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Design and applications of lean active resonator silencer
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maximum stable feedback gain and may produce unwanted control spillover effects Hence the gain. of a feedback ANC system needs to be carefully tuned Nevertheless implementing a feedback. controller using an analog circuit has the benefit that the hardware costs and system complexity is. relatively low One example of a successful implementation are active resonator silencer cassettes. ASCs which have been developed at the Fraunhofer IBP for more than two decades 4. The second section of this paper gives a brief overview on the characteristics the performance and. some practical applications of classical ASC units and arrays which have dimensions in the range. of decimeters The third section of the paper describes more resent work on ASCs which has been. focused on the design of much leaner ASCs with dimensions in the range of centimeters The fourth. section of the paper presents the key results on practical applications of lean ASC units and arrays in. a compact active noise control system for a partly opened sliding window 5 and a compact active. silencer system for a round ventilation duct 6 Finally the results are summarized and possible future. work on lean ASC units is discussed,2 CLASSICAL ASC. An ASC comprises a loudspeaker mounted in an air thigh casing a microphone which is mounted. in close vicinity of the loudspeaker membrane and an analog electronic controller circuit with. adjustable gain The basic components of a classical ASC are shown in Figure 1a In essence a. passive ASC is a mass spring damper system The loudspeaker membrane is the mass and the. membrane suspension and the enclosed air volume in the casing form the spring Viscoelasticity in. the membrane suspension and other effects introduces some damping to the resonant system. Figure 1b shows that the absorption coefficient of the passive ASC control off has a relatively sharp. peak at around 125 Hz which is the resonance frequency of the spring mass damper system Closing. the feedback loop around the microphone loudspeaker pair control on amplifies the natural. membrane movement around the ASC resonance frequency Figure 1b shows that for the absorption. coefficient of the activated ASC the peak around 125 Hz is much wider in frequency and has a much. higher amplitude As reference Figure 1b also shows the absorption characteristics of a porous. absorber which is not efficient at low frequencies. Figure 1 a 3D drawing of an ASC b sound absorption coefficient normal incidence angle of a porous. absorber and an ASC with open and closed control loop. 2 1 Classical ASC applications, In the past ASCs have been successfully applied as low frequency absorbers in various technical. noise control applications 7 8 9 Figure 2 shows three concepts for ASC silencer systems The first. concept shown in Figure 2a comprises an ASC flush mounted in the wall of a duct with porous. absorption material mounted on the opposing duct wall This concept is suitable for air ventilation. ducts with relatively low flow speeds and normal climate conditions The second concept shown in. Figure 2b is similar to the first but is specifically designed for the application in the exhausts of. heating systems where burners produce low frequency noise In such a system the ASC is mounted. in a branch of the duct in order to take it out of flow and to reduce the impact of high exhaust gas. temperatures It needs to be considered that this influences the effective resonance frequency of the. ASC A foil is used to protect the electro mechanical components from heat and condensate The third. concept shown in Figure 2c comprises an ASC mounted in a labyrinth wall opening Such openings. in walls of buildings and enclosures are often needed to provide pressure equalization natural. ventilation suck in fresh air and expel exhaust gases. Figure 2 Sketch of ASCs concepts for a an active duct silencer b an active silencer for heating. systems c an active labyrinth wall opening, Another typical application of ASC Systems are splitter silencers for larger ventilation systems. As illustrated in Figure 3a for this application usually arrays of ASCs are employed in combination. with passive absorber splitters As shown in Figure 3b at low frequencies the insertion loss IL of. the splitter silencer with ASCs is higher than for an equivalent splitter silencer employing only porous. absorbers In order to get a good broadband performance it is important to combine ASC systems with. appropriate porous absorbers Towards high frequencies the performance of splitter silencers is. limited by the width of the gap between the splitters The results in Figure 3b also illustrate an. interesting effect the attenuation maximum of the ASCs varies with the microphone position upstream. or in the center axis of the ASC loudspeakers This has to do with the slight change in the delay. between the acoustic signal picked up by the microphone and the loudspeaker reaction to it This. changes the open loop transfer function of the closed feedback control loop. Figure 3 a 3D Sketch of ASC array splitters in a ventilation duct b The IL according to ISO 7235. position of the ASC detection microphone is varied. Figure 4 shows pictures of commercial applications of the active silencers concepts discussed. above All these practical systems provide significantly higher noise attenuation at low frequencies. than porous absorbers of the same size The main drawback is the relatively high costs of such silencer. systems Therefore they have only be successful in applications where restrictions in installations. space outweighed the increased costs, Figure 4 Pictures of practical active silencer applications a ASCs top of a HVAC server unit. b a compact active silencer at the exhaust side of a heating system c splitter silencer in a wind tunnel. In all ASC applications it is important to fine tune the feedback gain of the ASCs in order to. achieve good low frequency performance and to limit high frequency spillover effects In. appropriately designed and tuned silencer systems passive porous absorbers counter balance active. control spillover effects of the ASCs,3 LEAN ASC DESIGN.
