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1 Welcome Contents Back 1 Active silencers for air-conditioning units P. Leistner, H.V. Fuchs 1. Introduction The noise emission of air-conditioning units can be reduced directly at the fan during the design stage by standard constructive and technological measures. However, for demanding applications the sound level achievable by primary noise control exceeds the required values and thus additional silencers are needed. The high acoustic requirements for such silencers culminate at the low frequencies with a minimum of space available and only marginal pressure loss allowed. Additionally, the geometric appearance and design of the unit have to be retained. To resolve this difficult but frequently arising conflict a new series of air-conditioning units, e.g. for computer rooms, telecom networks or museums, is equipped with equally novel active silencers. Their attenuation principle is based on an acoustic resonance system tuned to low frequencies, the efficiency of which is significantly increased by an electro-acoustic reinforcement system. 2. Sound attenuation and system integrity High attenuation at low frequencies is required when noise problems of heating, ventilating and air-conditioning (HVAC) systems have to be solved with silencers or sound absorbers. Furthermore, acoustic components have to fulfill specific demands concerning size, hygienic, thermal and chemical integrity as well as installation and running costs. A variety of solutions have become available during the development of Alternative Fibreless Absorber (ALFA) modules at IBP [1], Fig. 1. Several acoustic resonance systems may be tuned to the low frequency range and can be combined to yield broadband attenuators, e.g.: Welcome Contents Back 1

2 Contents First Page Back 2 Membrane Absorbers A combination of Helmholtz- and plate resonators as all-metal splitter silencers for high hygienic demands and / or harsh environments, Fig. 2, Angular Stack Silencers A compound of plate resonators forming an angular draft cladding to the interior walls of exhaust stacks and ventilating ducts, Fig. 3, Microperforated panel or foil absorbers A multitude of Helmholtz resonator surface structures with inherent friction loss as light-weight silencers and absorbers for high temperatures and vibrational load, These examples illustrate combinations of high attenuation at low frequencies with other special virtues according to very specific demands. The outstanding benefit of Active Silencer Cassettes as another member of the ALFA-family is their high broadband sound attenuation achievable with a minimum of silencer length. Therefore, it fits the compact design of an air-conditioning unit with only a marginal extension of its size. 3. Active silencer cassettes The basic system of Active Silencer Cassettes [2,3] is represented by an acoustic resonator which mainly consists of a loudspeaker membrane (masslike) and the air volume (spring-like) of the rear casing, Fig. 4. The attenuation of the resonator, which is tuned to low frequencies, increases considerably if the sound pressure close to the membrane is detected by a microphone and if the microphone voltage is inverted, amplified and fed back to the loudspeaker coil. This electro-dynamically driven resonant structure is designed to minimize the acoustic impedance of the front side of the cassette. Since the loudspeaker and the rear volume essentially define the resonance frequency an adaptation to a given noise spectrum is possible. Furthermore, the amplification can be varied to adjust the impedance close to its optimum according to any individual application. The upper limit in frequency as well as sound intensity of this absorption reinforcement system is set by the restricted mobility of the membrane and by the electro-acoustic stability of the feedback loop. Contents First Page Back 2

3 Contents First Page Back 3 The electrical installation of the active cassettes requires only the connection to a DC-power supply (24 V). The power consumption depends on the sound intensity of the incident noise and is in the range between 2 and 8 W. Thin acoustically transparent layers, e.g. perforated sheet metal, fibre fabric or plastic foil serve as a protective cover of the components and reduce the flow noise generated by the flow over the cassette. These compact cassettes can be regarded as the basic element of a modular silencer system for combined and optimized wall linings or splitter silencers in HVAC-ducts. 4. Application at openings of air-conditioning units The geometric integration is one of the major problems of silencers for air inlets and outlets of air-conditioning (AC) units. As mentioned before, several aspects such as compactness, design and minimization of volume have to be considered to retain the original performance and appearance of the unit. Under these conditions short silencer arrangements with broadband attenuation and sufficiently large air gaps are particularly required. A typical application of an AC unit is shown in Fig. 5 (Conditioner Himod, Liebert Hiross). Here the air in the room enters through the inlet at the top of the unit, passes the conditioning components and discharges into a double floor. At the selected measuring point in front of the unit the solid floor panels suppress acoustic contributions of the downstream side. Therefore in this case a silencer is needed at the inlet opening only, but with minimal height and with the given cross-section of the unit. The measurements are carried out in a semi-anechoic chamber. The measured sound pressure level spectrum in Fig. 6 at the selected point (Fig. 5) clearly illustrates the dominance of low frequencies. Especially the peak level at the blade passing frequency of about 200 Hz is perceptible as an annoying tonal noise. On the basis of these results the silencers were designed as shown in Fig. 7. In a vertical frame with a height of 300 mm the active silencer is arranged in three lines whereby each line consists of three active cassettes. All the cassettes have the same components, and thin foam layers are placed at their opposite side. Due to these passive absorbers for attenuation at middle and high frequencies an asymmetrical broadband silencer is achieved. The sound pressure level with the silencer built in (Fig. 6) convincingly shows the broadband attenuation as well as the reduced tonality problem at 200 Hz. During Contents First Page Back 3

