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Simple sound transmission loss measurements using a modified impedance tube technique
SIMPLE SOUND TRANSMISSION LOSS MEASUREMENTS USING A MODIFIED IMPEDANCE TUBE TECHNIQUE A. R. WHArMOREand M. V. LOWSON
Department of Transport Technology. Loughborough Unicersity of Technology, Loughborough, Leics (Great Britain) (Received: 16 May, 1973) S UM M A R Y
A method of comparing the sound transmission characteristics of various materials, and combinations of materials, is presented, using a modified impedance tube technique. The procedure proved to be relatively quick and inexpensive in comparison with standard reverberation suite tests, and is therefore particularly usefid for the qualitative ranking of multiple samples. The limitations of the technique are discussed in some detail, and particular emphasis is given to the problems of small sample size and method of mounting in the apparatus.
INTRODUCTION
The transmission loss of acoustic barriers is normally measured via tests in a reverberation suite. The specification for such tests, ~ requires the use of large panels built into a wall between two reverberant rooms, and a random incidence sound field for the acoustic measurements. As is well known, results from such tests typically follow a curve similar to that shown in Fig. 1. At sufficiently low frequencies, response is governed by stiffness; at intermediate frequencies the transmission loss is governed by panel mass, while at the highest frequencies, the effects of 'coincidence' dominate. However, for many practical acoustic barriers the most important region is the intermediate, or mass law, range of transmission loss. Results of acoustic tests in this range are therefore of particular significance in design. In addition, under typical conditions, acoustic barriers do not have to repulse ideal random fields, and empirical testing has suggested that a 'field incidence' distribution, which retains some of the directionality from the acoustic source, is more appropriate. 293
Applied Acoustics (6) (1973)--O Applied Science Publishers Ltd. England. 1973--Printed in Great Britain
294
A. R. W'HATMORE M. V. LOWSON
4.5
f I'Q °
23 I.iJ
.I
Z
o
63
100
200
400
800
16C0
3200
FREQUENCY * SURFACE WEIGHT--Hz ,Lbs/SQ.FT.
Fig. 1.
Theoretical results for t r a n s m i s s i o n loss according to mass law (after Beranek2).
The technique for acquiring transmission loss data, suggested in this paper, uses a normal incidence sound field, which, in general terms, gives as good an approximation to the practical field incidence curve as does the random acoustic field. This feature is shown in Fig. 2, where, over the important mass law range, the direct relation between normal, random, and field incidence transmission loss data is presented. The graph shows a difference of approximately 5 dB between each case, over most of the frequency range. Furthermore, the results from different reverberation suites do not always agree. Figure 3 is based on work by Brown, 3 and shows that even a standardised test procedure can give a scatter of up to 6 dB for the same type of panel. Thus, for many design problems, reverberation room methods for transmission loss testing would appear to be unnecessarily complex and expensive. It is natural, therefore, to seek a simpler technique. We have therefore developed, at Loughborough University of Technology, a modified impedance tube technique for normal incidence testing of small samples. As discussed above, it is not thought that the use of a normal incidence sound field for transmission loss measurements imposes any severe limitation on the utility of
295
SIMPLE SOUND TRANSMISSION LOSS MEASUREMENTS
30 N
/
2S
s
x'
t~l 0, --I
20
1S
'S
I
o
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o
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,°. . . . . . . . . . . . ..°
v1 v}
-2
I--
0
lOO
200
400 803 FREQUENCY-Hz
16(30
32[J0
Fig. 2. Transmission loss of a 0.035 in aluminium panel measured in different facilities• - Results obtained at Mira (Brown3). - . . . . . . . Results obtained at Liverpool University. ........ Results obtained at Salford University.
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~~
I
~
°
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Fig. 3.
¢,
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10,000
Typical s o u n d transmission loss curve (figures apply to thin panels).
296
A. R. W H A T M O R E , M. V. L O W S O N
the results. The most important difference between the two methods appears to be sample size. In reverberation suite testing large panels are used, whilst the impedance tube technique only involves samples of about 6 in diameter. Therefore, one might expect differences between transmission loss data taken by the two techniques, and this would be particularly noticeable at the low frequency end of the audio spectrum where transmission loss is governed by sample stiffness. Clearly, for a given material or composite the edge clamping and sample size used in the impedance tube measurement would result in a larger value of sample stiffness, in comparison with the same specimen tested in a reverberation suite. This suggests that it would be better to use an impedance tube technique as a method of comparing the acoustic effectiveness of one sample with another, rather than trying to extract absolute transmission loss data. In addition, the value of an impedance tube test can be realised when many samples are to be tested, as the method is quick and relatively cheap in comparison with standard reverberation suite procedures. To date, most of the data published on transmission loss measurements have been based on full scale tests, although some work by Walker, 4 using a modified impedance tube technique, has yielded good results. He performed transmission loss tests similar to those outlined in this paper, and obtained good agreement between experiment and theory for ~ in lead sheet, and good correlation with large-scale reverberation room tests, for -~ in hardboard, using samples of about 4 in diameter. Walker's other tests on somewhat heavier specimens were less convincing, probably due to the particular type of samples tested, and the water tank method of mounting used in his apparatus. To date our results--obtained using the modified impedance tube technique--are encouraging.
