High Speed Trains external noise : a review of measurements and source models for the TGV case up to 360km/h P.-E Gautier(1), F; Poisson (1), F; Letourneaux (2) (1)SNCF, Paris, France; (2)SNCF, Vitry, France Abstract External noise tests at high speed, carried out for different trains in different countries (France, Germany, Italy, Japan, Korea Spain…) provided pass-by levels which, when combined with array measurements, enabled a better understanding of noise sources on high speed trains. In the TGV case, in parallel with tests at operating commercial speeds, various acoustical measurements were carried out, either within dedicated programs (German French cooperation programme “Deufrako” projects labeled “K “and “K2”, 350 kph measurement campaign a few years ago on TGV Duplex-type train sets, and more recently, 360 kph test on TGV Réseau type train sets), or at the occasions of very high speed campaigns. A review of the obtained results is made for the different train set types, with comparison with results for similar measurements on other train systems when available. The results show a significant importance of rolling noise up to speeds up to 380 km/h. A review of acoustic antenna results and their interpretation in terms of source identification is also carried out, which shows that source identification is still an open point to some extent. Identified sources are used as input for acoustical models for trains pass-by prediction. Two types of models (models for impact studies in environment, or models for the prediction of global noise reduction potential obtained with reductions on individual sources) are shortly presented. In the latter case, emphasis is put on the latest generation of models, recently developed for the SILENCE project and applied to TGV. A special case will concern the identification of options for reducing rolling noise where predictions are presented and analyzed with respect to line tests of such devices. Introduction Noise from high speed trains is a sensitive issue, as high speed train lines are built either in densely populated areas, or conversely in zones where the pre existing noise was very low. As soon as the 1990’s, high interest was then given to measuring of pass-by noise from high speed trains, and understanding the different sources contributions in order to help reducing global external noise. More recently, train pass-by noise was considered as an “interoperability parameter” and thence limited in the high speed and conventional rail Technical Specification for Interoperability (TSI) [1] concerning the subsystem Rolling stock.. The goal was to limit train pass- by noise for all trains in Europe at creation level to lower the cost of implementing noise barriers. As a consequence, for the past ten years, a number of pass-by noise measurement campaigns at high speed were carried out for different trains in different countries (France, Germany, Italy, Japan, Spain…) : “Deufrako cooperation K” [2] and “K2” [3], “NOEMIE”, “V350” and “V360” SNCF measurement campaigns. From the data gathered in each configuration, regression laws have been extracted and source characterization attempted. A review of the pass-by noise of high-speed trains in Europe is first presented. Results of a measurement campaign carried out with a TGV with high roughness wheels are then discussed, which puts an emphasis on the potentially higher than believed so far contribution of rolling noise Then, various characterizations of the noise sources through antenna measurement campaigns are presented. The source models are also addressed through the limitations of the existing array processing methods. Implementation of these sources models into prediction codes are also presented, with an emphasis of the last generation of models. Results in terms of sources reduction potentials are finally discussed, with a glimpse over experimental results obtained with rolling noise reduction experimental tests Equivalent sound pressure levels Review of the pass-by noise levels for the European high-speed trains A summary of results obtained during measurement campaigns carried out the past ten years is presented in table 1. The spread of results for different series of TGV’s is around 1.5 dB(A), when analyzed on the same track. Moreover, various European high speed trains (TGV, ICE, ETR…) show very close values when measured at the same site: identical values for 300 km/h and above, and up to 2 dB(A) at 250 km/h. It then can be asserted that, the overall dispersion of the pass-by noise of different types of high speed trains is narrow. Pass-by noise values Test site Train speed (kph) measured at 25m in dB(A) TSI+ tracks 250 300 320 350 