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Design of a test facility for transmission loss measurement
Applied Aeoustics 18 (1985) 315-323
Design of a Test Facility for Transmission Loss Measurement G. Papanikolaou and A. Trochides School of Engineering, University of Thessaloniki, Thessaloniki (Greece) (Received: 4 May, 1984)
SUMMARY This paper describes the design, construction and perJormance of a test Jacility Jor transmission loss measurement. The receiving portion of the jacility consists oJ a 118 m 3 reverberation room, while the source room is a 1.0 m 3 pit with high sound insulation. The partition being measured makes up the top of the pit. A series oJ measurements concerning the perlormance of the test Jaeilio, are presented and discussed.
I N T R O D UCTI O N The sound insulation properties of panels and partitions are of great importance in several areas of acoustics. Sound insulation data are essential to architects and noise control engineers in their planning concerning building acoustics or noise control in industry. The sound insulation properties of partitions are characterized by their transmission loss. The transmission loss is mainly obtained by the two room method, where the specimen is placed in the c o m m o n wall between source and receiving room. According to the standards the TL is defined by: S TL = N R + 1 0 1 o g ~
(1)
where N R is the noise reduction between the source room and the 315 Applied Acoustics 0003-682X/85/$03.30 ,K Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain
316
G. Papanikolaou, A. Trochides
receiving room, S is the sound transmitting area and A the total absorption of the receiving room. The most important requirement for the application of this method is that the sound field in both rooms must be sufficiently diffuse. Thus, for the measurement of transmission loss, two adjacent reverberation chambers are required. There is a series of difficulties and disadvantages with large facilities. The most important are their high cost, the difficulty of changing their acoustical characteristics to suit the intended application and their unsuitability for measuring small specimens compared with full-sized building partitions. To avoid the above disadvantages, the possibility of constructing a small test facility is very attractive. The use of small facilities especially for testing small panels and partitions seems very convenient. This paper describes the design and construction of a small test facility for transmission loss measurement. Various measurements concerning the performance characteristics, advantages and limitations of the test facility are presented and discussed.
D E S I G N A N D C O N S T R U C T I O N OF T H E TEST FACILITY The test facility was designed to yield reliable transmission loss results for small panels and partitions. Figure 1 shows a schematic cross-sectional view of the measuring system. The source room consists of a 1.0 m 3 pit (enclosure) (Fig. 2). The pit walls are triple-leaf construction with high airborne-sound insulation. The receiving portion of the system consists of a 118 m a reverberation room of the acoustical laboratory of the University of Thessaloniki with a microphone sweeping a circular path of 2 m and inclined at 30 ° to the horizontal (Fig. 3). The chamber is constructed according to the international standards and was tested during an international cyclic programme of sound absorption measurements. Figure 4 shows a cross-sectional view of the pit. The pit is 1.20 m deep with an averaging cross-section measuring about 1.0m. It has a total volume of about 1.0 m 3 and all walls are splayed to improve its modal distribution per one-third octave band as compared with parallelepiped. The walls of the pit are a three leaf construction, with no connections, in order to achieve high airborne-sound insulation. As shown in Fig. 4, two
Design of a test facility j b r transmission loss measuremen t PRINTER
~
317
RECORDER
ANALYZER
POWER AMP.
POWER AMP.
,xx
/ XXXXXXXXXXXXXXXXXXXXXX\\XXx.
Fig. 1.
ROTATING MICROPHONE
/
XNX.X, XXXXXXXXX\\\
\XX
\
\\
\
X X\
\
Schematic view of the measuring system.
Fig. 2.
View of the pit.
\
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H
318
G. Papanikolaou, A. Trochides
Fig. 3.
The receiving portion of the system.
of the leafs (inner and outer) are made of 18 mm plywood and the intermediate of 4 0 m m concrete. First, the plywood boards were assembled and all joints were glued, fastened with screws and acoustic sealant was applied along the edges to ensure a good seal against noise leakage. The gap between the two plywood walls was filled with sound absorptive fibre material and then the 4 0 m m concrete wall was steei panel rJbber
i plywoo~ concrete
absorptive m a t e r i a l --
" :,x~,xxxxx"~\xxx\\xxx\x\\~, \ \ \ \ x x x x x \ \ x x x x \ \ ~ x x x x x x x x x \ x x x \ \ x x x ~ x x x x ~ x x ~
Fig. 4.
