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

Structure-Borne Sound Isolation of Acoustic Test Chambers: In-Situ
Validation
Abhay RAJMANE
G+H Schallschutz GmbH, Germany

ABSTRACT
Acoustic test chambers are used as controlled environment for measuring sound power level ofdiverse equipment ranging from household equipment over automobile components to wholeautomobiles, etc. They are also used as standard environment for diverse acoustic an d vibration
experiments. The background noise level inside an acoustic test chamber is a crucial factor during thesound power measurement of silent specimen or during a test on sensitive equipment. To achieve anegligible background noise level, it is inevitable to structurally isolate the acoustic test chamber fromthe surrounding structure. The isolation from structure-borne sound is performed using vibration
isolator systems. These systems minimize the ingress of structure-borne sound in the floor and walls ofan acoustic test chamber and thereby minimize the radiated airborne sound in the chamber. The elastic
isolation is designed such that the natural frequency of the spring -mass system is well below thecut-off frequency of the respective acoustic test chamber.
To validate the elastic isolation of acoustic test chambers, the natural frequency of these chambers
is experimentally measured on multiple installations. Performing an experimental modal analysis, thenatural frequencies and mode shapes of different modes of the test chambers are extracted. It isvalidated that the measured natural frequency practically coincides with designed fundamentalfrequency. The experimental results from several acoustic test chambers are presented in this work.
Keywords: Structure-borne-sound Isolation, Elastic Isolation, Anechoic Chamber

1. INTRODUCTION
Acoustic test chambers are used as controlled environment for measuring sound power level ofdiverse equipment ranging from household equipment over automobile componen ts to wholeautomobiles, etc. They are also used as standard environment for diverse acoustic and vibration
experiments. Acoustic test chambers are broadly classified as Anechoic Chambers and Reverberationchamber. The background noise level inside an acoustic test chamber is crucial factor during the soundpower measurement of silent specimen or during a test on sensitive equipment.
To achieve the negligible background noise levels, the acoustic chambers are built with hightransmission loss elements. The background noise level cannot be reduced below a certain limit byfurther increasing the airborne transmission loss of walls and ceiling. The direct airborne transmission
reaches its limit and secondary sound transmission paths dominate the sound pressure level inside thechamber. The secondary path includes the ingress of structure-borne sound in the structure andradiation of airborne sound inside chamber. To minimize the background noise level further, it isinevitable to structurally isolate the acoustic test chamber from the surrounding structure.
The isolation from structure-borne sound is performed using vibration isolation products e.g. G+H
MAFUND Rubber Sheets, VIBREX Spring Strips and metallic spring isolators. These systems
minimize the ingress of structure-borne sound in the floor and walls of an acoustic test chamber andthereby minimize the radiated airborne sound in the chamber. The elastic isolation is designed suchthat the natural frequency of the spring-mass system is well below the cut-off frequency of therespective acoustic test chamber.
Two different measurement approaches are explained in this paper. In the first approach, the efforts
are dedicated towards computing the difference in structure-borne-sound level between fundament andbaseplate. In the second approach, the natural frequency of the baseplate is measured using
experimental modal analysis. The sequential application of the two methods allows us to identify a1336

dedicated demand for a retrofit solution. First, we generally detect or exclude issues with the vibration
damping system of an isolated measurement chamber and investigate furthermore the detuning in caseof an improper design.

2. APPROACH 1: STRUCTURE-BORNE-SOUND TRANSMISSION LOSS
2.1 Measurement philosophy and set-up
Normally the air-borne sound transmission loss is measured by exciting one side of sound isolating
element and calculating difference in sound power level on either side of isolating element, on theguidelines of laboratory standard DIN EN ISO 10140-2 (1). The analogous approach is used here as
described in Figure 1. The elastically isolated baseplate is the receiving side and the fundament is thesource side. The fundament is excited with a standardized tapping machine and the structure-borne
sound velocity level is measured on the fundament and on the elastically isolated baseplate as shown in
Figure 1.

Figure 1 – Experimental set-up
2.2 Measurement results
The measurements are taken on a baseplate of an anechoic chamber. The dimensions of thebaseplate are L x W x H = 7.7 x 5.3 x 0.2 m, the total mass including anechoic chamber is 32 t resulting
in a designed natural frequency of the setup of approximately 4 Hz. The measurement results ofstructure-borne-sound velocity level are plotted in Figure 2. It is observed that the measured velocity
level on receiving side is almost same as background noise level on receiving side. Th e curve indicatesthat the structure-borne-sound isolation is working as expected, but the efficiency of structure-borne
isolation cannot be absolutely quantified as in the case of airborne sound.

