Sound transmission loss of glass and windows in laboratories with different room design

Applied Acoustics 25 (1988) 269-280

Sound Transmission Loss of Glass and Windows in Laboratories with Different Room Design

A.

Cops

Acoustics and Heat Conduction Laboratory, Department of Physics, Catholic University of Louvain, Belgium

& D. Soubrier Belgian Building Research Institute, Brussels, Belgium (Received 11 January 1988; revised version received 18 April 1988; accepted 21 April 1988)

A BS TRA C T Systematic research has been carried out in two Belgian laboratories on the sound transmission loss ( S T L ) ~?l"glasspanels and windows to investigate the influence of room design. The samples used were three glass panels with the same dimensions, but different thicknesses. Research has also been done on glass panels of the same thickness inserted into three d(fferent frames of P VC, aluminium and wood, which are normally used in practice. The different glass panels and windows were fixed in a niche between the transmitting and receiving room with the same edge dimensions. Systematic S T L measurements have been done with the conventional two-room method in both laboratories attd the sound intensity method has been used at the AHCL. The windows are of the type which can be opened and the airtightness, to get an optimum STL, has been controlled and improved (f neeessao,. The design ~[" the niche opening in the two laboratories is d(fJerent and this results in some systematic discrepancies in the STL. Also the influence of loudspeaker and microphone positions has been investigated. 269

Applied Acmtstics 0003-682X/88/$03.50 fi" 1988 Elsevier Science Publishers Ltd, England. Printed in Great Britain

270

A. ('op,s, D. Souhrier

1 INTRODUCTION To achieve optimum sound insulation against outdoor sound sources, it is important to know exactly the different sound transmission paths of the facades in the design of buildings. Generally the windows are the weakest parts and to optimize the sound insulation of facades it is important to improve the quality of windows. Therefore il is valuable to know the sound transmission loss (STL) of windows and window parts with high precision and to detect eventual leaks. These measurements can be done with the sound intensity method. 1-7 This kind of measurement on glass and windows has been done in the AHC-laboratory. Research with the conventional method has been executed in the two laboratories in order to compare the STL obtained between rooms with different volumes and size and niches with different depth and size also. Some clear systematic differences in STL values have been observed at low, medium and high frequencies, mainly influenced by the niche size and depth. Measurements have been done with different loudspeaker and microphone positions. The spread on the sound transmission loss values is rather large at 1o~ frequencies. In considering the airtightness of the three windows of PVC~ wood and aluminium, which were of an openable type, two of them were not free of leaks, and had to be adjusted.

2 M E A S U R I N G METHOD Measurements have been done with the conventional two-room method described in ISO Standard 1408 and with the sound intensity technique which is gaining acceptance as a viable alternative to the standard two-room method, especially in cases where complicated structures are concerned.~ ~ By definition the STL of a structure (in dB) is given by: R = 10 log (Pi/PO

i I)

where Pi and Pt are the incident and the transmitted sound powers. For the determination of the incident sound power, the intensity method is identical to the classic two-room method and Pi is determined by the sound pressure measurement in the diffuse field of the source room. In standard form the incident sound power is based on: P~ = ( p ~ / 4 p c ) S

12)

with Pl the sound pressure in the source room, p the density of air, c the velocity of sound in the air and S the structure surface. The transmitted

271

Sound transmission loss of glass and windows

sound power for the classic two-room method is determined from the sound pressure in the diffuse receiving room based on the relationship: (3)

P, = ( p22/4pc)A 2

where A 2 is the total surface of the receiving room. Substitution of eqns (2) and (3) into eqn (1) and conversion to the decibel scale yields the standardized formulation for the two-room method: 8 R = Lp, - L~,, + 10log(S/A2)

(dB)

(4)

For the intensity method the transmitted power is determined from the surface averaged sound intensity 11 as: (51

P~ = I~S

Substituting eqns (2) and (5) into eqn (1) yields the formulation of the intensity method: R = Lp,

-

LI,-

6

(dB)

[6)

The procedure to estimate the energy density in the reverberant rooms involves spatial averaging of the sound pressure level in the diffuse field of the rooms. Waterhouse 9 pointed out that in a reverberation room there is an increase in energy density at the boundaries. Thus estimates of the total room sound energy based on measurements of the sound pressure level in the central portion of the reverberant rooms will be too low. As a result of this work sound power measurement standards now include a correction for this effect. The real sound power in the source and receiving rooms may be accounted for by introducing the Waterhouse correction factor ~- ,o in eqns (2) and (3), which gives: Pj = [p2(1 + 2A ,/8 V~)4pc]S Pt =

