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The choice of frequency weightings and RT correction for measurements in a simple test for sound insulation
Applied Acoustics 16 (1983) 401-407
The Choice of Frequency Weightings and RT Correction for Measurements in a Simple Test for Sound Insulation
L. C. Fothergill Department of the Environment, Building Research Establishment, Building Research Station, Garston, Watford WD2 7JR, Herts (Great Britain)
(Received: 28 July, 1982)
SUMMARY A simple Jorm o j test f o r airborne sound insulation could involve generating a broad band pink noise in the source room and measuring the source and receiving room levels with a meter containing frequency weighting networks. In practice, the actual source room spectrum shape will depend on room absorption characteristics and this could lead to an error. The effect o f this has been examined by computer simulation. It is also necessary to make some f o r m o f R T correction to the receiving room level and two possible f o r m s have been simulated. The results of the simulations have been compared with 1SO 717 ratings.
INTRODUCTION
There is considerable interest in devising a simplified test to measure the insulation against airborne sounds between dwellings. Many forms of test have been suggested 1 -4 but the most popular idea is to use a broad band noise spectrum in the source room and to measure the source and receiving r o o m levels with a sound level meter containing one or more frequency 401 c Controller, HMSO, London, 1983.
402
L. C. Fothergill
weighting networks. It is also necessary to make some form of reverberation time (RT) correction to the receiving room level to make the result representative of furnished conditions. To be of practical use it must be possible to relate results from the simplified test to values obtained using the field test measuring method described in ISO 140 (Part 4) and the rating procedure described in ISO 717 (1982). The reference curve used in ISO 717 resembles the A weighting curve shape and consequently the ratings are highly correlated with dB(A) insulation values. Here, dB(A) insulation refers to results obtained from the usual ISO 140 one-third octave measuring method and 0'5s RT correction but with the source and corrected receiving room levels converted to dB(A) values on the assumption of a pink source room spectrum; the insulation being taken as the difference between the two dB(a) levels. A simplified test on the lines described above, using A weighting in the sound level meter, can be designed to give results which approximate to the dB(A) insulation, but there are practical difficulties involving the source room spectrum and the RT correction. For the source room spectrum a loudspeaker could be set to produce a pink noise spectrum over the range 100 Hz-3150 Hz in a room having typical reverberation time characteristics. However, when the loudspeaker is moved to another room the spectrum shape may change. It is not practical to correct the spectrum in every r o o m so an error will be introduced. The reverberation time correction, whether obtained by a steady state or decay measurement, could only represent some form of average value over all or part of the frequency range instead of individual one-third octave values, so this is another source of error. The objective of this paper is to investigate the two sources of error described above and to see how simulated results compare with actual IS O 717 ratings. In particular, the effect of using C weighting instead of A weighting for the source room measurement is examined and the difference between an RT correction based on the average low frequency RT is compared with one based on the full range. The study has been made by computer simulation of the relevant effects on one-third octave data collected during a survey of the sound insulation between modern dwellings built in the UK. 5 Data for party walls between 467 pairs of rooms were used, representing many different designs and construction methods. The RTs of the source rooms were not measured so in all cases they were assigned the same values as the corresponding receiving room.
Weight&gs and R T correction for a test for sound insulation
403
EFFECT OF FREQUENCY W E I G H T I N G IN SOURCE R O O M As an example of the test procedure, assume the loudspeaker has been set up to produce a pink spectrum in a typical room. When the loudspeaker is moved to another room having different absorption characteristics the spectrum shape will change but changes at low frequency will not have much effect on the measured A weighted level. In the receiving room the spectrum will approximate to an inverse A weighting shape and so, when it is measured with an A weighted meter, all frequencies will make a significant contribution. Consequently, low frequency changes in source room spectrum shape will affect the receiving room dB(A) level more than the source room dB(A) level, resulting in errors. Using C weighting in the source room overcomes this problem but it is necessary to know how the C-A result compares with the A-A value. If the source room spectrum remained reasonably fiat there would be a systematic difference of about 2 dB, the C-A values being the larger. In order to see if C-A results were affected less than A-A results by changes in source room spectrum, simulated C-A and A-A results were calculated for each pair of rooms. These results were denoted by ACA1 and AAAI, respectively. For comparison ideal reference values, denoted by 6CA and fiAA, were calculated with the source room spectrum kept 'flat', i.e. corrected for each room. To prevent the effect of receiving room RT errors confounding the result, in both cases individual one-third octave corrections were applied before weighting.
