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Sound insulation of domestic roofing systems: Part 1
AppltedA~ou.stics13(1980) 109-120
SOUND
INSULATION OF DOMESTIC SYSTEMS: PART 1
ROOFING
KENNETH R. COOK
App~edPhysies Department, Royal MelbournelnstituteofTechno~gy, Melbourne (Australia) (Received: 14 May, 1979)
SUMMARY
The three parts of this paper are to be published in three issues of Applied Acoustics. The present part discusses the experimentaljacilities of the measuring laboratory and gives details of tests conducted on ceiling specimens. In Part 2 the work on various types of roof component will be presented. In Part 3 will follow the results oJ"tests carried out on the roof and ceiling components combined. It will also consider the ability of the roofing system to provide sound insulation, as a component of the whole building envelope, Jrom typical external noise spectra. The availability of sound insulation values obtained by laboratory measurements does provide the building industry with guidance in building design, even though valuesjor actual buildings may va~3~ considerably from laboratory-measured values.
INTRODUCTION
In normal domestic dwelling design, often little attention is paid to the ability of the roofing system to provide adequate sound insulation of the building interior from airborne external noises. Even though treatments may be applied to the basic roofing system, the purpose of such treatments is primarily to increase the thermal comfort of the building's occupants or to provide adequate moisture-proofing. The ultimate aim of this laboratory project is to investigate the sound insulation provided by various practical domestic roofing systems. It is also desired to determine the part played by the roofing system in the sound insulation provided by the whole building envelope against common airborne external noise sources. As a prelude, however, it is considered relevant to investigate the insulation due separately to each of the two roofing-system components--that is, the ceiling and the roof. Various types of each of these Were tested and the effect on transmissionloss values by the addition of common infills studied. 109 Applied Acoustics 0003-682X/80/0013-0109/$02.25 (C) Applied Science Publishers Ltd, England, 1980 Printed in Great Britain
[ 10
KENNETH R. COOK
EXPERIMENTAL FACILITIES
The measuring facilities are located at the Division of Building Research, CSIRO, Highett (Victoria, Australia). The laboratories are shown in plan and section in Fig. 1. The boundaries of each room are of 300 mm reinforced concrete. Upper room, S, used as the sending room, is of 202.6m 3 effective volume and 224.4m 2 total boundary surface area and contains no diffusing elements. Lower room, N, is of 103.3m 3 volume and has a total boundary surface area of 135.0m 2, Diffusion is increased in this room by ten particle-board elements, each of about 2-1 m z onesided area and suspended in random fashion. The aperture between these transmission rooms allows insertion of the test specimen. As Fig. 1 shows, the floor of room S and the ceiling of room N are formed by the one 300 mm of reinforced concrete.
Compliance oJ rooms with standards The two relevant standards for the measurement of transmission loss are ISO/R 140 a and ISO/R 354. 2 Both rooms comply with these standards. However, for lower room N, investigations showed that, for third-octave bands below 160 Hz an insufficient number of room modes exist to provide an adequately diffuse sound field as recommended by ASTM E90. 3 The size of the aperture and test specimen comply with clause 4.1 of IS O/R 140 and, because of the 305 mm aperture depth it is possible to avoid deep apertures on either side of the sample. Tests were carried out to determine the limit of validity of measured transmission loss values due to possible flanking transmission. Suitable isolating structures were placed in the aperture but such tests which will be described in Part 3 where they are more relevant.