Cross-sectional view of the pit.
Design of a test facility Jor transmission loss measurement
319
constructed by pouring concrete in a preconstructed gap within the absorptive material. The necessary electrical wiring was fed through a side wall and the small hole in the wall was plugged with acoustic sealant. The measuring microphone was mounted on a light adjustable suspension system. The noise source consists of a high efficiency, wide range loudspeaker located on the floor of the pit. The panel being measured is placed at the top of the pit on a rubber seal and is clamped tightly.
P E R F O R M A N C E TESTS A N D M E A S U R E M E N T S Several tests and measurements were carried out to evaluate the performance of the facility.
Coincidence frequency The panel used for our measurements was a 1.5 mm steel panel often used as a reference. The panel's thickness, surface mass density and Young's modulus are given below. The coincidence frequency of the panel can be calculated as: 1
J ; - 1.8h
(Hz)
(2)
where c is the sound speed in air (340 m/s), h is the panel thickness (m), p is the panel density (kg/m 3) and E is the Young's modulus (N/m 2) of the panel material. The calculated frequency agrees well with the observed coincidence TL(dB) 50 40 30 2O I0 63
Fig. 5.
125
250
500
1000 2000 4000
8000(Hz)
Transmission loss of a 1.5 m m steel plate.
G. Papanikolaou, A. Trochides
320
frequency. In Fig. 5 the 1.5 m m steel panel has a calculatedj~ of 8.5 kHz and an estimated Jc of about 8 kHz from the measured one-third octave band data. Pit effects at low frequencies
The sound field in the pit cannot be considered r a n d o m at low frequencies because of its small dimensions. For all TL measurements there is a pronounced decrease in the 250 Hz one-third octave band and an increase in the 163 Hz one-third octave band. The decrease is due to pit resonances and the increase to the absence of resonances. The normal models j , of the pit can be determined only approximately (because of splayed pit surfaces) from: Jl,
=(2)[(rtx/lx)2 + (ny/ly)2 + (rtz//z)2] 1'2
(3)
nx, ny, n == 0 , 1,2,3 . . . . Inserting 1.0, 0'8 and 1"20 m for Ix, ly and/=, respectively, we obtain the normal modes for the pit. The calculations indicate the presence of modes in the 250 Hz one-third octave band, so that the pit pressure is elevated. On the other hand, the absence of vertical modes in the 160 Hz one-third octave band results in a low pit pressure and, consequently, an increase in TL. In order to investigate the influence of the exciting signal on the TL, a series of measurements were carried out using a less tonal signal, i.e. band limited pink noise. The results are shown in Fig. 6. As one would expect, the resonance effects in the low frequency range are almost eliminated. TL(dB) 50 40 o-
-o- ,o"
30 20 10
Fig. 6.
Results of transmission loss measurements using different exciting signals. (a) Third-octave excitation. (b) Band limited pink noise.
Design of a test Jacility for transmission loss measurement
321
Cut-off frequency The main disadvantage of small test chambers is their high cut-off frequency, which means that one cannot obtain reliable sound insulation results at low frequencies. In order to establish the lower frequency limit of the pit, the sound pressure level in the pit was measured continuously, once with the top of the pit open and then closed with the test panel. The results are shown in Fig. 7. These results indicate that the lower cut-off frequency for the test facility is about 400 Hz. i,
t
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1
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-
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d~ F]ec~,fier
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Lower L;m Freq: -~ ~
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mmtsec. Paper
0
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20
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- ! Multiply Freq. Scale by: . . . . . . . . .
Fig. 7.
Zero l.e'~l:
~7~' ~ ~
11612/2112
Sound pressure level in the pit. (a) Open pit. (b) Closed pit.
Transmission loss measurements The test facility was used to obtain transmission loss data for a 1.5 m m thick, 120 cm x 120 cm steel panel. The results are shown in Fig. 8. The calculated field incidence transmission loss for a 1.5 m m thick steel panel (surface density, 11.4 kg/m 2) is 33 dB at 1000 Hz. The measured data yield about 32 dB which is in good agreement. The transmission loss exhibits TL(dB) 50 ';0 30 20
O;
Fig. 8.
i 125
i 250
, 500
J lO00
i 2000
i 4000
i ~O00(Hz)
Results of transmission loss measurements.