4 Contents First Page Back 4 these measurements made on the conditioner the air flow of the unit was adjusted to about 9000 qm/h and its reduction by the silencer amounts to less than 3 %. With the corresponding air speed of 10 m/s in the spacing between the silencers the proper function of the active cassettes could be demonstrated in the presence of air flow. 5. Concluding remarks The innovative approach with active silencers is based on the electro-acoustic support of the resonant vibrations of a mass-spring-system. The high broadband attenuation at low frequencies results from the adjustment of an impedance which approximates the theoretical optimum for grazing sound incidence. Due to the use of inexpensive standard components in a compact metal casing the Active Silencer Cassettes can be adapted and easily implemented to solve various noise control problems. The achievable attenuation at low frequencies with very short silencers represents the major advantage especially at openings of AC-units, Fig. 8. Several silencer frames can be arranged in line for further noise reduction where the necessary space is available. Continuous detection of the electric power consumption may be used to monitor the function of the active cassettes. Apart from this, the modular system of the silencer enables its integration into the service concept of HVAC systems. 6. Joint development of marketable products The R&D results with active silencers for AC-units as described above represent only the first step towards a marketable product. Usually, far too many promising noise control solutions after their detailed scientific investigation fail to comply with the harsh market requirements. For instance, hundreds of sophisticated active noise control systems are treated in thousands of high-grade international publications, but only negligibly few of them were actually commercialized so far. Of course, many obstacles have to be surmounted to transfer R&D results into a market governed mostly by small and medium-size enterprises. Here, the Fraunhofer concept [4], Fig. 9, of a close and continuous alliance of researchers and industrial partners stands for joint developments of marketable products. On a Platform Innovative Acoustics (PIA) with over 10 German manufacturers and a Contents First Page Back 4

5 Contents First Page Back 5 growing number of open-minded applicants a bridge is being built from research into practice. This platform releases synergies and saves resources of all partners so that even very small companies [4] can successfully exploit recent developments of novel acoustic components and technologies, see the case history described in [5]. 7. Summary The noise emission from air openings of innovative and compact air-conditioning units, e.g. for computer rooms, telecom networks or museums, is reduced by the application of equally novel active silencers. The practical requirements are directed especially towards high attenuation at low frequencies with a short and compact silencer arrangement as well as marginal pressure loss. With a top frame, which comprises a special configuration of active silencer cassettes, the broadband level reduction at the dominating low frequencies exceeds 10 db. The operating principle of the cassettes is based on an acoustic resonance system tuned to low frequencies, the efficiency of which is significantly enhanced by employing electro-acoustic reinforcement. The easy installation of the cassettes as well as the permanent monitoring of their function allows an integration into the service concept of the air-conditioning units. Contents First Page Back 5

