The measurement of structure-borne sound transmission using impulsive sources

AppliedAcoustics15(1982)355-361

THE

MEASUREMENT TRANSMISSION

OF STRUCTURE-BORNE SOUND USING IMPULSIVE SOURCES

R. J. M. CRAft<

Department of Building, Heriot-Watt University, Edinburgh, Scotland (Great Britain) (Received: 7 December, 1981)

SUMMARY

A simple method to measure structure-borne sound transmission is described. Measurement is made of the level difference in the acceleration between two structural elements using a plastic headed hammer as a noise source. The method is at least as accurate as conventional measurements made under steady-state conditions using continuous noise sources and can be carried out with less instrumentation on site and in about a tenth o f the time. The portability of the source greatly simplifies the measurements as a hammer can be used to hit structures in a wide variety o f positions whereas shakers can only be used in limited situations. In addition, attaching a shaker to a wall can damage the wall surface whereas, with care, a hammer hit will not.

INTRODUCTION

One of the difficulties with carrying out acoustic field measurements on building structures is that it is very difficult to directly excite a building structure. Conventional sources which are used to excite structures, such as shakers, have to be large and massive if any significant amount of energy is to be input into the structure as a small shaker will be unable to move a wall that may weigh several tons. This makes the source difficult to move around and difficult to attach to a wall without causing damage to the wall finish. The result is that structural sources tend to be used only for specialised applications. One solution to the problem is to use impulsive sources to excite the structure, This is essentially what is done when a standard tapping machine is used to excite a floor but the limitation of the tapping machine is that it can only be used to tap in a downward direction so that it cannot be used on walls or on ceilings. Floors are 355 Applied Acoustics 0003-682X/82/0015-0355/$02.75 ~ Applied Science Publishers Ltd, England, 1982 Printed in Great Britain

356

R.J.M. CRAm

frequently covered with carpets and the presence of even a thin carpet can seriously affect the power input into the floor at the higher frequencies. In addition, the moving parts of a tapping machine produce a considerable amount of acoustic noise and this can interfere with the measurements. One impulse source that has none of these problems is a simple plastic headed hammer. A hammer is light, portable and cheap and can be used to excite almost all structures at almost any position. In addition, the plastic head causes no damage to most decorative finishes. The only disadvantage of a hammer as a source is that the strength of a hammer hit will vary with each hit so that the power input will be different with every hit. However, if, as is usually the case, it is the level difference between the source wall and another wall that is required, measurements can be made on ttae source wall and the receiving wall simultaneously to give the level difference. De Tricauld 1 has shown that the transmission loss of walls can be obtained using impulsive sources. In this paper similar techniques are used to give the attenuation from one wall or floor to another or from a wall to a room.

METHODOF MEASUREMENT It has been shown by de Tricauld 1 that for two subsystems where the sound fields are diffuse the level difference, D, between any point in each subsystem measured under steady-state conditions with a continuous noise source is the same as the level difference between the integral of the two signals measured using impulsive sources, provided the integration is carried out for the entire duration of the noise source. This can be written as:

D = 101°g~

a~dta~dt

(1)

where a is the transducer signal and could represent either the acceleration of the wall or the sound pressure in a room. For exactly the same level difference it is necessary that the directivity of the sources be the same. This condition is satisfied for shakers and impulsive hammers which are both point sources. If the level, L, for an impulsive source is defined as: L = 10log

f0Ta2 ~o~dt

(2)

where t = 0 is the start of the impulsive source and t = Tis a time when the level has

MEASUREMENT OF STRUCTURE-BORNE SOUND TRANSMISSION

357

been reduced to a very low level, then further calculations can be carried out as if a steady-state source had been used. This is the same integration that is carried out under linear averaging except that, in addition, the integral is divided by the time of integration. If the integration time is kept the same for both integrals then the level difference will be correct. The level found from eqn. (2) is also the value, Lax, used in environmental noise measurements so that the difference in the level, Leq, for a continuous noise source is the same as the difference in the level, Lax, for any sources. As the level difference for any hammer hit is the same as the level difference for any other hammer hit at any given set of measuring and source positions, only one hammer hit is required at any given source position. If the exact nature of the noise source that will be present in the building is unknown, or, if statistical energy analysis is being used for analysis, then the measurements are required for an 'average' source. It is therefore desirable to carry out measurements with as many source positions as possible. This can be achieved by carrying out the measurement integration over a long time period whilst hitting the wall a large number of times, with each hit being in a different position. For the measurements that were carried out, three different measurement positions were used for each subsystem. For a period of 30s for each pair of measuring positions the source wall was continuously hit, with each hit being at a different position on the wall. In this way, about a hundred different source positions were measured. cceterometer

