The significant differences in disturbance, when evaluated with regard to the average noise levels, shows that the noise level is a decisive factor with regard to disturbance. Measures to limit the disturbance reaction due to ventilation noise should, therefore, naturally be directed in the first instance at lowering the noise level.

The extent to which a ventilation noise is perceived as disturbing depends not only on its dB(A) level, but also on the spectral distribution and the pres­ence of tones or intermittent components in the noise. From an experiment carried out on respondents exposed to ventilation noises with different charac­teristics in a simulated office room, it emerged that the highest acceptable level was about 7 dB higher for ventilation noise with a superimposed tone at 30 Hz than for other types of noise.8 In another experiment, it was found that the tolerance level was much higher for a tone than for a noise at 100 Hz, whereas the opposite tendency applied at 1000 Hz.**

Earlier experiments indicate clearly that a lowered sound pressure level can be an effective measure to reduce the inconvenience reactions due to a ventilation noise, provided that it is targeted at the most critical frequency range from the point of view of influence or that the measure results in a gen­eral lowering over the entire spectral range of the ventilation noise, Elimination of Different Ventilation Noise Sources

Efforts to reduce the noise from a ventilation system may be concentrated on measures concerning the fan, the fan room, the fan ducts, and the supply and exhaust air terminals.1’9 The Fan

Fans with poorly designed or excessively simple straight blades should be replaced with quality fans with lower noise generation. As the accumu­lation of dirt on impellers often causes imbalance, leading to vibration and

Unnecessary noise, these should be cleaned regularly. Imbalance, whether due to dirt or to other causes, should be corrected by adjustment. Defective bearings should be adjusted or replaced. Struts and sharp edges in front of an impeller should be avoided. Impact noise insulation should be intro­duced between fan room and floor structure and between fan room and connecting ducts. The fan unit should be enclosed, with satisfactory air­borne sound insulation as the objective. The Fan Room

The roof and walls of the fan room should be lined internally with absor­bent materials in order to reduce the sound level in the fan room. The wall in­sulation should be sufficient to reduce the transmission of sound to adjoining silent: premises. The Fan Ducts

Straight, internally smooth ducts should be avoided as these give very little noise reduction. Fan noise can pass virtually unobstructed. Silencers should be installed inside the ducts by covering the walls with absorbent material. In this way noise in the higher-frequency range may be reduced. The low-frequency components of the fan noise are more difficult to at­tenuate. Very thick absorbent linings are needed to reduce such noise. Al­tering the area of the duct produces a damping of the noise because parts of the sound are reflected back into the duct. This kind of damping, which may also be achieved at the openings of a ventilation duct, is most effective for low-frequency noise. By inserting internally smooth bends in the duct, damping of the noise may be obtained. The larger the duct widths, the better the damping for low frequencies. Narrow ducts dampen very little. Sound-absorbent bends may produce very sharp re­ductions in noise level. High frequencies are dampened most easily with duct bends. To obtain damping in the low-frequency range with bends, wide ducts are required. Larger spaces with absorbent walls, “absorption chambers,” built into the ductwork, also give effective damping in the lower frequency ranges. The ventilation duct often consists of large noise-generating surfaces which may need to be insulated or enclosed. Counter noise may be an alternative method of reducing the noise level in a ventilation duct. However, this method is relatively costly compared with other technical solutions. The Supply and Exhaust Air Terminals

There is usually a certain damping of the fan noise at the opening of the duct. The damping is greatest if the opening consists of a pipe projecting clear of the wall. The noise radiation is also lowest if it is on the level of the roof or the wall in a corner of the room. Excessive air velocities in the opening may also cause noise in the terminal device, as may inappropriately shaped devices with sharp edges, etc. Supply and exhaust air terminal devices may be fitted with silencers or absorbents.