6 Contents First Page Back 6 8. References [1] Fuchs, H.V.: Alternative fibreless absorbers new tools and materials for noise control and acoustic comfort. (in print) Acta Acustica 87 (2001). [2] Krüger, J.; Leistner, P.: Noise reduction with actively absorbing silencers. Applied Acoustics 51 (1997) 1, [3] Krüger, J., Leistner, P.: Wirksamkeit und Stabilität eines neuartigen aktiven Schalldämpfers. Acta Acustica 84 (1998) 4, [4] Fuchs, H.V.; Eckoldt, D.; Hemsing, J.: Alternative Sound Absorbers for Industrial Use: Acousticians on the Quest for Alternative Attenuators. VGB Power Tech 79 (1999), [5] Fuchs, H.V.: From advanced acoustic research to novel silencing procedures and innovative sound treatments. (in print) Acta Acustica 87 (2001). [6] HiSAS The semi active silencer. Technical Handbook, Liebert-Hiross S.p.A., Contents First Page Back 6

7 Contents First Page Back 7 1 Porous Recycling Glass 2 Membrane Absorber 3 Compound Panel Absorber 4 Broadband Compact Absorber 5 Angular Stack Silencer 6 Cleanable Reactive Silencer 7 Active Quarter-wavelength Absorber 8 Active Silencer Cassette 9 Microperforated Panel Absorber 10 Microperforated Suspended Ceiling 11 Microperforated Foil Absorber 12 Microperforated Automotive Absorber Fig. 1: Alternative Fibreless Absorber ALFA materials and elements as developed and marketed together with 11 industrial partners [1] Contents First Page Back 7

Contents First Page Back 8 Charateristics Structure Applications tunable (< 50 Hz) silencer splitters all-metal enclosures robust / resistive partitions sealed all-around linings easy cleaning

8 Contents First Page Back 8 Charateristics Structure Applications tunable (< 50 Hz) silencer splitters all-metal enclosures robust / resistive partitions sealed all-around linings easy cleaning baffles low pressure drop screens easy recycling shields Fig. 2: Technological characteristics, physical structure and possible applications of the membrane absorber boxes [1] Contents First Page Back 8

Contents First Page Back 9 Charateristics Structure Applications stainless steel sheets stack-integrated exhaust stacks ventilating ducts space-saving power plants non-clocking mineral fibre plants

9 Contents First Page Back 9 Charateristics Structure Applications stainless steel sheets stack-integrated exhaust stacks ventilating ducts space-saving power plants non-clocking mineral fibre plants easy cleaning waste burning plants low frequencies chemical plants no pressure drop industrial plants Fig. 3: Technological characteristics, physical structure and possible applications of the angular stack silencer [1,4] Contents First Page Back 9

10 Contents First Page Back 10 protective cover microphone loudspeaker 24 V amplifier casing Fig. 4: Schematic sketch of a single active silencer cassette Contents First Page Back 10

11 Contents First Page Back 11 air inlet silencer AC unit (fan, air-filter, heat-exchanger etc.) 2 m measuring point (example) 1.5 m air outlet double floor Fig. 5: Schematic sketch of the air-conditioning unit with air intake from the room and outlet into a double floor Contents First Page Back 11

12 Contents First Page Back sound pressure level [db] without silencer with silencer k 2k 4k 8k frequency [Hz] Fig. 6: Measured sound pressure level (measuring point see Fig. 5) with and without inlet silencer (see Fig. 7) Contents First Page Back 12

13 Contents First Page Back 13 air inlet active triple cassette absorptive layer mm silencer AC unit Fig. 7: Schematic sketch of the active inlet silencer Contents First Page Back 13

14 Contents First Page Back 14 Fig. 8: Illustration of different types of air-conditioning units with active inlet silencers [6] Contents First Page Back 14

15 Contents First Page Back 15 PHASES STEPS FRAUNHOFER Research & Development Adaptation & Production Optimization & Marketing Problem Solution Test Verification Prototype Patent Suitability Design Resistance Durability Demonstration Presentation Quality Quantity Price Competition Penetration Co-operation Diversification INDUSTRY Fig. 9: Fraunhofer concept for the joint development of marketable products [5] Contents First Page Back 15

Development of a Reactive Silencer for Turbo Compressors

Development of a Reactive Silencer for Turbo Compressors Jan Smeulers Nestor Gonzalez TNO Fluid Dynamics TNO Fluid Dynamics Stieltjesweg 1 Stieltjesweg 1 2628CK Delft 2628CK Delft jan.smeulers@tno.nl nestor.gonzalezdiez@tno.nl