MIiASLTRIN(.I Sh Sll MS
COIl l'('111iona / ,',),',I~'111
For the majority of experimental work the signal source was a random noise generator limited to the chosen third-oeta'~,e frequency band. This signal, boosted by a 25 W power amplifier, was applied to a 25 W loudspeaker mounted on a square hardboard baffle. The loudspeaker input power was visually monitored by a voltmeter in an effort to maintain constant power when moving between different microphone positions. The signal to the condenser microphone was fed to, in a nearby control room, an audio-frequency spectrometer, an associated band-pass filter and a high-speed level recorder. With the aid of a duplicate receiving system fed fi-om a second microphone placed in the other transmission room it was possible to record the two microphone signals simultaneously. Because of large variations in levels with time it was l\)und necessary to incorporate a statistical distribution analyser for each time-averaging
SOUND INSULATION OF DOMESTIC ROOFING SYSTEMS: PART ]
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K E N N E T H R. C O O K
of each level between I00 and 315 Hz, when sampling continued for at least two minutes. For higher frequencies the signals were time averaged for at least 20 sec. For each day of measurement a comparison was made of the responses of the two microphone sets for each third-octave band by bringing the two microphones close together in the sound field of one room. From ISO/R 140, the equation for the determination of transmission loss, TL, becomes: TL = k I - L 2 + lOloglo(S/A2)dB
(1)
where L 1, L 2 are space time average levels (dB); S is the specimen area (m2); A 2 is the total equivalent absorption area of the receiving room, in turn given by: A 2 = 55.3 V/cTm 2
(2)
where V is the receiving room volume (m3); t' is the speed of sound in air (m/sec) and T is the room reverberation time (sec). Space-averaging was achieved by using five microphone positions in each room then arithmetically averaging the decibel values. For reverberation time measurement a similar type of loudspeaker remained in receiving room N. Sound pressure decays were read from level-recorder traces with a protractor. The mean value of T f o r each microphone position was found from five decays and the decays measured for each of the five microphone positions.
Real-time analysis system During later experimental work use was made of a real-time analyser/computer when the signal was excited broad-band. Augmenting the power by inclusion of a 300 W amplifier and a dodecahedral assembly of high-frequency loudspeakers, the amplifiers were energised by tape-recorded broad-band random noise. The spectrum was adjusted to give approximately pink airborne reverberant sound in the source room. To find the time-averaged level in each third-octave band a 32 sec integration time was used. The great advantage of this system was the vastly improved speed of sampling and the resulting reduction of errors in measured values due to any possible variation in temperature, relative humidity and pressure of the air in the room. One slight disadvantage of using this method does appear due to the non-simultaneous sampling of the sound fields in the two rooms. However, this was reduced by sampling the field in one room, then in the other room, then repeating this cycle of events to yield mean values. For reverberation-time measurements the sound pressure was allowed to decay broad-band and the decays were analysed in each third-octave band component. Analyser programming was such that any decays of double slope or of excessive curvature were excluded. The mean decay rate, ddB/sec, was determined from at least five acceptable decays for each of the five microphone positions. The value of A x in eqn. (1) was then found from:
SOUND INSULATION OF DOMESTIC ROOFING SYSTEMS" PART
A 2 ~-
0"921 Vct/cm 2
1
113 (3)
The frequency range for investigation over the whole of this project encompassed all third-octave bands between 100 and 10 000 Hz. CEILING SAMPLES
Discussion is now confined to the acoustic performance of the ceiling component of a domestic roofing system. A report on the combination of the ceiling and roof components is deferred to Part 3 of this series of papers. A rather typical ceiling in Australian homes comprises an untreated Australianhardwood frame and plasterboard cladding, so this formed the basic ceiling sample. The code names for ceiling samples used for testing are shown in Appendix 1. Figure 2 shows the constructional details of this ceiling, sample C/A, constructed in accordance with the then-current Australian standards. '~s The 9-mm plasterboard cladding of gypsum paperboard, comprising a plaster core between two layers of strong paper, had a surface density of 7.3 kg/m 2. It was fastened every 150 mm to battens by countersunk-head galvanised nails with any recesses between sheets filled with jointing plaster. The wooden perimeter of the ceiling sample was 11.5 m 2 but the value of S in eqn. ( 1) was taken as 11.2 m 2, representing the open area within the edge of the aperture. Sound leaks between sample and perimeter were prevented by the use of a hardrubber strip, glass-fibre and rubber hose.
