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Low Frequency Noise from Air Handling Units — ABB Points the Way Forward

environment is low frequency noise, usually coming from air-handling systems. Its effects can be very debilitating to workers exposed to it. Just how debilitating it can be remains the subject of intense study, particularly in Scandinavia. Nonetheless, since it is commonly agreed that such noise at the very least will lead to worker efficiency losses, there is a market for solutions. ABB is developing a way to cut out this noise in ventilation and air conditioning systems. To date, silencer systems have tended to be heavy pieces of equipment, depending mostly on using fibrous sound muffling material. The higher the frequency of noise the more of this fiber needs to be used. Since it is packed into the system it will often block the air flow channels, leading to a loss of pressure in the air unit and therefore reduced performance. Another disadvantage is that traditional methods can do nothing about low-frequency noise. ABB has now developed an advanced silencer that not only reduces high-frequency noise with less dampening material but also for the first time offers a way to get rid of low-frequency noise. The new silencer takes an active approach to attacking lowfrequency noise. This involves a microphone fitted ahead of the silencer unit that records the lowfrequency sound. The sound is then fed into a controller, where the phasing of the sound waves is manipulated digitally. The controller kills the low-frequency sound by producing a precise opposite of the sound wave formation. A second microphone, fitted downstream of the silencer, is used to monitor that the sound has been cleaned up. The system is the first to offer a way to eliminate sounds below 150 hertz. Higher-frequency sounds are tackled using dissipative dampening techniques. The difference here is that fibrous material is replaced by a plastic muffling material. This is less bulky, meaning that dampening is achieved with a lower pressure drop than at present. A further benefit of using the plastic material is that no micro-fibers are blown into the air – a particularly important feature for units used in clean environments like hospitals, laboratories and clean rooms. The advanced silencer, which will be marketed beginning in autumn 2001, is designed to work with the latest air units made by ABB but can be retrofitted to older systems already installed in buildings.