ACOUSTIC INTRINSIC PERFORMANCES OF NOISE BARRIERS: ACCURACY OF IN SITU MEASUREMENT TECHNIQUES

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1 Twelfth International Congress on Sound and Vibration ACOUSTIC INTRINSIC PERFORMANCES OF NOISE BARRIERS: ACCURACY OF IN SITU MEASUREMENT TECHNIQUES Francesco Asdrubali, Giulio Pispola and Francesco D Alessandro Department of Industrial Engineering, University of Perugia, Via G. Duranti, Perugia, Italy. fasdruba@unipg.it Abstract Laboratory and in situ methods have been used for measuring intrinsic performances of noise barriers, as prescribed by the European standard series EN The use of in situ techniques is promising, but their accuracy has to be duly verified, even in comparison with well-known standardized procedures. Sound insulation and reflection properties have been measured through a MLS-based technique in an outdoor test field. The paper analyzes the procedures that mainly influence the accuracy: correction for wave spreading and time windowing. Repeatability of the in situ method for sound insulation is satisfying and its results look consistent with simple prediction models. Nevertheless, in situ data can be overestimated at low frequencies, due to the overlapping of the transmitted and diffracted components. The method has to be carefully employed when the sample shows apertures as slits or holes, unless a different kind of sound propagation is assumed at the receiving side. A good agreement was found between in situ and laboratory sound insulation data, while in situ and laboratory absorption properties show poorer correlation. INTRODUCTION The certification of acoustic intrinsic properties (sound absorption, insulation and diffraction) of traffic noise barriers is nowadays a very interesting subject, both from a technical and an economical point of view. Thanks to the recent promulgation of new national noise legislation, huge investments are expected in the next years for noise abatement along the Italian road network. Knowledge of the intrinsic properties of anti-noise devices (as noise barriers and diffracting caps) in actual use and in simulated test fields can be very useful to 1

2 the manufacturer, to the designer and also to the purchaser in order to quantify the performances of a system or to make comparisons between different products. The intrinsic properties, indeed, provide a more reliable characterization of noise barriers rather than the extrinsic ones (i.e. Insertion Loss), that strongly depend on several different factors. Two kinds of measurement methods are commonly employed to evaluate the intrinsic properties of noise barriers: laboratory methods, using a diffuse sound field in a reverberation room, and in situ methods. The latter better simulate the real operating conditions of traffic noise barriers, which are usually exposed to perpendicular or slightly oblique rather than diffuse sound fields. The paper focuses on these latter methodologies. IN SITU ACOUSTIC INTRINSIC PROPERTIES In situ methods use pseudo-random deterministic signals, called Maximum Length Sequences (MLS), to reconstruct the impulse response of a system with high background noise immunity [1]. Impulse responses of the measurement chain + barrier system in different points and the response of the measurement chain system are subsequently recorded. The first are called overall responses, while the latter free-field one, as reflecting surfaces close to the receiver (apart from the ground) are avoided during their acquisition. The overall impulse responses provide information on reflected or transmitted responses, while the free-field ones allow the extraction of the incident responses. Intrinsic properties are finally calculated as ratios of power spectra. European Technical Specification CEN/TS 1793 part 5 [2] specifies the procedures to measure two intrinsic quantities: Reflection Index RI [3] and Sound Insulation Index SI and the corresponding single-number ratings DL RI and DL SI. In the framework of an agreement with the Italian Ministry for Environment, four samples of noise barriers made of novel sustainable materials have been tested at the University of Perugia in a dedicated outdoor test field (Table 1). A satisfying repeatability has been observed for SI measurements (standard deviation lower than 1 db), while it appears poorer for RI (deviation from the mean even approaching 5%), in particular for deeply profiled and highly reflective surfaces. Table 1 Characteristics and single-number ratings of the tested noise barriers. Sample code Profile Materials DL RI [db] DL SI [db] METF flat Polyester fibres-lined steel panels 6 33 METP parabolic Recycled foam-lined steel panels 3 27 PLAST flat Recycled plastic panels with glass 5 36 wool absorbing layer CONC flat Concrete panels with rock wool layer and natural fibres

