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Offices with high sound insulation
OFFICES
WITH
HIGH
SOUND
INSULATION
R. D. FORD, P. LORD and A. W. WALKER
The Acoustics Group, Department of Pure and Applied Physics, The University of Salford, Lancashire, Great Britain (Received: 10 August, 1967)
SUMMARY
Two prototype offices were built from lightweight materials using the "floating box" principle. Measurements show that both the impact and the airborne sound insulation between the offices is comparable with Grade 1 (U.K.) insulation between dwellings. This type of construction is particularly suited to frame buildings where floor loading can present serious problems. Cost analysis indicates that it compares very favourably with other systems giving comparable insulation. INTRODUCTION
The work described in this paper arose from a specific requirement but the principles are generally applicable and a "case history" may help to underline those principles. The problem was to provide executive offices on one of the upper floors of a new high-rise office block with both airborne and impact sound insulation comparable to that between dwellings. The traditional way to do this would be to use a 9-inch brick wall between offices, but a wall mass of 90 lb/ft 2 could not be supported by the steel frame of the building. Comparatively lightweight materials therefore had to be used. These materials should not be too expensive, nor take up too much space, nor require any particular skill in handling. In order to prove the design, and to iron out snags off-site, a prototype was built consisting of an executive office and a secretary's office. The impact and airborne sound insulation between the offices was measured under a variety of test conditions. THE DESIGN OF THE OFFICES
The overall approach to the problem is very similar to that found in the building system developed by Conbox Ltd., of Denmark, 1 in which the living units are in AppliedAcoustics--Elsevier Publishing Company Ltd., England--Printed in Great Britain
22
R. D. FORD, P. LORD, A. W. WALKER
the form of concrete boxes spaced from one another and separated from the main structure by vibration-isolating pads. In the system to be discussed, however, much lighter materials are employed for the walls and ceilings: the floor is still concrete and the "boxes" float inside the main building. The general layout of an office suite is shown in Fig. I. High sound insulation is required both between offices and between the executive office and the corridor. The doors are the weakest links in both of these walls and, although it has been shown that satisfactory insulation can be provided, it has now been decided that the layout should be modified to ease the problem. Windows in the walls opposite to the corridor are not really a weak link because there are, in addition, windows in the external building facade forming a double glazed unit with a cavity exceeding 8 inches. facade
External I
•
i
|
Executive office
[orridor Fig. 1. The layout of the prototype offices. Woodwool is one common building material which answers many of the basic requirements for high sound insulation, because it possesses structural stiffness but not sufficient to give rise to a marked coincidence effect when used in a double-leaf partition. When the raw woodwool slabs have been built up onto a timber framework the complete room surface can be acoustically sealed by a good quality plaster. It is advantageous to leave the cavity sides unfaced because this provides a little absorption in the cavity, but even then it is safer to include an absorbent quilt of, for example, mineral wool, to reduce cavity resonances. There can, of course, be no flanking transmission because the two leaves are mounted on the two separate floating floors. The sound reduction index (SRI) at 500 Hz of woodwool, doubleleaf partitions is empirically related to the superficial mass M and cavity width d by 2: SRI = 16 + 20 log(Md) dB. Fig. 2 is a graphical interpretation of this formula.