FORM OF REVERBERATION TIME CORRECTION For a practical simplified test a single figure RT correction derived from a steady state or decay method 6 could be applied to the A weighted receiving room level. Two forms of RT correction have been investigated. First, an RT correction (with respect to 0.5 s) obtained by averaging all sixteen individual one-third octave RT values was used to simulate measuring the average decay rate of a broad band pink spectrum. The results were denoted by AAA2 and ACA2. However, with the usual ISO 717 procedure the rating is usually determined by a few low frequency bands and so only the low frequency RT is relevant. To simulate this the mean of the first eight RT values was used to provide the correction, again with respect to 0.5 s. The results were denoted by AAA3 and ACA3.
404
L. C. Fothergill
ANALYSIS AND RESULTS The change in source room level due to change in RT was calculated using the term 10 log Ti/Tic, where T i is the RT of the room at frequency i and Tic is the corresponding value in the calibration room. The calibration room RTs are listed in Table 1. They are from an actual room and were chosen because they approximate to the mean values of the sample used in the analysis. These mean values are also shown in Table 1. In order to investigate the importance of the source room frequency weighting, the AAA1 and ACAI values were compared with the ideal reference values using the following statistics: XA = E(~AA - AAA)/N YA = [E(3AA - A A A ) 2 / ( N - 1)] 1/2 XC = Z(aCA - ACA)/N YC = [Y{3CA - A C A ) Z / ( N - 1)] 1:2 where N is the number of walls (467). The X values demonstrate any bias due to the simplified methods and the Yvalues, which have the form of standard deviations, demonstrate the size of the errors. The results of the test concerning source room spectrum variation without the RT error are shown in row 1 of Table 2. The X values show that both AAA1 and ACA1 overestimate the reference values by about 0.4 dB. However, the Y values show that the ACA1 values come from a population with a smaller spread about ACA than the AAA1 values about AAA. The difference is significant at the 5 per cent level. About 95 per cent of the values obtained by the AAA 1 method should be within + 1.6 dB and - 2.4 dB of the true value while 95 per cent of the ACA1 values should be within + 0 . 9 d B and - 1 . 7 d B of the true value. In order to examine the difference between the A values and corresponding ISO 717 ratings, the following statistics were evaluated: PA = Z(D - AAA)/N QA = [E(D - PA - A A A ) 2 / ( N - 1)] t/2 PC = E(D - ACA)/N QC = [Y~(D - PC - A C A ) 2 / ( N - 1)] 1/2 Where D is the ISO 717 rating D.r,w for the same data as the A values. The
Weightings and R T correction for a test for sound insulation
405
TABLE 1 Reverberation Time Data
Frequency
(1) R T (Cal.)
