The heterodyne filter, developed at the RIT, Stockholm, Sweden, is a laboratory transmission system for the speech frequency range. It can be used as a band pass filter with continuously variable low frequency cut‐off and high frequency cut‐off of very steep attenuation characteristics, 1–2 c.p.s. per db and 60‐db attenuation outside the band pass range. The two cut‐off frequency settings can be chosen independently in the frequency range 0–4000 c.p.s. One frequency dial on the panel controls the band width, another controls the low pass cut‐off. A band suppression action can also be obtained. In this case the band width is restricted to 2 alternative values. The mid‐frequency will be continuously variable, 0–4000 c.p.s. The heterodyne filter is meant for articulation and hearing tests in acoustic research work but can also be used as a wave analyzer with continuously variable band‐width of 50 c.p.s.‐4000 c.p.s. in the frequency range of 20–12,000 c.p.s.