3 EFFECTS OF WAVE SPREADING CORRECTION The procedures that mainly influence the results provided by the in situ methods are the correction for sound wave spreading and the time windowing. A correction for wave spreading has to be performed, as the sound intensity decreases with the distance d SM of the receiver from the source. According to the definitions given in CEN/TS [2], sound wave propagation from the source is assumed to be spherical, which means to assume the sound source as a point one. Two different wave spreading correction methods are adopted: for RI in the time domain through multiplication of the impulse responses by the time, while for SI in the frequency domain by means of a multiplication of the power spectra by the squared path lengths ( geometrical correction). In order to analyze the efficacy of the spherical propagation assumption in the time domain, free-field impulse responses have been recorded at several distances between source and microphone (sampling frequency: 512 Hz, MLS order: 16, averages: 8). In Figure 1 ( the absolute amplitudes of the first peaks are correlated to their corresponding arrival times: t sample stands here for the time instant calculated multiplying the first peak sample number by the sampling frequency. This recorded time does not coincide with the physical one estimated from the sound speed and the measured distance, as evident from Figure 1 (. First peak absolute amplitude [Pa] h =.8*t corr h =.1*t sample recorded time ( corrected time ( regression ( regression ( Time [s] First peak arrival equivalent distance dpeak [mm] 18 d peak =.993 d SM Measured source-microphone distance d SM [mm] Figure 1 Effect of the source-microphone distance on the free-field impulse responses: first peak amplitude vs. arrival time, first peak equivalent distance vs. measured distance. Indeed, the measurement chain, as noticed by De Geetere [4], introduces a delay corresponding to a characteristic distance, that can be evaluated by correlating the measured distance d SM and the one equivalent (d peak ) to the first peak arrival time. Such distance is called source characteristic distance (SCD) [4] and it can be estimated (R 2 = ) around.175 m for the chosen measurement chain (see intercept in Figure 1 (). The recorded time t sample can then be corrected by means of the linear function: 3

4 t corr SCD tsample = c [s] (1) p where p indicates the regression slope and c the sound speed. This way the amplitude of the first peak becomes inversely proportional to the corrected time (R 2 =.9977), as shown in Figure 1 (, consistently with the spherical assumption. This does not occur employing the recorded time. As far as the frequency domain, solid lines in Figure 2 ( show the normalised mean third-octave band spectra of the incident components extracted at several distances d SM. It can be noticed that the wave spreading correction is needed to quantify the incident sound energy: indeed, the maximum relative error from the mean (dashed-and-dotted lines) is too high (above 6%) without any correction. From this point of view, the geometrical correction prescribed for SI seems to match the one prescribed for RI (signal multiplication by the time). On the other side, noticeable discrepancy appears on the mean spectra at low frequencies: the effect of such distortions on the final results should be further investigated. In any case the corrected time (Eq. (1)) has to be used in order to limit the error to values comparable to the geometrical correction in the whole range. Nonetheless, the fluctuations from the mean look higher in the low-frequency range as the measurement chain frequency response is poorer in this range and the hypothesis of spherical propagation may not be suitable when the distance d SM becomes comparable to the wavelength. geometrical spectrum correction ( recorded time signal multiplication (c) maximum relative error ( maximum relative error (c) 1.E+ corrected time signal multiplication ( without correction (d) maximum relative error ( maximum relative error (d) 1% 1.E+ geometrical spectrum correction ( corrected time signal multiplication ( maximum relative error ( without correction (c) maximum relative error ( maximum relative error (c) 1% 1.E-1 Normalized mean spectrum 1.E-2 1.E-3 1.E-4 1% 1% Maximum relative error Normalized mean spectrum 1.E-1 1.E-2 1.E-3 1% Maximum relative error 1.E-5 1.E % 1.E-4 1% Figure 2 The effect of the corrections for wave spreading on the frequency responses: incident components extracted from free-field impulse responses (d SM = m); transmitted components extracted from overall impulse responses (d M =.14 1 m). In Figure 2 ( the dependence of the transmitted power spectra on the barrierreceiver distance d M has been investigated. The sound source was placed at 1 m from one of the tested barriers (sample CONC); the microphone, aligned with the source, 4

