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Characterization of building equipment
Applied Acoustics 61 (2000) 273±283 www.elsevier.com/locate/apacoust
Characterization of building equipment Michel Villot * Acoustics Group, CSTB, 24 Rue Joseph Fourier, 38400 Saint-Martin-d'HeÁres, France Received 1 February 1999; received in revised form 4 October 1999; accepted 11 October 1999
Abstract The feasibility of a characterization method consisting of measurement of the structureborne noise generated by building equipment mounted in a laboratory test room and in calibrating the test room by using a reference structure-borne sound source, is presented. The experimental results obtained with dierent sources (motorized awnings and waste water pipes) mounted on dierent walls validates the method. In conclusion, some ideas on structure-borne noise prediction in buildings are proposed. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction There is a need to characterize building equipment so that products can be compared and the structure-borne noise they generate, when connected to building structures, can be estimated. Since sound pressure levels are commonly measured in building acoustics test facilities, a simple characterization method would consist of mounting the equipment onto a test wall and directly measuring the noise radiated by the supporting wall into a reverberant room (Fig. 1). However, the noise measured would also depend on the test wall and its structural environment. A reference source should then be used to calibrate the test facility, assuming the reference source and the equipment are both mechanical sources of the same type relative to the supporting wall (see explanation further on); the dierence between the noise levels generated by the equipment and by the reference source is then the characteristic wanted. In this paper, the feasibility of such a simple method is studied. Two types of building equipment are considered: waste water pipes and motorized awnings. They are light enough, compared to building structures, so that the force source assumption is allowed. This assumption is validated and its domain of applicability de®ned in the * Corresponding author. Tel.: +33-76-76-25-25; fax: +33-76-44-20-46. 0003-682X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0003-682X(00)00034-7
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Fig. 1. Principle of the method (1) Ð supporting wall; (2) Ð building equipment; (3) Ð reverberant room.
following section. In Section 3, the method is validated by characterizing the same sources mounted on two dierent walls; the results are then discussed. Finally, some ideas on structure-borne noise prediction in buildings are proposed (Section 4). 2. Domain of applicability Two types of mechanical sources are studied (see schematic drawings given in Figs. 2 and 3): . Two 10 cm diameter waste water pipes (one of cast iron and the other of PVC) with two ®xation points about 1.5 m apart on the supporting wall; the stationary water ¯ow rate was set at l, 2 or 4 1/s. . Two motorized awnings (called Nos. 1 and 2 in this paper) with also two ®xation points about 1.5 m apart; awning No. 2 is heavier than awning No. 1.
Fig. 2. Schematic drawing of waste water pipe installation.
M. Villot / Applied Acoustics 61 (2000) 273±283
275
Fig. 3. Schematic drawing of motorized awnings.
For the two types of source, the two ®xation points were supposed to be uncorrelated. Moreover the sources tested were likely to only generate forces normal to the supporting wall since only the low and mid frequency range was concerned as shown in Fig. 6 (moments are known to contribute increasingly with increase in frequency) and since the ®xation points were located more than one meter away from the supporting wall edges (moments can be important even at low frequencies when the source is close to a plate edge). It is obvious that these limits in frequency and distance have to be more precisely de®ned, either by calculation or experiment, to make the method operational. The sources were mounted on two types of walls: a 10 cm thick plaster block wall (90 kg/m2) and a 15 cm thick hollow concrete block wall (220 kg/m2). The velocity at each ®xation point can be written (see [1] for example) v v0 ÿ Ys :F
1
where v0 is the free velocity of the source, Ys its mobility and F the coupling force. The velocity v is also the velocity of the receiving structure YR :F and the coupling force can then be written: F v0 = YR Ys
2
If the source is much more mobile than the receiving structure jYR j << jYS j; (2) simpli®es as: F v0 =Ys
3
showing that the applied force is a characteristic of the source (called force source). The source type can be easily identi®ed by measuring the free velocity v0 (source disconnected from its supporting wall) and the velocity v at the coupling point (source connected); if jvj2 << jv0 j2 the source is a force source (v is often called blocked velocity).