Figure 2 – Experimental results of structure-borne-sound velocity level difference1337

2.3 Intermediate conclusions of Approach 1
The difference in structure-borne velocity level between source and receiving side is increasing
with increasing frequency. During these measurements, good scientific practice was observed whilechoosing a low self-noise and high sensitivity accelerometer. Despite this, the level on the receivingside is almost equal to background level. On the source side, the magnitude of excitation cannot beincreased further, because the construction of standardized tapping machine is fixed. Furthermore, thelevel of excitation on source side is largely dependent on the impedance of fundament. A fundament on
the upper floor of a multi-story building will have a higher source side excitation than a fundament thatis part of earth floor. Furthermore, because of dispersive nature of structure-borne-sound, amplitude ofthe excited wave is largely dependent on frequency and distance from tapping machine. Therefore, thesuccess of structure-borne-isolation cannot be quantitatively accessed using this approach.

3. APPROACH 2: MEASURING FUNDAMENTAL FREQUENCY OF VIBRATION
3.1 Measurement philosophy
The fundamental frequency of elastic isolations is designed based on the cut-off frequency fc of theisolated test chamber and the dominant excitation frequency outside the test chamber. The cut-off
frequency of a chamber depends on the lining depth in case of anechoic chamber or the volume of achamber in the case of a reverberation chamber. It defines the lowest measurable frequency band of theairborne sound inside the chamber. The dominant excitation frequency outside the chamber is imposedby installed aggregates outside of test chamber.
To clearly differentiate between rigid-body-vibrations and bending vibrations of the baseplate, adetailed modal analysis of the baseplate is performed. A virtual grid is marked on baseplate as shownin figure 3. The intersection points of this grid are excitation positions. One asymmetrically locatedgrid-point on the baseplate is selected as accelerometer position, which can be excited at all possible
frequencies in the frequency range of interest. The accelerometer is glued on the baseplate at theselected position for all impulse excitations.
The plate is excited by the impulse hammer at all grid positions and a FFT analysis is performed in
the frequency range from 0 Hz to 100 Hz. The inertance is defined as cross correlation between
acceleration and force (2). The frequency response spectrum of each impulse-response combination isplotted as magnitude of inertance and imaginary part of inertance. Using the quadrature picking
method (3), the natural frequency and mode shapes are extracted. The peak in the magnitude ofinertance indicates the natural frequency of the baseplate. The imaginary part of inertance at thecorresponding frequency indicates the relative modal displacement at the excitation position. A typicalresult of inertance is presented in figure 3. Using this information, the natural frequencies of vibrationof the baseplate and their corresponding mode shapes are extracted as shown in figure 5.
[(m/ s²)/ N]

Ac celerance (Magnitude)
Work ing : c7 : Input : FFT Analyzer

8M
6M
4M
2M
00
[(m/ N¯¹s²)^½]

2040

[Hz]

60

80

Ac celerance I (Imaginary Part)Work ing : c7 : Input : FFT Analyzer

8k
4k0-4k-8k0Imaginary grid on baseplate

2040

[Hz]

6080

Magnitude and imaginary part of inertance

Figure 3. Virtual grid on a baseplate and inertance results of experimental modal analysis

1338

3.2 Measurement results
Using this approach, the fundamental frequency of elastic isolation is measured in the case of threedifferent baseplates and they are compared with the respective design fundamental frequencies.
3.2.1 Measurements on a baseplate isolated using metallic spring system
The measurements are performed on a baseplate of an anechoic chamber, which is designed toperform acoustics measurements on automobile components. The anechoic chamber is built on areinforced concrete baseplate of 35cm thickness and isolated using a G+H FU metallic spring system.
The dimensions of the baseplate are L x W = 7.1 x 6.3 m. The chamber is built up by a combination ofsound damping panels and wedge absorbers. The mass of the baseplate including the anechoic
chamber is 57 t and the design natural frequency of the isolation is approx. 4 Hz. The measurement
grid on baseplate is shown in figure 4.