TL resultsJor basic" ceiling, C/A Using the conventional system equipment the results for this basic ceiling (without infill) are shown in Table 1. Such results are shown in full detail, as an example, and include the mean levels, L 1 and L 2, level differences, D, absorption area, A 2 and TL values. The 95~0 confidence limits, PE in D and in TL, are also shown and were found from: TL +_ to.ozsSvk/n I'2 d B
(4)
where to.o25 was found from the Student t distribution for ( n - 1) degrees of freedom and STLwas the standard deviation in measured TL values. Although not to be shown for subsequent sets of results, precision estimates of this type were made for all samples. The lower precision in values of D at low frequencies was due to lower modal density and to the difficulty in maintaining constant power between microphone positions. The lower precision at high frequencies was caused by the lack of omnidirectionality of the loudspeaker. The cladding exhibits the coincidence effect at 4 k H z while slight resonance behaviour is evident at 315Hz. Between 250 and 1250 Hz the sample followed the expected mass law behaviour, but with a slope of 5 dB per octave.
KENNETH R. COOK
114
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SOUND INSULATION OF DOMESTIC ROOFING SYSTEMS; PART 1
115
TABLE 1 CEIL1NG C/A: TRANSMISSIONLOSSES
f (H-)
L~ (dB)
L2 (dB)
D (dB)
PE D (dB)
10 log (S/A2)
TL (dB)
PETI(dB)
10-5 13"8 14"8 18-3 18"9 19'3 22'6 24"1 25"3 27'0 29' 1 30-6 31 '7 32-2 32"3 28-3 24-4 29" 0 33" 1 37"4 39'3
2"5 2-0 2-2 1-2 1"4 1-2 1"9 0-6 0'4 1"0 1 "4 0'5 0'5 0"5 0"6 0'8 0'8 0-9 1-4 2'0 2"0
(dB) 100 125 160 200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000 6300 8000 10000
97'3 100'3 99-9 98"3 97-2 97"6 99"6 99"7 98-6 98'6 99'0 99"4 98'8 97'8 97'3 92"9 93" 1 90"0 85" 1 79"3 73-7
92"0 92-7 91"8 86'7 84.5 84"5 83"0 81"5 78'9 76"7 74"5 72'7 70" 1 67'8 66"8 65"7 69' 1 60" 5 50"7 39"7 31 "0
5"3 7'6 8'0 11-6 12-6 13-t 16'6 18"2 19.7 21'9 24.5 26.7 28.7 30"0 30'5 27"2 24-0 29-4 34-3 39"6 42-7
2"4 1"9 2"1 1" 1 1'4 l'2 1"9 0'6 0'4 1"0 1"4 0'5 0"5 0"5 0"5 0-8 0-8 0'9 1"4 2"0 2'0
5'2 6'2 6'8 6'7 6"3 6"2 6"0 5"9 5"7 5"1 4'6 3-9 3"0 2-2 1"8 1-1 0-4 - 0"4 - 1 "2 - 2"3 -3"4
Arithmetic mean T L values: 100-3150 Hz = 23-7dB. 125-4000 Hz = 24.5 dB. 100-10000 Hz = 25.8dB.
Subsequent tests on the sample, first 20 months after construction and again 8 months later, showed to a reasonable degree of confidence that the sample behaviour was not significantly affected by ageing and/or weathering.
Ceiling samples with infills Studies were made of the effect of adding various commercially available infills to the basic ceiling sample. The first was that of 50 mm glass fibre of 0.59 kg/m 2 surface density. The results for this sample, C/B, are shown graphically in Fig. 3 along with a comparison of sample C/A. Comparison tests showed that no significant changes in TL values had occurred up to 250 Hz, but that the infill had enhanced values above 250 Hz. N o change was evident in the 4 k H z coincidence dip. When sarking of 0.23-ram thick double-sided aluminium foil was laid across the ceiling joists the results for the corresponding sample, C/C, were as shown in Fig. 4. Below 250 Hz TL values are seen to be lower than those when no infill was present, although not significantly so. Between 500 and 2500 Hz the TL enhancements due to the foil are between 4 and 9 d B and range between 12 and 18dB for frequencies above 4 k H z . Finally, the results show that the addition of sarking has had the effect of significantly damping the flexural waves in the plasterboard cladding at 4 kHz.
