Acoustic Wave Filters; an Extension of the Theory

Extension of theory of acoustic wave filters to high frequencies.---In the simple form of acoustic wave filter theory already published, the wave-length was assumed long in comparison with the length of a section of the conduit. Experimentally additional bands appeared at higher frequencies and these are now discussed. Advantage is taken of the recurrence at higher frequencies of the vanishing of the acoustic impedance of a section of the conduit, and approximate values of the components of this impedance are established for the neighboring region of frequencies. Formulas for low-pass and single-band-pass filters, using these values, are found to give the experimental cutoffs of the additional bands with satisfactory accuracy. By appropriate design an additional band can be made practically to disappear. Tables showing agreement of experiment with theory are given. The theory is thus extended to higher frequencies but not to those for which additional resonance frequencies of the branches occur.