REDUCING THE INGRESS OF URBAN NOISE THROUGH VENTILATION OPENINGS

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1 REDUCING THE INGRESS OF URBAN NOISE THROUGH VENTILATION OPENINGS DJ Oldham 1*, MH de Salis 2 and S Sharples 3 1 Acoustics Research Unit, School of Architecture and Building Engineering, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, UK. 2 Vipac Engineers and Scientists Ltd., Unit E1-B,Centrecourt, North Ryde, NSW 2113 Australia. 3 Centre for the Built Environment, School of Environment and Development, Sheffield Hallam University, Sheffield S1 1WB, UK. ABSTRACT For buildings in busy urban areas affected by high levels of noise from road traffic the potential to use natural ventilation can be limited by excessive noise entering through ventilation openings. This paper is concerned with techniques to reduce noise ingress into naturally ventilated buildings while minimizing airflow path resistance. A combined experimental and theoretical approach to the interaction of airflow and sound transmission through ventilators for natural ventilation applications is described. A key element of the investigation have been the development of testing facilities capable of measuring the airflow and sound transmission losses for a range of ventilation noise control strategies. A method is proposed for quantifying the acoustic performance of different strategies to enable comparisons and informed decisions to be model leading to the possibility of a design methodology for optimising the ventilation and acoustical performance of different strategies. INDEX TERMS Natural ventilation, noise control, road traffic, urban noise. INTRODUCTION Interest in the use of renewable energy sources to achieve ventilation for new and existing buildings has been stimulated by concerns regarding sustainability and the need to conserve finite energy reserves (Liddament, 1996). However, the small pressure differentials available to drive natural ventilation systems in buildings requires a system to have inherently low airflow resistance in order to achieve adequate ventilation rates. Opening large areas of the building facade can achieve low airflow resistance but this will significantly decrease the noise insulation of the building fabric. Natural ventilation systems will offer little resistance to the ingress of externally generated noise and hence the option of natural ventilation will often be restricted to buildings in areas of low ambient noise levels. Increasing pressures to exploit natural ventilation, however, necessitate an examination of measures which will render it a viable option in a wider range of areas including those with higher background noise levels, typically due to road traffic. This paper is concerned with techniques to reduce noise ingress into naturally ventilated buildings while minimizing airflow path resistance. It is primarily concerned with large non- * Contact author djoldham@liv.ac.uk 878

2 domestic buildings of open plan design as small domestic buildings tend to be closed plan causing difficulties with airflow paths THE ACOUSTICAL AND AIRFLOW PERFORMANCE OF APERTURES The standard equation for Sound Reduction Index (SRI) of a composite panel consisting of an element of area A A and Sound Reduction Index SRI A in a wall of area A W and Sound Reduction Index SRI W is: SRI W + A A 10 W A ( db) = 10 log ( A + ) (1) W AA SRIW 10 + A 10 SRI A 10 Equation (1) shows that the effective sound insulation of a composite façade is a function of the sound reduction indices and relative areas of each component. Typically for a naturally ventilated building the composite façade Sound Reduction Index SRI A+W will tend to be dominated by the poor performance of the ventilation aperture (determined by SRI A ) Airflows in naturally ventilated buildings result when a pressure differential, P, is created across a building's facade by wind and /or buoyancy forces. The equation of flow for a thin orifice plate shown below has been shown experimentally to be valid for a large opening of simple geometry, such as would be employed as a ventilation aperture in a building façade: Q = A ( P ) 0.5 m 3 s 1 (2) where A is the area of the orifice plate in m 2. Although the airflow performance of an aperture is determined only by its absolute area, its acoustical performance is a function of its relative area and the relative areas and sound insulating properties of other elements of the façade. The acoustical performance of a façade containing an aperture is also frequency dependent and thus the effect of an aperture on a façade will depend upon the nature of the solid elements of the façade and the spectral characteristic of the noise source. As road traffic is the major source of exterior noise in urban areas, the technique adopted in this work to enable the effect of noise control strategies on the acoustical and airflow performance to be evaluated together was to use a single figure SRI calculated by logarithmically summing the attenuated A weighted traffic noise spectrum and subtracting it from the sum of the un-attenuated spectrum and to base airflow performance on a standard façade area (1 m 2 ) but to present it as a function of the percentage of façade area occupied by the ventilation opening. This approach can be used to quantify the required percentage open area of a façade for a given flow rate with a certain pressure difference and the resultant effective sound reduction index for road traffic. NOISE CONTROL STRATEGIES Noise control engineering is now an established profession with a rich literature describing the application of a range of techniques arising from several decades of applied research (see, for example, Bies and Hansen, 1996). However, few of these techniques have been applied to control noise ingress through a simple aperture. A number of potential techniques for reducing noise ingress via a ventilation aperture have previously been reviewed by the authors (de Salis, Oldham and Sharples, 2002). Data exists concerning both acoustical and airflow performance for some of these techniques which can be employed to model their 879

