Hans-Martin Tröbs, Stefan Schoenwald, Armin Zemp. Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Acous- tics/Noise ...
Comparison of different measurement methods of radiation efficiency of lightweight structures Hans-Martin Tröbs, Stefan Schoenwald, Armin Zemp
Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Acoustics/Noise Control, CH-8600 Dübendorf, Switzerland. Summary
Currently there is no standardized measurement method for determining the ratiation e ciency of building elements. The new draft of the EN 12354-1 [1] for the prediction of sound transmission in buildings suggests a correction of the sound reduction index for lightweight structures below the critical frequency by using the radiation e ciency of the building element. To evaluate dierent existing methods, namely the intensity and sound pressure method, for measuring the required data for the radiation e ciency calculation, tests were conducted at a wooden panel at the Laboratory for Acoustics and Noise Control at Empa Dübendorf. The panel was excited by airborne and structure borne sound. The surface vibration velocity was measured with a scanning laser vibrometer. This also allows the radiation e ciency to be calculated by using the Rayleigh integral. A good agreement between the investigated measurement methods and the analytical solution could be shown. PACS no. 43.40.Dx, 43.55.Rg
1.
Introduction
The EN 12354-1 describes a calculation method for the prediction of the sound insulation of a building element in the building taking into account for anking sound transmission. For building anking transmission, at the junction only that energy is transferred that is related to resonant modes. The coupling of the sound eld in the rooms with the structure is determined in the model by the sound reduction index of the building elements. However, below the critical frequency, also forced transmission, that is not related to the resonant modes, occurs. Therefore, in the fourth draft of EN 12354-1 from 2013 there is a correction of the Sound Reduction Index R of a building element below the coincidence frequency. The radiation e ciency for airborne and structural excitation, σa and σs of the building element (see equation 1) is used to remove the nonresonant transmission component. This correction is particularly important in lightweight components for an accurate prediction if the coincidence frequency is in the relevant frequency range. The resulting Sound Reduction Index R∗ for resonant transmission is used in the EN 12354-1 prediction model as an input quantity for the prediction of the apparent sound insulation. The apparent
sound insulation includes direct transmission through the separating element between the adjacent rooms as well as anking sound transmission. R∗ = R + 10log
(
σa σs
)
[dB]
(1)
Currently there is no standardized measurement method to determine this radiation e ciency. The radiation e ciency is de ned in equation 2. Wrad denotes the radiated power by the surface area S , ρ0 the density of air, c0 the speed of sound in air and ⟨v 2 ⟩ the spatially averaged mean square vibration velocity of the surface. σ=
Wrad ρ0 · c0 · S · ⟨v 2 ⟩
[−]
(2)
In order to assess various methods, a comparison of the sound pressure and intensity method was conducted in the Laboratory for Acoustics and Noise Control at the Empa Dübendorf on a light wooden particle board. The plate was excited by airborne sound as well as by a shaker. The measurement results and analytical predictions are compared and discussed. 2.
Specimen properties and measured quantities
(c) European Acoustics Association
The specimen, a particle board with the dimensions of 1.23 m x 1.48 m x 0.016 m and a weight per unit
FORUM ACUSTICUM 2014 7-12 September, Krakow
Tröbs, Schoenwald, Zemp: Radiation efficiency
Radiation index 10log σ [dB]
10 5 0 0 −5 −10 −10 −20 −15 −20 −30 −25 −30 −40
Figure 1. Photograph of plate in anechoic room (room a). ′
area of m = 10.8 kg/m2 was installed in a standard window opening between a reverberant room (room b) and an anechoic room (room a), shown in gure 1. At its perimeter, the panel was mounted between wooden battens and 3 mm felt, the gaps were sealed afterwards with acrylic sealing. The spatially averaged mean square vibration velocity ⟨v 2 ⟩ was measured in a grid of 9 x 11 and 25 x 31 measurement points with a scanning laser vibrometer. The surface vibration velocity was evaluated with a Fast Fourier Transform (FFT) using a 2 Hz resolution and ltered afterwards in one third octave bands. The sound power W was determined by scanning with an intensity probe using a 12 mm spacer as well as by applying the reverberation room method from the sound pressure and the reverberation time in the room in one third octave bands. 3.
Intensity room b Intensity room a Sound pressure room b 63
125
250 500 1000 Frequency [Hz]
2000
4000
Figure 2. Radiation index for shaker excitation and velocity grid 9 x 11 points. Comparison of intensity- and sound pressure method in reverberant room (room b) and anechoic room (room a).
Figure 3. Schematic drawing of DCM approach [2].
Measurement results
Figure 2 shows the estimated radiation index over the relevant frequency range for the intensity and sound pressure method. The comparison shows a good agreement between the dierent methods for shaker excitation. The dierences below 160 Hz are likely due to spacer length respectively a too low diusivity of the sound eld. The measurement uncertainty illustrated as error bars is indicated for one data set for better visibility. The estimated uncertainty was in a comparable range for all the dierent data sets. In general, the accuracy of both methods is dependent on the room conditioning. For the sound pressure method the sound eld should be as homogeneous as possible. For the intensity method ideally a direct sound eld with all waves propagating away
from the surface of the specimen should exist. Therefore, the sound pressure method was applied in the reverberant room (room b)only, whereas the intensity measurements were performed in both rooms. To improve the conditions for the intensity measurement in the reverberant room, additional absorption was placed on the wall opposite to the test opening. 4.
