Realization of high-performance bandpass filter by impedance-mirroring

• Novel elastic wave bandpass filters are designed by impedance-mirroring.
• The filters are realized only with a finite periodic impedance-mirrored unit cells.
• Non-oscillatory transmission and sharp roll-off are achieved by the proposed filters.
• The filter performance was verified by torsional-wave experiments in a bar.

Ideal bandpass filters must have sharp roll-off and high transmission over a target pass band simultaneoulsy, but it is still difficult to make such filters of a reasnoable size. To achieve such performance, we propose a novel finite phononic crystal filter made of impedance-mirrored elements each of which constists of two layers having different impedances. For broad bandpass filtering, one of the two layers is selected to be one eighth wavelength long at the center frequency of the passband and the other layer, one quarter wavelength long with a matched impedance. For narrow bandpass filtering, the broad bandpass filter is slightly modified in such a way that the concept of the impedance-mirroring is applied globally over the finite periodic structure filter, not within each unit cell. To check the performance of the proposed filters, experiments using torsional elastic waves in a rod were conducted.