3 performance. However, for others there is a lack of data and a measurement programme has been undertaken by the authors to provide missing data and fuller details of this work have been presented elsewhere (Oldham, de Salis and Sharples, 2001). In obtaining measured data the technique of acoustic intensimetry was employed in order to make direct measurements of the sound power transmission characteristics of the inlet/outlet systems and airflow measurements were made using fan based systems to establish air flows through models of the air inlet and ducting configurations. Figure 1. Effect of Distance of Simple Screen from Aperture on Measured Insertion Loss. Figure 2. Effect of Distance of Simple Screen from Aperture on Measured Airflow Characteristics 880

4 Results obtained for a simple noise control device consisting of a screening plate in front of an aperture are shown in Figures 1 and 2. It can be seen that at low frequencies some acoustical benefit can gained by moving the plate closer to the aperture but in the higher frequency range little variation is noted with distance from the aperture. However, the flow rate resulting from a given pressure differential is not reduced significantly as the plate is moved closer to the hole until the separation distance is very small. Insertion loss results for larger ventilators may be predicted by scaling the frequencies down by the scaling ratio. Most conventional noise control treatments (e.g. screening using louvres) are not effective at low frequencies. When these devices are used to combat road traffic noise their effectiveness at reducing the A weighted sound pressure level is limited. This suggests the need to use hybrid systems consisting of techniques to act on both low and high frequencies. Active noise control is most effective at low frequencies and as a result, for a noise source such as road traffic, it has to be employed in combination with other techniques which are most effective at high frequencies (Oldham, de Salis and Sharples, 2001). Results of experiments for an inlet system incorporating a significant length of ductwork (2 metres) are shown in Figure 3 and it can be seen that the performance obtained is comparable to that for temporally invariant noise sources. Figure 3. Attenuation of Recorded Traffic Noise by Active Noise Control System no active noise control: with active noise control. DESIGN GUIDANCE Figure 4 illustrates a design methodology has been developed involving the presentation of acoustic and airflow data in such a way as to enable the designer to make an informed choice The airflow in cubic metres per hour per metre squared of façade area per Pascal 0.5 may be calculated as a function of the proportion of ventilator area to total façade area. A chart of sound insulation to road traffic noise against flow rate may then be compiled. This can be applied where a specified flow rate and sound insulation are required. The necessary open area for an assumed pressure differential or the required pressure differential for a given open area to achieve the design conditions can be determined. 881

5 Figure 4 shows the projected performance of apertures to normally incident sound when screened by the absorbent backed plates described above and incorporating active noise control. Up to 4 dba increase in attenuation to road traffic is achieved when compared to the purely passive attenuator. This is significant as the maximum possible increase in performance would be 7 dba to bring the performance up to that of the homogeneous wall. As an example of the use of the design technique consider a simple building of dimensions 20m x 10m x 5m with an air change requirement of 5 per hour and a design pressure differential of 4 Pa. The corresponding value of Q is 5 x 1000/(100 * 2) = 25 m 3 hr -1 /m 2 Pa 0.5. It can be seen from Figure 4 that this value of Q can be achieved for 0.9% open area of the façade with a sound reduction of approximately 35dB for simple screens close to the aperture and approximately 37.5dB with the addition of active noise control. Figure 4. Acoustic and Airflow Design Chart for Simple Screen and Active Noise Control System in Front of Aperture. CONCLUSIONS Natural ventilation system need not necessarily result in poor acoustic insulation of a building. A simple ventilator design such as a screen in front of an aperture could provide up to 16 dba greater Sound Reduction Index than a wall façade containing an aperture while transmitting flow rates commensurate with requirements for sensible cooling and typically available natural pressure differentials. Combination of this strategy with one that is more effective for low frequencies such as active noise control may be able to provide the above flow rates and an SRI to road traffic approaching that of the homogeneous wall. A method of presenting the acoustical performance of a ventilator in conjunction with airflow performance data has been proposed which would enable a designer to select an approach that will satisfy both attenuation needs and other design requirements. 882

6 ACKNOWLEDGEMENTS The financial support of the Engineering and Science Research Council of the United Kingdom in the course of this work is gratefully acknowledged. REFERENCES Bies DA and Hansen CH 1996 Engineering Noise Control, Theory and Practice. E and FN Spon. De Salis MH, Oldham DJ and Sharples S 2002, Noise control strategies for naturally ventilated buildings. Accepted for publication in Building and Environment. Oldham DJ, de Salis MH Sharples S. 2001, Reduction of noise ingress for naturally ventilated buildings. Proceedings of the 8th International Congress on Sound and Vibration. Hong Kong, China, pp Liddament ML A Guide to Energy Efficient Ventilation. AIVC. 883