A (sub)millimetric source is obtained by difference-frequency mixing in a nonlinear crystal. It allows investigation of spectral absorption between 4 and 40 cm−1 with a new two-beam spectroscopic mounting. The complete mounting is described. The measurement method is given in detail. Compressed gaseous nitrogen-induced absorption measurements are used to calculate relaxation times and spectral moments. Comparisons are made with other measurements in close frequency regions, and theoretical models and an empirical line-shape model.