HIGH
SOUND
INSULATION
23
If 2-inch slabs of woodwool are chosen to form the partitioning and these are faced with approximately 1 inch of plaster, the total surface mass, including timber studding, is something like 20 lb/ft 2. With a 2-inch mineral wool quilt suspended in a 6-inch cavity, Fig. 2 shows that the sound reduction index at 500 Hz should be 55 dB. Initially, single doors of high sound insulation were tried but they could not compare with the very high wall insulation. Such insulation could only be maintained by using two doors in series, one in each box. 80 711 5O om
J
J
4~
°lID ~
~ 3o g 2O 10
5
10 0 Total mass, M, in lldlt 2
50
Fig. 2. The sound reduction index at 500 Hz of double-leaf woodwool partitions. The insulation of the ceiling was not quite as critical because there is a fairly large cavity and a substantial floor above that. Nevertheless, the tops of the boxes were constructed from a single 2-inch slab of woodwool rough-plastered on the underside to seal it. An absorbent ceiling was suspended a few inches below the real ceiling. Once the walls and ceiling had been erected and plastered, the floor, which was formed from pre-cast, reinforced concrete units, was covered with a sand and cement screed and finished with rubber tiles. The floors are not continuous between offices or between an office and the corridor. The gaps in the floor and in the door surround were filled with a fairly soft synthetic foam rubber. Applied Acoustics,
1 (1968) 21-28
24
R . D . FORD, P. LORD, A. W. WALKER
Information can be found in the published works of Jackson, King and Maguire, 3 Payne, 4 Brownsey 5 and Witham 6"~ to enable satisfactory isolation mounts to be specified. The resonant frequency,f, of the system can be determined approximately by f = 3.13,N/~M~ where dc is the compressive deflection of the mount in inches and M is a function of the mass it is supporting. Certain other factors must be taken into consideration in order to obtain a really satisfactory isolator. For instance, the fact that the dynamic stiffness increases with increasing frequency must be allowed for: there must not be too great a static deflection; and the shape of the mount is important if dynamic stability is to be achieved. Fortunately, dynamic stability is not such a serious problem in this case as it would be, for instance, when dealing with the isolation of rotating machinery. The large office has a floor area of 14 ft x 20 ft and, with the construction described above, the total weight is approximately 20,000 lb. Calculation showed that, by using synthetic rubber of cross section 2 in x 2 in running along the longer sides of the office, a resonant frequency of 10 Hz could be obtained for the whole office on the mounts. This means that at 20 Hz and above there is positive gain in insulation. This influences the airborne sound insulation as well as impact sound due to the reduction of flanking paths.
RESULTS
A prototype office suite was built according to the layout shown in Fig. 1. In order to test the partitioning the apertures which had been left for the doors were initially blocked with pieces of woodwool and plastered so as to provide the same insulation in these areas. The sound source was a random noise generator feeding a 15-inch loudspeaker through one-third octave filters. The receiver was a capacitor microphone feeding a microphone amplifier and one-third octave filters. This enabled the sound pressure level difference AL in the two rooms to be measured at a number of positions. The results were converted to the normalized level difference D using: T D = AL + 10 log 0.---5 where T is the reverberation time of the receiving room. The values for D as a function of frequency are shown in Fig. 3 and compared with the grade 1 insulation curve. The average is 55 dB. The woodwool was removed from the door aperture between the offices and replaced by a nominal 30 dB door. The normalized sound level difference was
HIGH
SOUND
INSULATION
25
measured and gave an average value o f 42 dB. This illustrates very clearly the disastrous results which can arise from using a d o o r with inadequate insulation in walls with high insulation. A n extra 30 dB d o o r was introduced between the offices thus forming a doubled o o r system with an air space 6 inches wide. The average normalized level difference in this case was 50 dB and is c o m p a r e d with the grade 1 curve in Fig. 3. 70
~-50
-
I
Id)
•=, 30
° o~
/co) 2O
t..
10
0
I00
200
400
800 Frequency
1600
3ZOO
in IIz.
Fig. 3. The airborne sound insulation between the prototype offices: (a) with the doorways blocked up; (b) with the double door system fitted; (c) with wooden wedges by-passing the rubber mounts; (d) grade 1 (U.K.) curve for airborne sound insulation between dwellings. The impact insulation was tested when the double d o o r system was installed by placing a standard tapping machine on the floor o f the secretary's r o o m and measuring the noise in octave bands produced in the executive office. Again, the measured levels, L, were corrected to give the normalized octave band levels, N, using: N = L -
T 10 log 0--.-.~
where T is the reverberation time o f the receiving room. Applied Acousticx, I 0 9 6 8 ) 21- 28
26
R. D. FORD, P. LORD, A. W. WALKER
The values for N as a function of frequency are shown in Fig. 4 and they come well within the grade 1 impact insulation requirements. Although the insulation has been shown to come up to grade l standards, it has not been proved that floating the rooms has made a significant contribution. Unfortunately, once the rooms had been built it was incredibly difficult to shortcircuit the isolating mounts. Ideally the rooms should have been jacked up so that the rubber could be removed, but in the circumstances this proved impossible. The
70 60
f (b) \
"-~5 0 ~ -a~~ ;
~-~
(el
,g
~'~'~
~o
\
2O 10
125 Fig. 4.
250
500 1000 2000 Frequency in Hz.
