We propose a photoacoustic detector with a differential Helmholtz resonant photoacoustic cell for use in studies of the broadening and shifts of the molecular rotational-vibrational absorption lines due to collisions with selectively excited molecules of a buffer gases. The design and specifications of the photoacoustic laser spectrometer including the laser source for exciting buffer gas molecules at a fixed frequency, tunable narrow-band diode laser for recording absorption line shape and characteristics, as well as the measurement technique are described.