PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

DESIGNING A BETTER PLENUM WINDOW OR BALCONY FORHIGHER NOISE REDUCTION AGAINST OUTDOOR NOISELouisa LY CHEUNG1; Gary YUEN2; Calvin CHIU3; David BK YEUNG4; Y.G. TONG5; Ching
CHAN6;
1,2,3,4,6
5

Ramboll Hong Kong Limited, Hong Kong

Universiti Tun Hussein Onn Malaysia, Malaysia

ABSTRACT
Plenum windows have been used as innovative measures to reduce traffic noise in recent decade. Nowadays,
they are also used in mitigating more serve traffic noise, railway noise or even fixed noise which requiredifferent noise reduction characteristic and enhanced performance.
To enhance the noise reduction performance of the plenum windows and balconies, different kinds ofabsorption treatment like micro-perforated absorbers and perforated absorption panels had already been used
and tested. They could provide around 1dB of improvement. Meanwhile, window designers are finding
alternative ways to further enhance the performance, such as by changing the physical design and dimensions.
The most common changes are to increase the overlapping length of the plenum of the window or by adding
a solid parapet in front of the plenum balcony door.
Numerous designs of windows and balconies with different dimensions have been tested, with different
overlapping length, different acoustic treatment and even different outer window or balcony door design. The
tests were done in full scaled mockup using linear array of loudspeakers mimicking traffic noise source. The
effects in noise reduction of implementing these design changes to plenum windows/balconies are being
studied by comparing the measurement results in this paper.
Keywords: Plenum Window, Façade Device

1. INTRODUCTION
Hong Kong is a vibrant city with a very dense population. The demand of residential space isalways high. More and more residential buildings are built in response. At the meantime, theballooning population also brings more traffic to the city. These newly planned residential projects
are facing more and more challenging traffic noise problems.
In recent decade, a few different plenum windows have been used in different residential projects
to mitigate different kind of environmental noise. Offsite mockup tests are commonly used todetermine the noise attenuation performance of specially design acoustic windows and balcon ies inHong Kong.
In order to find ways to further improve the noise attenuation of plenum windows, three different
plenum windows were tested with different overlapping length and other window designs, ie. sidehung and top hung window and the results is presented and discussed in this paper.

1lycheung@ramboll.com
gyuen@ramboll.com
3 cchiu@ramboll.com
4 dyeung@ramboll.com
5ygtong@uthm.edu.my
6 mchan@ramboll.com
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2. CURRENT PLENUM WINDOW DESIGN
The plenum window is formed by placing a sliding panel as a noise screen behind an opened
window to block the direct path of the noise from outside to entering the indoor habitable space. Thesliding panel is wider than the width of the window opening to create an overlapping of a minimumof 100mm. A 100mm separation between the outer window and the sliding panel is maintained toallow natural ventilation. Currently, most sliding panel are having an overlapping of 100mm to 300mm
and fitted with micro-perforated absorber on the side facing outward. Sometimes, absorption materials
are also added to the top and the vertical sides of the plenum. The MPA and absorption materials areincorporated to increase the absorption of the plenum to further reduce the sound energy beingtransmitted through the plenum windows.
Previous laboratory and mockup testing have been done to study the incorporation of
microperforated absorber (MPA) to the sliding panel and adding absorption material to the air gap of
the plenum.(1)3.

DETERMINATION OF NOISE REDUCTION

To describe the traffic noise attenuation performance of plenum windows, insertion loss andtransmission loss are the two major terms used in Hong Kong(1,2). In determining the insertion loss,
the performance of the window is compared with the performance of a conventional window, ie. aside hung swing window of the same size or smaller size. The Transmission Loss (TL) is determinedby comparing the measured indoor and outdoor noise level difference and weighted with normalized
traffic noise spectrum(3).
3.1 Mockup Test
Full scaled off site mockup tests were carried out to study the impact of the change in design
parameters. The mockup was constructed in a factory site in Mainland China, with a 3 -storey building
fitted with the test window at the façade of tests rooms with size similar or smaller to the room thatthe window is designed to be built with. Line source was used as the sound source to mimic thecarriageway generating traffic noise to residential buildings. White noise was used as testing signal
and the measured results were weighted with the normalized traffic noise spe ctrum as stated inBSEN1793-3(3) to determine the traffic noise attenuation of the specially designed plenum window.

Figure 1 Set up of the Off-site mockup test. The array of loudspeakers is placed parallel to the testing
façade, which create a 0º line source angles

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4.

IMPROVEMENT MEASURESIn designing the special acoustic windows for mitigating environmental noise in residentialbuildings, architects and the clients take the leading role. It is a pre-requisite to provide the minimum
natural light and ventilation by the window. Followed by the aesthetic concern and construction
constraints, there is little room for huge changes made to the window design dimensions. The usabilityof the windows, such as with a very long sliding panel taking up footprint and being not able to fully
open the outer window for maintenance and cleaning, is concerned during design.
There are few changes that can be possibly done currently. Increasing the overlapping length,
adding more absorption material to the plenum have the least impact in changing the design. Switching
the outer window design from side hung to top hung are huge changes which may turn over the whole
façade design.

5.