T h e impact insulation between the prototype offices: (a) with both offices s u p p o r t e d on
the rubber mounts; (b) with wooden wedges by-passing the rubber mounts; (c) with a separate floating floor in the secretary's room; (d) grade l (U.K.) curve for impact insulation between dwellings. best that could be managed consisted of driving wooden wedges in between the supporting steel frame and the undersides of the concrete floors. When this had been done the impact and airborne sound insulation were measured again and are shown in Figs. 3 and 4. The airborne insulation has been decreased on average by 3 dB and the impact insulation by 10 dB. It seems quite certain that the insulation would decrease even more if the rubber could be by-passed completely.
27
HIGH SOUND INSULATION
During the impact measurements it seemed that much energy was travelling from the floor directly into the walls of the secretary's room and was then being radiated into the cavity and through the wall of the executive office. To check this theory the wooden wedges were left beneath the secretary's office but removed from beneath the executive office. A small area of floating floor was then placed in the secretary's room and the impact insulation retested. The measured levels are shown in Fig. 4 and exhibit a marked improvement over the other tests. Even these levels were not due to true impact transmission but rather to the airborne transmission of the noise level produced in the secretary's room by the impacts. This method of impact insulation is, of course, a one-way action since it is the floating floor which is stopping the energy travelling into the walls, but in this instance it is not important because it is the executive, not the secretary, who required maximum insulation.
CONCLUSIONS
The "floating box" principle appears to have application in office building where high sound insulation requirements are a priority and where floor loading could present serious problems. Cost analysis indicates that the system is economically feasible if compared with the alternative of using conventional materials, i.e., brick and concrete and then strengthening the steel structure. With the double-door system the prototype offices gave an insulation equivalent to grade 1 dwellings. It is, however, clear that double doors are inconvenient, and for the production units the architects preferred to modify the layout to that shown in Fig. 5, in which the doors are not so critical.
Externa[ facade lr' •---qr
Executive office
r~-
Secretary's office Lobby
Corridor Fig. 5. The modified layout for the production units. Applied Acoustics, I (1968) 21-28
28
R. D. FORD, P. LORD, A. W. WALKER
The impact test with the floating floor suggests that when one office in particular needs a high degree of insulation it is n o t necessarily correct to construct all of the offices in floating box form. In this instance it would seem better to have the executive office as a floating box but to float only the floors in the secretary's office and in the corridor. This should result in the same high a i r b o r n e sound insulation as before, because flanking transmission is still eliminated, plus very high insulation for the executive office from impacts in the secretary's office or corridor.
ACKNOWLEDGEMENTS The a u t h o r s wish to acknowledge the generous co-operation of Littlewoods Ltd. and MacLellan R u b b e r Ltd. in the construction of the experimental office suite.
REFERENCES 1. V. L. JORDAN,Sound Insulation in Multiple Flat Dwellhlgs o f Diflbrent Construction, Navrex Conference, London, June 15-16, 1965. 2. De Nederlandse Houtwolcementplaat ; Warmte en Koude lsolatie, p. 7.
3. A. JACKSON,A. J. KINGand C. R. MAGUIRE,Determination of the static and dynamic elastic properties of resilient material, Proc. LE.E., 101, part 2, no. 83, Oct. 1954. 4. A. R. PAVNE,Shape factors and functions in rubber engineering, The Engineer, Feb. 27. 1959. 5. C. M. BROWr~SEV,The assessment of performance of antivibration materials, Proe. I.E.E., 109, part A, supp. no. 3, 1962. 6. W. m. A. WITHAM,The design of frictionally located rubber Icearings for bridges, paper presented at an I.E.D. conference, The Engineering Designer, Dec. 1964. 7. W. A. A. WlTHAM,Dynamic design method for vibration isolation, Eng. Mater. DesLe,, August, 1965.
