Fume hoods do not generate noise but amplify noises generated in the exhaust system. The potential noise level from a fume hood increases as the hood gets wider. According to Saunders (1993), "These system noises come from a variety of causes: air going in excess of 2,000 fpm through the ducting; turbulence caused by the improper treatment of ducting turns or transitions to smaller or larger ducting; unbalanced blower impeller wheels or frayed and cracked blower/motor belts; assorted junk left in the ducting during construction. In-line dampers cause noise when they are partially closed." [Saunders, 1993]
The Handbook of Facilities Planning (1990) suggests the following to minimize fume hood noise:
·Fume hood plenum velocity should be approximately half the fume hood slot velocity, and the slot velocity should not exceed 1,200 fpm.
·The fume hood collar must have a one-inch radius between the chamber and the duct, and the air velocity must not exceed 1,400 fpm at this point. Provide a 12-inch diameter sleeve for a six-foot hood, and two 10.5-inch sleeves for an eight-foot hood or larger.
·Air velocity in the duct must not exceed 1,600 fpm unless dust and other solid particles must be exhausted.
·Exhaust fans should be located as far from supply air as practical to reduce noise transmission.
·Fan speed should not exceed 1,800 rpm to reduce noise generation.
·Radial-blade, centrifugal fans should be used; they produce less air noise than backward-inclined airfoil fans. Forward-curved fan blades move more air with less fan wheel blade velocity, i.e., "tip speed" and may even be less noisy.
·A vibration isolator, such as a flexible, neoprene-coated glass fiber cloth, must be installed between the fan and the duct.
General laboratory noise minimization
Noise attenuation — case study
