Prototype development of hybrid

active/passive silencer for ventilation noise

Everyday noise and health

Noise problems are common in industrial and domestic applications. Common sources of indoor low frequency noise are, for example, related to Heating Ventilation and Air-Conditioning (HVAC) systems installed in buildings. These systems are, for example, installed in factories, office buildings and public buildings as well as in schools and hospitals in order to obtain a high indoor air quality, however, a drawback with HVAC systems is that they may reduce the acoustical comfort indoors. Fan noise is transmitted through the ventilation duct system and radiates throughout the building.


 Different kinds of ventilation systems are usually also installed in supermarkets and as standard in new constructed dwelling houses and as add on ventilation kits to old buildings. Accordingly, low frequency noise is frequent in our everyday life and we are usually exposed to low frequency noise during long periods of the day. At night the ventilation noise is usually more dominant since the level of the background noise masking the ventilation noise is usually lower during this period. This may result in that we turn of the ventilation system, resulting in reduced air quality.

A long exposure to low frequency noise can be a health hazard, for example, cause fatigue, stress and reduce the human performance. From an economic point of view this is negative. According to companies in the ventilation industry -manufacturers and contractors- low frequency noise produced by HVAC systems is a growing problem in building applications. Demands concerning the reduction of low frequency noise generated by ventilation systems will continue to grow to insure an acceptable acoustic indoor comfort in our buildings.

Traditional solutions

Today, conventional silencers for HVAC systems are based on a passive noise control approach. Porous sound absorbing material is used, and the material covers the interior surface of the duct or forms parallel baffles inside the duct.


The noise attenuation is dependent preliminary on the length of the silencer, the thickness and the flow resistance of the sound absorbing material, the air passage and the wavelength of the sound. The passive silencers have the advantage that they produce high broadband noise attenuation, from mid- up to high frequencies. On the other hand, for low frequency noise the passive silencers are fairly ineffective, resulting in low frequency duct born noise propagates to different parts of the building. Due to the long acoustic wavelength of the low frequency noise more efficient passive silencers for low frequency noise tend to be very large, which results in bulky and unpractical large silencers. Another drawback with passive silencers is the pressure loss over the silencers. A high pressure loss requires more powerful fans, resulting in increased running cost for the system which is, for example, negative from an economical point of view. To obtain a high attenuation of low frequency noise in HVAC systems more cost efficient methods are needed.


Active Noise Control solutions and benefits

Active Noise Control (ANC) is a complement to the passive noise control. In several applications it has been proven that ANC is a useful method to low frequency noise problems. Applications were ANC systems have been successful are, e.g., reduce noise in ducts, cabin noise reduction in turboprop and jet aircraft as well as different vehicles and marine applications. In HVAC applications hybrid passive/active silencers are needed to enable noise attenuation over a broad frequency range. Other advantages with an ANC based silencer is that the physical size and weight of the silencer can be reduced compared with only using a large passive silencer. This is preferable from a practical installation point of view as well as in the manufacturing and transportation of these silencers. Hybrid passive/active silencers result in that the physical space limitation in buildings is no longer an issue when installing ventilation systems.


The research team at BTH is working with the development of a low cost hybrid silencer consisting of a module based system design and a digital feedfoward control system that fit the ventilation industries requirements. This module based low cost hybrid silencer is based on standard ventilation duct parts and passive silencers which is an advantage from the manufacture side. The module based design results in a very flexible silencer; the active part can be combined with different kinds of passive silencers and circular and square duct configurations. In these cases the passive silencer can be adapted to fit the specific control situation. The loudspeaker and microphones can also easily be reached outside the duct enabling easy maintenance of the system. The flexible design is unique for low cost hybrid silencers and such silencers have not been reported from other researchers. A research area in the active control of propagating sound in ducts is focusing on the control of higher order modes. To control the higher order modes a multiple channel active noise control system based on several loudspeakers and error sensors are needed.  Current research in algorithm development focuses, for example, on tracking rapidly changing noise characteristics and reducing the calculation load for the control system with a view to be able to handle a large number of loudspeakers and error sensors for efficiently control the higher order modes.

Project leader: Sven Johansson

Project supervisor: Lars Håkansson











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