WITH
HIGH
SOUND
INSULATION
R. D. FORD, P. LORD and A. W. WALKER
The Acoustics Group, Department of Pure and Applied Physics, The University of Salford, Lancashire, Great Britain (Received: 10 August, 1967)
SUMMARY
Two prototype offices were built from lightweight materials using the "floating box" principle. Measurements show that both the impact and the airborne sound insulation between the offices is comparable with Grade 1 (U.K.) insulation between dwellings. This type of construction is particularly suited to frame buildings where floor loading can present serious problems. Cost analysis indicates that it compares very favourably with other systems giving comparable insulation. INTRODUCTION
The work described in this paper arose from a specific requirement but the principles are generally applicable and a "case history" may help to underline those principles. The problem was to provide executive offices on one of the upper floors of a new high-rise office block with both airborne and impact sound insulation comparable to that between dwellings. The traditional way to do this would be to use a 9-inch brick wall between offices, but a wall mass of 90 lb/ft 2 could not be supported by the steel frame of the building. Comparatively lightweight materials therefore had to be used. These materials should not be too expensive, nor take up too much space, nor require any particular skill in handling. In order to prove the design, and to iron out snags off-site, a prototype was built consisting of an executive office and a secretary's office. The impact and airborne sound insulation between the offices was measured under a variety of test conditions. THE DESIGN OF THE OFFICES
The overall approach to the problem is very similar to that found in the building system developed by Conbox Ltd., of Denmark, 1 in which the living units are in AppliedAcoustics--Elsevier Publishing Company Ltd., England--Printed in Great Britain
22
R. D. FORD, P. LORD, A. W. WALKER
the form of concrete boxes spaced from one another and separated from the main structure by vibration-isolating pads. In the system to be discussed, however, much lighter materials are employed for the walls and ceilings: the floor is still concrete and the "boxes" float inside the main building. The general layout of an office suite is shown in Fig. I. High sound insulation is required both between offices and between the executive office and the corridor. The doors are the weakest links in both of these walls and, although it has been shown that satisfactory insulation can be provided, it has now been decided that the layout should be modified to ease the problem. Windows in the walls opposite to the corridor are not really a weak link because there are, in addition, windows in the external building facade forming a double glazed unit with a cavity exceeding 8 inches. facade
External I
•
i
|
Executive office
[orridor Fig. 1. The layout of the prototype offices. Woodwool is one common building material which answers many of the basic requirements for high sound insulation, because it possesses structural stiffness but not sufficient to give rise to a marked coincidence effect when used in a double-leaf partition. When the raw woodwool slabs have been built up onto a timber framework the complete room surface can be acoustically sealed by a good quality plaster. It is advantageous to leave the cavity sides unfaced because this provides a little absorption in the cavity, but even then it is safer to include an absorbent quilt of, for example, mineral wool, to reduce cavity resonances. There can, of course, be no flanking transmission because the two leaves are mounted on the two separate floating floors. The sound reduction index (SRI) at 500 Hz of woodwool, doubleleaf partitions is empirically related to the superficial mass M and cavity width d by 2: SRI = 16 + 20 log(Md) dB. Fig. 2 is a graphical interpretation of this formula.