116
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SOUND INSULATION OF DOMESTIC ROOFING SYSTEMS: PART 1
117
Following this test the glass fibre infill was removed but the sarking left across the joists. Sarking is often used in roofing systems to provide both thermal insulation and moisture-proofing. The results for this sample, C/D, are also shown in Fig. 4. They indicate that sarking has little significant effect on TL values below 3150 Hz, but that the coincidence effect has been reduced. Acoustic tiles are sometimes used on ceiling undersides as acoustic treatment for rooms, so measurements were made on sample C/E formed by screwing to the underside of the basic ceiling, C/A, woodfibre tiles of 3.8 kg/m 2 surface density. Figure 5 shows the results for this sample and for sample C/A. Only small changes in TL values below 2 k H z are evident, but the coincidence dip is now seen to have been changed from 4 kHz to 2 kHz. In a real dwelling the level differences, due to the increase in the receiving-room equivalent absorption area, are higher. Suppose in a real dwelling acoustic tiles were later added to a ceiling already containing glass fibre infill. Such a specimen, sample C/F, was constructed and the results obtained are also shown in Fig. 5 (but only at 250 Hz and above, since no significant changes were found to occur below 250Hz). The additional infill increased the slope of the curve and enhanced the high-frequency TL values by up to 8 dB.
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118
KENNETH R. COOK
To increase T L values in the mass-controlled region 74 m m mineral-wool batts were used as the infill. The high surface density of 6.2 kg/m 2 is not normally used in domestic-dwelling applications. Figure 6 shows the results for this sample, C/G, along with sample C/A and for the ceiling sample, C/B, containing the lower surface density glass-fibre. It shows that except for the low frequency region it is possible to achieve relatively high T L values for a ceiling by the use of a single infill. In addition, such an infill provides substantial thermal insulation. 70
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Increase in cladding thickness The cladding surface density was doubled by nailing a second 9 mm layer of plasterboard to the original, forming sample C/AA. The results for this sample and for C/A are shown in Fig. 7, The panel resonance was lowered to 160 Hz, but with the coincidence effect remaining at 4 kHz. TL valueswere enhanced by 3 to 6 dB between resonance and coincidence and by about 8 dB above coincidence.
SOUND INSULATION OF DOMESTIC ROOFING SYSTEMS: PART 1
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This paper has indicated the effects on t r a n s m i s s i o n loss values of the addition of infills to a basic ceiling sample. G e n e r a l l y the infitl has had little effect on T L values in the low frequency region. In order to remove the 4 kHz coincidence dip a relatively heavy infill is required. A p o i n t of practical interest is the effect on s o u n d insulation of the a d d i t i o n of infills to a ceiling when that ceiling is c o m b i n e d with a roof to form a roofing system. C o m m e n t on such an effect is deferred until Part 3 of this series of papers when tests on roofing-system samples will be presented. REFERENCES 1. ANON.Field and Laboratory Measurements of Airborne and Impact Sound Transmission. 1SO/R 140-1960 (E), International Organisation for Standardisation. Ist edn, 1960. 2. ANON. Measurement of Absorption Coefficients in a Reverberation Room. ISO/R 354-1963 (E), International Organisation for Standardisation. ist edn, 1963. 3. ANON.Standard Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions. ASTM E90-75. American Society for Testing and Materials, 1975. 4. ANON.Light Timber Framing Code. AS CA38-1971, Section 5, Standards Association of Australia, 1971. 5. ANON. Recommendations for Co-ordinated Preferred Dimensions in Building. AS 1234-1972, Standards Association of Australia, 1972.
120
KENNETH R. COOK
APPENDIX ] : ('()DES FOR CEILING SAMPLES
Code
C/A C/B C/C C/D C/E C/F C/G C,AA
Description Basic ceiling of 9 mm plasterboard cladding As above, but with infill of 50 mm glass-fibre blanket between joists As above, plus 0-23 mm double-sided aluminium foil laid across joists As CIA. with 0.23 mm double-sided aluminium foil laid across joists As C/A, with 13 mm wood-fibre acoustic tiles screwed to cladding underside As above, plus 50 mm glass-fibre blanket between joists As C/A, plus 74 mm mineral-wool batts between joists As C/A, but with second 9 mm plasterboard cladding nailed to initial layer.
