| | | The noise level of a fan must be taken into consideration when designing a ventilation system. The affect of a sound source (fan) on the rooms that need ventilation and the neighbourhood can be estimated using the following information: The noise is primarily created by the fan, possibly also by ducting, and other components like filters, heaters, shutters etc. if the air flow speed is too high (whistling noises). Therefore a maximum air flow speed of 7 m/s is recommended. At the same time noise transmission by fan or other components must be reduced as much as possible when installing. The maximum noise emission recommendations should not be exceeded. Reduction in noise can be achieved by installing the noise source as far away as possible or by use of attenuators. Generally the noise level should be kept as low as possible at its source, that means selecting low noise fans. Room absorbtion (Figure 8) Each room has a specific attenuation characteristic. It depends on the state of the walls, floor, ceiling, furniture and of course the size. The sound pressure level LPA is different at each position of a room. The figure is always lower than the sound power level LWA of the noise source. With room size and average absorbtion coefficient the average room absorbtion can be calculated (in m2 Sabine). Directivity factor Q The directivity factor takes into account at what position the noise source and the listener are with in a room. Noise angle 45°, Q = 4 Noise angle 0°, Q = 8 Room attenuation AL Is the difference between sound power and sound pressure level (VDI 2081). | | To adapt a sound power level figure to what the human ear hears it can be converted into a sound pressure level. Stated with spherical sound level propagation (free-field conditions) the reduction can be calculated in relation to the distance form the noise source (figure 4). For an exact noise figure within a room the absorbtion capacity of the room is of much higher importance. Sound emission levels affecting the neighbourhood. The following recommended sound levels for neighbourhoods should not be exceeded. | | Figure 4 Difference between sound power and pressure level at a distance: | | Figure 6 Addition of several noise sources of equal intensity | | |
| | | Example classroom Room volume: 72 m3 average absorbition coefficient: 0.1 a m average aborbtion area. :Sabine 14 m2 Room position 1, grille in the middle of the room Noise angle 0°, Q = 8 Distance 1.8 m AL = 2.5 (dB) Room position 2, grille in room corner Noise angle 45°, Q = 4 Distance 4 m AL = 5 (dB) | | |