Classical ASCs have dimensions in the range of decimeters i e 250x250x200 mm3 This limits the. range of applications to which they can be applied For example the concept of an ASC in an labyrinth. wall opening shown in Figure 2c would only be feasible for relatively thick wall constructions but not. for thin walled lightweight enclosures In addition the production costs of classical ASC has proven. to be too high for a wider commercialization Hence in order to widen the range of possible. applications and reduce manufacturing costs resent work on ASCs has been focused on the design of. much leaner ASCs with dimensions in the range of centimeters i e 50x50x50 mm3 As indicated in. Figure 5 the initial idea was to replace a classical ASC by an 5x5 array of lean ASCs which has the. same effective area but only requires a quarter of the installation height Scaling the dimensions of. the ASC down by almost an order of magnitude comes with a number of design challenges This. section addresses the design challenges and discusses the characteristics of the lean ASC units and. ASC line arrays The concept of a 5x5 ASC array has not yet been implemented. Figure 5 a Schematics of a classical ASC and b Schematics of a lean ASC design and a concept for. a 5x5 array of lean ASCs with equivalent effective area as a classical ASC. 3 1 Scaling and design of ASC arrays, As discussed in Section 2 a passive ASC is a mass spring damper system Hence reducing the size. of the loudspeaker and reducing the air volume enclosed by the casing increases the resonance. frequency Furthermore small loudspeakers have a lower stroke limit which limits the maximum. sound power output at low frequencies This is contrary to the idea of using ASCs for the reduction. of low frequency noise In order to investigate concepts and components for lean ASCs a test set up. has been designed which implements the ASC concept shown in Figure 2a The set up comprises a. duct with a rectangular cross section of 50x50 mm with a primary noise source at one end and an. anechoic termination at the other Figure 6a shows with Type A and Type C two examples of lean ASC. designs that were investigated Figure 6b shows the ILs for different types of ASC designs Compared. to the classical ASCs shown in Figure 1 the resonance frequency of the lean ASCs is about three. to four times higher Nevertheless for some designs an IL of over 10dB is achieved in the frequency. range between 125 Hz and 500 Hz All lean ASC designs investigated produce control spillover effects. in the frequency range between 800 Hz to 1000 Hz This may be improved by optimizing the. characteristics of the feedback loop control circuit. Type A Type C, Figure 6 a Pictures of two lean ASC designs b results of the IL for three types of lean ASC designs. Based on the results for single ASCs the ASC design type C was selected for studies on ASC. line arrays comprising a total of four individual ASCs in a decentralized Multi Input Multi Output. MIMO feedback control scheme Figure 7a shows a picture of the ASC line array The results for. the ILs with one two three or all four ASCs switched on simultaneous are presented in Figure 7b. The interaction between the individual ASCs reduces the stability of the MIMO system Hence the. feedback gains were set such that the MIMO system with all four ASCs switched on remained stable. Therefore the IL results for one active ASC are about 5 dB lower than those for a single type C ASC. shown in Figure 6b Nevertheless using all four ASCs simultaneously yields an IL of more than 20 dB. in the frequency range between 125 Hz and 315 Hz However the more ASCs are used simultaneously. the higher are the spillover effects in the frequency range around 800 Hz This indicates that MIMO. ASC systems need to be handled with care and that further investigations on the stability of the MIMO. feedback loop are required for additional improvement of the performance. Figure 7 a Picture of a lean ASC line array b results of the IL for one two three or four active ASCs. 3 1 1 Application of activated carbon powder in the back volume. In order to arrive at an even smaller installation height of the ASCs without shifting the ASC. resonance frequency further up the stiffness of the back volume in the casing needs to be reduced In. classical ASCs porous fibre material in the back volume is used to shift the resonance frequency. and the effective frequency range down by a couple of Hz One possible approach to drastically. reduce the stiffness of the back volume is to evacuate the air However the low static atmospheric. pressure in the back volume would require an entirely different transducer design since conventional. loudspeakers are not designed to operate at high static pressure differences 10. Another efficient more practical approach is the application of activated carbon in the back volume. Activated carbon is available in different forms e g as granulates and powders Investigations and. studies on the acoustic absorption characteristics of activated carbon have shown that fine loose. activated carbon powder is the best filling to reduce the stiffness of the back volume of the lean ASCs. 11 Figure 8a shows a prototypical loudspeaker casing with activated carbon powder filling The. loudspeaker is mounted with the outside of the membrane facing into the loudspeaker casing This is. because otherwise the fine activated