The Latest Developments Of An Attenuator For Naturally Ventilated Buildings

To be presented at inter.noise 2004, The 33rd International Congress and Exposition on Noise Control Technology Prague, 22-25 August

C. Field
Arup Acoustics, Level 10, 201 Kent Street, Sydney, NSW, Australia, 2000

Chris.field@arup.com.au

Abstract [082] This paper presents the updated and improved design of an attenuator for naturally ventilated buildings. The first version of the device was presented at Internoise 96, as part of PhD research carried out at the University of Sydney. Since the first design, many improvements have been made to enhance the sound transmission loss performance of the device. The latest device has been designed to be installed in the façades of naturally ventilated buildings to facilitate natural airflow into and out of buildings, whilst attenuating external noise entering buildings. The device consists of quarter wave resonators arranged into arrays, tuned to particular frequencies of noise. Transmission loss test results for the device are presented, indicating a weighted sound reduction index, Rw = 22 dB.

1. Introduction

During doctoral research on the attenuation of noise entering buildings through ventilation openings, a passive noise control prototype was developed to reduce the level of external noise entering buildings whilst allowing natural ventilation. The first working prototype was completed in 1997. Since 1997, the design of the ventilator attenuator has been optimized to maximize natural airflow into buildings and minimize the intrusion of noise. This paper presents the development of the device, the latest transmission loss testing results, and pressure drop test results from testing in a laboratory wind tunnel at the University of Sydney. A comparison of pressure drop to other existing devices is also presented.

2. Applications of the device

The ventilator attenuator can be used where natural ventilation is required, but is specifically suited to noisy environments, or where acoustic privacy is required.

During the initial development stage of the device, the intended use was in residential buildings located close to busy roads, railways, or under aircraft flight paths. After the first working prototype was tested, further applications were identified. Recent draft revisions to building ventilation requirements in international building codes [1,2] have resulted in emphasis towards sufficient natural ventilation in residential, educational and commercial buildings. Therefore, applications for the ventilator attenuator can be summarized as follows:

• In facades of existing or new residential or school buildings to facilitate natural ventilation and to prevent buildup of pollutants generated by sources within buildings which may affect the health of occupants or lead to damage of buildings.
• For fresh air intake/exhaust paths to commercial office buildings, found in environmentally conscious building design using the “stack” effect for ventilation.
• In air transfer voids/paths for mechanical ventilation systems in ceiling voids and in doors to corridors in private offices where speech privacy is required.

3. Latest prototype design details

3.1 Developments in the Prototype Design

Since the first working prototype was produced in 1997, the design of the ventilator attenuator has been optimized to be installed in a standard double brick cavity wall. Figure 1 below shows a photograph of the first prototype installed in a residence for acoustic testing in 1997. Figures 2 and 3 show the latest prototype, which has been reduced in size to replace two standard bricks in a double brick cavity wall.

Figure 1: First prototype

Figure 2: Latest prototype (one piece only)	Figure 3: Latest prototype with casing

3.2 Tube Lengths and Arrangement

Arrays of quarter wave resonators are tuned to frequencies of sound in the range of 500 Hz to 2 kHz. The lengths of the tubes can be adjusted, however, to suit the source of noise to be attenuated. For example, for installation near busy roads, the resonator lengths can be tuned to target traffic noise frequencies specific to the site.

The resonator arrays are arranged in order of increasing length, with the smallest tubes located closest to the side of the façade closest to the source of noise, as this represents the arrangement which minimizes the pressure drop across the device.

3.3 Ventilation Opening

The shape of the ventilation opening has been designed with aerodynamic considerations taken into account, minimizing pressure drop and to therefore facilitate airflow into buildings. The total open area of each device is approximately 5400 mm2. The ventilation opening path is kinked so that the direct line of sight from inside to outside is blocked. The open area of the ventilation opening represents 17 % of the total area of the device. The device has been designed in modular form so that multiple devices can be joined together to facilitate higher airflow rates into buildings where required.

4. Acoustic testing of Latest prototype 

4.1 Details of the Device for Testing

The device tested was a ventilator array consisting of six 230 mm high x 150 mm wide x 300 mm deep ventilator attenuator modules glued together to form a 2 x 3 array. The casings of the modules were of 4.5 mm thick white acrylic and the attenuators within each module were made of clear injection moulded polyurethane.

