ANNOYANCE AND PLEASANTNESS ASSESSMENT FOR RAILWAYS NOISE BARRIERS BY MEANS OF IMMERSIVE VIRTUAL REALITY

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1 ANNOYANCE AND PLEASANTNESS ASSESSMENT FOR RAILWAYS NOISE BARRIERS BY MEANS OF IMMERSIVE VIRTUAL REALITY F. Aletta, M. Masullo Scientific Head L. Maffei Interdepartmental Center Built Environment Control, RiAS Second University of Naples, Aversa, Italy Abstract The paper presents the preliminary stage of a research about the effect of a multisensory perception on the perceived annoyance and pleasantness of railways noise barriers. The aim of this study is to investigate if a different design of the barrier can influence the noise perception of the population by making the sound source (train pass-by) visible or not, while the same audio signals are held. At the same time, visual and aesthetic aspects are investigated. Immersive Virtual Reality (IVR) technologies and a paired comparison structure were used to prepare the test. As the audio signals were held constant during the experiments, it seemed likely to ascribe the noticed differences of the assessment to the visual stimulus. 1. INTRODUCTION The EU noise policy and its recent Directives on the Assessment and Management of Environmental Noise [1] are driving to an increasing use of barriers for noise railways mitigation, in order to protect the health and improve the quality of life of the people living nearby. As a consequence, the noise barrier industry should be to attain every time more effective products. Moreover the subjective perception of noise annoyance and pleasantness may be influenced by different multisensory factors beyond the objective noise reduction [2] [3]. One of the main aspects to consider is the visual stimulus [4] [5] [6]; the finding that a transparent barrier (visible source) would be perceived as more inefficient is often contradicted in many papers [7] [8] showing that when the barrier is opaque (non-visible source) the listener expects its loudness to be significantly increased. By using new immersive virtual reality techniques, this preliminary research aims to investigate how auditory and visual interaction influence the subjects perception and preference. The same audio signal for the attenuated scenarios was kept while the visual stimulus was changed during the tests. 2. METHODOLOGY OF THE EXPERIMENTAL INVESTIGATION Two passing of trains were recorded by means of a portable device M-Audio Microtrack 24/96 and binaural headphones Sennheiser HDC 451 close to the train station of S.Marcellino-Frignano in Italy. The measurements were accomplished by an operator who looked perpendicularly at the railway line at an height of 1,65 m

2 above the ground for two train pass-by: the first one passed on track 3 (Fig.1) at distance d = 22,5 m from the receiving point with the train coming from left to right respect to the operator, while the second one passed on track 4 (d = 33,0 m) from right to left. Figure 1 Scheme of the scenario (the barrier doesn t really exist) Afterwards these two signals were filtered by mean of one octave band filters which considered the effect of the noise barrier intersection for each relative position source-receiver. These filters were implemented on an audio editing software and applied to the original sound tracks. Then the four signals (original and attenuated) were calibrated in the anechoic chamber by an artificial dummy head, in order to reproduce the same auditory conditions of the in situ situation. At the same time, a 3D model of the real scenario was created (Fig.3). Finally both the auditory and visual component of the scenarios were uploaded to a virtual reality software. For the test session a lab room was prepared with a workstation and the IVR system was implemented by a 3d-helmet visor and a tracker for the head s movement (Fig.2). C B A Figure 2 The immersive virtual reality system (workstation-a, 3d helmet visor-b and tracker-c) Six scenarios (1: track 3 no barrier, 2: track 3 opaque barrier, 3: track 3 transparent barrier, for the first train s passage; 4: track 4 no barrier, 5: track 4 opaque barrier, 6: track 4 transparent barrier, for the second s train passage ) were presented to a sample of 10 subjects (5 Male, 5 Female) with normal hearing and vision capacity. For the opaque barrier and the transparent barrier scenarios the same attenuated audio signals (pass-by on track 3, first train and pass-by on track 4, second train) were used, while for the no barrier scenarios (pass-by on track 3 and 4) the original ones were used.

3 Figure 3 The three visual conditions (no barrier, opaque barrier and transparent barrier for the first train, track 3) For the subjects assessment a paired comparison structured questionnaire was prepared [9] [10] [11]; after a training test for the IVR system the test started and the subjects were asked to answer the three following questions: 1. Which of the two scenarios you have just seen is the loudest? 2. Which of the two scenarios you have just seen is the more visually pleasant? 3. Which of the two scenarios you have just seen do you prefer overall? The subjects had to express their difference/preference on a 9-point scale [11]: Equal Weak diff. Moderate diff. Moderate + diff. Strong diff. Strong + diff. Very strong diff. Very strong + diff Max diff. Each scenario was compared with the others, so the subjects had to express an assessment about 15 pairs of scenarios according to the following scheme (Fig.4). PAIRED ANALYSIS track 3 track 4 track 3 O track 4 O track 3 T track 4 T track 3 : : : : : : track 4 1 : : : : : track 3 O 6 2 : : : : track 4 O : : : track 3 T : : track 4 T : Figure 4 The scheme used for the paired comparison The 15 pairs were also presented according to a balanced sequence. 3. RESULTS ANALYSIS After the comparison among all the scenarios, all the scores from the 9-point scales were calculated for the three questions. The maximum score resulting from the subjects assessment was assumed as top-reference score and all the other scores were normalized to it (%) [10] [11]. As regards the perceived loudness (1 st question, Fig.5a), a clear decaying trend is found for both the tracks 3 and 4, even if the audio signals are the same for the opaque and transparent barriers, while concerning the visual pleasantness (2 nd question, Fig.5b) the opaque barrier scenarios seem to be the less effective.

