The design of an opto–acoustic detector (OAD) with a Helmholtz cavity is described. Data on the OAD frequency and phase parameters measured at a pressure of 0.5 Torr for pure CH3OH for the lasing line of a CO2 laser 9P(16) and the steepness of the Volt––Watt characteristic for pure CH3OH and CH3OH rarefied with air are presented. The limitting sensitivity and minimum detectable concentration of the spectrophone are estimated. The following data have been obtained: Cmin= 2.9E10-6 for CH3OH for the lasing line of the CO2 laser 9P(16), and Cmin= 5.8E10-4 for SO2 for the lasing line of the CO2 laser 9R(18). Some measurements of the collisional broadening of the absorption lines of the CH3OH , CH3CN , and SO2 molecules are presented. These values are (14 ± 2)MHz/Torr, (17±3)MHz/Torr, and (12 ± 2)MHz/Torr, respectively.