Boundary acoustic wave device

A boundary acoustic wave device comprising:

• a multilayer structure including a first medium having piezoelectric characteristics, a non-electroconductive second medium, and a third medium through which slow transverse waves propagate at a lower acoustic velocity than slow transverse waves propagating through the first and second media, the first medium, the third medium and the second medium being stacked in that order;
• and
• an IDT disposed between the first medium and the third medium;
• wherein
• the IDT includes a metal layer made of a metal having a density ρ in the range of about 3000 kg/m3 to about 21500 kg/m3;
• and
• the IDT has electrode fingers arranged at a pitch of λ and has a thickness H1 satisfying the relationship of approximately 0.006λ≦H1≦0.2λ, and the third medium has a thickness H2 satisfying the relationship of approximately H1<H2≦0.7λ;
• and
• the thickness H1 of the IDT is set such that an acoustic velocity of boundary acoustic waves is less than or equal to the lowest value of the acoustic velocities of longitudinal waves, fast transverse waves and slow transverse waves.