SOUND
INSULATION OF DOMESTIC SYSTEMS: PART 1
ROOFING
KENNETH R. COOK
App~edPhysies Department, Royal MelbournelnstituteofTechno~gy, Melbourne (Australia) (Received: 14 May, 1979)
SUMMARY
The three parts of this paper are to be published in three issues of Applied Acoustics. The present part discusses the experimentaljacilities of the measuring laboratory and gives details of tests conducted on ceiling specimens. In Part 2 the work on various types of roof component will be presented. In Part 3 will follow the results oJ"tests carried out on the roof and ceiling components combined. It will also consider the ability of the roofing system to provide sound insulation, as a component of the whole building envelope, Jrom typical external noise spectra. The availability of sound insulation values obtained by laboratory measurements does provide the building industry with guidance in building design, even though valuesjor actual buildings may va~3~ considerably from laboratory-measured values.
INTRODUCTION
In normal domestic dwelling design, often little attention is paid to the ability of the roofing system to provide adequate sound insulation of the building interior from airborne external noises. Even though treatments may be applied to the basic roofing system, the purpose of such treatments is primarily to increase the thermal comfort of the building's occupants or to provide adequate moisture-proofing. The ultimate aim of this laboratory project is to investigate the sound insulation provided by various practical domestic roofing systems. It is also desired to determine the part played by the roofing system in the sound insulation provided by the whole building envelope against common airborne external noise sources. As a prelude, however, it is considered relevant to investigate the insulation due separately to each of the two roofing-system components--that is, the ceiling and the roof. Various types of each of these Were tested and the effect on transmissionloss values by the addition of common infills studied. 109 Applied Acoustics 0003-682X/80/0013-0109/$02.25 (C) Applied Science Publishers Ltd, England, 1980 Printed in Great Britain
[ 10
KENNETH R. COOK
EXPERIMENTAL FACILITIES
The measuring facilities are located at the Division of Building Research, CSIRO, Highett (Victoria, Australia). The laboratories are shown in plan and section in Fig. 1. The boundaries of each room are of 300 mm reinforced concrete. Upper room, S, used as the sending room, is of 202.6m 3 effective volume and 224.4m 2 total boundary surface area and contains no diffusing elements. Lower room, N, is of 103.3m 3 volume and has a total boundary surface area of 135.0m 2, Diffusion is increased in this room by ten particle-board elements, each of about 2-1 m z onesided area and suspended in random fashion. The aperture between these transmission rooms allows insertion of the test specimen. As Fig. 1 shows, the floor of room S and the ceiling of room N are formed by the one 300 mm of reinforced concrete.
Compliance oJ rooms with standards The two relevant standards for the measurement of transmission loss are ISO/R 140 a and ISO/R 354. 2 Both rooms comply with these standards. However, for lower room N, investigations showed that, for third-octave bands below 160 Hz an insufficient number of room modes exist to provide an adequately diffuse sound field as recommended by ASTM E90. 3 The size of the aperture and test specimen comply with clause 4.1 of IS O/R 140 and, because of the 305 mm aperture depth it is possible to avoid deep apertures on either side of the sample. Tests were carried out to determine the limit of validity of measured transmission loss values due to possible flanking transmission. Suitable isolating structures were placed in the aperture but such tests which will be described in Part 3 where they are more relevant.
MIiASLTRIN(.I Sh Sll MS
COIl l'('111iona / ,',),',I~'111
For the majority of experimental work the signal source was a random noise generator limited to the chosen third-oeta'~,e frequency band. This signal, boosted by a 25 W power amplifier, was applied to a 25 W loudspeaker mounted on a square hardboard baffle. The loudspeaker input power was visually monitored by a voltmeter in an effort to maintain constant power when moving between different microphone positions. The signal to the condenser microphone was fed to, in a nearby control room, an audio-frequency spectrometer, an associated band-pass filter and a high-speed level recorder. With the aid of a duplicate receiving system fed fi-om a second microphone placed in the other transmission room it was possible to record the two microphone signals simultaneously. Because of large variations in levels with time it was l\)und necessary to incorporate a statistical distribution analyser for each time-averaging
SOUND INSULATION OF DOMESTIC ROOFING SYSTEMS: PART ]
~A'
L,3 3225
/
42.30
Room'S
"
I
505-.