4
276
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Fig. 4. Narrow band spectra of free and blocked velocities; motorized awning No. 1 on a plaster block wall.
Inequality (4) was systematically veri®ed for the 4 sources tested; the force source assumption was allowed if a minimum dierence of 20 dB between the two velocity levels was obtained. Fig. 4 shows the measured narrow band spectra of both free and blocked velocities in case of motorized awning No. 1 mounted on the 10 cm plaster block wall; a dierence of more than 20 dB can be seen over the whole frequency range. Fig. 5 shows the same spectra in case of the cast iron pipe mounted on the same plaster block wall; at low frequencies (below 250 Hz), the velocity level dierence Lv0 ÿ Lv becomes smaller than 10 dB, showing that the pipe is no longer a force source compared to the plaster block wall. In this case, the force applied also depends on the properties of the receiving structure and the simple reference force source method fails to calibrate the test wall; a much more complicated method,
M. Villot / Applied Acoustics 61 (2000) 273±283
277
Fig. 5. Narrow band spectra of free and blocked velocities; cast iron pipe (1 l/s) on a plaster block wall.
involving the measurement of the wall input mobility and the equipment internal mobility and free velocity would then be needed. 3. Feasibility tests 3.1. Experimental validation The 3 sources verifying the force source assumption (the PVC pipe and the two motorized awnings) were tested, mounted on two walls (plaster blocks and hollow concrete blocks) radiating in a reverberant room; the corresponding (spatially averaged) sound pressure levels Lpe were measured in the room.
278
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Fig. 6. Dierence Lpe ÿ Lprf (a) PVC waste water pipe (2 l/s); (b) motorized awning No. 1; (c) motorized awning No. 2. (1) 15 cm hollow concrete blocks; (2) 10 cm plaster blocks.
In a second step, the two walls were mechanically driven ÿby an electrodynamic shaker used as reference source and the corresponding levels L0prf were again measured. To make sure that the force applied by the shaker was the same on every wall, the force was also measured (using a force sensor) and the pressure level normalized to the force. Lprf L0prf ÿ Lf
5
The characteristic dierence Lpe ÿ Lprf was then calculated for the 3 sources mounted on the two walls (Fig. 6); the spectra obtained are practically independent of the
M. Villot / Applied Acoustics 61 (2000) 273±283
279
supporting wall. In order to evaluate how acceptable these results are, the Aweighted single number value of the pressure levels obtained with the two walls were compared (see Table 1); the 1.5 dB(A) maximum dierence obtained is acceptable and validates the method. 3.2. Discussion The results presented simply show the feasibility of the method; however, some details have to be more precisely de®ned to make the whole procedure operational. First of all, the noise measured in the room is not only the structural noise of the source but also the noise acoustically radiated by the source (on the emission side) and transmitted through the wall. This extraneous noise has to be measured (for example by disconnecting the source from its supporting wall and by measuring the noise on the receiving side) to check its importance and it eventually has to be subtracted from the total noise measured (source connected). Fig. 7 shows the noise measured in the case of a PVC pipe with a ¯ow rate of 2 1/s, connected to the 10 cm plaster blocks and then disconnected; above 1/3 octave 630 Hz, the total noise is about 3 dB higher than the transmitted noise, showing that the structural noise is of the same level as transmitted noise (note also the relatively high structure-borne noise levels obtained around 400 Hz and corresponding to the critical frequency of the wall). Secondly, the levels Lpe and the reference levels Lprf . presented were obtained from averaging source positions on the wall: three positions for the awnings, only one position for the waste water pipes (the whole installation could not be moved) and ®ve different positions of the reference source. In order to quantify the corresponding standard deviation, the pressure levels produced by 20 reference source positions randomly distributed over a 10 cm thick concrete block wall (250 kg/m2) of size 5 2.5 m were measured. Fig. 8 shows the standard deviation obtained; below 200 Hz, unacceptable values up to 4 dB are obtained, due to the modal behavior of the wall. An easy way to suppress this wall input mobility problem consists of applying the reference source at the same locations on the wall as the equipment ®xation points; the characteristic dierence Lpe ÿ Lprf should then be independent of the location of the excitation points. Thirdly, since Lprf is normalized to the applied force, the characteristic dierence Lpe ÿ Lprf represents in fact the force level of the source tested, measured indirectly; this indirect method is easier than the direct measurement which would need the insertion of a force sensor at the ®xation points of the real source. However, the original idea of the method was to use a reference source in order to avoid any force measurement at all. The ideal mechanical reference source should deliver the same Table 1 Figures
6a
6b
6c
Wall (1) Wall (2)
71.0 70.5
81.5 80.0
84.5 83.5
280
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Fig. 7. Normalized pressure level Lpe (expressed as power level); PVC pipe (2 l/s) on a plaster block wall, connected and disconnected.