(7)

[P2( 1 + fi-A2//8 V2)4pc]A2

{8)

where 2 is the wavelength, A 1, A e and V 1, V2 the internal surface areas and the volumes of the source and receiving rooms, respectively. Substituting eqns (7) and (8) into (1) and conversion to the decibel scale yields the adapted formulation for the classic two-room method: R=Lp,-Lp,+

_

l (1 +2A,/8I/,) I O I o g ( S / A 2 ) + IO og[~ + ).Az/,8V2 )

(dB)

(9)

When both rooms have equal surfaces and equal volumes this last term in the equation vanishes. For the STL measured with the intensity method the transmitted sound

A. Copes',D. Soubrier

272

power given by eqn (5) need not be corrected for the Waterhouse factor. Thus substituting eqns (5) and (7) into eqn (1) yields: N

.io-6+lOlog(l+),A,/8V1)

R=Lp~-101og

(dB)

(10)

i=1

where I i is the radiated sound intensity by the ith surface of the panel and N is the number of equally subdivided surfaces of the panel. More comments on the Waterhouse correction on STL measurements are given in Ref. l 1. The Waterhouse correction gives an increase in the STL of about 2 dB at 100Hz to 0"5dB at 500Hz, resulting in a better agreement with the conventional method. 6

3 E X P E R I M E N T A L WORK In both laboratories measurements have been executed on the same glass panels with thickness 6mm, (6-12-6)mm, (10-12-4)mm and dimensions 1.480m x 1.230m and windows with glass panels of the same thickness inserted in PVC, wood and aluminium frames. The dimensions of the glass panels inserted in the frames are: 1"260m x l'015m for the PVC frame, 1"280m x 1.030m for the wooden frame and 1.327m x 1.077m for the aluminium frame. The niche dimensions in which the glass panels and windows are inserted are 1-500 m x 1.250 m.

3.1 Influence of airtightness of the windows Before starting measurements on the STL of the different windows airtightness tests have been done following the European Standard EN 42:12 In Table 1 the results of the airtightness measurements giving the flow rate in m3/h, for overpressures from 50 up to 600 Pa are presented. The principal parameters in this table are: --the two different laboratories where the measurements have been done are indicated by AHCL (Acoustics and Heat Conduction Laboratory) and BBRI (Belgian Building Research Institute). --three types of openable frames in PVC, wood and aluminium which are normally used in practice. --three types of glass panels with thickness 6mm, (6-12-6)mm and (10-12-4) mm. Moreover, it has to be mentioned that the tightness tests in both laboratories have been done with the same equipment and by the same operator. From

Sound transmission loss of glass and windows

273

the remarks in Table 1 it is clear that, except for the PVC windows, all the other windows had to be improved for tightness before useful measurements of the STL could be started. The results as finally presented in Table 1 are much better than the conformity agreements prescribed in the European Standard UEATC.13 3.2 Influence of design of the sound transmission rooms

The main difference between the test facilities in the two laboratories is the design of the transmitting walls between the source room and the receiving room in which the test objects have been inserted. In the test facility of the AHCL, the transmitting wall is of double construction with a total thickness of 42 cm and the niche is staggered as described in DIN-52210.1'* In the test facility of the BBRI the transmitting wall is a single one, the niche has a flat construction and the depth is 22cm. The walls are of concrete and the measured glass panels and windows are fixed as shown in Fig. 1 at a position 2:1 within the niche.15 In both cases the transmitting walls have at least a 10 dB higher STL over the complete frequency range of interest than the test object, which means that the measured STL values of the test construction are not influenced by flanking. Figure 2 represents the STL ofa 6-mm glass panel measured in the A H C L and the BBRI test facilities. Small but systematic deviations occur at the lower and mid frequencies, but they are opposite to each other. These systematic deviations are mainly caused by the well known niche effect. The 1

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2

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Mineral wool

Concrete

2

Plaster

(a)

String

Resilient material

(b)

Fig. 1. (a) Mounting of a specimen in the flat opening between the sound transmitting rooms of the Belgian Building Research Institute. 1, Concrete blocks; 2, plaster material; 3, wooden lists of 25 x 25 ram; 4, mastic Perennator TX200IS; 5, testing element. (b) Mounting of a specimen in the staggered opening between the sound transmitting rooms of the Acoustics and Heat Conduction Laboratory, following DIN-52210;I~ Dimensions in mm.

