Disclosed herein is an acoustic device with improved acoustic performance. The acoustic device includes: a housing that forms the appearance of the acoustic device; a magnet that generates magnetic force; a yoke that includes paramagnetic material that concentrates the magnetic force; a voice coil that vibrates due to the magnetic force when an electric signal having sound information is applied; a diaphragm that comes into close contact with the voice coil and that vibrates and generates sound in response to the vibration of the voice coil; a plate that is located between the diaphragm and the magnet; and a locking ring that is located on the plate and that fastens part of the diaphragm, wherein the sectional surface of the locking ring has a rectangular shape the height of which is greater than the width thereof.