An acoustic system comprising: a duct that has a function of causing a fluid to flow therein and has a tubular shape; a sound source; and at least one membrane-type resonance body, wherein the at least one membrane-type resonance body has at least one membrane-shaped member that is closed and formed as a part of a wall of the duct and vibrates in response to sound, and a frame attached to a rear surface side of the membrane-shaped member so as to form a closed or substantially closed space on the rear surface side of the membrane-shaped member, the at least one membrane-type resonance body being configured to cause acoustic resonance by a structure including the at least one membrane-shaped member and the closed or substantially closed space, and to suppress sound that is transmitted from the sound source into the duct and radiated from a downstream end portion of the duct, wherein the sound source is either an internal sound source or an external sound source, wherein, in case of the internal sound source, said internal sound source is disposed inside the duct or at an outer peripheral portion of the duct, which communicates with an inside of the duct, and in case of the external sound source, said external sound source is at a distance within a wavelength at a frequency of the acoustic resonance on the outside from an upstream end portion of the duct, wherein, for at least one membrane-shaped member or at least one membrane-type resonance body, in a case where a wavelength determined from a frequency at which a sound pressure of sound generated by the sound source is maximum is denoted by λ and an integer of 0 or more is denoted by m, a center of the membrane-shaped member is positioned at a distance which is larger than (m × λ/2 - λ/4) and is smaller than (m × λ/2 + λ/4) from a position of the sound source, and wherein in a case where a Young's modulus of the membrane-shaped member is denoted by E (Pa), a thickness is denoted by t (m), a thickness of the closed or substantially closed space is denoted by d (m), and a circle equivalent diameter of a region where the membrane-shaped member vibrates is denoted by Φ (m), hardness E × t 3 (Pa × m 3) of the membrane-shaped member is 21.6 × d -1.25 × Φ 4.15 or less. a duct that has a function of causing a fluid to flow therein and has a tubular shape; a sound source; and at least one membrane-type resonance body, wherein the at least one membrane-type resonance body has at least one membrane-shaped member that is closed and formed as a part of a wall of the duct and vibrates in response to sound, and a frame attached to a rear surface side of the membrane-shaped member so as to form a closed or substantially closed space on the rear surface side of the membrane-shaped member, the at least one membrane-type resonance body being configured to cause acoustic resonance by a structure including the at least one membrane-shaped member and the closed or substantially closed space, and to suppress sound that is transmitted from the sound source into the duct and radiated from a downstream end portion of the duct, wherein the sound source is either an internal sound source or an external sound source, wherein, in case of the internal sound source, said internal sound source is disposed inside the duct or at an outer peripheral portion of the duct, which communicates with an inside of the duct, and in case of the external sound source, said external sound source is at a distance within a wavelength at a frequency of the acoustic resonance on the outside from an upstream end portion of the duct, wherein, for at least one membrane-shaped member or at least one membrane-type resonance body, in a case where a wavelength determined from a frequency at which a sound pressure of sound generated by the sound source is maximum is denoted by λ and an integer of 0 or more is denoted by m, a center of the membrane-shaped member is positioned at a distance which is larger than (m × λ/2 - λ/4) and is smaller than (m × λ/2 + λ/4) from a position of the sound source, and wherein in a case where a Young's modulus of the membrane-shaped member is denoted by E (Pa), a thickness is denoted by t (m), a thickness of the closed or substantially closed space is denoted by d (m), and a circle equivalent diameter of a region where the membrane-shaped member vibrates is denoted by Φ (m), hardness E × t 3 (Pa × m 3) of the membrane-shaped member is 21.6 × d -1.25 × Φ 4.15 or less.