0'94 1"69 2'25 2"72 3'75 4'69 .....

10-12-4

0"70 1"27 t-69 2'02 2"58 3-28 3"75

6-12-6

Wood a (ram)

0-75 t'08 t-36 1'69 2'16 2"58 3"05

6

0-47 0'94 1-22 1"45 t'83 2'11 2'49

!~-12-4

0"66 1"03 1-31 1"45 1"83 2-16 2-44

6-12-6

PVC b (ram)

0'6t 0-92 1"13 1'27 1-59 1'83 2-06

6 " - " 0-23 (/'28 0-40 0'45 0"54

--- " ---" 0"23 0'28 0'40 0"45 0"49

10-12-4 6-12-6

ALU" (ram)

0"61 0'94 1"15 1-22 1'55 1'92 2-72

6

p

50 100 t50 201) 300 400 500

( Pal

0"89 0"94 1-22 t'41 1"88 2"25 2"77

10-12-4 0-47 0-89 1"03 I'27 1"74 2"06 2-39

6-12-6

Wood" (mm)

....

1-45 2'35 2"77 3"49 4'31

6

P V C h (mm)

BBRI

t'17 t-78 2-25 2.67 3"33 3"71 +22

0"92 1"4l 1.74 2"02 2.44 2'86 3"61

1¢)-12-4 6-12-6

" Flow rate lower than the accurac? of the m e a s u r i n g equipment. b PVC window tested as delivered no i m p r o v e m e n t s necessary. ': Window frames did not c o n l b r m to the technical agreement prescription: i m p r o v e m e n t s have been made. '~ W i n d o w tested as delivered, ~' Window fi-amcs did not conform to the ~,echnical agreement prescriptions; i m p r o v e m e n t s have been made.

50 100 150 200 300 400 500

(Pa)

p

,4 HCL

TABLE 1 M e a s u r e m e n t s of the Total Flow Rate of Air (m3/h) T h r o u g h the W i n d o w s

10-12-4 0"80 0-75 1-3I 0"98 1.74 t-27 1"92 1'55 2 " 3 5 1"88 2'81 2-16 3. t4 2'49

6

0"70 0-94 1'27 1'50 1'78 2'06 2'39

6-12-6

ALU' (mm)

0-75 0-94 1"22 1"45 1'78 2"11 2.72

6

t~ "-.a

Sound transmission loss of glass and windows

275

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Fig. 2, Sound transmission loss of a glass panel with thickness 6ram, measured in ( + - - +) the AHCL and (O ©) the BBRI facility.

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125

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250

t

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1000

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Fig. 3. Sound transmission loss of a glass panel with thickness 6 mm measured in the AHCL facility: (× - x ) staggered niche: ( 0 - - - 0 ) flat niche.

higher values of the STL in the mid frequencies obtained in the AHCL facility are a result of the staggered niche instead of the flat niche constructed in the BBRI facility. This has been clearly shown in an experiment done in the AHCL laboratory when the staggered niche has been changed to a flat one at both sides of the glass panel. The results of the STL values on the 6-mm glass panel with both types of niches are shown in Fig. 3. There is a clear and systematic decrease in STL values in the mid frequencies for the flat niche configuration. At lower frequencies the results obtained in the AHCL facility are lower compared to the STL values in the BBRI laboratory. The reason is that deeper niches have a tendency to give lower STL values at lower frequencies, s In Fig. 4 a comparison of STL measurements between the two laboratories is given for a wooden window with double glass of thickness (10-12-4)ram. The same

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Fig. 4. Sound transmission loss of a wooden window with glass of thickness (1012-4)mm measured in ( + +) the AHCL facility and ( D - - t T ) the BBRI facility.