322
G. Papanikolaou, A. Trochides
mass-controlled behaviour in the frequency range between 400 and 6000 Hz, even though the measured slope is somewhat lower than the theoretical 6 dB per octave. This result agrees with previous experimental data2 obtained by a similar method. The insertion loss, 1L, of the steel panel was also measured, l f L o is the sound pressure level in the reverberation room with no panel and L the sound pressure level with the panel on the pit, then: IL =
L o -
L
(4)
The results are shown in Fig. 9. IL(dB) 50 4O 30 20 tO 63
Fig, 9.
7~5 2~ 56o 7obo 2obo ~o6o 8dooc,z) Results of insertion loss measurements.
Assuming a reverberant field in the pit and combining eqns (1) and (4) yields: S TL = IL + AL + 101og~ (5) where AL is the sound pressure level difference in the pit with and without a panel. The above equation can be used for converting the IL data into TL data. This equation is valid only for panels without absorptive liners and above 1 kHz because of the small dimensions of the pit. Figure 7 shows the term AL measured continuously per one-third octave band. Using these values of AL and the factor 10 log S / A , Fig. 9 can be corrected to yield the TL values given in Fig. 8. CONCLUDING REMARKS In the work described in this paper the performance of a small test facility for transmission loss measurement was investigated.
Design of a test Jacility Jor transmission loss measurement
323
Based on the reported measurements it can be concluded that this test facility can be used for small specimens. The accuracy of the results seems to be adequate for most practical purposes, provided care is exercised below the cut-off frequency of the pit. To obtain more reliable results below the cut-off frequency, band limited pink noise, instead of third-octave excitation, can be used. Further work is in progress to improve the performance of the facility. It concerns the sound insulation properties of the pit walls in order to establish an upper limit for valid TL measurement and the use of the facility for measuring panels with sound absorptive liners facing the pit.
REFERENCES 1. L. L. Beranek, Noise and vibration control, McGraw-Hill Book Company Inc., 1971, p. 276. 2. K. S. Nordby, Measurement and evaluation of the insertion loss of panels, Noise Control Engineering (January, 1978), p. 22.
Design of a Test Facility for Transmission Loss Measurement G. Papanikolaou and A. Trochides School of Engineering, University of Thessaloniki, Thessaloniki (Greece) (Received: 4 May, 1984)
SUMMARY This paper describes the design, construction and perJormance of a test Jacility Jor transmission loss measurement. The receiving portion of the jacility consists oJ a 118 m 3 reverberation room, while the source room is a 1.0 m 3 pit with high sound insulation. The partition being measured makes up the top of the pit. A series oJ measurements concerning the perlormance of the test Jaeilio, are presented and discussed.
I N T R O D UCTI O N The sound insulation properties of panels and partitions are of great importance in several areas of acoustics. Sound insulation data are essential to architects and noise control engineers in their planning concerning building acoustics or noise control in industry. The sound insulation properties of partitions are characterized by their transmission loss. The transmission loss is mainly obtained by the two room method, where the specimen is placed in the c o m m o n wall between source and receiving room. According to the standards the TL is defined by: S TL = N R + 1 0 1 o g ~
(1)
where N R is the noise reduction between the source room and the 315 Applied Acoustics 0003-682X/85/$03.30 ,K Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain
316
G. Papanikolaou, A. Trochides
receiving room, S is the sound transmitting area and A the total absorption of the receiving room. The most important requirement for the application of this method is that the sound field in both rooms must be sufficiently diffuse. Thus, for the measurement of transmission loss, two adjacent reverberation chambers are required. There is a series of difficulties and disadvantages with large facilities. The most important are their high cost, the difficulty of changing their acoustical characteristics to suit the intended application and their unsuitability for measuring small specimens compared with full-sized building partitions. To avoid the above disadvantages, the possibility of constructing a small test facility is very attractive. The use of small facilities especially for testing small panels and partitions seems very convenient. This paper describes the design and construction of a small test facility for transmission loss measurement. Various measurements concerning the performance characteristics, advantages and limitations of the test facility are presented and discussed.