Aperture
I _ _
Room N
P
Section A- A Fig. 1.
T r a n s m i s s i o n rooms.
]II
112
K E N N E T H R. C O O K
of each level between I00 and 315 Hz, when sampling continued for at least two minutes. For higher frequencies the signals were time averaged for at least 20 sec. For each day of measurement a comparison was made of the responses of the two microphone sets for each third-octave band by bringing the two microphones close together in the sound field of one room. From ISO/R 140, the equation for the determination of transmission loss, TL, becomes: TL = k I - L 2 + lOloglo(S/A2)dB
(1)
where L 1, L 2 are space time average levels (dB); S is the specimen area (m2); A 2 is the total equivalent absorption area of the receiving room, in turn given by: A 2 = 55.3 V/cTm 2
(2)
where V is the receiving room volume (m3); t' is the speed of sound in air (m/sec) and T is the room reverberation time (sec). Space-averaging was achieved by using five microphone positions in each room then arithmetically averaging the decibel values. For reverberation time measurement a similar type of loudspeaker remained in receiving room N. Sound pressure decays were read from level-recorder traces with a protractor. The mean value of T f o r each microphone position was found from five decays and the decays measured for each of the five microphone positions.
Real-time analysis system During later experimental work use was made of a real-time analyser/computer when the signal was excited broad-band. Augmenting the power by inclusion of a 300 W amplifier and a dodecahedral assembly of high-frequency loudspeakers, the amplifiers were energised by tape-recorded broad-band random noise. The spectrum was adjusted to give approximately pink airborne reverberant sound in the source room. To find the time-averaged level in each third-octave band a 32 sec integration time was used. The great advantage of this system was the vastly improved speed of sampling and the resulting reduction of errors in measured values due to any possible variation in temperature, relative humidity and pressure of the air in the room. One slight disadvantage of using this method does appear due to the non-simultaneous sampling of the sound fields in the two rooms. However, this was reduced by sampling the field in one room, then in the other room, then repeating this cycle of events to yield mean values. For reverberation-time measurements the sound pressure was allowed to decay broad-band and the decays were analysed in each third-octave band component. Analyser programming was such that any decays of double slope or of excessive curvature were excluded. The mean decay rate, ddB/sec, was determined from at least five acceptable decays for each of the five microphone positions. The value of A x in eqn. (1) was then found from:
SOUND INSULATION OF DOMESTIC ROOFING SYSTEMS" PART
A 2 ~-
0"921 Vct/cm 2
1
113 (3)
The frequency range for investigation over the whole of this project encompassed all third-octave bands between 100 and 10 000 Hz. CEILING SAMPLES
Discussion is now confined to the acoustic performance of the ceiling component of a domestic roofing system. A report on the combination of the ceiling and roof components is deferred to Part 3 of this series of papers. A rather typical ceiling in Australian homes comprises an untreated Australianhardwood frame and plasterboard cladding, so this formed the basic ceiling sample. The code names for ceiling samples used for testing are shown in Appendix 1. Figure 2 shows the constructional details of this ceiling, sample C/A, constructed in accordance with the then-current Australian standards. '~s The 9-mm plasterboard cladding of gypsum paperboard, comprising a plaster core between two layers of strong paper, had a surface density of 7.3 kg/m 2. It was fastened every 150 mm to battens by countersunk-head galvanised nails with any recesses between sheets filled with jointing plaster. The wooden perimeter of the ceiling sample was 11.5 m 2 but the value of S in eqn. ( 1) was taken as 11.2 m 2, representing the open area within the edge of the aperture. Sound leaks between sample and perimeter were prevented by the use of a hardrubber strip, glass-fibre and rubber hose.