force whatever the supporting wall and the excitation positions are. An interesting and easy to use source, called a mini tapping machine [2] is often used in building acoustics (particularly in Germany). This source is an electrodynamic tapping machine with one 22 g hammer tapping at a 10 Hz frequency; the source is small and can be manually applied to a wall. This mini tapping machine was tested at CSTB by measuring the applied force level with the source located at several randomly distributed positions on 4 types of walls: an 18 cm concrete wall (460 kg/m2), a 20 cm hollow concrete block wall
M. Villot / Applied Acoustics 61 (2000) 273±283
281
Fig. 8. Standard deviation of pressure levels Lprf with 20 positions of shaker on a concrete block wall.
(275 kg/m2), a 10 cm concrete block wall (250 kg/m2) and a 5 cm plaster block wall (50 kg/m2). The mean value and the standard deviation of a total of 20 measurement results are shown in Fig. 9; the standard deviation is quite acceptable (1.5 dB) below 200 Hz, but rather large above (between 2.5 and 3.5 dB). A smaller standard deviation would probably be obtained if the results were restricted to one type of wall; unfortunately, statistical results cannot be given in this case, the number of measurements performed per wall being too small. 4. Structure-borne noise prediction in buildings Suppose equipment has been characterized by the method proposed in this paper, using a given reference source (the mini tapping machine for example) and is known
282
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Fig. 9. Force applied by mini tapping machine on 4 types of walls (see text); mean value and standard deviation.
by its characteristic dierence Lpe ÿ Lprf measured in the laboratory. If the structure-borne noise generated by the reference source in a given con®guration in a building is known (level Lprf;situ ) then the structure-borne noise generated by the equipment in the same con®guration can be calculated as Lpe;situ Lpe ÿ Lprf Lprf;situ
6
Now, a method of predicting structure-borne noise in buildings generated by the ISO tapping machine already exists in the European standard EN 12354 [3]. In this
M. Villot / Applied Acoustics 61 (2000) 273±283
283
standard, the (in situ) impact noise level calculated includes direct and ¯anking transmissions and is estimated from the (laboratory) normalized direct impact sound level Ln of the ¯oor (generated by the ISO tapping machine on the bare ¯oor), the reduction of impact sound level L of the ¯oor covering, the sound reduction index R of the ¯oor and the dierent ¯anking walls and the vibration reduction index Kij of the dierent junctions. A similar calculation could be made for any structure-borne reference source, assuming a data base of direct sound levels generated by the reference source mounted on dierent walls is available (this time, data obtained by averaging several reference source positions are needed, since the exact location of the equipment is not known). The in situ structure-borne noise Lpe;situ generated by the equipment could then be estimated using Eq. (6). Acknowledgements This work was partly supported by the French Plastic Pipe Association (STRPVC), Pont a Mousson (cast iron pipes), SOMFY (motorized awnings) and the Conseil Regional RhoÃne-Alpes; part of the measurements were performed by students from INSA Lyon, France. References [1] Cremer L, Heckl M. Structure-borne sound. Berlin, New York, Heidelberg: Springer-Verlag 1973. [2] Klein hammerwerk, system GoÈsele (mini tapping machine distributed by Norsonic) 1991. [3] European standard EN 12354. Estimation of acoustic performance of buildings from the performance of products. Part 2: impact sound insulation between rooms 1996.