276

A. Cops, D. Soubrier

systematic differences as discussed in Fig. 2 occur. The same tendencies of differences occur for all measurements on other glass panels and windows but they are not shown here. From these comparisons the importance of standardization of niches for STL measurements is clearly shown. Other differences of minor importance between the measuring facilities are the volumes of the transmission rooms. In both cases the source and receiving rooms are equal. Those of the ACHL facility have volumes of 87 m 3 but different shape and those of the BBRI facility consist of volumes of 50 m 3 with identical shape. The smaller the test rooms, the less accurate the STL measurements are at the lower frequencies. It is clearly shown in Ref. 5 that the influence of loudspeaker and microphone positions on the 95% confidence limits of the STL increases with decreasing frequencies, for measurements obtained on a glass panel in the AHCL facility. These confidence limits on the STL measurements are even larger as shown in Ref. 15 from STL measurements between the smaller rooms of the BBRI facility. Figure 5 represents an example of the mean values of the STL measured on a PVC window with a glass panel of thickness (10-12-4)mm. The 95% confidence limits are the results of 20 different loudspeaker positions in the source room.

3.3 Glass panels versus windows As already mentioned, measurements of the STL have been done on three glass panels with different thickness of 6 mm, (6-12-6) mm and (10-12-4) mm and on three windows of PVC, wood and atuminium in which panels with ~ 50~--~-.--~---W-T--r-T -T---"~--T ~--T'--'--U-~'"

T

r

C~

20~ !

Fig. 5. Sound transmission loss of a PVC window with a glass panel of thickness (1012-4)mm, measured in the BBRI facility: ( O - - O ) mean values obtained from measurements with 20 loudspeaker positions: (+ - +) 95% confidence limits•

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~0~

I

125

25O

500

1000

i_i

20OO

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z,O00

iHz!

Sound transmission loss of glass and windows

277

these same thicknesses have been inserted. As an example, the results of the (6-12-6)-mm thick glass are shown in Fig. 6. At low frequencies some small discrepancies occur between the results obtained for the different windows, with the lowest values for the aluminium window. At mid and higher frequencies the different windows give approximately the same results. Higher differences occur if the results of the STL of the glass panel are compared with those of the windows. In the mid frequencies the panel results are at least 4 dB lower and above the coincidence frequency the panel results are the better. A possible explanation for this is the higher sound radiation of the glass panel with the larger surface at the mid frequencies resulting in lower STL values compared to the windows. At frequencies above the coincidence frequency the lower values for the STL of the windows can be explained by an inevitable tightness problem of openable windows, even if the tightness results are better than prescribed. ~3 It has to be mentioned that the STL values on 6-mm and (10-12-4)-mm thick glass and glass in windows show the same discrepancies. This p h e n o m e n o n occurred in both laboratories. 3.4

Intensity

versus

conventional

method

The results described in the previous sections have all been obtained with the conventional two-room method. In the A H C L facility also the well-known sound intensity technique has been used especially for measuring the

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1000

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2000

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~.000 f (Hz)

Fig. 6. Sound transmission loss of the double glass panel with thickness (6-126)ram compared with the results on PVC, wooden and aluminium windows, in the AHCL test facility: ( A - ~ ) glass panel: ( x - - x I PVC window; ( E l " D) wooden window; ( 0 - 0 ) aluminium window.

278

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Fig. 7. STL values of the PVC window with the 6-ram thick glass panel obiained with the intensity' method in the AHCL lest facility: ( ~ {~) nlain frame; (~ ~1 openable frame: ( ~ ~ ) glass panel: (O ()) overall values.

i i

0 i_ _fl_~... _j i . Z _ ~ _ _ ~ . _ L 125 250 500

d

I I _ L _ & & _a 1000 2000 4000 +{P~'

Fig. 8. STL of the wooden window ~sith the 6-ram thick glass panel obtained ~ith the intensity method in the AHCI. test lacility: ([i] [~) irlain frame; I F ~ ! openablc f'rarne: (/', ...... z^,O glass panel; (71) 7 J overall values,

separate parts of the windows. Discussions on the measuring accuracy due to finite difference approximation, instrumentation mismatch, near-field effects, field reactivity, scanning or fixed point measurements and the position of the microphone probe ~,ersus the radiating structure, appear iri an extended form in Refs. 6 and 16. The most important advantage of the sound intensity method over tile conventional one is the possibility of measuring the STL of individual parts of composite structures like windows or other facade elements. The dilferent parts of the windows, namely tile main tYame, the openable frame which contains the glass panel and the glass panel itself have been scanned separately to obtain the sound radiation. Tile main frame and the openable fralne were subdivided into eight equal surfaces and the radiated sound intensity level was obtained by scanning over a well defined time, The glass panel was subdivided into 30 equal parts and the same scanning time per area was used. The measurements have been done with the centre point of the probe at 5Cln from the measuring object. I::igures 7 and 8 represent, respectively, the STL results obtained for the three ditferent parts of the P V ( window with the 6-mm thick glass and the wooden window with the 6-ram

