PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

An indirect method for the measurement of impact sound insulation
Simon BAILHACHE1; Stéphanie COLIN2; Michel VILLOT3
1,23

CSTB, FranceMV-Expert, FranceABSTRACTThis paper describes a way of measuring impact sound insulation for heavy structures without using thestandard tapping machine. Instead, another source of known contact force - e.g. an impact hammer orelectrodynamic shaker - is connected to the building element under test. The sound pressure level is measured
in the receiving room and then corrected from the difference in force level between the standard tapping
machine and the substitution source to obtain the impact sound pressure level. An application of this indirect
method is the laboratory determination of the impact sound performance of walls, which can be used as an
input for prediction of impact noise of isolated heavy stairs as well as of structure-borne noise from service
equipment in situ; the standard tapping machine is indeed not suitable for exciting vertical building elements.
In this paper, the indirect method is described, then applied to a floor element and the results are comparedto the impact sound pressure level directly determined using the standard tapping machine on the same floor.
Several measurements are performed using different numbers of source and microphone positions. The
corresponding results are presented and discussed.
Keywords: Impact sound, Measurement, Indirect methods

1. INTRODUCTION
Since 2017, standardization group CEN/TC126/WG1/TG3 is working on a new laboratory
measurement method of the impact sound insulation of walls. The practical applications of suchmeasurements may not be straightforward, since the concept of impact sound insulation usuallyapplies to impacts produced on building floors. However, the impact sound insulation performance ofwalls can be used as input data in predictions of impact sound when the impacts are generated onstairs connected to a heavy structure. Indeed, this quantity is introduced in EN ISO 12354-2 Annex F(1) to define the impact sound reduction ΔL of heavy isolated stair landings and of lightweight stairs
connected to heavy walls as follows:ο‫ܮ‬௟௔௡ௗ௜௡௚Ȁ௦௧௔௜௥௦ ൌ ‫ܮ‬௡ǡ଴ǡ௪௔௟௟ െ ‫ܮ‬௡ǡ௟௔௡ௗ௜௡௚Ȁ௦௧௔௜௥௦

(1)

In equation 1, Ln,0,wall is the normalized impact sound pressure level (in dB ref. 2.10 -5 Pa) of thewall to which the landing or stairs are connected, while L n,landing/stairs is the normalized impact soundpressure level when the impacts are produced to the considered isolated landing or lightweight stairs.
These quantities are related to the excitation produced by the standard tapping machine, which can
only be used on horizontal elements. The determination of L n,0,wall thus requires a specificmeasurement method using a different impact source of known applied force.
Previous work also highlighted another possible use of L n,0,wall in predictions of structure-borne
sound from service equipment when a force source assumption can be made (2). Measurements wereperformed using a special tapping machine adapted to walls that require s to be fixed into the element
under test.
1

Simon.bailhache@cstb.fr
Stephanie.colin@cstb.fr
3
Michel.jean.villot@gmail.com
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In this paper, a different method using an impact hammer is proposed. Lighter and easilytransportable, this impact source however suffers from two major disadvantages in comparison withthe tapping machine: its non-calibrated excitation and a limited impact area. The first issue can beeasily dealt with by applying a correction to the measured sound pressure level depending on the force
level of the excitation – hence the “indirect” character of the proposed method. The second point may
require an increased number of source positions in order to reach comparable accuracy, due to modal
behavior of the wall.
In this study the method is applied to a concrete floor, so that the results can be compared to thoseobtained with the standard tapping machine. The experimental protocol is first described in section 2.
Results are presented in section 3 and the influence of the number of hammer positions is discussed.

2. EXPERIMENT PROTOCOL
2.1 General principle
The building element under test is an 18 cm thick concrete floor connected to concrete walls. The
54 m3 room located below is considered as receiving room. The standardized sound pressure level
using the tapping machine (noted L’ nT,TM ) is measured in the range 50-5000 Hz following
EN ISO 16283-2 (3). The standard uncertainty values given in EN ISO 12999-1 (4) are used.
In addition, a second measurement of the standardized sound pressure level is performed byreplacing the tapping machine by a 1.5 kg impact hammer (Kistler type 9728A2000) equipped with arather hard tip. Repeated impacts are produced to obtain quasi-stationary excitation over averaging
periods of 15 s. For each hammer position i, the measured sound pressure level L p,hammer,i (correctedfrom background noise) is used to determine the sound power to sound pressure transmission function
D TF,i as defined in ISO 10848-1 (5).
‫்ܦ‬ிǡ௜ ൌ ‫ܮ‬௣ǡ௛௔௠௠௘௥ǡ௜ െ ‫ܮ‬ௐǡ௛௔௠௠௘௥ǡ௜(2)