DCM
When using the Discrete Calculation Method (DCM) [2], the plate is subdivided into individual point sound sources with the spacing d and equivalent circular area Si around the measurement point as illustrated in gure 3.
FORUM ACUSTICUM 2014 7-12 September, Krakow
Tröbs, Schoenwald, Zemp: Radiation efficiency
10
0
−10
−20 Airborne DCM Shaker DCM Airborne Intensity Shaker Intensity
−30
−40
63
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250 500 1000 Frequency [Hz]
2000
The sound power radiated individually by each piston source can be estimated from the vibration velocity amplitude. From the phase relationship between the pistons their interaction is taken into account. If the spacing between the sources is su ciently small, the sound power radiated by the panel can be estimated. The spatial resolution d between the measurement points must be smaller than half of the wavelength at the coincidence frequency. Figure 4 shows the eatimated radiation index as a funtion of the frequency for the intensity method as well as for the DCM. The illustration shows a good agreement of intensity method and DCM. The DCM estimates the radiated sound power and radiation e ciency from FFT-velocity input data, before the nal results are band ltered. Therefore the radiation ef ciency of single modes can be determined correctly, whereas other methods may be restricted by the low diusity and modal overlap of the sound eld in the room and the panel. Another advantage of the DCM is the reduce in uence of background noise on the estimation of radiation e ciency. Therefore, no specially conditioned chamber is required as no acoustical measurements are necessary. Comparison of DCM and Leppington theories
Figure 5 shows the radiation index for all the investigated methods. The diagram shows a good match between DCM and the predictions according to Leppington [3] for an all-side clamped plate. For this boundary condition the radiation e ciency is doubled when compared to a simply supported plate. Therefore, the radiation index increased by 3 dB.
0
−10
−20 Airborne DCM Shaker DCM Leppington rigid baffle Leppington clamped +3dB Prediction Air prEN 12354−1
−30
−40
4000
Figure 4. Comparison of the radiation index for airborne and shaker excitation using intensity method and DCM.
5.
Radiation index 10log σ [dB]
Radiation index 10log σ [dB]
10
63
125
250 500 1000 Frequency [Hz]
2000
4000
Figure 5. Comparison of the radiation index using airborne and shaker excitation for DCM and theories according to Leppington as well as according to the fourth draft of EN 12354-1.
The prediction of the radiation e ciency for airborne excitation according to the fourth draft EN 12354-1 requires the total loss factor η of the panel (see equation 3), where σf is the radiation e ciency by forced excitation and σ the radiation e ciency for resonant radiation, i.e. for shaker excitation. fc represents the coincidence frequency. The structural reverberation times were determined according to EN ISO 10848-1 [4] and by using the Power Injection Method [5]. This method is based on an energy analysis of the ratio between injected power and lost power.
σair =
6.
σf + rσ ; 1+r
r=
πfc σ 4f η
(3)
Correction value for the Sound Reduction Index
Figure 6 presents the correction values from measurements compared to an empirical estimate according to the fourth draft of EN 12354-1. It is obvious that the corrections, calculated from measurement data, are lower than the suggested 8 dB of fourth draft of EN 12354-1. The suggested 8 dB correction leads to an overestimation of the sound reduction index and to a too optimistic assessment of the acoustic performance of the investigated building element and the corresponding sound insulation of the anking paths which are predicted with this input data.
FORUM ACUSTICUM 2014 7-12 September, Krakow
Tröbs, Schoenwald, Zemp: Radiation efficiency
30 DCM Intensitymethod 25x31 Prediction prEN 12354−1 Correction prEN 12354−1, 8dB below fc
10log(σa / σs) [dB]
25 20 15 10 5 0 −5 −10
63
125
250 500 1000 Frequency [Hz]
2000
4000
Figure 6. Comparison of the correction calculated from measured data for the sound reduction index below fc and the empirical estimation of 8 dB according to fourth draft EN 12354-1. 7.
Conclusions
The results of the sound pressure and intensity method for the determination of the radiation e ciency of the investigated particle board correspond well. The DCM for determining the radiation e ciency for airborne and structure borne excitation based on the surface vibration velocity of the panel also shows a good agreement with the measurement methods mentioned previously. Furthermore, a good comparability of the DCM and the analytical prediction according to Leppington could be shown. The correction values for the Sound Reduction Index R∗ , calculated according to the fourth draft of EN 12354-1, was smaller than the suggested empirical estimate of 8 dB. This lead to an overestimation of the resonant Sound Reduction Index. This resulted in an underestimation of the anking path contribution of such a panel on the building side.
Acknowledgement The measurements of sound intensity and sound pressure were carried out by using a multi-channel analyser MKII of Müller-BBM VibroAkustik Systeme and the corresponding Software PAK 5.8. The authors would like to acknowledge the support by the supplier.
References [1] CEN/TC 126/WG 02: Fourth draft of EN 12354-1. Doc. Number: N 0330, CEN-AFNor [2] N. Hashimoto: Measurement of sound radiation ef ciency by the discrete calculation method. Applied Acoustics 62 (2001) 429-446.
[3] F.G. Leppington at al: Resonant and non-resonant acoustic properties of elastic panels. I. The radiation problem. Proc. R. Soc. Lond. A 406 (1986) 139-171. [4] EN ISO 10848-1: Acoustics - Laboratory measurement of the anking transmission of airborne and impact sound between adjoining rooms - Part 1: Frame document. CEN (2006) [5] D.A. Bies, S. Hamid: In situ determination of loss and coupling loss factors by the power injection method. Journal of Sound and Vibration 70(2) (1980) 187-204.