RESULTS AND DISCUSSION

Two side hung type plenum window was measured to study the effects of increasing overlapping
length. A top hung type plenum window was tested to demonstrate the benefits of using top hungwindow as the outer window instead of a side hung outer window. Traffic noise weighted transmission
losses (TLtraffic ) are used to represent the traffic noise attenuation of the tested windows. Window 1 inTable 1 was tested with 2 line source angles while window 2 and window 3 in Table 2 and Table 3
was tested with 1 line source angle.
5.1 Increasing overlapping
Table 1 shows the measured transmission losses weighted with traffic noise spectrum of window 1
with an outer window of 1165mm(H) x 600mm(W) tested with increasing overlapping length from300mm to 565 mm. Table 2 shows the measured results of window 2 with larger opening size,1600mm(H) x 700mm(W), with 450mm and the very wide 800mm overlapping. The difference ofresults is also shown in Table 1. Each transmission losses measured were compared with that of the300mm and presented in form of the delta of transmission losses.

Figure 2 Window 1 tested with increasing overlapping. The view from outside shows the outdoor
microphone location and the largest overlapping length
From Table 1, it can be observed that increasing the overlapping length, from 300 mm to 565mm
in a steps of around 50mm, gives better improvement to the TLs when the line source is at 45º withthe building façade. The TLs increased with the increase in overlapping length. The improvement
ranges from 0.4 to 0.9 dB(A) with a parallel line source and that with a 45º inclined source ranges
from 0.6 to 1.2dB.
Table 2 shows the increases in TL traffic of window 2 by increasing the overlapping from 450mm to800mm. The 800mm overlapping is the maximum overlapping length that can be allowed within the2.4m wide room. Window 2 was tested with different absorption settings. The tremendous increase in3508

overlapping length at window 2 gives surprising increases in TL traffic s, especially when there are MPA
incorporated on the sliding panel. The improvement ranges from 2.2dB(A) to 4.2dB(A).
Table 1 Dimensions, different settings, measured traffic-weighted TLs and differences of TLs of window 1,a side hung plenum window with 100 air gaps
Window 1

Overlap(mm)MPA

Absorption

Source

TLtraffic

materialAngledB(A)∆TLtraffic

300

16.6

365

16.9

0.4

17.1

0.6

17.2

0.6

415

0º

465
515

Yes, with 40mm

Yes, at ceiling

17.5

0.91165(H) x565

airgap, absorptive

and vertical

17.4

0.8600(W)300

mounting box,

side of the

16.8

365

NRC 0.5

opening

45º415

Non-465

favorable

515
565

17.4

0.6

17.7

1.0

17.5

0.7

17.8

1.1

17.9

1.2

Table 2 Dimensions, different settings, measured traffic-weighted TLs and differences of TLs of window 2,a side hung plenum window
Window 2Overlap(mm)Gap (mm)

4501600mm(H) x100700mm(W)800MPAAbsorption
MaterialLinesourceangle

TLtraffic∆TLtraffic

dB(A)

dB(A)

No

No

10.3

--

Yes

No

11.4

--

No

Yes

11.2

--

Yes

Yes

12.1

--

No

No

13.1

2.8

Yes

No

15.6

4.2

No

Yes

13.4

2.2

Yes

Yes

15.7

3.6

0º

5.2 Using top hung window as outer window
A 700mm wide top hung type plenum window was used in the tests. The window opening size, theline source setting, overlapping length and the absorption material provisions were the same aswindow 2 in previous section. hence, the results of window 2 and window 3 can be compared directly.
The measured results are tabulated in Table 3. The measured TLtraffic s of a top hung window (window3) are at least 4dB(A) larger than those of a side hung window (window 2) of the same sizeį ŕũŦġdifference ranges from 4dB(A) to 5.1dB(A). This brings a very good improvement as a small designchange. Like window 2, the improvement with the presence of an MPA is larger than those cases3509

without this relatively large absorption surface.

Figure 3 The view of window 3, a top hung plenum window, from inside
Table 3 Dimensions, opening width, absorption materials, measured traffic-weighted TLs and differences ofTLs of window 3, a top hung plenum window
Window

Dimensions

OverlappingAbsorption

TLtraffic∆TLtraffic

(mm)

(mm)

material

dB(A)

dB(A)

No

No

10.3

--

Yes

No

11.4

--

No

Yes

11.2

--

1600(H) x

Yes

Yes

12.1

--

700(W)

NoNo

14.4

4.1

Yes

No

16.4

5.1

No

Yes

15.1

4.0

Yes

Yes

17.0

4.9Side Hung450

Top Hung450MPA

6. CONCLUSION
Mockup tests were done to study the effect of increasing the overlapping length of the plenum
windows and by changing the outer window from a side-hung swing window to a top hung windowof the same size. Increasing the overlapping length can bring up to 1.2 dB(A) by increasing theoverlapping from 300mm to around 650mm. it can bring a maximum of 4.2dB(A) improvement when
the overlapping is increase from 450mm to 800mm. changing the outer window from side hungwindow to top hung window can improve the TL traffic by at least 4dB(A). The tested results suggested
more feasible ways to further improve the noise attenuation performance of plenum windows when avery large noise attenuation is required.

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7. REFERENCE
1. Tong YG, Tang SK, Kang J, Fung A, Yeung MKL. Full scale field study of sound transmission acrossplenum windows. Appl 2015; 89:244–53.
2. Cheng T, Cheung LLY, Yeung DBK. The Effects of Acoustic Treatment on Plenum Windows in Reducing
Outdoor Noise in Residential Buildings. INTER-NOISE and NOISE-CON Congress and Conference
Proceedings; 26-29 August 2018; Chicago, USA 2018. p. 3318–25.
3. British Standards Institution. BS EN 1793-3:1998 Road traffic noise reducing devices. Test method for
determining the acoustic performance. Normalized traffic noise spectrum. 1998.

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