a primary speaker for reproducing the left ear signal and a primary speaker for reproducing the right ear signal are arranged at a predetermined interval in a first direction,

a secondary speaker for reproducing the left ear signal is arranged at the same position as the primary speaker for reproducing the left ear signal in the first direction, and

a secondary speaker for reproducing the right ear signal is arranged at the same position as the primary speaker for reproducing the right ear signal in the first direction.

(6) The acoustic device according to any of (1) to (4), in which

the two speakers arranged back-to-back include a primary speaker and a secondary speaker for forming directivity,

a primary speaker for reproducing the left ear signal and a primary speaker for reproducing the right ear signal are arranged at a predetermined interval in a first direction,

a secondary speaker for reproducing the left ear signal is arranged at a position closer to the primary speaker for reproducing the right ear signal than the primary speaker for reproducing the left ear signal in the first direction, and

a secondary speaker for reproducing the right ear signal is arranged at a position closer to the primary speaker for reproducing the left ear signal than the primary speaker for reproducing the right ear signal in the first direction.

(7) The acoustic device according to (6),

in which the secondary speaker for reproducing the left ear signal and the secondary speaker for reproducing the right ear signal are the same speaker.

(8) The acoustic device according to any of (1) to (4), in which