D E S I G N A N D C O N S T R U C T I O N OF T H E TEST FACILITY The test facility was designed to yield reliable transmission loss results for small panels and partitions. Figure 1 shows a schematic cross-sectional view of the measuring system. The source room consists of a 1.0 m 3 pit (enclosure) (Fig. 2). The pit walls are triple-leaf construction with high airborne-sound insulation. The receiving portion of the system consists of a 118 m a reverberation room of the acoustical laboratory of the University of Thessaloniki with a microphone sweeping a circular path of 2 m and inclined at 30 ° to the horizontal (Fig. 3). The chamber is constructed according to the international standards and was tested during an international cyclic programme of sound absorption measurements. Figure 4 shows a cross-sectional view of the pit. The pit is 1.20 m deep with an averaging cross-section measuring about 1.0m. It has a total volume of about 1.0 m 3 and all walls are splayed to improve its modal distribution per one-third octave band as compared with parallelepiped. The walls of the pit are a three leaf construction, with no connections, in order to achieve high airborne-sound insulation. As shown in Fig. 4, two
Design of a test facility j b r transmission loss measuremen t PRINTER
~
317
RECORDER
ANALYZER
POWER AMP.
POWER AMP.
,xx
/ XXXXXXXXXXXXXXXXXXXXXX\\XXx.
Fig. 1.
ROTATING MICROPHONE
/
XNX.X, XXXXXXXXX\\\
\XX
\
\\
\
X X\
\
Schematic view of the measuring system.
Fig. 2.
View of the pit.
\
\\
H
318
G. Papanikolaou, A. Trochides
Fig. 3.
The receiving portion of the system.
of the leafs (inner and outer) are made of 18 mm plywood and the intermediate of 4 0 m m concrete. First, the plywood boards were assembled and all joints were glued, fastened with screws and acoustic sealant was applied along the edges to ensure a good seal against noise leakage. The gap between the two plywood walls was filled with sound absorptive fibre material and then the 4 0 m m concrete wall was steei panel rJbber
i plywoo~ concrete
absorptive m a t e r i a l --
" :,x~,xxxxx"~\xxx\\xxx\x\\~, \ \ \ \ x x x x x \ \ x x x x \ \ ~ x x x x x x x x x \ x x x \ \ x x x ~ x x x x ~ x x ~
Fig. 4.
Cross-sectional view of the pit.
Design of a test facility Jor transmission loss measurement
319
constructed by pouring concrete in a preconstructed gap within the absorptive material. The necessary electrical wiring was fed through a side wall and the small hole in the wall was plugged with acoustic sealant. The measuring microphone was mounted on a light adjustable suspension system. The noise source consists of a high efficiency, wide range loudspeaker located on the floor of the pit. The panel being measured is placed at the top of the pit on a rubber seal and is clamped tightly.
P E R F O R M A N C E TESTS A N D M E A S U R E M E N T S Several tests and measurements were carried out to evaluate the performance of the facility.
Coincidence frequency The panel used for our measurements was a 1.5 mm steel panel often used as a reference. The panel's thickness, surface mass density and Young's modulus are given below. The coincidence frequency of the panel can be calculated as: 1
J ; - 1.8h
(Hz)
(2)
where c is the sound speed in air (340 m/s), h is the panel thickness (m), p is the panel density (kg/m 3) and E is the Young's modulus (N/m 2) of the panel material. The calculated frequency agrees well with the observed coincidence TL(dB) 50 40 30 2O I0 63
Fig. 5.
125
250
500
1000 2000 4000
8000(Hz)
Transmission loss of a 1.5 m m steel plate.
G. Papanikolaou, A. Trochides
320
frequency. In Fig. 5 the 1.5 m m steel panel has a calculatedj~ of 8.5 kHz and an estimated Jc of about 8 kHz from the measured one-third octave band data. Pit effects at low frequencies
The sound field in the pit cannot be considered r a n d o m at low frequencies because of its small dimensions. For all TL measurements there is a pronounced decrease in the 250 Hz one-third octave band and an increase in the 163 Hz one-third octave band. The decrease is due to pit resonances and the increase to the absence of resonances. The normal models j , of the pit can be determined only approximately (because of splayed pit surfaces) from: Jl,
=(2)[(rtx/lx)2 + (ny/ly)2 + (rtz//z)2] 1'2
(3)
nx, ny, n == 0 , 1,2,3 . . . . Inserting 1.0, 0'8 and 1"20 m for Ix, ly and/=, respectively, we obtain the normal modes for the pit. The calculations indicate the presence of modes in the 250 Hz one-third octave band, so that the pit pressure is elevated. On the other hand, the absence of vertical modes in the 160 Hz one-third octave band results in a low pit pressure and, consequently, an increase in TL. In order to investigate the influence of the exciting signal on the TL, a series of measurements were carried out using a less tonal signal, i.e. band limited pink noise. The results are shown in Fig. 6. As one would expect, the resonance effects in the low frequency range are almost eliminated. TL(dB) 50 40 o-
-o- ,o"
30 20 10
Fig. 6.
