Active control of noise and vibration

Ventilation system

Ventilation, heating and air-conditioning systems are installed in factory and office buildings as well as public buildings, e.g., in hospitals. Different kinds of ventilation systems are also often installed in our dwelling houses. Low frequency noise is often annoyance and can also cause fatigue, stress and reduce the human performance. According to a large Swedish ventilation system manufacturer low frequency noise in ventilation systems is an important problem. A long exposure of low frequency noise can be a health hazard. Low background noise level is, however, very important Ventilation_taketfor well being and maintaining a high standard of acoustical comfort. Consequently, the noise produced by the ventilation systems is a growing problem in building applications.

Today, the conventional silencers for reducing ventilation noise are based on porous sound absorbing material. The conventional passive silencers consist of that sound absorbing material covers the interior surface of the duct or forms parallel baffles inside the duct. The attenuation of the noise is dependent preliminary on the silencer length, the thickness and the flow resistance of the absorbing material, the air passage and the wavelength of the sound . The passive silencers have the advantage that they have high attenuation over a broad frequency range over approximately 250 Hz, on the other hand, for noise below approximately 250 Hz the performance of the passive silencers are fairly ineffective. From other applications it has been proven that Active Noise Control (ANC) can be used to solve low frequency noise problems. It has been shown that the ANC systems efficiently attenuate low frequency noise where passive methods has been either ineffective or tends to be very large and bulky. On the other hand ANC systems are ineffective at higher frequencies, but in this frequency range the passive silencers perform very well. However, a silencer based on a combination of active and passive control methods results in high noise reduction over a broader frequency range, from the low frequency range up to the high frequency range. Other advantages are that the ANC based silencer results also in reduced size, weight and volume, which are preferable in practical installation point of view, as well as in consideration of the production and transportation of these silencers.



The aim of the project is to attenuate ventilation noise by using active noise control technologies. Low frequency noise is a growing problem in industrial and domestic applications. Generally, ventilation, heating and air-conditioning systems produce low frequency noise problems as the conventional passive silencers do not enable a sufficient attenuation at these low frequencies. VentilationsrörActive noise control systems efficiently improves the attenuation of low frequency noise. However, important constrains given by the industry in this application are; an active noise control system should be easy to adapt to an existing ventilation system, an active noise control system should essentially be based on standard ventilation parts and a ventilation mechanic should be able to mount an active noise control system. An important objective in the project is the development of a module based active noise control system based on standard ventilation parts which enables sufficient low frequency attenuation and adapts to the industries conditions.

The approach to use active noise control enables a non-bulky solution to low frequency ventilation noise problems. Preliminary active ventilation noise control experiments show promising results.

Furthermore, the components of an active noise control system -microphones, loudspeaker and control unit- enable the design of a module based active noise control system. The microphones and the loudspeaker may be incorporated into microphone modules respectively a loudspeaker module based on standard ventilation ducts and parts. The module design is likely to be a suitable active noise control solution for the ventilation industry. Furthermore, the module based active noise control structure seems also to be beneficial with respect to maintenance of the active control system, e.g. at malfunction of the active noise control system uncomplicated and inexpensive replacements of standard system modules are likely to solve the problem.

In cooperation with Lindab it is our wish to develop a module based active noise control technology for the reduction of low frequency ventilation noise.

The purpose with the project is the development of algorithms and methods for active noise and vibration control in the applications: Active control of noise in ventilation systems . The project focuses towards technologies that enable a sufficient base for product development at a technological level close to the research front in these applications. During the two years we have the following partial goals:

• Design a robust experimental set-up for the evaluation of active noise control in ventilation systems with different configurations.

• Experimental investigation of the acoustical properties of ventilation systems with important duct configurations.

• Develop a prototype module based active noise control system for a ventilation system.

• Implementation and initial evaluation of the prototype module based active noise control system in an experimental set-up of a ventilation system.





Project manager Dr. Lars Håkansson

Research responsible Prof. Ingvar Claesson



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