(2) Mean R T
100 125 160 200 250 315 400 500 630 800 1 000 1 250 1 600 2 000 2 500 3 150
0.8 0.9 1.1 1.4 1.7 1.8 1.8 1.9 1.9 2.0 2.2 2.2 2.1 1.9 1.7 1.5
1.0 1.0 1.1 1-3 1.4 1.5 1.7 1"8 1.9 1.9 1.9 1.9 1.8 1.7 1.6 1.4
Column (1) contains the R T values for the calibration room chosen for the analysis Column (2) contains the mean R T values for the sample used in the simulation
P parameters are a measure of the systematic difference between D and the simulated values. The Q parameters are a measure of the dispersion of the simulated values about their own means. The results for this test are also shown in row 1 of Table 2. AAA1 is typically 1.5 dB smaller than D while ACA1 is typically 0.3 dB larger. The effect of the two forms of reverberation time correction were TABLE 2 Values of Test Parameters for Three Forms of R T Correction
RT correction One-third octave (1) Broad band (2) LF band (3)
XA
YA
XC
YC
PA
QA
PC
QC
-0-41 - 1.04 -0.09
0.99 1.86 1-11
-0-37 - 1.00 -0.04
0.66 1.56 0.91
1.52 0.88 1.84
0.91 1.26 0.86
-0.29 -0.93 0-03
0.74 0.95 0.75
406
L. C. Fothergill
investigated using the same statistical tests. The results are shown in Table 2, rows 2 and 3. The ACA values are superficially closer to the ideal reference values than the AAA values for both forms of correction. Closest agreement with the reference value is obtained using the low frequency correction on the ACA results (ACA3). With this method 95 per cent of the ACA3 results should be within + 1.9 dB and - 1.7 dB of the reference value. The P and Q values show that the ACA3 results also give the best agreement with the Dn~r~.values. In fact, 95 per cent of these simple test values should be within ___1.5 dB ofD, T,w.For the next best result, AAA3, 95 per cent of results will be within + 1.7dB of D,T,w -- 1-8 dB. These results show that a simplified method using C weighting in the source room, A weighting in the receiving room and an RT correction based on the average low frequency decay time can give results which are little effected by changes in the source room spectrum and are in good agreement with ISO 717 ratings. The next best result comes from A-A measurements, again combined with a low frequency RT correction. However, for this case there is a systematic difference between the simulated results and DnT,w of nearly 2dB.
CONCLUSIONS Various forms of a simplified test for sound insulation have been compared by computer simulation. In particular, the use of A-A and C-A weightings in the source and receiving rooms have been examined and two forms of RT correction have been tested. The results were compared with ideal values calculated assuming that the source room spectrum was adjusted in every source room to keep it pink, and with one-third octave RT corrections applied to the source room spectrum before A weighting. The simulated results were also compared with ISO 717 ratings calculated for the same data. The analysis shows that a method using C weighting in the source room and A weighting in the receiving room, combined with an RT correction derived from the average low frequency decay time, gave results closest to the ideal value and very close to the ISO 717 ratings. The next best results came from an A-A method with the low frequency RT correction. These results were systematically about 2 dB smaller than the ISO 717 values. The C-A approach has the disadvantage that the sound level meter
Weightings and RT correction Jor a test Jor sound insulation
407
must have both weightings and the operator must select the correct one. However, this is partially offset because a simple band limited pink noise spectrum could be used to determine the RT correction instead of a special inverse A weighting shaped spectrum that would be required for an A - A method, such as is described in reference 2. In practice, there seems to be little difference between the two methods provided an RT correction based on low frequency measurements is used. Further experimental work is required to test these findings and also to determine the optimum arrangement of loudspeaker and microphone positions.
ACKNOWLEDGEMENT The work described has been carried out as part of the research programme of the Building Research Establishment of the Department of the Environment and this paper is published by permission of the Director.
REFERENCES 1. D. H. Stephens, Measurement of sound insulation with a sound level meter, Applied Acoustics, 9 (1976), 131-8. 2. ASTM, Tentative recommended practice for determining a single number rating of airborne sound isolation in multi-unit building specifications, ASTM E 597-77T. 3. J. Roland, Airborne isolation measurements by impulse techniques, Noise Control Engineering (Jan./Feb., 1981). 4. G. L. Fuchs and N. Stasysjyn, Airborne and impact noise level criteria for buildings, Applied Acoustics, 12 (1979), 187-94. 5. E. C. Sewell and W. E. Scholes, Sound insulation performance between dwellings built in the early 1970s. Current Paper 20/78, Building Research Station, Watford, WD2 7JR. 6. L. C. Fothergill, An investigation of simple methods for assessing reverberation time, Applied Acoustics, 15 (1982), 11-29.