MODIFIED I M P E D A N C E TUBE A P P A R A T U S
Figure 4 is a diagram showing the modified impedance tube apparatus developed. The basic system consists of a standard B & K impedance tube, Type 4002, normally used for the measurement of absorption coefficient and acoustic impedance. This unit has been modified by the addition of another tube, of similar dimensions, containing a polyurethane foam wedge to provide an anechoic termination. This arrangement absorbs 99 per cent of the incident sound energy within the impedance tube working range, i.e. between 150 Hz to 3.5 KHz. The sample to be tested is mounted between the two sections, as shown in Fig. 4. Sound pressure level measurements are taken at the face of the sample, in the source tube, and just before the top of the wedge inside the receiver tube, the difference between the two levels, in dB, representing the sample transmission loss at that particular frequency. Test frequencies covering the apparatus operating range are selected such that one data
SIMPLE SOUND TRANSMISSION LOSS MEASUREMENTS
297
point falls within each one-third octave band, between 100 Hz and 4 KHz, which enables a representative transmission loss graph to be drawn over the range of interest. Auxiliary equipment consists of a B & K oscillator, Type 1024, and a B & K microphone amplifier, Type 2607, with associated ½ in microphone and cathode follower. Also connected to the receiving tube monitoring system is a B & K filter unit, Type 2020, which is automatically tuned to the oscillator frequency. This enables the test frequency to be picked out amongst the general background noise within the receiver tube, which may sometimes be high in comparison with the test frequency for samples having a large sound transmission loss. micre~hone
t er'o~ne'2~n min:~f
J ibr~q ass
x'k '
~ itrr~eudaenCx¢
,,..:.......~[:.!
Ioudspe,-'~er
.~ar~le
•".".. i .." -.-:-....:...:.. ..
rec-~ver ~
~ou rce tub
'." ~.': " "~'. ~'-" .'-i • '. • " :" " " : i'.'. ?. : :.
Fig. 4.
Lesi~ient
_..~o_lno] i
Modified impedancetube apparatus for measurementof sample transmissionloss.
The major difficulty with this type of measurement is satisfactorily mounting the sample in the apparatus. The two most important features to consider are minimisation of mechanical coupling between the source and receiver tubes and reduction of sound leakage from around the periphery of the sample. These difficulties have been overcome by the use of resilient sealing strips, but as this is a relatively simple method, there should still be considerable scope for improvement.
DISCUSSION OF RESULTS
Figure 5 shows field incidence transmission loss data for steel and lead sheets as a function of the frequency times surface weight. The solid lines have been taken from reverberation suite tests, and the experimental results from the impedance tube, plotted as separate data points. A correction of 5 dB has been applied to this data to convert from normal to field incidence. In each case the mean line through the
298
A. R. WHATMORE, M. V. LOWSON
impedance tube data shows good agreement with the results from the reverberation suite tests, over most of the frequency range. However, at the lo~er end of the spectrum considerable scatter may be observed, this error probably being due to the effects of small sample size and edge fixing. Figure 6 shows a selection of other experimental transmission loss data obtained so far. Three different types of acoustic barrier mat are illustrated. Sample A consists of two relatively massive layers, separated by expanded polystyrene, which gives the specimen a high overall stiffness. [n samples B and C, which are more ~E
2)
I
z c
i
I 0~2
1..5 3,2 6,3 1:5 2 .~ FREOUENCY(KHz) X SURFACE wEIGHT(LBS/SQFTI ~i"
"8
50
Fig. 5. C o m p a r i s o n of modified impedance tube and reverberation suite data for lead and steel sheet. -- ;< - × -- "< - modified impedance tube data points for lead sheet; . . . O . . . O . . . O modified impedance tube data points for steel sheet.
flexible, the interface layers are of cotton waste and polyurethane foam. respectively. All three combinations have approximately the same weight. With these particular types of sample, i.e. a sandwich construction, differences in sound transmission loss will be governed by the mechanical coupling between the two outer massive layers. The test results are given in Fig. 6 and show that, out of the three samples tested, the polyurethane foam is the most effective in reducing mechanical coupling, whilst the rigidity of the expanded polystyrene layer in sample A makes it relatively poor as a spacer material. The cotton waste composition falls close to the polyurethane foam, but the compactness of the cotton fibres appears to increase the mechanical coupling between the two outside layers, and hence reduces the acoustic effectiveness of this particular type of barrier mat.