Characterization of building equipment Michel Villot * Acoustics Group, CSTB, 24 Rue Joseph Fourier, 38400 Saint-Martin-d'HeÁres, France Received 1 February 1999; received in revised form 4 October 1999; accepted 11 October 1999
Abstract The feasibility of a characterization method consisting of measurement of the structureborne noise generated by building equipment mounted in a laboratory test room and in calibrating the test room by using a reference structure-borne sound source, is presented. The experimental results obtained with dierent sources (motorized awnings and waste water pipes) mounted on dierent walls validates the method. In conclusion, some ideas on structure-borne noise prediction in buildings are proposed. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction There is a need to characterize building equipment so that products can be compared and the structure-borne noise they generate, when connected to building structures, can be estimated. Since sound pressure levels are commonly measured in building acoustics test facilities, a simple characterization method would consist of mounting the equipment onto a test wall and directly measuring the noise radiated by the supporting wall into a reverberant room (Fig. 1). However, the noise measured would also depend on the test wall and its structural environment. A reference source should then be used to calibrate the test facility, assuming the reference source and the equipment are both mechanical sources of the same type relative to the supporting wall (see explanation further on); the dierence between the noise levels generated by the equipment and by the reference source is then the characteristic wanted. In this paper, the feasibility of such a simple method is studied. Two types of building equipment are considered: waste water pipes and motorized awnings. They are light enough, compared to building structures, so that the force source assumption is allowed. This assumption is validated and its domain of applicability de®ned in the * Corresponding author. Tel.: +33-76-76-25-25; fax: +33-76-44-20-46. 0003-682X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0003-682X(00)00034-7
274
M. Villot / Applied Acoustics 61 (2000) 273±283
Fig. 1. Principle of the method (1) Ð supporting wall; (2) Ð building equipment; (3) Ð reverberant room.
following section. In Section 3, the method is validated by characterizing the same sources mounted on two dierent walls; the results are then discussed. Finally, some ideas on structure-borne noise prediction in buildings are proposed (Section 4). 2. Domain of applicability Two types of mechanical sources are studied (see schematic drawings given in Figs. 2 and 3): . Two 10 cm diameter waste water pipes (one of cast iron and the other of PVC) with two ®xation points about 1.5 m apart on the supporting wall; the stationary water ¯ow rate was set at l, 2 or 4 1/s. . Two motorized awnings (called Nos. 1 and 2 in this paper) with also two ®xation points about 1.5 m apart; awning No. 2 is heavier than awning No. 1.
Fig. 2. Schematic drawing of waste water pipe installation.
M. Villot / Applied Acoustics 61 (2000) 273±283
275
Fig. 3. Schematic drawing of motorized awnings.
For the two types of source, the two ®xation points were supposed to be uncorrelated. Moreover the sources tested were likely to only generate forces normal to the supporting wall since only the low and mid frequency range was concerned as shown in Fig. 6 (moments are known to contribute increasingly with increase in frequency) and since the ®xation points were located more than one meter away from the supporting wall edges (moments can be important even at low frequencies when the source is close to a plate edge). It is obvious that these limits in frequency and distance have to be more precisely de®ned, either by calculation or experiment, to make the method operational. The sources were mounted on two types of walls: a 10 cm thick plaster block wall (90 kg/m2) and a 15 cm thick hollow concrete block wall (220 kg/m2). The velocity at each ®xation point can be written (see [1] for example) v v0 ÿ Ys :F
1
where v0 is the free velocity of the source, Ys its mobility and F the coupling force. The velocity v is also the velocity of the receiving structure YR :F and the coupling force can then be written: F v0 = YR Ys
2
If the source is much more mobile than the receiving structure jYR j << jYS j; (2) simpli®es as: F v0 =Ys
3
showing that the applied force is a characteristic of the source (called force source). The source type can be easily identi®ed by measuring the free velocity v0 (source disconnected from its supporting wall) and the velocity v at the coupling point (source connected); if jvj2 << jv0 j2 the source is a force source (v is often called blocked velocity).
4
276
M. Villot / Applied Acoustics 61 (2000) 273±283
Fig. 4. Narrow band spectra of free and blocked velocities; motorized awning No. 1 on a plaster block wall.
