An acoustic device includes a first chamber, a first through hole, a vibration structure, and a separation structure. The first chamber includes a first end and a second end. The first through hole is defined at the first end of the first chamber. The vibration structure is disposed at the second end of the first chamber and is configured to transmit an acoustic wave away from the first chamber. The separation structure is disposed within the first chamber and divides the first chamber into a first sub-chamber and a second sub-chamber. The separation structure defines a second through hole connecting the first sub-chamber and the second sub-chamber.