299
S[MPLE SOUND TRANSMISSION LOSS MEASUREMENTS SAMPLE A
exDq~ 1
polystyrene bq
pOlyure,hane foQm
•bres ~--
3
J
9 g
SAMPLE C ~ massive
SAMPLE B
jJ
2
Jj
J
J
J I
f
7
o 100
200
400
800
1600
3200
5000
FREQUENCY-Hz
Fig. 6.
Normal incidence transmission loss (dB) versus frequency (Hz) for three composite barrier mats.
CONCLUDING REMARKS
For the lead and steel samples used in the experiment, the modified impedance tube technique yields results which are in good agreement, over the mass law region, with data obtained from standard reverberation suite tests (Fig. 5). In addition, for the three physically-different samples tested, a quantitative difference in sound transmission characteristics has been demonstrated (Fig. 6). A main criticism of this test procedure is the unrepresentative stiffness imposed by the apparatus upon the sample. This would be particularly important if the final form of the material were to be used as a large panel, in which case a reverberation suite test is the obvious choice for transmission loss measurements. However, if the final form and overall stiffness of the various samples are essentially unknown, the impedance tube offers a very good alternative method for sample ranking.
REFERENCES 1. BS 2750: 1956. Recommendations for Field and Laboratory Measurement of Airborne and
Impact Sound Transmission in Buildings.
300
A. R. VCHATMORE, M. V. LOWSON
2. L. L. BERANEK, The transmission and radiation of acoustic waves by structures, Proc. Inst. ,~Iech. Engrs. 173 (1959) p. 12. 3. M. BROWN. The Measurement of Airborne Sound Transmission Loss, 1969. 4. C. WALKER, 1967. Optimisation of Parameters Necessary in the Design of Plasterboard Lightweight Partitions Having a High Transmission Loss, M.Sc. Thesis, ISVR, Southampton University.
Department of Transport Technology. Loughborough Unicersity of Technology, Loughborough, Leics (Great Britain) (Received: 16 May, 1973) S UM M A R Y
A method of comparing the sound transmission characteristics of various materials, and combinations of materials, is presented, using a modified impedance tube technique. The procedure proved to be relatively quick and inexpensive in comparison with standard reverberation suite tests, and is therefore particularly usefid for the qualitative ranking of multiple samples. The limitations of the technique are discussed in some detail, and particular emphasis is given to the problems of small sample size and method of mounting in the apparatus.
INTRODUCTION
The transmission loss of acoustic barriers is normally measured via tests in a reverberation suite. The specification for such tests, ~ requires the use of large panels built into a wall between two reverberant rooms, and a random incidence sound field for the acoustic measurements. As is well known, results from such tests typically follow a curve similar to that shown in Fig. 1. At sufficiently low frequencies, response is governed by stiffness; at intermediate frequencies the transmission loss is governed by panel mass, while at the highest frequencies, the effects of 'coincidence' dominate. However, for many practical acoustic barriers the most important region is the intermediate, or mass law, range of transmission loss. Results of acoustic tests in this range are therefore of particular significance in design. In addition, under typical conditions, acoustic barriers do not have to repulse ideal random fields, and empirical testing has suggested that a 'field incidence' distribution, which retains some of the directionality from the acoustic source, is more appropriate. 293
Applied Acoustics (6) (1973)--O Applied Science Publishers Ltd. England. 1973--Printed in Great Britain
294
A. R. W'HATMORE M. V. LOWSON
4.5
f I'Q °
23 I.iJ
.I
Z
o
63
100
200
400
800
16C0
3200
FREQUENCY * SURFACE WEIGHT--Hz ,Lbs/SQ.FT.
Fig. 1.
Theoretical results for t r a n s m i s s i o n loss according to mass law (after Beranek2).
