“…These microphones convert the sound signal into an electric signal, which is filtered and detected by a lock-in amplifier. Photoacoustic spectroscopy is widely used for the detection of several gases in the concentration range of ppbv and sub-ppbv Berrou et al, 2010;Sthel et al,2011;Harren et al,2008;Thomas, 2006;Elia et al, 2009;Sorvajärvi et al, 2009;Sigrist et al, 2008;Angelmahr et al,2006;Filho et al, 2006;Schramm et al, 2003;Harren et al, 2000;Miklos et al, 2001;Gondal, 1997;Repond & Sigrist, 1996;Sigrist, 1994aSigrist, , 1994b. There are some types of trace gas detection systems based on continuous wave (CW) CO 2 laser, optical parametric oscillator (OPO) in combination with photoacoustic spectroscopy and quantum cascade laser (QCL).…”