except Belgium TGV Thalys Belgium 88.5 92 93 France 85.5 90 92 Germany 85.5 TGV Duplex France 87 91 92 95 TGV Atlantique France 90.5 94.7 TGV Réseau France 89 91.5 94 (330kph) 97 ICE3 France 87.5 90 91.5 Germany 85.5 89 92 AVE Spain 86 90 91 ETR480 Italy 90.5 ETR500 Italy 88 90.5 TSI limits project - 91 93 - Table 1: pass-by noise values of high speed trains measured at 25m Influence of the state of the wheels During the “V360” measurement campaign, pass-by noise of a TGV-Réseau was recorded at different stages of the measurement campaign in the same location. It appeared that following track works at a few defined dates during the test campaign, some ballast dust might have been run over by the wheels, the roughness of which significantly increased on the following days. The measured noise values were then increased by 1.5 to 2.5 dB(A) after each track work episode, and the influence of the increase of the measured pass-by level can be noticed throughout the whole investigated speed range : 250 to 360 kph (see table 2). Pass-by noise values Train speed (kph) at 25m (dB(A)) 250 300 330 TGV Réseau 89 91.5 94 TGV Réseau 93 95.5 97 “corrupted wheel state” Table 2: Influence of the wheel surface quality on pass-by noise (TGV Réseau) It can also be inferred from the latter observation that the transition speed between rolling noise and aerodynamic noise for the TGV Réseau is higher than often previously claimed, when it had been said to lie around or under 300kph. Following that hypothesis, the influence of rolling noise created by the wheel roughness up to 360 kph would not have been so significant: for the TGV Réseau case, the upper values of measured pass-by noise (for corrupted wheel state) went up to 97 dB(A), whereas values measured on the TGV Duplex in a former campaign were not higher than 94 dB(A) at 350 kph. These observations also still confirm in this case the applicability up to, speeds around 350 km/h of the “30 logV” regression rule (1), which was shown in [3] and which is characteristic of rolling noise dominated behaviour. TGV pass-by noise versus speed In the last SNCF measurement campaign, a TGV POS, composed of Duplex power cars and 8 single floor coaches was measured for speeds ranging from 100 kph to 380 kph. The 25m measured LAeq,tp are presented figure 1. Figure 1: LAeq,tp for TGV POS from 200 kph to 380 pkh A linear regression was performed between the increase of LAeq,tp and the logarithm of the train speed. The general equation is: LAeq,tp(V) - LAeq,tp(V0) = K log(V/V0) (1) with V the train speed, V0 the reference train speed. Between 200 kph and 380 kph, the regression coefficient is K=30.4 with a correlation coefficient R2 equal to 0.93. This regression coefficient has already been checked within the frame of the Deufrako projects [2] [3] and reconsidered in [4]. It is nearby the value of 30 commonly used in the prediction formula for rolling noise which is widely used to extrapolate noise emission of classical trains. It confirms that the contribution of the rolling noise, which is the main noise source for conventional speeds, remains high up to speeds around 360 km/h for a TGV train set which complies with the TSI limits at 300 km/h. Then, a significant reduction of the pass-by noise of a TGV train set running at commercial speed (~320 kph) can only be reached by acting both on the aerodynamic sources and the rolling noise sources, as presented in [4]. Source characterization Acoustic array measurements A number of acoustic array measurements have been performed on different trains [2],[3], [4],[7],[8]…They usually enable, from the noise map recovered on the measurement vertical plane, a qualitative identification of the noise sources on the train. Improvement of the acquisition and processing equipment power of in the last years allows to use and process easily much more microphones. SNCF is presently using a star shape antenna of 72 microphones located near the track. The beam forming technique is still used after the removing of the Doppler effect to characterize the sources. Noise maps can be presented for each third octave band and narrow band spectrum can be extracted for a given position. Other array shapes have also been used [8] as well as “acoustic mirrors” in Japan [7].. An example of measurement on TGV Duplex at 350 km/h is given below in Figure 2. Figure 2 - example of a TGV Duplex noise maps obtained with an antenna of microphones Above: third octave band 500 Hz / Below: third octave band 4000 Hz The low frequency bands show the contribution of aerodynamic noise around the driver’s window recess and around the first bogie region (top), whereas rolling noise appears on the lower part of the figure.