HIGH
SOUND
INSULATION
23
If 2-inch slabs of woodwool are chosen to form the partitioning and these are faced with approximately 1 inch of plaster, the total surface mass, including timber studding, is something like 20 lb/ft 2. With a 2-inch mineral wool quilt suspended in a 6-inch cavity, Fig. 2 shows that the sound reduction index at 500 Hz should be 55 dB. Initially, single doors of high sound insulation were tried but they could not compare with the very high wall insulation. Such insulation could only be maintained by using two doors in series, one in each box. 80 711 5O om
J
J
4~
°lID ~
~ 3o g 2O 10
5
10 0 Total mass, M, in lldlt 2
50
Fig. 2. The sound reduction index at 500 Hz of double-leaf woodwool partitions. The insulation of the ceiling was not quite as critical because there is a fairly large cavity and a substantial floor above that. Nevertheless, the tops of the boxes were constructed from a single 2-inch slab of woodwool rough-plastered on the underside to seal it. An absorbent ceiling was suspended a few inches below the real ceiling. Once the walls and ceiling had been erected and plastered, the floor, which was formed from pre-cast, reinforced concrete units, was covered with a sand and cement screed and finished with rubber tiles. The floors are not continuous between offices or between an office and the corridor. The gaps in the floor and in the door surround were filled with a fairly soft synthetic foam rubber. Applied Acoustics,
1 (1968) 21-28
24
R . D . FORD, P. LORD, A. W. WALKER
Information can be found in the published works of Jackson, King and Maguire, 3 Payne, 4 Brownsey 5 and Witham 6"~ to enable satisfactory isolation mounts to be specified. The resonant frequency,f, of the system can be determined approximately by f = 3.13,N/~M~ where dc is the compressive deflection of the mount in inches and M is a function of the mass it is supporting. Certain other factors must be taken into consideration in order to obtain a really satisfactory isolator. For instance, the fact that the dynamic stiffness increases with increasing frequency must be allowed for: there must not be too great a static deflection; and the shape of the mount is important if dynamic stability is to be achieved. Fortunately, dynamic stability is not such a serious problem in this case as it would be, for instance, when dealing with the isolation of rotating machinery. The large office has a floor area of 14 ft x 20 ft and, with the construction described above, the total weight is approximately 20,000 lb. Calculation showed that, by using synthetic rubber of cross section 2 in x 2 in running along the longer sides of the office, a resonant frequency of 10 Hz could be obtained for the whole office on the mounts. This means that at 20 Hz and above there is positive gain in insulation. This influences the airborne sound insulation as well as impact sound due to the reduction of flanking paths.
RESULTS
A prototype office suite was built according to the layout shown in Fig. 1. In order to test the partitioning the apertures which had been left for the doors were initially blocked with pieces of woodwool and plastered so as to provide the same insulation in these areas. The sound source was a random noise generator feeding a 15-inch loudspeaker through one-third octave filters. The receiver was a capacitor microphone feeding a microphone amplifier and one-third octave filters. This enabled the sound pressure level difference AL in the two rooms to be measured at a number of positions. The results were converted to the normalized level difference D using: T D = AL + 10 log 0.---5 where T is the reverberation time of the receiving room. The values for D as a function of frequency are shown in Fig. 3 and compared with the grade 1 insulation curve. The average is 55 dB. The woodwool was removed from the door aperture between the offices and replaced by a nominal 30 dB door. The normalized sound level difference was
HIGH
SOUND
INSULATION
25
measured and gave an average value o f 42 dB. This illustrates very clearly the disastrous results which can arise from using a d o o r with inadequate insulation in walls with high insulation. A n extra 30 dB d o o r was introduced between the offices thus forming a doubled o o r system with an air space 6 inches wide. The average normalized level difference in this case was 50 dB and is c o m p a r e d with the grade 1 curve in Fig. 3. 70
~-50
-
I
Id)
•=, 30
° o~
/co) 2O
t..
10
0
I00
200
400
800 Frequency
1600
3ZOO
in IIz.
Fig. 3. The airborne sound insulation between the prototype offices: (a) with the doorways blocked up; (b) with the double door system fitted; (c) with wooden wedges by-passing the rubber mounts; (d) grade 1 (U.K.) curve for airborne sound insulation between dwellings. The impact insulation was tested when the double d o o r system was installed by placing a standard tapping machine on the floor o f the secretary's r o o m and measuring the noise in octave bands produced in the executive office. Again, the measured levels, L, were corrected to give the normalized octave band levels, N, using: N = L -
T 10 log 0--.-.~
where T is the reverberation time o f the receiving room. Applied Acousticx, I 0 9 6 8 ) 21- 28
26
R. D. FORD, P. LORD, A. W. WALKER
The values for N as a function of frequency are shown in Fig. 4 and they come well within the grade 1 impact insulation requirements. Although the insulation has been shown to come up to grade l standards, it has not been proved that floating the rooms has made a significant contribution. Unfortunately, once the rooms had been built it was incredibly difficult to shortcircuit the isolating mounts. Ideally the rooms should have been jacked up so that the rubber could be removed, but in the circumstances this proved impossible. The
70 60
f (b) \
"-~5 0 ~ -a~~ ;
~-~
(el
,g
~'~'~
~o
\
2O 10
125 Fig. 4.
250
500 1000 2000 Frequency in Hz.
