PURPOSE: To improve sound absorbing effect by forming a rubber plate of a Helmholtz resonator constituted of a partition plate, a perforated plate provided on one side of the partition plate, and a cavity part provided between the partition plate and perforated plate.

CONSTITUTION: A rubber plate 11 is formed of a Helmholtz resonator. The rubber plate 11 is therefore formed of a partition plate 12, a perforated plate 14 with numerous holes 13 provided on one side of the partition plate 12, and a cavity part 15 provided between the partition plate 12 and the perforated plate 14. A Helmholtz type muffling box is formed by the cavity part 15 between the partition plate 12 and the perforated plate 14, and the frequency of noise generated by a fan is the blade passing frequency. Accordingly, when the resonance frequency of the resonator is set in the such a way as to be this blade passing frequency, noise based on the blade passing frequency can be absorbed.