Physical Characteristics

Ventilation noise originates primarily from fans and the air turbulence gener­ated inside ducts and around supply air and exhaust air terminal devices. The appearance of the noise is, of course, affected by factors such as the speed of rotation and the power of the fan, and by how the fan is stabilized or in other ways acoustically insulated. The noise level and the frequency characteristics are also largely determined by the velocity of the air inside ducts and around terminal devices, where factors such as the dimensions and appearance of the ducts and terminal devices may play a decisive role in the appearance of the noise.

The description of the physical characteristics of ventilation noise is based on more reliable knowledge than the description of the human effect. Miscon­ceptions about the levels and frequency characteristics of ventilation noise are still common. This in turn has sometimes led to wrong suggestions about the measures that should be taken in order to eliminate the effects of a ventilation noise exposure.

The links between levels of exposure and inconvenience caused by ventilation noise are described in an investigation carried out on office workers.2 Technical measurements and analyses of the ventilation noise at 155 typical office workplaces were in this study combined with assessments by the office workers of the level of disturbance that they experienced, the effect on working performance, fatigue, stress-related pain, and headaches. The average noise level was about 40 dB(A) at two of the workplaces, while it was about 35 dB(A) at two others. It emerged from the narrow-band analyses that the sound pressure levels of the infrasound were not in any event of an order that this type of sound frequencies (below 20 Hz) could contribute to any disturbance effects. Any steps taken to counter the sound frequen­cies of the ventilation noise under 50 Hz, i. e., the point of intersection between the threshold curve of auditory perception and the spectral level distribution curve of the ventilation noise, would thus be ineffective in these cases. This conclusion is based on the fact that the sound pressure levels of the ventilation noise frequency under 50 Hz were significantly below the threshold curve of auditory perception, and as such were not audible. This situation is considered to be representati ve of to­day’s ventilation noise in offices. The same results were obtained in a study by Paakkonen.3 It must be pointed out, however, that levels above the perception threshold in the infrasound range can, of course, be generated from heavy — duty ven­tilation systems, for example in factories, stores, and department stores.

It should also be pointed out that the levels from ventilation noise in, for ex­ample, workshop premises are often lower than those emanating from other sources, for example machines of various kinds. It is not uncommon for the venti­lation noise from industrial premises to cause more disturbance in adjacent offices than in the industrial premises themselves. The fact that ventilation noise propa­gates in this manner is due to its pronounced low-frequency character. The more low-frequency components in the noise, the greater the propagation. Sound radia­tion from industrial premises, via duct openings in facades and roofs, may in this manner also cause disturbances in nearby residential accommodations.