The technique for acquiring transmission loss data, suggested in this paper, uses a normal incidence sound field, which, in general terms, gives as good an approximation to the practical field incidence curve as does the random acoustic field. This feature is shown in Fig. 2, where, over the important mass law range, the direct relation between normal, random, and field incidence transmission loss data is presented. The graph shows a difference of approximately 5 dB between each case, over most of the frequency range. Furthermore, the results from different reverberation suites do not always agree. Figure 3 is based on work by Brown, 3 and shows that even a standardised test procedure can give a scatter of up to 6 dB for the same type of panel. Thus, for many design problems, reverberation room methods for transmission loss testing would appear to be unnecessarily complex and expensive. It is natural, therefore, to seek a simpler technique. We have therefore developed, at Loughborough University of Technology, a modified impedance tube technique for normal incidence testing of small samples. As discussed above, it is not thought that the use of a normal incidence sound field for transmission loss measurements imposes any severe limitation on the utility of
295
SIMPLE SOUND TRANSMISSION LOSS MEASUREMENTS
30 N
/
2S
s
x'
t~l 0, --I
20
1S
'S
I
o
Z
o
10
pY
..°°.0 . . ° ~ °°.°"
,°. . . . . . . . . . . . ..°
v1 v}
-2
I--
0
lOO
200
400 803 FREQUENCY-Hz
16(30
32[J0
Fig. 2. Transmission loss of a 0.035 in aluminium panel measured in different facilities• - Results obtained at Mira (Brown3). - . . . . . . . Results obtained at Liverpool University. ........ Results obtained at Salford University.
t~
3O
"o
~u l
~_
!
°° u
~~
I
~
°
._c o
I/ I /
~
,o
I 100
Fig. 3.
¢,
I 11~0 FREQUENCY-H_
10,000
Typical s o u n d transmission loss curve (figures apply to thin panels).
296
A. R. W H A T M O R E , M. V. L O W S O N
the results. The most important difference between the two methods appears to be sample size. In reverberation suite testing large panels are used, whilst the impedance tube technique only involves samples of about 6 in diameter. Therefore, one might expect differences between transmission loss data taken by the two techniques, and this would be particularly noticeable at the low frequency end of the audio spectrum where transmission loss is governed by sample stiffness. Clearly, for a given material or composite the edge clamping and sample size used in the impedance tube measurement would result in a larger value of sample stiffness, in comparison with the same specimen tested in a reverberation suite. This suggests that it would be better to use an impedance tube technique as a method of comparing the acoustic effectiveness of one sample with another, rather than trying to extract absolute transmission loss data. In addition, the value of an impedance tube test can be realised when many samples are to be tested, as the method is quick and relatively cheap in comparison with standard reverberation suite procedures. To date, most of the data published on transmission loss measurements have been based on full scale tests, although some work by Walker, 4 using a modified impedance tube technique, has yielded good results. He performed transmission loss tests similar to those outlined in this paper, and obtained good agreement between experiment and theory for ~ in lead sheet, and good correlation with large-scale reverberation room tests, for -~ in hardboard, using samples of about 4 in diameter. Walker's other tests on somewhat heavier specimens were less convincing, probably due to the particular type of samples tested, and the water tank method of mounting used in his apparatus. To date our results--obtained using the modified impedance tube technique--are encouraging.
MODIFIED I M P E D A N C E TUBE A P P A R A T U S
Figure 4 is a diagram showing the modified impedance tube apparatus developed. The basic system consists of a standard B & K impedance tube, Type 4002, normally used for the measurement of absorption coefficient and acoustic impedance. This unit has been modified by the addition of another tube, of similar dimensions, containing a polyurethane foam wedge to provide an anechoic termination. This arrangement absorbs 99 per cent of the incident sound energy within the impedance tube working range, i.e. between 150 Hz to 3.5 KHz. The sample to be tested is mounted between the two sections, as shown in Fig. 4. Sound pressure level measurements are taken at the face of the sample, in the source tube, and just before the top of the wedge inside the receiver tube, the difference between the two levels, in dB, representing the sample transmission loss at that particular frequency. Test frequencies covering the apparatus operating range are selected such that one data
SIMPLE SOUND TRANSMISSION LOSS MEASUREMENTS
297
point falls within each one-third octave band, between 100 Hz and 4 KHz, which enables a representative transmission loss graph to be drawn over the range of interest. Auxiliary equipment consists of a B & K oscillator, Type 1024, and a B & K microphone amplifier, Type 2607, with associated ½ in microphone and cathode follower. Also connected to the receiving tube monitoring system is a B & K filter unit, Type 2020, which is automatically tuned to the oscillator frequency. This enables the test frequency to be picked out amongst the general background noise within the receiver tube, which may sometimes be high in comparison with the test frequency for samples having a large sound transmission loss. micre~hone
t er'o~ne'2~n min:~f
J ibr~q ass
x'k '
~ itrr~eudaenCx¢
,,..:.......~[:.!