The ventilator array was 540 mm wide x 540 mm high overall and was installed in an opening size of 550 mm wide x 550 mm high in a plasterboard filler wall. The ventilator array was framed on both sides of the wall with an architrave of 90 mm wide x 19 mm thick timber.

The device used for testing is shown in Figure 4.

4.2 Testing Procedure and Results

Testing was carried out in accordance with AS 1191-2002 in 2002, which has recently been revised as ISO 717.1 2004 [3]. Results of testing are shown in Figure 5.

The test results indicate significant sound reduction from 400 Hz to 4 kHz, the frequencies to which the resonators are tuned. Significant low frequency attenuation is also achieved due to the small dimensions of the ventilation opening relative to the wavelength of incident sound.

The relatively poor sound reduction performance in the 200 Hz third octave band can be attributed to excitation of the frequency of resonance of the device, which can be considered to be similar in behaviour to an expansion chamber [4]. From the dimensions of the device, the resonance frequency of the device was calculated to be approximately 210 Hz, which is consistent with the reduced performance of the device in the 160 Hz and 200 Hz third octave bands (assuming a reasonably low Q factor for the device). 

The weighted sound reduction index, corresponding the results given in Figure 5 is Rw = 22 dB (or Dn,e,w = 37 dB). For comparison, the expected performance of closed and well-sealed single glazing is Rw = 25 dB, whilst an open window in a residential façade would provide a maximum sound reduction of 10 dB. Therefore the performance of the ventilator attenuator is comparable to a closed window whilst still providing natural airflow into buildings.

5. Pressure drop testing of latest prototype

Pressure drop testing was also carried out to ensure that the ventilator attenuator facilitated satisfactory natural airflow into buildings. According to the latest developments in the UK Building Regulations, when the new approved Building Regulations - Document F [1] is issued (probably early 2005), there will be reference to UK Building Bulletin 93 [2] and clarification of the ventilation rate at which internal classroom ambient noise levels needs to be achieved. As natural ventilation of school classrooms is a targeted application of the ventilator attenuator, comparison of the pressure drop across the device to achieve minimum airflow requirements is necessary.

Current ambient noise level limits in Building Bulletin 93 are set at a minimum airflow rate of 3 Litres/s/person. 

Figure 6 shows measured pressure drop with flow rate through one ventilator attenuator device measured at in the wind tunnel laboratory at the University of Sydney.

Figure 6: Pressure drop test results

The pressure drop results given in Figure 6 indicate that each ventilator attenuator provides sufficient airflow for up to 10 occupants of a classroom with a reasonably low pressure drop. Therefore a total of three devices would be adequate for a classroom with 30 occupants. The relatively low pressure drop to achieve airflow into buildings can be demonstrated by direct comparison to two devices with similar ventilating functions. Figure 7 shows pressure drop for various flow velocities (ie flow rates corrected for open areas of the respective devices to account for the devices having different open areas for ventilation) through the device compared to a Greenwood Overframe Window Ventilator (open area 4000 mm2), a Passivent Aircool Ventilator (open area 100,000 mm2) and a typical 300 mm deep acoustic louvre used for exhaust outlets in plant rooms (30 % open area).

The pressure drops given in Figure 7 indicate that the ventilator attenuator provides a significantly lower pressure drop for a given airflow velocity than the other devices with similar ventilating applications.

6. Conclusion

The design of a ventilator attenuator, first presented at Internoise 96 as part of PhD research, has been developed and refined over the past eight years to the point of commercial production.

Results from sound reduction testing of the latest prototype device indicate an Rw = 22 dB. Pressure drop testing of the device shows that the ventilator attenuator provides a relatively low pressure drop compared to other natural ventilation devices.

Acknowledgements

The author gratefully acknowledges the assistance of Chris Matthews for test results and graphics supplied for this presentation. The author also wishes to acknowledge the ongoing involvement of Honoury Professor Fergus Fricke from the University of Sydney.

References

[1] The UK Building Regulations – Approved Document F, Ventilation, HMSO, 1995.

[2] Department for Education and Skills, Building Bulletin 93 – Acoustic Design of Schools, 2003.

[3] ISO 717.1 2004 Acoustics – Rating of sound insulation in buildings and of building elements Part 1: Airborne sound insulation

D.A. Bies and C.H. Hansen, Engineering Noise Control – Theory and Practice, E & F.N. Spon, 1998, pp 353