An acoustic device comprising: a speaker unit (31, 32); and a three-dimensional knitted fabric, wherein the three-dimensional knitted fabric is provided at a position substantially facing the speaker unit (31, 32) and is tensioned in a surface direction, and configured to convert an output sound of the speaker unit to an acoustic wave of a surface sound source, wherein at least one of: fabrics including a knitted fabric, a woven fabric, and a non-woven fabric that are two dimensional or three dimensional; synthetic leather; genuine leather; a synthetic resin film; and a bead foam is combined with the three-dimensional knitted fabric characterized in that: the three-dimensional knitted fabric is configured to convert the output sound of the speaker unit (31, 32) by vibration of yarns constituting a pair of ground knitted fabrics constituting the three-dimensional knitted fabric and by vibration of a connecting yarn extending back and forth between the ground knitted fabrics, and the device being configured to let the acoustic wave propagate to an outside, wherein a plate-shaped bead foam having a predetermined area is combined with the three-dimensional knitted fabric, the three-dimensional knitted fabric is disposed in the tense state so as to correspond to a hole portion formed at a position, of the bead foam, substantially facing the speaker unit (31, 32), and the bead foam, the hole portion, and the three-dimensional knitted fabric serve as a resonance part configured to vibrate air inside the hole portion to make the output sound of the speaker unit (31, 32) resonate. a speaker unit (31, 32); and a three-dimensional knitted fabric, wherein the three-dimensional knitted fabric is provided at a position substantially facing the speaker unit (31, 32) and is tensioned in a surface direction, and configured to convert an output sound of the speaker unit to an acoustic wave of a surface sound source, wherein at least one of: fabrics including a knitted fabric, a woven fabric, and a non-woven fabric that are two dimensional or three dimensional; synthetic leather; genuine leather; a synthetic resin film; and a bead foam is combined with the three-dimensional knitted fabric characterized in that: the three-dimensional knitted fabric is configured to convert the output sound of the speaker unit (31, 32) by vibration of yarns constituting a pair of ground knitted fabrics constituting the three-dimensional knitted fabric and by vibration of a connecting yarn extending back and forth between the ground knitted fabrics, and the device being configured to let the acoustic wave propagate to an outside, wherein a plate-shaped bead foam having a predetermined area is combined with the three-dimensional knitted fabric, the three-dimensional knitted fabric is disposed in the tense state so as to correspond to a hole portion formed at a position, of the bead foam, substantially facing the speaker unit (31, 32), and the bead foam, the hole portion, and the three-dimensional knitted fabric serve as a resonance part configured to vibrate air inside the hole portion to make the output sound of the speaker unit (31, 32) resonate. the three-dimensional knitted fabric is configured to convert the output sound of the speaker unit (31, 32) by vibration of yarns constituting a pair of ground knitted fabrics constituting the three-dimensional knitted fabric and by vibration of a connecting yarn extending back and forth between the ground knitted fabrics, and the device being configured to let the acoustic wave propagate to an outside, wherein a plate-shaped bead foam having a predetermined area is combined with the three-dimensional knitted fabric, the three-dimensional knitted fabric is disposed in the tense state so as to correspond to a hole portion formed at a position, of the bead foam, substantially facing the speaker unit (31, 32), and the bead foam, the hole portion, and the three-dimensional knitted fabric serve as a resonance part configured to vibrate air inside the hole portion to make the output sound of the speaker unit (31, 32) resonate.