I

v | Hammer

~

...

vre-amplifier Tape recorder Digital frequency anatyser

RECORDINfi ON S I T E

LABORATORYANALYSIS

Fig. 1. Instrumentation used for impulse measurements. The instrumentation used for the tests is shown in Fig. 1. The signals from the two accelerometers, or, for radiation from a wall, the source accelerometer and a microphone, were amplified and then recorded on the tape recorder. For analysis the signals were played back from the tape recorder into the real time digital frequency analyser which carried out the integration of all frequencies. The two preamplifiers were also used to filter out signals below 80 Hz and above 4000 Hz and this maximised the signal to noise ratio. The signal to noise ratio is important because the rms level is usually at least 30 dB below the peak level.

358

R.J.M. CRAIK RESULTS

In order to compare the results of impulse sources with more conventional continuous sources, a building was chosen that has already been studied. 2 Although measurements were made over a large part of the building, all the different joint types studied can be seen in the floor plan and section shown in Fig. 2. The numbers are the subsystem numbers and are the same as in those used in the earlier paper. 2

26

Room ] Room 1 % 2

70

27

I

1

55 A / / / / / / / / / / / / / / PLAN Fig. 2.

SECTION

Plan and section of part of the building studied.

For a comparison between the continuous and impulse source results the acceleration level difference was converted into a subsystem energy level difference 3 which was normalised for the damping of the receiving subsystem. The result has the units of a coupling loss factor and would be equal to it if there were only the two subsystems to consider. This gives: D,

=

rl2E2/E 1 ~ ~12

(3)

where D, is the normalised energy level difference, ~/1z is the coupling loss factor, ~2 is the total loss factor of the second subsystem and E is the subsystem energy level. The normalised level difference is larger than the coupling loss factor due to power flow into subsystem 2 from other subsystems but this difference is small and rarely larger than 3dB. The difference is almost constant for any joint type so that, provided the same normalisation is carried out, the results from different parts of the building can be compared. The predicted values that are given were found by computing the predicted energy level difference using a statistical energy level analysis model. 2 The energy level difference was then normalised in the same manner as the measured values. This gives a slightly different predicted value to that given in reference 2 which gave the predicted coupling loss factor. As the damping of the structure can be readily found 4 this leaves the ratio of the energy levels as the only unknown in eqn. (3). The continuous source measurements

MEASUREMENT OF S T R U C T U R E - B O R N E

359

S O U N D TRANSMISSION

were made with either a shaker or a tapping machine as the noise source. Measurements of acceleration or pressure were made first in one subsystem, then the other. No fixed number of positions was measured but rather measurements were made until the accuracy was better than + 2 d B at the one-third octave band 250 Hz. 5 The exact number of positions for each subsystem was typically 6 to 8 for the rooms and 10 to 20 for the walls and floors. Results from different parts of the building that were similar were averaged to give an average value for the normalised level difference in order to remove differences caused by random sampling. The standard deviation was also calculated to give a measure of the spread of results obtained by each technique• The results in Figs 3 to 7 show the mean measured coupling loss factor, plus or minus one standard

'~

100 [

10C

.~

~5

9C~

I

• ....... . . . . . . . . . . . . .

""',

.... 2

I~ , 125

250

500

1K

2K

HZ

Fig. 3. N o r m a l i s e d e n e r g y level difference for transmission f r o m wall 54 to floor 26. - C o n t i n u o u s noise source (20 joints); . . . . Impulsive noise source (10 joints); . . . . . . Predicted results,

•.............

/-/fS2"~'"--~,

{ I8 ° ' 1 2 5 . . . 7'50. . .soo .

1K

'

'

2~

'

.z

Fig. 4. N o r m a l i s e d energy level difference for transmission f r o m wall 54 to wall 70. - C o n t i n u o u s noise~ source (10 joints); . . . . I m p u l s i v e n o i s e s o u r c e ( 8 j o i n t s ) ; . . . . . . Predicted results. 100

'7~

. ..........'" .........,

..... ~d 10(

80

<

z i

i

i

i

i

125

250

500

1K

2K

Hz

Fig. 5. N o r m a l i s e d e n e r g y level difference for transmission f r o m wall 54 to wall 55. - C o n t i n u o u s noise source (3 joints); . . . . Impulsive noise (3 joints); . . . . . . Predicted results.

