A Continuously Variable Filter for Acoustic Applications
Gunnar Fant · The Journal of the Acoustical Society of America · 1950
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.