The Choice of Frequency Weightings and RT Correction for Measurements in a Simple Test for Sound Insulation
L. C. Fothergill Department of the Environment, Building Research Establishment, Building Research Station, Garston, Watford WD2 7JR, Herts (Great Britain)
(Received: 28 July, 1982)
SUMMARY A simple Jorm o j test f o r airborne sound insulation could involve generating a broad band pink noise in the source room and measuring the source and receiving room levels with a meter containing frequency weighting networks. In practice, the actual source room spectrum shape will depend on room absorption characteristics and this could lead to an error. The effect o f this has been examined by computer simulation. It is also necessary to make some f o r m o f R T correction to the receiving room level and two possible f o r m s have been simulated. The results of the simulations have been compared with 1SO 717 ratings.
INTRODUCTION
There is considerable interest in devising a simplified test to measure the insulation against airborne sounds between dwellings. Many forms of test have been suggested 1 -4 but the most popular idea is to use a broad band noise spectrum in the source room and to measure the source and receiving r o o m levels with a sound level meter containing one or more frequency 401 c Controller, HMSO, London, 1983.
402
L. C. Fothergill
weighting networks. It is also necessary to make some form of reverberation time (RT) correction to the receiving room level to make the result representative of furnished conditions. To be of practical use it must be possible to relate results from the simplified test to values obtained using the field test measuring method described in ISO 140 (Part 4) and the rating procedure described in ISO 717 (1982). The reference curve used in ISO 717 resembles the A weighting curve shape and consequently the ratings are highly correlated with dB(A) insulation values. Here, dB(A) insulation refers to results obtained from the usual ISO 140 one-third octave measuring method and 0'5s RT correction but with the source and corrected receiving room levels converted to dB(A) values on the assumption of a pink source room spectrum; the insulation being taken as the difference between the two dB(a) levels. A simplified test on the lines described above, using A weighting in the sound level meter, can be designed to give results which approximate to the dB(A) insulation, but there are practical difficulties involving the source room spectrum and the RT correction. For the source room spectrum a loudspeaker could be set to produce a pink noise spectrum over the range 100 Hz-3150 Hz in a room having typical reverberation time characteristics. However, when the loudspeaker is moved to another room the spectrum shape may change. It is not practical to correct the spectrum in every r o o m so an error will be introduced. The reverberation time correction, whether obtained by a steady state or decay measurement, could only represent some form of average value over all or part of the frequency range instead of individual one-third octave values, so this is another source of error. The objective of this paper is to investigate the two sources of error described above and to see how simulated results compare with actual IS O 717 ratings. In particular, the effect of using C weighting instead of A weighting for the source room measurement is examined and the difference between an RT correction based on the average low frequency RT is compared with one based on the full range. The study has been made by computer simulation of the relevant effects on one-third octave data collected during a survey of the sound insulation between modern dwellings built in the UK. 5 Data for party walls between 467 pairs of rooms were used, representing many different designs and construction methods. The RTs of the source rooms were not measured so in all cases they were assigned the same values as the corresponding receiving room.
Weight&gs and R T correction for a test for sound insulation
403
EFFECT OF FREQUENCY W E I G H T I N G IN SOURCE R O O M As an example of the test procedure, assume the loudspeaker has been set up to produce a pink spectrum in a typical room. When the loudspeaker is moved to another room having different absorption characteristics the spectrum shape will change but changes at low frequency will not have much effect on the measured A weighted level. In the receiving room the spectrum will approximate to an inverse A weighting shape and so, when it is measured with an A weighted meter, all frequencies will make a significant contribution. Consequently, low frequency changes in source room spectrum shape will affect the receiving room dB(A) level more than the source room dB(A) level, resulting in errors. Using C weighting in the source room overcomes this problem but it is necessary to know how the C-A result compares with the A-A value. If the source room spectrum remained reasonably fiat there would be a systematic difference of about 2 dB, the C-A values being the larger. In order to see if C-A results were affected less than A-A results by changes in source room spectrum, simulated C-A and A-A results were calculated for each pair of rooms. These results were denoted by ACA1 and AAAI, respectively. For comparison ideal reference values, denoted by 6CA and fiAA, were calculated with the source room spectrum kept 'flat', i.e. corrected for each room. To prevent the effect of receiving room RT errors confounding the result, in both cases individual one-third octave corrections were applied before weighting.
