Patent ID: 8242861

Claim:
An acoustic wave filter device having a ladder circuit configuration comprising: a plurality of series arm resonators connected in series with one another at a series arm connecting an input terminal and an output terminal; a parallel arm resonator disposed at a parallel arm connected between the series arm and a ground potential; a piezoelectric substrate made of a piezoelectric monocrystal; and electrodes provided on the piezoelectric substrate; wherein an anti-resonant frequency of at least one of the plurality of series arm resonators is different from that of remaining ones of the plurality of series arm resonators; one of the plurality of series arm resonators having the lowest anti-resonant frequency has a resonant frequency located in a passband and an electromechanical coupling coefficient k 2 less than an average of electromechanical coupling coefficients of all of the plurality of series arm resonators; and a propagation direction ψ obtained when a crystalline cutting plane of the piezoelectric substrate and an acoustic wave propagation direction are expressed as Euler angles (φ, θ, ψ) is such that an electromechanical coupling coefficient of one of the plurality of series arm resonators having the lowest anti-resonant frequency is less than an average of electromechanical coupling coefficients of all of the plurality of series arm resonators.