Imaginary grid on baseplate

G+H FU metallic spring system

Figure 4 – Measurement on a baseplate insulated using metallic spring system
The results of modal analysis are presented in figure 5. The measured fundamental frequency ofwhole-body-motion is f 1 = 4 Hz and it coincides with the designed natural frequency. Furthermore, thetilting mode natural frequency (f 2=19.5 Hz) and first bending mode natural frequency (f 3=31Hz) are
extracted from the modal analysis as shown in figure 5.
f1 = 4 Hz

f2 = 19.5 Hz

f3 = 31 Hz

Figure 5 – Natural frequencies and mode shapes of base plate, metallic spring system1339

3.2.2 Measurement on a baseplate isolated using VIBREX spring strips system
As a second object, an acoustic vehicle chassis dynamometer with f c = 100 Hz is selected. The
chassis dynamometer is built as a room-in-room concept and the weight of the inner room including
baseplate, massive walls and ceiling is approximately 365 t. The dimensions of the baseplate are L x W
x H = 13.3 x 10.6 x 0.4 m. The baseplate including inner structure of the anechoic chamber is isolated
from the fundament using G+H VIBREX spring strips system as shown in figure 6. The design natural
frequency of isolation is approximately 10 Hz. To perform the modal analysis, a virtual grid is marked
on baseplate at approximate distances of 2 m, as shown in Figure 6.

Imaginary grid on baseplate

G+H VIBREX spring strips

Figure 6 – Measurement on a baseplate isolated using VIBREX spring strips system
The measurement results are presented in figure 7. The first natural frequency of the baseplate isobserved at 11.75 Hz. From the mode shape, it is observed that it is rigid-body-translatory mode invertical direction. The small difference between designed frequency and measured frequency is causedby the missing mass of the wedge absorbers and the ventilation system inside the anechoic chamber at
the measurement stage. The tilting-mode frequency is observed at 17.75 Hz and first bending vibration
mode is at 28.6 Hz as shown in figure 7.
f1 = 11.75 Hz

f2 = 17.75 Hz

f3 = 28.6 Hz

Figure 7 – Natural frequencies and mode shapes of base plate, VIBREX spring strips system1340

3.2.3 Measurement on a baseplate isolated using G+H MAFUND rubber sheets system
The third anechoic chamber is designed for measuring the sound power level of household
equipment with f c = 100 Hz. Because of stringent requirements on the background noise level, theanechoic chamber is built with the room-in-room type construction. The baseplate carrying theinternal cladding with a flat panel system is isolated from its building using a G+H MAFUND rubber
sheet system as shown in figure 8. The dimensions of the baseplate are L x W x H = 6 x 5.8 x 0.25 m.
The mass of baseplate including the anechoic chamber is 38 t resulting in a designed natural frequencyof the isolation of approximately 21 Hz. To perform the modal analysis, a virtual grid is marked on thebaseplate with approximate distances of 1 m, as shown in Figure 8.

Imaginary grid on baseplate
G+H MAFUND rubber sheets
Figure 8 – Measurement on a baseplate isolated using MAFUND rubber sheets system
The natural frequencies of vibration of the baseplate and their mode shapes are presented in Figure
9. The first fundamental frequency is observed at 22Hz and the first tilting frequency is observe d at 26Hz. The first bending mode of vibration is observed at 35 Hz and the breathing mode of the baseplateis observed at 46 Hz as shown in figure 9. Considering the missing mass of the absorbing system inside
the anechoic chamber at the time of measurement, the fundamental frequency of spring-mass system
practically coincides with the design natural frequency.
f1 = 22 Hz

f2 = 26 Hz

f3 = 35 Hz

f4 = 46 Hz

Figure 9 – Natural frequencies and mode shapes of base plate, MAFUND rubber sheets system1341

4. CONCLUSIONSTaking measurements of structure-borne-sound velocity levels of source and receiving side andcomputing the difference, it can be qualitatively checked, if the isolation of a baseplate from its
fundament is properly designed. However, a quantitative measurement of the actual efficiency cannot
be done.
A second approach shows that the fundamental frequency of spring-mass system can be accuratelymeasured. It is verified for three different types of vibration isolating systems that the measuredfundamental frequency practically coincides with the designed natural frequency of a given
baseplate-isolation-system. The robustness of both methods allows us to offer the combined package
as a service to identify the existence of vibration isolation issue, followed by a classification of theseverity of a system’s detuning. Based on this quantification, we can rectify the tuning of a given
spring-foundation-chamber setup to its desired parameters

REFERENCES
1.DIN EN ISO 10140-2:2010-12, Acoustics - Laboratory measurement of sound insulation of building
elements - Part 2: Measurement of airborne sound insulation
2. Ewins D. J., Modal Testing: Theory Practice and Application
3. Svend Gade et. al., How to determine modal parameters of simple structures, Brüel & Kjaer Application
Note 35601342