FORM OF REVERBERATION TIME CORRECTION For a practical simplified test a single figure RT correction derived from a steady state or decay method 6 could be applied to the A weighted receiving room level. Two forms of RT correction have been investigated. First, an RT correction (with respect to 0.5 s) obtained by averaging all sixteen individual one-third octave RT values was used to simulate measuring the average decay rate of a broad band pink spectrum. The results were denoted by AAA2 and ACA2. However, with the usual ISO 717 procedure the rating is usually determined by a few low frequency bands and so only the low frequency RT is relevant. To simulate this the mean of the first eight RT values was used to provide the correction, again with respect to 0.5 s. The results were denoted by AAA3 and ACA3.
404
L. C. Fothergill
ANALYSIS AND RESULTS The change in source room level due to change in RT was calculated using the term 10 log Ti/Tic, where T i is the RT of the room at frequency i and Tic is the corresponding value in the calibration room. The calibration room RTs are listed in Table 1. They are from an actual room and were chosen because they approximate to the mean values of the sample used in the analysis. These mean values are also shown in Table 1. In order to investigate the importance of the source room frequency weighting, the AAA1 and ACAI values were compared with the ideal reference values using the following statistics: XA = E(~AA - AAA)/N YA = [E(3AA - A A A ) 2 / ( N - 1)] 1/2 XC = Z(aCA - ACA)/N YC = [Y{3CA - A C A ) Z / ( N - 1)] 1:2 where N is the number of walls (467). The X values demonstrate any bias due to the simplified methods and the Yvalues, which have the form of standard deviations, demonstrate the size of the errors. The results of the test concerning source room spectrum variation without the RT error are shown in row 1 of Table 2. The X values show that both AAA1 and ACA1 overestimate the reference values by about 0.4 dB. However, the Y values show that the ACA1 values come from a population with a smaller spread about ACA than the AAA1 values about AAA. The difference is significant at the 5 per cent level. About 95 per cent of the values obtained by the AAA 1 method should be within + 1.6 dB and - 2.4 dB of the true value while 95 per cent of the ACA1 values should be within + 0 . 9 d B and - 1 . 7 d B of the true value. In order to examine the difference between the A values and corresponding ISO 717 ratings, the following statistics were evaluated: PA = Z(D - AAA)/N QA = [E(D - PA - A A A ) 2 / ( N - 1)] t/2 PC = E(D - ACA)/N QC = [Y~(D - PC - A C A ) 2 / ( N - 1)] 1/2 Where D is the ISO 717 rating D.r,w for the same data as the A values. The
Weightings and R T correction for a test for sound insulation
405
TABLE 1 Reverberation Time Data
Frequency
(1) R T (Cal.)