In equation 2, LW,hammer,i is the structure-borne sound power level (in dB ref. 10 -12 W) applied atthe ith hammer position.
Applying a power substitution method, the transmission function can be used to calculate the soundpressure level of the tapping machine L p,TM,i as follows:‫ܮ‬௣ǡ்ெǡ௜ ൌ ‫ܮ‬ௐǡ்ெǡ௜ ൅ ‫்ܦ‬ிǡ௜(3)For low-mobility structures, EN 15657 (6) allows to approximate the installed structure-borne
sound power level L W as follows:‫ܮ‬ௐ ൎ ͳͲ  ቀܴ݁൫ܻோǡ௘௤ ൯ቁ ൅ ‫ܮ‬ி௕ǡ௘௤(4)

In equation 3, Y R,eq is the equivalent mobility of the receiver (in m/(N.s)) and L Fb,eq is theequivalent blocked force level (in dB ref. 10 -6 N) of the source.
These relationships can be simplified as follows:‫ܮ‬௣ǡ்ெǡ௜ ൌ ͳͲ  ቀܴ݁൫ܻோǡ௘௤ ൯ቁ ൅ ‫ܮ‬ிǡ்ெǡ௜ ൅ ‫ܮ‬௣ǡ௛௔௠௠௘௥ǡ௜ െ ቀͳͲ  ቀܴ݁൫ܻோǡ௘௤ ൯ቁ ൅ ‫ܮ‬ிǡ௛௔௠௠௘௥ǡ௜ ቁ(5)Which further reduces to a simple force level correction:‫ܮ‬௣ǡ்ெǡ௜ ൌ ‫ܮ‬௣ǡ௛௔௠௠௘௥ǡ௜ ൅ ‫ܮ‬ிǡ்ெǡ௜ െ ‫ܮ‬ிǡ௛௔௠௠௘௥ǡ௜

(6)

The corrected sound pressure level then needs to be averaged over all hammer positions andcorrected from the reverberation time of the receiving room for comparison to the results of thestandard measurement.

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2.2 Source and microphone positions
For measurements using the tapping machine, 6 source positions are considered. As specified in
EN ISO 16283-2, this allows for only one measurement for each source position. One microphone
position is considered and made different for each source position.
For measurements using the impact hammer, the same 6 source positions are considered. Additional
measurements are then performed by adding other source positions up to a total of 15.

3. RESULTS
3.1 Excitation
The force levels measured by the impact hammer for the 15 source positions are represented inFigure 1. The force level of the tapping machine, considered as independent from its location on thefloor, is also represented for comparison. These values are obtained from calculations according to
the analytical model described in (7).
These results show that the force level of the impact hammer is well repeatable except for one
source position (no. 4), due to weaker operator action. The force level of the impact hammer drops at
800 Hz and higher frequencies, whereas the force level of the tapping machine continues to increase.
Nevertheless, it remains well above background noise for all source positions.

Figure 1 – Force levels of the impact hammer and of the tapping machine1325

3.2 Impact sound pressure level
The corrected sound pressure level of the impact hammer is represented in Figure 2 as a functionof the number of source positions considered. The corresponding single-number ratings calculated
according to ISO 717-2 (8) are given in Table 1.
From these results, it can be seen that the corrected sound pressure level may vary slightly
depending on the number of positions considered. With an increasing number of positions consideredin the average calculation, the corrected sound pressure level rapidly reaches a stable value. The
maximum difference between the stabilized values (15 positions considered) and the results for only
one position considered is approximately 5 dB at 200 Hz. As of 9 positions considered, the differenceswith 15 positions considered are less than 1 dB for all frequency bands except at 63 Hz (1.2 dB).
When considering the single-number ratings, all results are within a 1-dB range. From 7 positions
considered, the values remain constant.