carbon powder would enter into the gap between the coil and. magnet of the voice coil motor Figure 8b shows the spectra of the sound power levels generated by a. loudspeaker mounted in a reference casing and a casing which only has a quarter of the reference. volume with and without activated carbon powder filling. Figure 8 a prototypical loudspeaker casing with activated carbon powder filling b spectra of. the sound power levels generated c picture of the resulting lean ASC design. The results for the back volumes with carbon powder filling show that the resonance frequency of. the loudspeaker system shifts towards lower frequencies by about one octave Also in the frequency. range between about 250 Hz and 400 Hz the loudspeaker systems with small filled back volume and. large empty back volume generate sound pressure levels of about the same magnitude Using an. activated carbon powder filling allows reducing the height of the lean ASC back volume to about. 20 mm Applying a loudspeaker with slim driver design allows to design a lean ASC unit with a total. installation height of about 25 mm Figure 8c shows the resulting lean ASC design. 4 LEAN ASC APPLICIONS, Scaling the dimensions of the ASC units down by an order of magnitude widens the range of. possible applications This section presents the key results of two studies in which lean ASC units are. used to build up compact active silencer systems,4 1 Sliding window with active gap. Open windows are probably the easiest and most common way to ventilate rooms However open. windows even if only partly opened do not provide efficient sound insulation Hence especially in. noisy urban outdoor environments opening a window can cause unwanted acoustic transmissions to. the inside A concept to tackle this problem are windows with acoustically muffled ventilation gaps. Based on the ASC application concept shown in Figure 2c lean ASC units where used to design an. active noise control system for a partly opened sliding window 5 Figure 9a shows the 3D drawing. of a sliding window On the right hand side the opening frame is extended with an L shaped profile. which overlaps with the wall and ranges over the entire height of the window Figure 9b illustrates. that this extension forms a labyrinth gap when the window slides partly open At the end of the. extension profile a line array consisting of 20 lean ASCs is installed to actively reduce low frequency. noise transmission through the gap to the inside Figure 9c shows one segment of the ASC line array. which itself consist of four individual ASC units, Figure 9 a 3D drawing of a sliding window b illustration of the labyrinth gap with ASCs c picture.
of a segment of the ASC line array, After preliminary studies a prototypical life size window was constructed and installed in a sound. transmission chamber as shown Figure 10a The results for the sound transmission loss STL. measurements for the closed window the open window without labyrinth profile the open window. with passive labyrinth profile and the open window with labyrinth profile and activated ASCs are. presented in Figure 10b The STL of the open window without acoustic treatment has a significantly. lower STL than the closed window The STL of the open window with passive labyrinth is similar to. the STL of the window without acoustic treatment except for frequencies above 1800 Hz where the. passive treatment in the labyrinth gap becomes effective. Figure 10 a prototypical window installation in the test chamber b results of the STL measurements. In the frequency range up to 400 Hz the STL of the open window with ANC is almost as high as. the STL of the closed window The weighted STL of the open window with ANC is about 4 dB higher. than that of the open window without acoustic treatment However the relatively poor STL in the. frequency range between 400 Hz and 2500 Hz limits the value of the weighted STL However overall. the results of the experimental studies on the sliding windows with active gap are promising. Shortcomings that result in the poor STL in the frequency range between 400 Hz and 2500 Hz may be. overcome by developing passive acoustic treatments that compensate the active control spillover. effects and cover the high frequency range more efficiently. 4 2 Active silencer for a round ventilation duct, Due to increasing requirements on energy efficiency and air tightness of buildings forced. ventilation systems are increasingly popular even for domestic dwellings Hence there is an. increasing demand to reduce noise from Heating Ventilation and Air Conditioning HVAC systems. As discussed in the introduction silencers for the control of low frequency noise require a relatively. large installation space By extending the ASC application concept shown in Figure 2a lean ASC. units where used to design a compact active silencer system for a ventilation duct 6 The test set up. comprises a duct of circular cross section with a diameter of 130 mm with a primary noise source at. one end and an anechoic termination at the other Figure 11a shows the basic ASC set up including a. 20 mm thick porous absorption layer Figure 11b shows a view into the duct. Figure 11 a Sketch of the ASC set up b picture of the view into the duct. In the study various configurations of the active silencer components have been investigated most. notably the application of multiple control loudspeakers Figure 