Acoustic wave device and multiplexer

An acoustic wave device comprising:

• a first input/output terminal and a second input/output terminal;
• and
• a plurality of acoustic wave resonators connected to a path connecting the first input/output terminal and the second input/output terminal;
• wherein
• each of the plurality of acoustic wave resonators includes an interdigital transducer (IDT) electrode on a substrate having piezoelectricity;
• the substrate includes:
• a piezoelectric film on a surface of which the IDT electrode is provided;
• a high acoustic velocity film made of at least one of aluminum nitride, lithium tantalate, lithium niobate, quartz, alumina, sapphire, magnesia, silicon nitride, silicon carbide, zirconia, cordierite, mullite, steatite, forsterite, spinel, sialon, aluminum oxide, silicon oxynitride, diamond-like carbon, diamond, and silicon, or made of a material which includes any one of aluminum nitride, lithium tantalate, lithium niobate, quartz, alumina, sapphire, magnesia, silicon nitride, silicon carbide, zirconia, cordierite, mullite, steatite, forsterite, spinel, sialon, aluminum oxide, silicon oxynitride, diamond-like carbon, diamond, and silicon as a main component;
• and
• a low acoustic velocity film located between the high acoustic velocity film and the piezoelectric film and made of any one of glass, silicon oxide, silicon oxynitride, lithium oxide, and tantalum oxide, or made of a material which includes any one of glass, silicon oxide, silicon oxynitride, lithium oxide, and tantalum oxide as a main component;
• and
• in a case where an electrode finger wavelength that is about twice a pitch between a plurality of electrode fingers parallel or substantially parallel to each other in the IDT electrode is denoted by λ (m), a film thickness of the piezoelectric film is denoted by t1 (m), a film thickness of the plurality of electrode fingers is denoted by t2 (m), t1/λ is denoted by a film thickness-wavelength ratio A of the piezoelectric film, t2/λ is denoted by a film thickness-wavelength ratio B of the plurality of electrode fingers, a density of the piezoelectric film is denoted by x (g/cm3), a density of the IDT electrode is denoted by y (g/cm3), and (A/x+B/y) is denoted by a mass coefficient M, a mass coefficient M of an acoustic wave resonator connected closest to the first input/output terminal among the plurality of acoustic wave resonators is equal to or more than about 0.95×0.054 and equal to or less than about 1.05×0.054.