TL resultsJor basic" ceiling, C/A Using the conventional system equipment the results for this basic ceiling (without infill) are shown in Table 1. Such results are shown in full detail, as an example, and include the mean levels, L 1 and L 2, level differences, D, absorption area, A 2 and TL values. The 95~0 confidence limits, PE in D and in TL, are also shown and were found from: TL +_ to.ozsSvk/n I'2 d B
(4)
where to.o25 was found from the Student t distribution for ( n - 1) degrees of freedom and STLwas the standard deviation in measured TL values. Although not to be shown for subsequent sets of results, precision estimates of this type were made for all samples. The lower precision in values of D at low frequencies was due to lower modal density and to the difficulty in maintaining constant power between microphone positions. The lower precision at high frequencies was caused by the lack of omnidirectionality of the loudspeaker. The cladding exhibits the coincidence effect at 4 k H z while slight resonance behaviour is evident at 315Hz. Between 250 and 1250 Hz the sample followed the expected mass law behaviour, but with a slope of 5 dB per octave.
KENNETH R. COOK
114
//
--[ ~, Joists tO0x 58 458etoc
X
Hangers 200x58
~8'
Additional joists I00x75
Corner
brace 50x25
X
/
,,
104o
1040 " ~
---~,,-
3200
SechonB-B'
N Battens50x 25
\I'°SLoo,0 Fig. 2.
Plasterboard ceiling.
SOUND INSULATION OF DOMESTIC ROOFING SYSTEMS; PART 1
115
TABLE 1 CEIL1NG C/A: TRANSMISSIONLOSSES
f (H-)
L~ (dB)
L2 (dB)
D (dB)
PE D (dB)
10 log (S/A2)
TL (dB)
PETI(dB)
10-5 13"8 14"8 18-3 18"9 19'3 22'6 24"1 25"3 27'0 29' 1 30-6 31 '7 32-2 32"3 28-3 24-4 29" 0 33" 1 37"4 39'3
2"5 2-0 2-2 1-2 1"4 1-2 1"9 0-6 0'4 1"0 1 "4 0'5 0'5 0"5 0"6 0'8 0'8 0-9 1-4 2'0 2"0
(dB) 100 125 160 200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000 6300 8000 10000
97'3 100'3 99-9 98"3 97-2 97"6 99"6 99"7 98-6 98'6 99'0 99"4 98'8 97'8 97'3 92"9 93" 1 90"0 85" 1 79"3 73-7
92"0 92-7 91"8 86'7 84.5 84"5 83"0 81"5 78'9 76"7 74"5 72'7 70" 1 67'8 66"8 65"7 69' 1 60" 5 50"7 39"7 31 "0
5"3 7'6 8'0 11-6 12-6 13-t 16'6 18"2 19.7 21'9 24.5 26.7 28.7 30"0 30'5 27"2 24-0 29-4 34-3 39"6 42-7
2"4 1"9 2"1 1" 1 1'4 l'2 1"9 0'6 0'4 1"0 1"4 0'5 0"5 0"5 0"5 0-8 0-8 0'9 1"4 2"0 2'0
5'2 6'2 6'8 6'7 6"3 6"2 6"0 5"9 5"7 5"1 4'6 3-9 3"0 2-2 1"8 1-1 0-4 - 0"4 - 1 "2 - 2"3 -3"4
Arithmetic mean T L values: 100-3150 Hz = 23-7dB. 125-4000 Hz = 24.5 dB. 100-10000 Hz = 25.8dB.
Subsequent tests on the sample, first 20 months after construction and again 8 months later, showed to a reasonable degree of confidence that the sample behaviour was not significantly affected by ageing and/or weathering.
Ceiling samples with infills Studies were made of the effect of adding various commercially available infills to the basic ceiling sample. The first was that of 50 mm glass fibre of 0.59 kg/m 2 surface density. The results for this sample, C/B, are shown graphically in Fig. 3 along with a comparison of sample C/A. Comparison tests showed that no significant changes in TL values had occurred up to 250 Hz, but that the infill had enhanced values above 250 Hz. N o change was evident in the 4 k H z coincidence dip. When sarking of 0.23-ram thick double-sided aluminium foil was laid across the ceiling joists the results for the corresponding sample, C/C, were as shown in Fig. 4. Below 250 Hz TL values are seen to be lower than those when no infill was present, although not significantly so. Between 500 and 2500 Hz the TL enhancements due to the foil are between 4 and 9 d B and range between 12 and 18dB for frequencies above 4 k H z . Finally, the results show that the addition of sarking has had the effect of significantly damping the flexural waves in the plasterboard cladding at 4 kHz.
