Band pass filter device

A band pass filter device comprising:

• a first band pass filter including a first acousto-electric transducer and first electro-acoustic transducers disposed on opposite sides of the first acousto-electric transducer and adapted for connection in common to an input signal source, said first acousto-electric transducer having pairs of fingers;
• a second band pass filter including a second electro-acoustic transducer connected electrically to said first acousto-electric transducer and second acousto-electric transducers disposed on opposite sides of the second electro-acoustic transducer and connected in common to an output terminal, said second electro-acoustic transducer having pairs of fingers, and
• said first acousto-electric transducer having the same structure as that of said second electro-acoustic transducer in which the number N of pairs of fingers in each of said first acousto-electric transducer and said second electro-acoustic transducer substantially is equal to 1.5/k.sup.2, where k.sup.2 represents the electromechanical coupling constant of a piezoelectric plate on which said fingers are formed and is expressed by k.sup.2 =2(v.sub.f -v.sub.m)v.sub.f, v.sub.f representing the velocity of a surface acoustic wave propagating along a free surface of said plate and v.sub.m representing the velocity of a surface acoustic wave propagating along a thin metal coated portion of the surface of said plate.