Sound transmission loss of glass and windows

279

thick glass. Over the whole frequency range of interest the main frame and the openable frame give better results than those of the glass panel. It is surprising that in the coincidence frequency region of the glass panel the STL values of the frames also drop to lower values. This occurs especially for the openable frames of the PVC and wooden windows, This is due to the strong connections with the glass panels. Also in these figures the overall calculated STL values of the windows are given. These values are in rather good agreement with the conventional method results which are not shown here. It is clear from these results that the intensity method is valuable for diagnostic purposes in complicated constructions.

4 CONCLUSIONS From the measurements on tightness and STL of glass panels and windows it is shown that the national and international standardization is of great importance. Not only the measuring methods, but also the complete design of transmission rooms have to be correctly prescribed. This is especially important for measurements on glass and windows, where niche depth, interior niche design, and fixing of panels and windows may give important discrepancies. Also systematic but important differences occur if comparisons are made between STL values of glass panels or windows. The recently developed intensity technique shows good agreement with the conventional two-room method. This technique is especially valuable for diagnostic purposes in composite walls, for detection of leaks and for flanking STL measurements. ACKNOWLEDGEMENT The authors wish to thank the Belgian Institute for Support of Scientific Research in Industry and Agriculture (IWONL-IRSIA) for the financial support of this project.

REFERENCES 1. Cops, A. & Minten, M., Comparative study between the sound intensity method and the conventional two-room method to calculate the sound transmission loss of wall constructions. Noise Control Engineering, 22 (1984) 104 11. 2. Roland, J., Villenave, M. & Martin, C., Intensim6trie acoustique application ~ila rechcrche des chemins de transmission du son. Report, Centre Scientifique et Technique du B~timent, Paris, 1984.

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3. van Zyl, B. G., Erasmus, P. J. & van der Merwe, G. J. J., A comparative study of the classic method and the sound intensity method for determining sound insulation. NPRL Report FIS 320, Council for Scientific and Industrial Research, Pretoria, Republic of South Africa, 1985. 4. Cops, A., Minten, M. & Myncke, H., The Use of Intensity Techniques in Building and Room Acoustics and Noise Control. Proceedings of the 5th FASE Symposium, Thessaloniki, 1985, pp. 67-85. 5. Cops, A., Minten, M. & Myncke, H., Influence of the Design of Transmission Rooms on the Sound Transmission Loss of Glass--Intensity versus Conventional Method. Noise Control Engineering Journal, 28 (1987) 121-9. 6. Minten, M., Cops, A. & Wijnants, F., The Sound Transmission Loss of a Single Panel Measured with the Two-Microphone and the Conventional Method Comparison with the Statistical Energy Analysis Model. Applied Acoustics, 22(4) (1987) 281-95. 7. van Zyl, B., Erasmus, P. & Anderson, F., On the Formulation of the Intensity Method for Determining Sound Reduction Indices. Applied Acoustics, 22 (1987) 213-28. 8. ISO Standard I-III, Measurement of sound insulation in buildings and o1 building elements, t987. 9. Waterhouse, R. V., Interference patterns in reverberant sound fields. J. Acoust. Soc. Am., 27 (1955), 247-58. 10. ISO Standard 3741, Determination of sound power levels of noise sources precision methods for broad-band sources in reverberation rooms, 1975. 11. Halliwell, R. & Warnock, A., Sound transmission loss: Comparison t~f conventional techniques with sound intensity techniques. J. Acoust. Soc. Am.. 77 (1985) 2094-103. 12. European Standard EN 42, Methods of testing windows- - Air permeability test, 1975. 13. UEATC prescription, Directive for the assessment of windows, 1974. 14. DIN-52210 part 2, Bauakustische Prfifungen ffir Luft- und Trittschalldfimmung, Berlin/K61n, Beuth Verlag, 1975. 15. Rapport Final, Pr6diction de l'Isolation Acoustique dans les B~timents, CST(" (Centre Scientifique et Technique de la Construction) and LAW (Laboratorium voor Akoestiek en Warmtegeleiding), Belgium, June 1987. 16. Cops, A. & Wijnants, F., Laboratory measurements of the sound transmissic~n loss of glass and windows---Sound intensity versus conventional method, Acoustics Australia, 16(2) (1988) 37 42.