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SOUND INSULATION OF DOMESTIC ROOFING SYSTEMS: PART 1
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Following this test the glass fibre infill was removed but the sarking left across the joists. Sarking is often used in roofing systems to provide both thermal insulation and moisture-proofing. The results for this sample, C/D, are also shown in Fig. 4. They indicate that sarking has little significant effect on TL values below 3150 Hz, but that the coincidence effect has been reduced. Acoustic tiles are sometimes used on ceiling undersides as acoustic treatment for rooms, so measurements were made on sample C/E formed by screwing to the underside of the basic ceiling, C/A, woodfibre tiles of 3.8 kg/m 2 surface density. Figure 5 shows the results for this sample and for sample C/A. Only small changes in TL values below 2 k H z are evident, but the coincidence dip is now seen to have been changed from 4 kHz to 2 kHz. In a real dwelling the level differences, due to the increase in the receiving-room equivalent absorption area, are higher. Suppose in a real dwelling acoustic tiles were later added to a ceiling already containing glass fibre infill. Such a specimen, sample C/F, was constructed and the results obtained are also shown in Fig. 5 (but only at 250 Hz and above, since no significant changes were found to occur below 250Hz). The additional infill increased the slope of the curve and enhanced the high-frequency TL values by up to 8 dB.
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To increase T L values in the mass-controlled region 74 m m mineral-wool batts were used as the infill. The high surface density of 6.2 kg/m 2 is not normally used in domestic-dwelling applications. Figure 6 shows the results for this sample, C/G, along with sample C/A and for the ceiling sample, C/B, containing the lower surface density glass-fibre. It shows that except for the low frequency region it is possible to achieve relatively high T L values for a ceiling by the use of a single infill. In addition, such an infill provides substantial thermal insulation. 70
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SOUND INSULATION OF DOMESTIC ROOFING SYSTEMS: PART 1
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This paper has indicated the effects on t r a n s m i s s i o n loss values of the addition of infills to a basic ceiling sample. G e n e r a l l y the infitl has had little effect on T L values in the low frequency region. In order to remove the 4 kHz coincidence dip a relatively heavy infill is required. A p o i n t of practical interest is the effect on s o u n d insulation of the a d d i t i o n of infills to a ceiling when that ceiling is c o m b i n e d with a roof to form a roofing system. C o m m e n t on such an effect is deferred until Part 3 of this series of papers when tests on roofing-system samples will be presented. REFERENCES 1. ANON.Field and Laboratory Measurements of Airborne and Impact Sound Transmission. 1SO/R 140-1960 (E), International Organisation for Standardisation. Ist edn, 1960. 2. ANON. Measurement of Absorption Coefficients in a Reverberation Room. ISO/R 354-1963 (E), International Organisation for Standardisation. ist edn, 1963. 3. ANON.Standard Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions. ASTM E90-75. American Society for Testing and Materials, 1975. 4. ANON.Light Timber Framing Code. AS CA38-1971, Section 5, Standards Association of Australia, 1971. 5. ANON. Recommendations for Co-ordinated Preferred Dimensions in Building. AS 1234-1972, Standards Association of Australia, 1972.
120
KENNETH R. COOK
APPENDIX ] : ('()DES FOR CEILING SAMPLES
Code
C/A C/B C/C C/D C/E C/F C/G C,AA
Description Basic ceiling of 9 mm plasterboard cladding As above, but with infill of 50 mm glass-fibre blanket between joists As above, plus 0-23 mm double-sided aluminium foil laid across joists As CIA. with 0.23 mm double-sided aluminium foil laid across joists As C/A, with 13 mm wood-fibre acoustic tiles screwed to cladding underside As above, plus 50 mm glass-fibre blanket between joists As C/A, plus 74 mm mineral-wool batts between joists As C/A, but with second 9 mm plasterboard cladding nailed to initial layer.