Ioudspe,-'~er
.~ar~le
•".".. i .." -.-:-....:...:.. ..
rec-~ver ~
~ou rce tub
'." ~.': " "~'. ~'-" .'-i • '. • " :" " " : i'.'. ?. : :.
Fig. 4.
Lesi~ient
_..~o_lno] i
Modified impedancetube apparatus for measurementof sample transmissionloss.
The major difficulty with this type of measurement is satisfactorily mounting the sample in the apparatus. The two most important features to consider are minimisation of mechanical coupling between the source and receiver tubes and reduction of sound leakage from around the periphery of the sample. These difficulties have been overcome by the use of resilient sealing strips, but as this is a relatively simple method, there should still be considerable scope for improvement.
DISCUSSION OF RESULTS
Figure 5 shows field incidence transmission loss data for steel and lead sheets as a function of the frequency times surface weight. The solid lines have been taken from reverberation suite tests, and the experimental results from the impedance tube, plotted as separate data points. A correction of 5 dB has been applied to this data to convert from normal to field incidence. In each case the mean line through the
298
A. R. WHATMORE, M. V. LOWSON
impedance tube data shows good agreement with the results from the reverberation suite tests, over most of the frequency range. However, at the lo~er end of the spectrum considerable scatter may be observed, this error probably being due to the effects of small sample size and edge fixing. Figure 6 shows a selection of other experimental transmission loss data obtained so far. Three different types of acoustic barrier mat are illustrated. Sample A consists of two relatively massive layers, separated by expanded polystyrene, which gives the specimen a high overall stiffness. [n samples B and C, which are more ~E
2)
I
z c
i
I 0~2
1..5 3,2 6,3 1:5 2 .~ FREOUENCY(KHz) X SURFACE wEIGHT(LBS/SQFTI ~i"
"8
50
Fig. 5. C o m p a r i s o n of modified impedance tube and reverberation suite data for lead and steel sheet. -- ;< - × -- "< - modified impedance tube data points for lead sheet; . . . O . . . O . . . O modified impedance tube data points for steel sheet.
flexible, the interface layers are of cotton waste and polyurethane foam. respectively. All three combinations have approximately the same weight. With these particular types of sample, i.e. a sandwich construction, differences in sound transmission loss will be governed by the mechanical coupling between the two outer massive layers. The test results are given in Fig. 6 and show that, out of the three samples tested, the polyurethane foam is the most effective in reducing mechanical coupling, whilst the rigidity of the expanded polystyrene layer in sample A makes it relatively poor as a spacer material. The cotton waste composition falls close to the polyurethane foam, but the compactness of the cotton fibres appears to increase the mechanical coupling between the two outside layers, and hence reduces the acoustic effectiveness of this particular type of barrier mat.
299
S[MPLE SOUND TRANSMISSION LOSS MEASUREMENTS SAMPLE A
exDq~ 1
polystyrene bq
pOlyure,hane foQm
•bres ~--
3
J
9 g
SAMPLE C ~ massive
SAMPLE B
jJ
2
Jj
J
J
J I
f
7
o 100
200
400
800
1600
3200
5000
FREQUENCY-Hz
Fig. 6.
Normal incidence transmission loss (dB) versus frequency (Hz) for three composite barrier mats.
CONCLUDING REMARKS
For the lead and steel samples used in the experiment, the modified impedance tube technique yields results which are in good agreement, over the mass law region, with data obtained from standard reverberation suite tests (Fig. 5). In addition, for the three physically-different samples tested, a quantitative difference in sound transmission characteristics has been demonstrated (Fig. 6). A main criticism of this test procedure is the unrepresentative stiffness imposed by the apparatus upon the sample. This would be particularly important if the final form of the material were to be used as a large panel, in which case a reverberation suite test is the obvious choice for transmission loss measurements. However, if the final form and overall stiffness of the various samples are essentially unknown, the impedance tube offers a very good alternative method for sample ranking.
REFERENCES 1. BS 2750: 1956. Recommendations for Field and Laboratory Measurement of Airborne and
Impact Sound Transmission in Buildings.
300
A. R. VCHATMORE, M. V. LOWSON
2. L. L. BERANEK, The transmission and radiation of acoustic waves by structures, Proc. Inst. ,~Iech. Engrs. 173 (1959) p. 12. 3. M. BROWN. The Measurement of Airborne Sound Transmission Loss, 1969. 4. C. WALKER, 1967. Optimisation of Parameters Necessary in the Design of Plasterboard Lightweight Partitions Having a High Transmission Loss, M.Sc. Thesis, ISVR, Southampton University.