5 was progressively placed at the receiving side at various distances d M. Transmitted components were isolated from the overall responses (sampling frequency: 512 Hz, MLS order: 16, averages: 32): unlike the incident components, a peak translation and relative amplitude attenuation were not apparent in this case. This means that the hypothesis of spherical propagation could not be totally correct at the receiving side. The analysis of the deviations of the spectra from the mean (dashed-and-dotted lines in Figure 2 () shows that is not straightforward to find the best correction method for the transmitted components. Indeed, the transmission paths do not necessarily coincide with the direct ones and the transmitted sound field can be regarded as a superposition of the sound waves generated by each portion of the barrier. Then a plane wave hypothesis (i.e., no wave spreading correction) may not be worse than the spherical one. This is likely to occur in particular for high-insulating noise barriers, as the one tested. EFFECTS OF TIME WINDOWING A time windowing procedure is essential to isolate the incident, reflected and the transmitted components from parasitic reflected and diffracted ones. A so-called Adrienne temporal window has been used for all the calculations, according to [2]. As far as sound insulation measurements, the influence of time windowing appears to be even stronger than that of the wave spreading correction. Indeed the diffracted component usually arrives before the complete extinction of the transmitted one: then, the transmitted energy may be not entirely included in the calculation implying an overestimation of sound insulation performances. Different measurement campaigns have been carried out on the sample CONC with and without rubber gaskets between the acoustic panels and between the panels and the posts; in the latter case, the presence of slits (see Figure 3 () was considerably worsening the insulating performance of the sample. The phenomenon ( sound leakage ) has allowed for examining the effect of the time windowing procedure. An approximate procedure has been developed to increase the time window length. The diffracted components were eliminated from the overall impulse responses recorded without the gaskets by subtraction of the responses recorded in the same positions with the gaskets; indeed the diffractions could be regarded as identical while the transmitted energy was practically negligible with the gaskets. In this way a wider time window could be used (51.3 ms instead of 8.2 ms): SI data determined employing the narrower window looks overestimated at low-mid frequencies (see Figure 3 (). Moreover, an increase of the time window length improves the lowfrequency limit of the method [2]. In order to take into account the presence of slits, the spherical propagation assumption could not be appropriate, as noticed by Watts et al. [1]. Then, an alternative geometrical correction factor has been developed assuming a spherical propagation at the source side and a semi-cylindrical one at the receiving side: 5

6 SI ( f ) 2 ( ) ( ) π St f dm d S = 1log 1 [db] 2 Si( f ) δ ( dm + tb + ds) (2) where S t and S i are respectively the transmitted and incident power spectra, d S is the source-barrier distance, t b is the barrier thickness and δ is the slit width (5 mm, in this case). Data calculated with the wider time window and with the modified correction factor (dotted line in Figure 3 () is consistent with that determined by other authors (e.g. [5]) for the transmission loss of slits in a wall, i.e. fairly low values of sound insulation with peaks at determined frequencies due to interference phenomena. 6 5 spherical propagation - T w =51.3ms spherical propagation - T w =8.2ms semicylindrical propagation at the receiving side - T w =51.3ms 4 SI [db] Figure 3 The effect of sound leakage on sound insulation measurements: detail of a slit between concrete panels of sample CONC; experimental results with different methods. SI data measured for sample CONC (sealing all the slits with gaskets) has been then compared with a prediction model for transmission loss (TL) of homogeneous panels [6]. Figure 4 ( shows the predicted TL calculated for the minimum (.11 m) and maximum (.21 m) thicknesses of the sample in comparison with experimental data obtained in front of different portions of the noise barrier. No geometrical spreading correction was here employed, as it would not imply an improvement of the transmitted energy estimation (see Figure 2 (). The physical properties used as input for the model are those of lightweight concrete (density: 14 kg/m 3 ; Young s modulus: 13 GPa; Poisson s coefficient:.2). Experimental data obtained through the equipment positioning prescribed in [2] (d S = 1 m and d M =.25 m) in front of the thicker portion of the barrier seems to be overestimated at low-mid frequencies when compared with the results of the prediction model (upper graphs in Figure 4 (). It should be noticed that the model low-frequency limit depends on the first resonance frequency of the panel and therefore on its thickness [6]. It was possible to achieve a good correspondence between experimental results and predicted data positioning the microphone and the source (d S =.13 m and d M =.16 m) in order to increase the difference between the arrival times of transmitted and diffracted components (see Figure 4 (). 6

