A silencing system (10a-10w) comprising: a tubular member penetrating a wall separating two spaces, and one or more silencers (21, 22, 22a, 22b, 23, 60, 62) that are disposed in the tubular member or on the outer peripheral surface of the tubular member, wherein the or each silencer (21, 22, 22a, 22b, 23, 60, 62) includes a cavity portion (30, 30a, 30b) that communicates with an interior space of the tubular member, and a total volume of the interior space of the tubular member and the cavity portion (30, 30a, 30b) of the or each silencer (21, 22, 22a, 22b, 23, 60, 62) is larger than a volume of an interior space of the tubular member alone, wherein a depth of the cavity portion (30, 30a, 30b) is less than λ / 4, where λ is a wavelength at a first resonance frequency of the tubular member, and the or each silencer (21, 22, 22a, 22b, 23, 60, 62) does not have a structure resonating at the first resonant frequency of the tubular member, wherein "0.05 ≤ Re[Bn] ≤ 0.8" and "0.0005≤Im[Bn]≤0.45" are satisfied in a case where a standardized effective modulus of elasticity in the interior space of the tubular member in which the silencers (21, 22, 22a, 22b, 23, 60, 62) are disposed is denoted by Bn, the standardized effective modulus Bn of elasticity is an average value in an octave band in which the first resonant frequency of the tubular member is present, an effective modulus of elasticity is such that in a region RA 0 in the interior space of the tubular member the modulus of elasticity is changed from the modulus of elasticity of air in a case where the one or more silencers (21, 22, 22a, 22b, 23, 60, 62) are disposed in the tubular member, and the width d of the region RA 0 in the axial direction of the tubular member is set to a length of 1/15 of a wavelength at the center frequency of the octave band in which the first resonant frequency of the tubular member is present, the standardized effective modulus Bn of elasticity is calculated from Bn = ξ / n where ξ is the effective impedance and n is the effective index of refraction, the effective impedance ξ and the effective index n of refraction are calculated from ξ = r 1 − 2 R + R 2 + T 2, n = − i log x + 2 πm kd where m is 0, T is an effective transmission coefficient, and is represented by T=T 0 ×exp(-i×k×d), where k is a wave number, R is a reflection coefficient of the tubular member, T 0 is a transmission coefficient of the tubular member, and r and x in are obtained from r = ∓ R 2 − T 2 − 1 2 − 4 T 2, x = 1 − R 2 + T 2 + r 2 T a tubular member penetrating a wall separating two spaces, and one or more silencers (21, 22, 22a, 22b, 23, 60, 62) that are disposed in the tubular member or on the outer peripheral surface of the tubular member, wherein the or each silencer (21, 22, 22a, 22b, 23, 60, 62) includes a cavity portion (30, 30a, 30b) that communicates with an interior space of the tubular member, and a total volume of the interior space of the tubular member and the cavity portion (30, 30a, 30b) of the or each silencer (21, 22, 22a, 22b, 23, 60, 62) is larger than a volume of an interior space of the tubular member alone, wherein a depth of the cavity portion (30, 30a, 30b) is less than λ / 4, where λ is a wavelength at a first resonance frequency of the tubular member, and the or each silencer (21, 22, 22a, 22b, 23, 60, 62) does not have a structure resonating at the first resonant frequency of the tubular member, wherein "0.05 ≤ Re[Bn] ≤ 0.8" and "0.0005≤Im[Bn]≤0.45" are satisfied in a case where a standardized effective modulus of elasticity in the interior space of the tubular member in which the silencers (21, 22, 22a, 22b, 23, 60, 62) are disposed is denoted by Bn, the standardized effective modulus Bn of elasticity is an average value in an octave band in which the first resonant frequency of the tubular member is present, an effective modulus of elasticity is such that in a region RA 0 in the interior space of the tubular member the modulus of elasticity is changed from the modulus of elasticity of air in a case where the one or more silencers (21, 22, 22a, 22b, 23, 60, 62) are disposed in the tubular member, and the width d of the region RA 0 in the axial direction of the tubular member is set to a length of 1/15 of a wavelength at the center frequency of the octave band in which the first resonant frequency of the tubular member is present, the standardized effective modulus Bn of elasticity is calculated from Bn = ξ / n where ξ is the effective impedance and n is the effective index of refraction, the effective impedance ξ and the effective index n of refraction are calculated from ξ = r 1 − 2 R + R 2 + T 2, n = − i log x + 2 πm kd where m is 0, T is an effective transmission coefficient, and is represented by T=T 0 ×exp(-i×k×d), where k is a wave number, R is a reflection coefficient of the tubular member, T 0 is a transmission coefficient of the tubular member, and r and x in are obtained from r = ∓ R 2 − T 2 − 1 2 − 4 T 2, x = 1 − R 2 + T 2 + r 2 T