Sound Level Control for Air Handling Units

People react very differently to the background noise of their environment depending on the activity they are performing. If the background noise is excessive, the sounds are perceived as unwanted noise. Focused intellectual work, for example, regularly requires a lower noise or background sound level than physical activity. The background sound emission of air handling units (AHU) is also perceived as either disturbing or acceptable, depending on the situation. This is particularly problematic in rooms with highly variable human activity, i.e. rooms in which phases of focused, noise-emphasising work and physical activity alternate. An example of such spaces are school classrooms, where noise pollution - also from air handling units - has to be avoided during lessons, while background noise is likely to be submerged and negligible during break times. The use of air handling units is often essential in order to maintain hygienic standards. In common rooms with several people in them, these devices can be perceived as disturbing during quiet periods. Therefore, there is a risk that they are switched off for the sake of convenience. However, as soon as the level of activity in the room increases, the unit is not necessarily put back into operation, which ultimately means that a pathogen-reducing air exchange then no longer exists: This situation has to be avoided. In this presentation, a concept is introduced with the objective of suppressing or reducing the sound emission of air handling units during idle periods (without putting the air handling unit out of operation) and allowing it during active periods. The sound level emitted by the air handling units shall be adapted to a desired background noise in the respective room depending on the situation. Such sound level adjustment has long been applied to devices and systems with loudspeakers. With this technology, also known as automatic volume control, the volume of a loudspeaker is increased or decreased depending upon the volume of the ambient noise. Unlike this well-known principle, the concept presented here permits the adaptation of a noise source (e.g. AHU) to the sound level present in a room. It consists of a microphone, an analyser and a controller and can be used in all common ventilation units (mobile air purifiers, central ventilation and air conditioning systems, etc.). The first findings will be presented at this presentation. The focus lies on the operating principle and the results of the measurements in a room with adjustable reverberation time and different positions of the noise sources and the sound sources due to the ventilation equipment.