Results of transmission loss measurements using different exciting signals. (a) Third-octave excitation. (b) Band limited pink noise.
Design of a test Jacility for transmission loss measurement
321
Cut-off frequency The main disadvantage of small test chambers is their high cut-off frequency, which means that one cannot obtain reliable sound insulation results at low frequencies. In order to establish the lower frequency limit of the pit, the sound pressure level in the pit was measured continuously, once with the top of the pit open and then closed with the test panel. The results are shown in Fig. 7. These results indicate that the lower cut-off frequency for the test facility is about 400 Hz. i,
t
I tB~Oel& K i a ~ r
1
-
-
3
Po~el,t;Jmeter Range:
d~ F]ec~,fier
/2~f ,5
Lower L;m Freq: -~ ~
HL Wr. Sp~ed: .~ SO
mmtsec. Paper
0
i--
20
Re¢. No.: Oaler
i0
OP 0124
S C
I
- ! Multiply Freq. Scale by: . . . . . . . . .
Fig. 7.
Zero l.e'~l:
~7~' ~ ~
11612/2112
Sound pressure level in the pit. (a) Open pit. (b) Closed pit.
Transmission loss measurements The test facility was used to obtain transmission loss data for a 1.5 m m thick, 120 cm x 120 cm steel panel. The results are shown in Fig. 8. The calculated field incidence transmission loss for a 1.5 m m thick steel panel (surface density, 11.4 kg/m 2) is 33 dB at 1000 Hz. The measured data yield about 32 dB which is in good agreement. The transmission loss exhibits TL(dB) 50 ';0 30 20
O;
Fig. 8.
i 125
i 250
, 500
J lO00
i 2000
i 4000
i ~O00(Hz)
Results of transmission loss measurements.
322
G. Papanikolaou, A. Trochides
mass-controlled behaviour in the frequency range between 400 and 6000 Hz, even though the measured slope is somewhat lower than the theoretical 6 dB per octave. This result agrees with previous experimental data2 obtained by a similar method. The insertion loss, 1L, of the steel panel was also measured, l f L o is the sound pressure level in the reverberation room with no panel and L the sound pressure level with the panel on the pit, then: IL =
L o -
L
(4)
The results are shown in Fig. 9. IL(dB) 50 4O 30 20 tO 63
Fig, 9.
7~5 2~ 56o 7obo 2obo ~o6o 8dooc,z) Results of insertion loss measurements.
Assuming a reverberant field in the pit and combining eqns (1) and (4) yields: S TL = IL + AL + 101og~ (5) where AL is the sound pressure level difference in the pit with and without a panel. The above equation can be used for converting the IL data into TL data. This equation is valid only for panels without absorptive liners and above 1 kHz because of the small dimensions of the pit. Figure 7 shows the term AL measured continuously per one-third octave band. Using these values of AL and the factor 10 log S / A , Fig. 9 can be corrected to yield the TL values given in Fig. 8. CONCLUDING REMARKS In the work described in this paper the performance of a small test facility for transmission loss measurement was investigated.
Design of a test Jacility Jor transmission loss measurement
323
Based on the reported measurements it can be concluded that this test facility can be used for small specimens. The accuracy of the results seems to be adequate for most practical purposes, provided care is exercised below the cut-off frequency of the pit. To obtain more reliable results below the cut-off frequency, band limited pink noise, instead of third-octave excitation, can be used. Further work is in progress to improve the performance of the facility. It concerns the sound insulation properties of the pit walls in order to establish an upper limit for valid TL measurement and the use of the facility for measuring panels with sound absorptive liners facing the pit.
REFERENCES 1. L. L. Beranek, Noise and vibration control, McGraw-Hill Book Company Inc., 1971, p. 276. 2. K. S. Nordby, Measurement and evaluation of the insertion loss of panels, Noise Control Engineering (January, 1978), p. 22.