12a shows the four principal control. configurations considered a a single ASC b two ASCs mounted opposite of each other at an angel of. 180 c three ASCs mounted at angles of 120 and d four ASCs mounted opposite of each other at. angles of 90 For simplicity only the signal of one single microphone sensor was fed back in phase. to all ASC loudspeakers simultaneously This reduces all configurations to a Single Input Single. Output SISO feedback control system The benefit of using two or more loudspeaker driven in phase. is that the first cross section mode of the duct at around 1547 Hz is not excited efficiently Using three. or more loudspeaker driven in phase results in an acoustic ring source 12 which does not efficiently. excite higher cross section modes of the duct Figure 12b shows the results of the IL for all four ASC. configurations With increasing numbers of ASC loudspeakers the IL in the low frequency range up. to about 600 Hz is increasing For the configuration with three and four loudspeakers an IL of over. 14 dB is achieved around 200 Hz However with increasing number of ASC loudspeakers the active. control spillover effects around 1000 Hz are also increasing For the configurations with one and two. ASC loudspeakers the passive porous absorption layer compensates the active control spillover For. the configuration with three ASC loudspeakers the control spillover effects are barely compensated. For the configuration with four ASC loudspeakers the control spillover exceed the IL provided by the. passive absorption layer resulting in a negative overall IL in the frequency range between 800 Hz and. 1000 Hz However the results of the experimental studies on active silencers for a round ventilation. duct with a SISO feedback control scheme are promising and encourage further investigations Within. this study the stability of the feedback control loops was systematically investigated by measuring. and analysing the open loop transfer functions of the feedback loops This analysis provided. interesting insights and hints for possible improvements Further studies may investigate a SISO. control system configuration that utilises the summed signal of all four ASC microphones and a. completely decentralised MIMO control scheme with four individual ASCs. Figure 12 a sketch of four control ASC configurations b results of the IL for ASC configurations. employing one two three or four ASC loudspeakers,5 CONCLUSIONS. This paper has given a brief overview on the characteristics the performance and some practical. applications of classical ASCs which have been developed at the Fraunhofer IBP over the last two. decades More resent work on lean ASCs has been presented especially issues with the downscaling. of the ASCs has been addressed and discussed in more detail Due to their compactness lean ASCs. have a wider application range than classical ASCs In this paper the key results of studies on. practical applications where lean ASCs are used in a compact active noise control system for a partly. opened sliding window and a compact active silencer system for a ventilation duct have been presented. The results are very promising However all studies on lean ASCs have shown that their performance. is limited by active control spillover effect in the frequency range between 800 Hz and 1000 Hz. especially for configurations where multiple ASCs are used simultaneously In order to improve the. performance further studies are needed to investigate the stability of SISO and MIMO ASC systems. The aim of the Fraunhofer IBP is to develop innovative commercially viable noise control concepts. and products based on lean ASCs,REFERENCES, 1 Schirmer W Hrsg Technischer L rmschutz VDI Verl D sseldorf 1996 S 186. 2 Cremer L M ser M Technische Akustik Engineering online library Springer Berlin 2003 S 156. 3 Gardonio P Rohlfing J Modular feed forward active noise control units for ventilation ducts The. Journal of the Acoustical Society of America 136 2014 Heft 6 S 3051. 4 Leistner P Krueger J Leistner M Hybride Schalld mpfer Hohe D mpfung bei tiefen Frequenzen. Heizung L ftung Klima Haustechnik HLH 47 1996 Heft 10 85 90 4. 5 Busse TA Anwendung Aktiver Schalld mpfer in einer Fensterl ftung Bachelorarbeit Universit t. Stuttgart Lehrstuhl f r Bauphysik Stuttgart 2016, 6 Hanft T Anwendung aktiver Schalld mpfer in einem runden Kanal Masterarbeit Universit t Stuttgart.
Institut f r Akustik und Bauphysik Stuttgart 2018, 7 Bay K Brandst tt P Kr mer M Modellierung aktiver Kompaktschalld mpfer DAGA 2006. 8 Bay K Leistner P Active sound insulation of wall and enclosure openings In 6th European Conference. on Noise Control Euronoise 2006 Tampere Finland 2006. 9 Bay K Leistner P Brandstaett P Applications of Active Resonators at Cooling Units of Rail Vehicles. In 37th International Congress and Exposition on Noise Control Engineering 2008 INTER NOISE. 2008 Shanghai China 2008, 10 Fraunhofer Gesellschaft zur F rderung der angewandten Forschung e V 80686 M nchen DE 2007. Tellerfeder Schallwandler Patentschrift EP3005725B1 17 10 2018. 11 Fraunhofer Gesellschaft zur F rderung der angewandten Forschung e V 80686 M nchen DE 2007. Akustisches System mit einem Geh use mit adsorbierendem Pulver Patentschrift EP3005726B1. 11 01 2017, 12 Swinbanks MA The active control of sound propagation in long ducts J Sound Vib 1973 27 3 411.

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