(2) Mean R T
100 125 160 200 250 315 400 500 630 800 1 000 1 250 1 600 2 000 2 500 3 150
0.8 0.9 1.1 1.4 1.7 1.8 1.8 1.9 1.9 2.0 2.2 2.2 2.1 1.9 1.7 1.5
1.0 1.0 1.1 1-3 1.4 1.5 1.7 1"8 1.9 1.9 1.9 1.9 1.8 1.7 1.6 1.4
Column (1) contains the R T values for the calibration room chosen for the analysis Column (2) contains the mean R T values for the sample used in the simulation
P parameters are a measure of the systematic difference between D and the simulated values. The Q parameters are a measure of the dispersion of the simulated values about their own means. The results for this test are also shown in row 1 of Table 2. AAA1 is typically 1.5 dB smaller than D while ACA1 is typically 0.3 dB larger. The effect of the two forms of reverberation time correction were TABLE 2 Values of Test Parameters for Three Forms of R T Correction
RT correction One-third octave (1) Broad band (2) LF band (3)
XA
YA
XC
YC
PA
QA
PC
QC
-0-41 - 1.04 -0.09
0.99 1.86 1-11
-0-37 - 1.00 -0.04
0.66 1.56 0.91
1.52 0.88 1.84
0.91 1.26 0.86
-0.29 -0.93 0-03
0.74 0.95 0.75
406
L. C. Fothergill
investigated using the same statistical tests. The results are shown in Table 2, rows 2 and 3. The ACA values are superficially closer to the ideal reference values than the AAA values for both forms of correction. Closest agreement with the reference value is obtained using the low frequency correction on the ACA results (ACA3). With this method 95 per cent of the ACA3 results should be within + 1.9 dB and - 1.7 dB of the reference value. The P and Q values show that the ACA3 results also give the best agreement with the Dn~r~.values. In fact, 95 per cent of these simple test values should be within ___1.5 dB ofD, T,w.For the next best result, AAA3, 95 per cent of results will be within + 1.7dB of D,T,w -- 1-8 dB. These results show that a simplified method using C weighting in the source room, A weighting in the receiving room and an RT correction based on the average low frequency decay time can give results which are little effected by changes in the source room spectrum and are in good agreement with ISO 717 ratings. The next best result comes from A-A measurements, again combined with a low frequency RT correction. However, for this case there is a systematic difference between the simulated results and DnT,w of nearly 2dB.
CONCLUSIONS Various forms of a simplified test for sound insulation have been compared by computer simulation. In particular, the use of A-A and C-A weightings in the source and receiving rooms have been examined and two forms of RT correction have been tested. The results were compared with ideal values calculated assuming that the source room spectrum was adjusted in every source room to keep it pink, and with one-third octave RT corrections applied to the source room spectrum before A weighting. The simulated results were also compared with ISO 717 ratings calculated for the same data. The analysis shows that a method using C weighting in the source room and A weighting in the receiving room, combined with an RT correction derived from the average low frequency decay time, gave results closest to the ideal value and very close to the ISO 717 ratings. The next best results came from an A-A method with the low frequency RT correction. These results were systematically about 2 dB smaller than the ISO 717 values. The C-A approach has the disadvantage that the sound level meter
Weightings and RT correction Jor a test Jor sound insulation
407
must have both weightings and the operator must select the correct one. However, this is partially offset because a simple band limited pink noise spectrum could be used to determine the RT correction instead of a special inverse A weighting shaped spectrum that would be required for an A - A method, such as is described in reference 2. In practice, there seems to be little difference between the two methods provided an RT correction based on low frequency measurements is used. Further experimental work is required to test these findings and also to determine the optimum arrangement of loudspeaker and microphone positions.
ACKNOWLEDGEMENT The work described has been carried out as part of the research programme of the Building Research Establishment of the Department of the Environment and this paper is published by permission of the Director.
REFERENCES 1. D. H. Stephens, Measurement of sound insulation with a sound level meter, Applied Acoustics, 9 (1976), 131-8. 2. ASTM, Tentative recommended practice for determining a single number rating of airborne sound isolation in multi-unit building specifications, ASTM E 597-77T. 3. J. Roland, Airborne isolation measurements by impulse techniques, Noise Control Engineering (Jan./Feb., 1981). 4. G. L. Fuchs and N. Stasysjyn, Airborne and impact noise level criteria for buildings, Applied Acoustics, 12 (1979), 187-94. 5. E. C. Sewell and W. E. Scholes, Sound insulation performance between dwellings built in the early 1970s. Current Paper 20/78, Building Research Station, Watford, WD2 7JR. 6. L. C. Fothergill, An investigation of simple methods for assessing reverberation time, Applied Acoustics, 15 (1982), 11-29.