Inequality (4) was systematically veri®ed for the 4 sources tested; the force source assumption was allowed if a minimum dierence of 20 dB between the two velocity levels was obtained. Fig. 4 shows the measured narrow band spectra of both free and blocked velocities in case of motorized awning No. 1 mounted on the 10 cm plaster block wall; a dierence of more than 20 dB can be seen over the whole frequency range. Fig. 5 shows the same spectra in case of the cast iron pipe mounted on the same plaster block wall; at low frequencies (below 250 Hz), the velocity level dierence Lv0 ÿ Lv becomes smaller than 10 dB, showing that the pipe is no longer a force source compared to the plaster block wall. In this case, the force applied also depends on the properties of the receiving structure and the simple reference force source method fails to calibrate the test wall; a much more complicated method,
M. Villot / Applied Acoustics 61 (2000) 273±283
277
Fig. 5. Narrow band spectra of free and blocked velocities; cast iron pipe (1 l/s) on a plaster block wall.
involving the measurement of the wall input mobility and the equipment internal mobility and free velocity would then be needed. 3. Feasibility tests 3.1. Experimental validation The 3 sources verifying the force source assumption (the PVC pipe and the two motorized awnings) were tested, mounted on two walls (plaster blocks and hollow concrete blocks) radiating in a reverberant room; the corresponding (spatially averaged) sound pressure levels Lpe were measured in the room.
278
M. Villot / Applied Acoustics 61 (2000) 273±283
Fig. 6. Dierence Lpe ÿ Lprf (a) PVC waste water pipe (2 l/s); (b) motorized awning No. 1; (c) motorized awning No. 2. (1) 15 cm hollow concrete blocks; (2) 10 cm plaster blocks.
In a second step, the two walls were mechanically driven ÿby an electrodynamic shaker used as reference source and the corresponding levels L0prf were again measured. To make sure that the force applied by the shaker was the same on every wall, the force was also measured (using a force sensor) and the pressure level normalized to the force. Lprf L0prf ÿ Lf
5
The characteristic dierence Lpe ÿ Lprf was then calculated for the 3 sources mounted on the two walls (Fig. 6); the spectra obtained are practically independent of the
M. Villot / Applied Acoustics 61 (2000) 273±283
279
supporting wall. In order to evaluate how acceptable these results are, the Aweighted single number value of the pressure levels obtained with the two walls were compared (see Table 1); the 1.5 dB(A) maximum dierence obtained is acceptable and validates the method. 3.2. Discussion The results presented simply show the feasibility of the method; however, some details have to be more precisely de®ned to make the whole procedure operational. First of all, the noise measured in the room is not only the structural noise of the source but also the noise acoustically radiated by the source (on the emission side) and transmitted through the wall. This extraneous noise has to be measured (for example by disconnecting the source from its supporting wall and by measuring the noise on the receiving side) to check its importance and it eventually has to be subtracted from the total noise measured (source connected). Fig. 7 shows the noise measured in the case of a PVC pipe with a ¯ow rate of 2 1/s, connected to the 10 cm plaster blocks and then disconnected; above 1/3 octave 630 Hz, the total noise is about 3 dB higher than the transmitted noise, showing that the structural noise is of the same level as transmitted noise (note also the relatively high structure-borne noise levels obtained around 400 Hz and corresponding to the critical frequency of the wall). Secondly, the levels Lpe and the reference levels Lprf . presented were obtained from averaging source positions on the wall: three positions for the awnings, only one position for the waste water pipes (the whole installation could not be moved) and ®ve different positions of the reference source. In order to quantify the corresponding standard deviation, the pressure levels produced by 20 reference source positions randomly distributed over a 10 cm thick concrete block wall (250 kg/m2) of size 5 2.5 m were measured. Fig. 8 shows the standard deviation obtained; below 200 Hz, unacceptable values up to 4 dB are obtained, due to the modal behavior of the wall. An easy way to suppress this wall input mobility problem consists of applying the reference source at the same locations on the wall as the equipment ®xation points; the characteristic dierence Lpe ÿ Lprf should then be independent of the location of the excitation points. Thirdly, since Lprf is normalized to the applied force, the characteristic dierence Lpe ÿ Lprf represents in fact the force level of the source tested, measured indirectly; this indirect method is easier than the direct measurement which would need the insertion of a force sensor at the ®xation points of the real source. However, the original idea of the method was to use a reference source in order to avoid any force measurement at all. The ideal mechanical reference source should deliver the same Table 1 Figures
6a
6b
6c
Wall (1) Wall (2)
71.0 70.5
81.5 80.0
84.5 83.5
280
M. Villot / Applied Acoustics 61 (2000) 273±283
Fig. 7. Normalized pressure level Lpe (expressed as power level); PVC pipe (2 l/s) on a plaster block wall, connected and disconnected.