•

J 125

.

,

i 250

,

.

i

500

.

=

i

1K

.

,

i

2K

,

,

Hz

Fig. 6. N o r m a l i s e d energy level difference for radiation f r o m wall 54 to r o o m 2. - C o n t i n u o u s noise source (12 walls); . . . . Impulsive noise source (10 walls); . . . . . . Predicted results.

360

R.J.M.

CRAIK

% 100

12s

2so

soo

~)

~K

~z

Fig. 7. N o r m a l i s e d e n e r g y level difference f o r t r a n s m i s s i o n f r o m f l o o r 26 to wall 54. Continuous noise s o u r c e (16 j o i n t s ) ; - - - I m p u l s i v e noise s o u r c e (10 j o i n t s ) ; . . . . . . P r e d i c t e d results.

deviation, calculated for both impulsive and continuous sources. Also shown in these Figures are the predicted values. Transmission from wall 54 to floor 26 can be seen in Fig. 3. The agreement between the two sets of results is good, although, inevitably, there is some difference. The spread of results for the impulsive source tends to be smaller, possibly because of the large number of source positions. The impulse source results also show better agreement with the predicted values with the dip and rise at 315 and 1000 Hz being less prominent than in the continuous source results. The results for transmission across a cross joint from wall 54 to wall 70 can be seen in Fig. 4. Again, there is a good agreement between the two sets of measurements although the impulse results tend to be lower at the mid-frequencies. At higher frequencies neither set of results agrees with the predicted results due to a limitation of the theory. Transmission from wall 54 to the small wall 55 can be seen in Fig. 5. As there were only three measured results the mean only has been shown. The agreement between the two methods is good. The mode count of the receiving wall is very low 2 resulting in poor agreement with the predicted results at low frequencies. Radiation from wall 54 to room 2 can be seen in Fig. 6. As with the structure to structure results, the agreement between the two methods is good with the spread of results from the impulse source being slightly less than for the shaker source. The agreement with the predicted curve is quite reasonable, although neither of the measured results gives the peak at the critical frequency (predicted at 291 Hz). Although a tapping machine is an impulsive source, for most practical purposes it can be treated as a continuous source. Unfortunately, the floor had to be tapped through a carpet and this effectively filtered out all the high frequencies so that measurements could not be made at all frequencies. Typical results for floor 26 to wall 54 (Fig. 7) show excellent agreement between the two techniques at all frequencies where measurements could be made. From these results it can be concluded that, although differences occur between the two measurement techniques where one averages many measuring positions for

MEASUREMENT OF STRUCTURE-BORNESOUND TRANSMISSION

361

a few source positions and the other averages m a n y source positions for a few measuring positions, the results are essentially the same with g o o d agreement. F o r the specific measurement procedures adopted the spread o f results f r o m the impulse sources tended to be less than the results for the continuous sources but this difference is slight. The greatest difference is in the time taken for the measurements. The total time was reduced by a factor o f 10 and the site time was reduced by a factor o f 20.

CONCLUSIONS The results o f this study have shown that impulsive sources can be used as structural sources for measurements in buildings. The techniques are at least as accurate as conventional measurement methods using continuous sources and can be carried out in a m u c h shorter time. The saving in time will depend on the instrumentation being used, but a factor o f ten would not be unreasonable. Even excluding the time saving, the increased versatility makes the use o f impulse sources preferable to continuous sources for m a n y applications.

ACKNOWLEDGEMENTS This work was financed by the Science and Engineering Research Council. The measurements using the impulsive sources were made with the assistance o f D. J. Mackenzie.

REFERENCES 1. P. DE TRICAULD, Impulsive techniques for the simplification of insulation measurements between dwellings, Applied Acoustics, 8 (1975), pp. 245-56. 2. R. J. M. CRAIK, The prediction of sound transmission through buildings using statistical energy analysis, Journal of Sound and Vibration, 82 (1982) (In press). 3. R. H. LYON,Statistical energy analysis of dynamical systems: Theory and applications, MIT Press, Massachusetts, USA, 1975. 4. R. J. M. CRAIK,Damping of building structures. Applied Acoustics, 14 (1981), pp. 347-59. 5. ANON,Guide to statistical interpretation of data. BS 2846. British Standards Institution, London, 1975.