photoacoustic analysis of at least one substance is protected against the harmful effects from the analyzed substance by sonically communicating the microphone with the excitation zone through a waveguide that is at least partly located within the excitation zone. The substance to be analyzed is supplied to the excitation zone, in which zone an acoustic signal is generated by an acoustic frequency modulation of an optical signal directed into the substance in the excitation zone. The substance is prevented from reaching the microphone by supplying a protecting gas to a portion of the waveguide proximate to the microphone. The waveguide is an acoustic resonator in which at least two antinodes of the acoustic signal are present, the resonator being excited in one antinode by the acoustic signal and the microphone being mounted in another antinode such that the waveguide transmits the generated acoustic signal from the substance through the protecting gas to the microphone without any significant weakening of the acoustic signal.