4 a Figure 5 Bar graphs for: a) Perceived noisiness and b) Visual pleasantness b When considering the general preference of the sample (3 rd question, Fig.7) the topscored scenarios are the transparent barriers ones and it is very significant to see that no real preference is noticed between the others: the opaque barrier option is as preferred as the no barrier option (indifference situation). Figure 6 Bar graph for General preference Below some graphs (Fig. 7) are presented as respect to the general attendance of the answers given by the sample: the percentage indicates how many times a scenario preponderated over the other in a comparison averaging the answers by the whole sample for the 15 comparisons. a b Figure 7 Answers attendance: a) perceived noisiness, b) visual pleasantness, c) general preference c Finally, a more detailed analysis was carried out in order to point out some aspects concerning the rating for the single comparisons (Fig. 8, 9, 10). The 3 vs. 3 O, 4 vs. 4 O, 3 vs. 3 T and 4 vs. 4 T comparisons for the perceived loudness (green boxes in Fig.8) were supposed to be check comparisons : in fact, 100% of the

5 sample said that the no barrier scenarios were louder than the opaque barrier and transparent barrier scenarios. In the same graph, in the 3 O vs. 3 T and the 4 O vs. 4 T comparisons (yellow circles) it is possible to see that the opaque barrier scenarios are perceived as louder than the transparent barrier scenarios. Figure 8 Direct comparisons for perceived noisiness (first scenario blue, second scenario red) From the analysis of the direct comparisons for the visual pleasantness (Fig.9) it is resulting that the transparent barrier scenarios are strongly preferred over the opaque barrier scenarios (yellow circles). Figure 9 Direct comparisons for visual pleasantness (first scenario blue, second scenario red) In the analysis of the direct comparisons for the general preference (Fig.10) it is possible to see that the no barrier scenario for track 3 is even preferred over the opaque barrier scenario, while for the 4 vs. 4 O comparison (track 4) there is an indifference assessment (red box on the left). Finally, in both the 3 O vs. 3 T and 4 O vs. 4 T comparisons (yellow circles) the transparent barrier scenarios are strongly preferred over the opaque barrier scenarios.

6 Figure 10 Direct comparisons for general preference (first scenario blue, second scenario red) The direct comparisons analysis is supposed to confirm the same trends of the general score, even if the ratings are different and sometimes less significant, probably due to the small size of the sample. 4. CONCLUSIONS This preliminary study shows that thanks to the IVR techniques a more accurate understanding of the impact of noise barriers can be performed. Regardless of the fact that the attenuated audio signals were the same for both the opaque and transparent options, the transparent noise barriers were better assessed as respect to the perceived loudness, the visual pleasantness and the general preference. Further investigation are needed to analyze the reaction of the sample if keeping the same visual component while changing the auditory stimulus. 5. REFERENCES [1] Directive 2002/49/EC of the European Parliament and of the Council relating to the Assessment and Management of Environmental Noise, 25 th June 2002 [2] Yamaguchi M., Horita H., Kuboki M., Toi T., Sound quality evaluation by physiological information under complex stimuli, Proceedings of Internoise 2009, Canada, August 2009 [3] Pheasant R. J., Horoshenkov K. V., Watts G. R., The role of audio-visual interaction on the perception of tranquility, Proceedings of Euronoise 2009, Scotland, United Kingdom, October 2009 [4] D. Menzel, E. Faccinelli, H. Fastl, Are absolute thresholds and loudness judgements influenced by different colours?, Proceedings of Euronoise 2008, France, 2008 [5] Joynt J.L.R., Kang J., The influence of preconceptions on perceived sound reduction by environmental noise barriers, Science of the Total Environment VOL. 408 (20), 2010, pp [6] Di Gabriele M., Maffei L., Başturk S., Masullo M., Esperienze di Realtà Virtuale Immersiva nella valutazione di impatto acustico, Atti del 38 Convegno nazionale Associazione Italiana di Acustica, Rimini, 8-10 giugno 2011 [7] Aylor D.E., Marks L.E., Perception of noise transmitted through barriers Journal of Acoustical Society of America VOL. 59, 1976, pp [8] Watts G.R., Chinn L., Godfrey N., The effect of vegetation in the perception of traffic noise, Applied Acoustics VOL. 56, 1999, pp [9] David H.A., The Method of Paired Comparisons Oxford University Press, New York, 1988 [10] Uni En Iso 5495, Analisi sensoriale - Metodologia - Metodo di comparazione a coppie, 2008 [11] Saaty T.L., Relative Measurement and its Generalization in Decision Making, Why Pairwise Comparisons are central in Mathematics for the Measurement of Intangible Factors - The Analytic Hierarchy/Network Process, Rev. R. Acad. Cien. Serie A. Mat. VOL. 102 (2), 2008, pp