T h e impact insulation between the prototype offices: (a) with both offices s u p p o r t e d on
the rubber mounts; (b) with wooden wedges by-passing the rubber mounts; (c) with a separate floating floor in the secretary's room; (d) grade l (U.K.) curve for impact insulation between dwellings. best that could be managed consisted of driving wooden wedges in between the supporting steel frame and the undersides of the concrete floors. When this had been done the impact and airborne sound insulation were measured again and are shown in Figs. 3 and 4. The airborne insulation has been decreased on average by 3 dB and the impact insulation by 10 dB. It seems quite certain that the insulation would decrease even more if the rubber could be by-passed completely.
27
HIGH SOUND INSULATION
During the impact measurements it seemed that much energy was travelling from the floor directly into the walls of the secretary's room and was then being radiated into the cavity and through the wall of the executive office. To check this theory the wooden wedges were left beneath the secretary's office but removed from beneath the executive office. A small area of floating floor was then placed in the secretary's room and the impact insulation retested. The measured levels are shown in Fig. 4 and exhibit a marked improvement over the other tests. Even these levels were not due to true impact transmission but rather to the airborne transmission of the noise level produced in the secretary's room by the impacts. This method of impact insulation is, of course, a one-way action since it is the floating floor which is stopping the energy travelling into the walls, but in this instance it is not important because it is the executive, not the secretary, who required maximum insulation.
CONCLUSIONS
The "floating box" principle appears to have application in office building where high sound insulation requirements are a priority and where floor loading could present serious problems. Cost analysis indicates that the system is economically feasible if compared with the alternative of using conventional materials, i.e., brick and concrete and then strengthening the steel structure. With the double-door system the prototype offices gave an insulation equivalent to grade 1 dwellings. It is, however, clear that double doors are inconvenient, and for the production units the architects preferred to modify the layout to that shown in Fig. 5, in which the doors are not so critical.
Externa[ facade lr' •---qr
Executive office
r~-
Secretary's office Lobby
Corridor Fig. 5. The modified layout for the production units. Applied Acoustics, I (1968) 21-28
28
R. D. FORD, P. LORD, A. W. WALKER
The impact test with the floating floor suggests that when one office in particular needs a high degree of insulation it is n o t necessarily correct to construct all of the offices in floating box form. In this instance it would seem better to have the executive office as a floating box but to float only the floors in the secretary's office and in the corridor. This should result in the same high a i r b o r n e sound insulation as before, because flanking transmission is still eliminated, plus very high insulation for the executive office from impacts in the secretary's office or corridor.
ACKNOWLEDGEMENTS The a u t h o r s wish to acknowledge the generous co-operation of Littlewoods Ltd. and MacLellan R u b b e r Ltd. in the construction of the experimental office suite.
REFERENCES 1. V. L. JORDAN,Sound Insulation in Multiple Flat Dwellhlgs o f Diflbrent Construction, Navrex Conference, London, June 15-16, 1965. 2. De Nederlandse Houtwolcementplaat ; Warmte en Koude lsolatie, p. 7.
3. A. JACKSON,A. J. KINGand C. R. MAGUIRE,Determination of the static and dynamic elastic properties of resilient material, Proc. LE.E., 101, part 2, no. 83, Oct. 1954. 4. A. R. PAVNE,Shape factors and functions in rubber engineering, The Engineer, Feb. 27. 1959. 5. C. M. BROWr~SEV,The assessment of performance of antivibration materials, Proe. I.E.E., 109, part A, supp. no. 3, 1962. 6. W. m. A. WITHAM,The design of frictionally located rubber Icearings for bridges, paper presented at an I.E.D. conference, The Engineering Designer, Dec. 1964. 7. W. A. A. WlTHAM,Dynamic design method for vibration isolation, Eng. Mater. DesLe,, August, 1965.