force whatever the supporting wall and the excitation positions are. An interesting and easy to use source, called a mini tapping machine [2] is often used in building acoustics (particularly in Germany). This source is an electrodynamic tapping machine with one 22 g hammer tapping at a 10 Hz frequency; the source is small and can be manually applied to a wall. This mini tapping machine was tested at CSTB by measuring the applied force level with the source located at several randomly distributed positions on 4 types of walls: an 18 cm concrete wall (460 kg/m2), a 20 cm hollow concrete block wall
M. Villot / Applied Acoustics 61 (2000) 273±283
281
Fig. 8. Standard deviation of pressure levels Lprf with 20 positions of shaker on a concrete block wall.
(275 kg/m2), a 10 cm concrete block wall (250 kg/m2) and a 5 cm plaster block wall (50 kg/m2). The mean value and the standard deviation of a total of 20 measurement results are shown in Fig. 9; the standard deviation is quite acceptable (1.5 dB) below 200 Hz, but rather large above (between 2.5 and 3.5 dB). A smaller standard deviation would probably be obtained if the results were restricted to one type of wall; unfortunately, statistical results cannot be given in this case, the number of measurements performed per wall being too small. 4. Structure-borne noise prediction in buildings Suppose equipment has been characterized by the method proposed in this paper, using a given reference source (the mini tapping machine for example) and is known
282
M. Villot / Applied Acoustics 61 (2000) 273±283
Fig. 9. Force applied by mini tapping machine on 4 types of walls (see text); mean value and standard deviation.
by its characteristic dierence Lpe ÿ Lprf measured in the laboratory. If the structure-borne noise generated by the reference source in a given con®guration in a building is known (level Lprf;situ ) then the structure-borne noise generated by the equipment in the same con®guration can be calculated as Lpe;situ Lpe ÿ Lprf Lprf;situ
6
Now, a method of predicting structure-borne noise in buildings generated by the ISO tapping machine already exists in the European standard EN 12354 [3]. In this
M. Villot / Applied Acoustics 61 (2000) 273±283
283
standard, the (in situ) impact noise level calculated includes direct and ¯anking transmissions and is estimated from the (laboratory) normalized direct impact sound level Ln of the ¯oor (generated by the ISO tapping machine on the bare ¯oor), the reduction of impact sound level L of the ¯oor covering, the sound reduction index R of the ¯oor and the dierent ¯anking walls and the vibration reduction index Kij of the dierent junctions. A similar calculation could be made for any structure-borne reference source, assuming a data base of direct sound levels generated by the reference source mounted on dierent walls is available (this time, data obtained by averaging several reference source positions are needed, since the exact location of the equipment is not known). The in situ structure-borne noise Lpe;situ generated by the equipment could then be estimated using Eq. (6). Acknowledgements This work was partly supported by the French Plastic Pipe Association (STRPVC), Pont a Mousson (cast iron pipes), SOMFY (motorized awnings) and the Conseil Regional RhoÃne-Alpes; part of the measurements were performed by students from INSA Lyon, France. References [1] Cremer L, Heckl M. Structure-borne sound. Berlin, New York, Heidelberg: Springer-Verlag 1973. [2] Klein hammerwerk, system GoÈsele (mini tapping machine distributed by Norsonic) 1991. [3] European standard EN 12354. Estimation of acoustic performance of buildings from the performance of products. Part 2: impact sound insulation between rooms 1996.