7 SI, TL [db] experiment: d SM =1.46m - T w =8.7ms (without correcton) experiment: d SM =.5m - T w =11.6ms (without correcton) prediction: t B =.11 m prediction: t B =.21 m impulse response [Pa] x d S =1 m, d M =.25 m ( d S =.13 m, d M =.16 m ( scaled time window ( scaled time window ( t x 1-3 sample [s] Figure 4 The effect of equipment positioning on sound insulation measurements: comparison between experiments and predictions; overall impulse responses. COMPARISON BETWEEN LABORATORY AND IN SITU METHODS The sound reduction index R and the absorption coefficient α S have been evaluated for the tested samples employing laboratory methods in reverberation rooms [2]. αs, 1-RI [-] SI, R [db] SI (geometrical spectrum correction) SI (corrected time signal multiplication) SI (without correction) R.3 sample METF (alph sample METF (1-RI) sample PLAST (alph sample PLAST (1-RI) Figure 5 Comparison between laboratory and in-situ measurement techniques: sound absorption properties; sound insulation properties. Figure 5 ( shows the comparisons among 1-RI and α S for samples METF and PLAST. Considering the anomalous peaks characterizing the behaviour of RI, it is rather difficult to properly compare the two quantities: in spite of this, they show a fairly good correspondence. It is necessary to state that the results provided by the two methodologies are not fully comparable [2, 7]. It has been observed [3] that, for deeply profiled and highly reflective surfaces, the in situ reflection measurements do

8 not give fully reliable results. Figure 5 ( shows the comparison between sound insulation index SI and sound reduction index R for the metallic barrier METF: a good correlation in the mass-law frequency range can here be noticed. SI has been calculated with different correction methods: while the effect of the geometrical correction appears substantially negligible, the time multiplication seems to reduce the results; the more the insulation gets higher, the more such decreasing effect is amplified, e.g. in case of thick concrete noise barriers. CONCLUSIONS Accurate measurement procedures are needed to evaluate intrinsic sound reflection and insulation performances of anti-noise devices as noise barriers. To this extent, insitu techniques look promising. Four samples have been tested in reverberation rooms (R and α S ) and in a dedicated outdoor test field (SI and RI), according to the European standard series EN Particular attention has been devoted to the basic hypothesis and procedures of the in-situ methods, e.g. wave spreading correction and time windowing. A spherical propagation assumption appears accurate for the incident responses, while poorer for the transmitted ones, in particular through high insulating samples or in presence of sound leakage through openings. Equipment positioning and time windowing have to be duly optimized, not just to properly isolate the needed responses from the parasitic ones, but even to avoid as much as possible their superposition. Good matching has been observed between SI data and a transmission loss prediction model, confirming the accuracy of the optimized method. Finally, comparisons among in-situ and corresponding laboratory quantities lead to similar results for sound insulation, but evidence some difficulties for sound reflection. REFERENCES [1]. G. R. Watts and P. A. Morgan, The use of MLS based methods for characterising the effectiveness of noise barriers and absorptive road surfaces, Proc. Inter-Noise 23, Korea, (23). [2]. European Committee for Standardisation, EN 1793, Road traffic noise reducing devices - Test method for determining the acoustic performance - Intrinsic characteristics ( ). [3]. F. Asdrubali, G. Pispola and G. Baldinelli, Optimization of in situ noise barrier intrinsic characteristics measurements, Proc. 11th International Congress on Sound and Vibration, St. Petersburg, Russia, (24). [4]. L. De Geetere, Analysis and improvement of the experimental techniques to assess the acoustical reflection properties of boundary surfaces, PhD thesis, Katholieke Universiteit Leuven, Leuven, Belgium (24). [5]. K.-T. Chen, Study of acoustic transmission through apertures in a wall, Applied Acoustics, 46, (1995). [6]. D. A. Bies and C. H. Hansen, Engineering Noise Control Theory and Practice. (Spon Press, 23). [7]. M. Garai and P. Guidorzi, Experimental verification of the European methodology for testing noise barriers in situ: sound reflection - airborne sound insulation, Proc. Inter-Noise 2, Nice, France, (2). 8