Energy Conservation by Active Noise Attenuation in Ducts

Abstract

This paper examines the possibility of using a combined active and absorptive attenuator in order to give good attenuation over a wide noise band from frequencies as low as 20 Hz while maintaining a very low pressure drop. A comparison is also made on the initial and operating costs of a combined active attenuator with that of a proposed silencer with a higher pressure drop. Active attenuators, although higher in initial cost, can show substantial cost savings over the life of the installation. The potential savings in energy can be considerable when the savings in power consumption per unit floor area is generalized to the office space within a big city.Silencers are normally used in air-conditioning ducts in order to reduce duct-borne fan noise. The presence of the conventional type of silencer, in certain cases, may present a significant obstruction to the airflow and increase the back pressure on the fan. Building services engineers may find that the space required for fitting a larger silencer is not available and may have to employ a silencer with a high pressure drop. This results in a low initial cost but also, less obviously, a high operating cost over the life of the installation. The silencer market has always been highly cost-conscious. Initial costs in purchasing silencers have often been considered far more important than operating costs. This explains why active noise attenuators, which are generally higher in initial cost than conventional silencers, have not been widely accepted, despite the fact that active attenuation of noise in ducts was established some fifty years ago. Over the years, active attenuation systems with one, two, or three loudspeakers have been proposed. The basis of these systems is to cancel unwanted noise by the addition of further noise, which is the inverse of the unwanted noise. The simplest effective system is the monopole attenuator, which comprises a microphone to detect the duct-borne noise and a loudspeaker to feed back the inverted sound, a The main advantages of active attenuators are significant low frequency performance and zero or very low back pressure. The low pressure drop across the active attenuator will result in long term savings in operating costs and in the electrical energy consumed by electrical equipment, Advances in technology have made the needed electronic transducers inexpensive and the active attenuators much more attractive than they have been in the past.It is generally accepted that, .for the foreseeable future, low-frequency attenuation below 500 Hz will be provided by active attenuators while high frequency attenuation will be more efficiently provided by absorptive passive attenuators in a combined system. Noise Control in Air-Handling SystemsNoise in air ducts is a common phenomenon in blocks of multi-story buildings which are fully air conditioned with fans and air-handling systems. Air-moving devices often create high levels of low frequency noise which readily propagate to the service areas via supply and extract duct work. Passive sil...

Part I - Effects of Diameter on Active Noise Control in Rectangular and Round Ducts

“…(Note that Guffey and Hickey (1983) report that using a single smaller duct with the same flow would change the pressure by the diameter ratio to the 4.5 power.) While splitters have been used to increase surface area of acoustically absorptive material in ducts for some time (CuUum, 1949;Beranek, 1960), there is only one study applying them to active noise control (Eghtesadi et al, 1986). As is discussed next, another primary consideration in applying active noise control is the width of the frequency bands.…”

Effects of Cross-Sectional Partitioning on Active Noise Control in Round Ducts

“…Whereas splitters have been used to increase surface area of acoustically absorptive material in ducts for some time, (12,13) there is only one study applying them to active noise control. (14) Further, each different axial vane splitting scheme would have different modes in the ducts. The acoustics of the ducts involved were not treated in this article.…”

Part II - Effects of Cross-Sectional Partitioning on Active Noise Control in Round Ducts

“…Eghtesadi et aL (1986) actually split an HVAC duct to reduce higher order mode concerns. Egafia et aL (1989) used an HVAC duct from a train car that was already split by design.…”

“…After reviewing the literature, no diameter effects studies or cross-sectional partitioning strategy studies were found except Eghtesadi et aL's (1986) study on energy conservation. The data obtained from such experiments would aid in devising simple ANC solutions for large duct broadband noise problems so often encountered in industry.…”

“…(Note that Guffey and Hickey (1983) report that using a single smaller duct with the same flow would change the pressure by the diameter ratio to the 4.5 power.) While splitters have been used to increase surface area of acoustically absorptive material in ducts for some time (Cullum, 1949;Beranek, 1960), there is only one study applying them to active noise control (Eghtesadi et al, 1986). …”