Figure 2 – Impact sound pressure level as a function of the number of hammer positions
Table 1 – Single-number ratings calculated according to ISO 717-2
Number of

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

L'nT,w (dB)

73

73

72

73

73

73

72

72

72

72

72

72

72

72

72

C I,100-2500 (dB)-12-12-12-12-13-13-12-12-12-12-12-12-12-12-12C I,50-2500 (dB)-12-12-12-12-12-13-12-12-12-12-12-12-12-12-12positions

The corrected sound pressure levels measured using 6, 10 and 15 positions of the impact hammer
and the sound pressure level for 6 positions of the tapping machine (considered as reference) arerepresented in Figure 3. Standard uncertainty limits according to ISO 12999-1 are also given for the1326

measurement using the tapping machine. The single-number ratings calculated according to ISO 7172 (6) are available in Table 2.
These results show that the corrected sound pressure level of the impact hammer and the soundpressure level of the tapping machine are very similar. Approximately half of the values of the impact
hammer corrected level (15 pos.) are within uncertainty limits of the tapping machine measurement
result. The other half are less than 1 dB from these limits, except at 100 Hz (1.2 dB gap), 4000 Hz(1.6 dB gap) and 5000 Hz (4.8 dB gap). Similar observations can be made for 6 and 10 impact hammer
positions considered, with results really close to those obtained for 15 positions considered.
Considering the single-number ratings, the stabilized L’ nT,w value obtained with the impact hammer
is 1 dB lower than the result obtained with the tapping machine, while the same values are obtained
for the spectrum adaptation terms.

Figure 3 – Impact sound pressure level measured with the impact hammer and with the tapping machine
Table 2 – Single-number ratings calculated according to ISO 717-2
Number ofpositions

Tappingmachine:
average

HammerHammerHammer(6 Pos.)(10 Pos.)(15 Pos.)

L'nT,w (dB)

73

73

72

72

C I,100-2500 (dB)-12-13-12-12C I,50-2500 (dB)-12-13-12-121327

4. CONCLUSIONSThe purpose of this study was to propose a measurement method of the impact sound insulation ofwalls. To this aim, the standard tapping machine being inapplicable on vertical elements, an impact
hammer is used as an impact source and the impact sound pressure level is estimated with an indirect
method: the sound pressure level measured in the receiving room is corrected from the difference inforce level between the hammer excitation and the tapping machine. In order to validate the principleof this simplified power substitution method, it was experimented on a concrete floor, where thestandard tapping machine can also be used and considered as reference case.
The results obtained with both methods are very similar, with slight variations depending on thenumber of hammer positions considered. Sufficient signal-to-noise ratio was obtained with the impact
hammer, even though problems could have been expected at high frequencies.
From these results, it seems possible to measure the impact sound insulation performance of wallswith the proposed method using an impact hammer. Due to a limited impact area, it may be necessaryto slightly increase the number of source positions in comparison to measurements with the standard
tapping machine. A minimum number of 10 positions seems a reasonable compromise.

ACKNOWLEDGEMENTS
This research work has been funded by CSTB Research and Development Department.

REFERENCES
1. EN ISO 12354-2. Building acoustics - Estimation of acoustic performance of buildings from theperformance of elements - Part 2: Impact sound insulation between rooms. 2017.
2. Bailhache S, Villot M. Predicting structure-borne noise from building service equipment using the impact
noise calculation model. Proc ICSV24; 23-27 July 2017; London, United Kingdom 2017.
3. EN ISO 16283-2. Acoustics - Field measurement of sound insulation in buildings and of building elements- Part 2: Impact sound insulation. 2015.
4. EN ISO 12999-1. Acoustics - Determination and application of measurement uncertainties in building
acoustics - Part 1: Sound insulation. 2014.
5. EN ISO 10848-1. Acoustics - Laboratory and field measurement of flanking transmission for airborne,
impact and building service equipment sound between adjoining rooms - Part 1: Frame document. 2017.
7. EN 15657. Acoustic properties of building elements and of buildings - Laboratory measurement ofstructureborne sound from building service equipment for all installation conditions. 2017.
7. Cremer L, Heckl M, Petersson B.A.T. Structure-borne sound: Structural vibrations and sound radiation ataudio frequencies. 3rd ed. Berlin, Germany: Springer; 2005. p.306-310.
8. ISO 717-2. Acoustics - Rating of sound insulation in buildings and of building elements - Part 2: Impact
sound insulation. 2013.

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