Acoustic wave device

An acoustic wave device comprising:

• a high-acoustic-velocity support substrate;
• a low-acoustic-velocity film provided on the high-acoustic-velocity support substrate;
• a piezoelectric layer provided on the low-acoustic-velocity film;
• and
• an interdigital transducer (IDT) electrode provided on the piezoelectric layer;
• wherein
• an acoustic velocity of a bulk wave propagating through the high-acoustic-velocity support substrate is higher than an acoustic velocity of an acoustic wave propagating through the piezoelectric layer;
• an acoustic velocity of a bulk wave propagating through the low-acoustic-velocity film is lower than the acoustic velocity of a bulk wave propagating through the piezoelectric layer;
• the low-acoustic-velocity film includes a first portion and a second portion that is located closer to the high-acoustic-velocity support substrate than the first portion;
• the first portion includes same or substantially same materials as materials included in the second portion;
• and
• ρ1 and ρ2 are different, where ρ1 is a density of the first portion of the low-acoustic-velocity film and ρ2 is a density of the second portion of the low-acoustic-velocity film.