Active Noise Control in Ventilation Systems Practical Implementation Aspects

Abstract: In many kinds of buildings, the ventilation is handled by a mechanical ventilation system. Such ventilation systems constitute a well known source of broadband noise. As awareness of the negative effects that subjection to low frequency noise can have on human well-being has increased, so too has the requirement for quieter ventilation installations. Traditionally, duct born noise is attenuated using passive resistive silencers. These passive silencers are valued for their ability to produce a high level of attenuation over a broad frequency range, however they tend to become large and bulky if designed for low frequency attenuation. The active noise control (ANC) technique is known for its ability to produce high levels of attenuation in the low frequency range even with a relatively moderate sized ANC system. On the other hand, ANC normally tends to be ineffective for higher frequencies. Accordingly, a combination of active- and passive techniques, i.e. the construction of a hybrid active/passive silencer, provides a duct silencer solution of manageable size which also covers the low frequency range. The ANC systems controller normally relies on adaptive digital signal processing. Even so, adequate levels of attenuation are not likely to be obtained if the installation of the ANC system is not designed to account for the physical factors that may degrade its performance. This thesis focuses on applying ANC in ventilation systems, with particular emphasis on analysis and installation design, for the purpose of reducing the influence of some of these degrading physical factors. The degrading factors which are of particular interest include: flow induced noise in the microphone signals, acoustic feedback between the control loudspeaker and reference microphone, and standing waves inside the ducts. With respect to installation design, focus is also placed upon industry requests on the ANC system. Taking this into consideration has led to a module based approach, in which the microphones and the loudspeaker are installed in separate modules based on standard duct parts. This thesis comprises four parts. The first describes initial investigations of potential microphone installations intended to reduce flow induced noise. The second part analyzes the influence of flow induced noise on the digital controller and presents further investigations of microphone modules. Further, results of measurements conducted in an acoustic laboratory according to an ISO-standard are presented. The attenuation produced by the ANC system was approximately 15-25 dB between 50-315 Hz even for airflow speeds up to 20 m/s. The third part of this thesis focuses on the possibility of using the passive silencer with which the ANC system is combined, to reduce acoustic feedback and standing waves. The fourth and final part investigates the possibility of using a passive silencer to reduce standing waves in the duct when the ANC system is not installed near the duct outlet.