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.

Another example is given in Figure 4 where the noise emitted by the electrical equipment of the head power car is highlighted around 1580 Hz.

Figure 4: noise map of a TGV POS running at 318 kph (1580 Hz) The analysis of results of array measurements on a frequency band at different speeds can give useful indications: the contribution of a source not depending on speed cant then be enhanced. Conversely, the emergence with speed of sources like aerodynamic noise can then be easily evidenced. The following examples show how at 320 km/h the contribution of aerodynamic noise can be identified for the 200 Hz and 400 Hz third octave bands (Figure 5), around the bogies and in the 250 and 315 Hz bands for the pantograph on the front motor-coach (Figure 6). Noise from equipments (fans) can be seen in 800 Hz to 1250 Hz bands. (Figure 7) Rolling noise clearly appears on the wheels regions at 2000 Hz, and 2500 Hz, even with some leakage to the latter band (Figure 8).

200 Hz Third octave band

400 Hz

Figure 5: noise map of a TGV POS running at 320 kph (200 Hz and 400 Hz third octave bands )

250 Hz

315 Hz

Figure 6: noise map of a TGV POS running at 320 kph (250 Hz and 315 Hz third octave bands )

800 Hz

1000 Hz

1250 Hz

Figure 7:noise map of a TGV POS running at 320 kph (800 Hz to 12500 Hz third octave bands )

2000 Hz

2500 Hz

Figure 8: noise map of a TGV POS running at 320 kph(2000Hz and 2500Hz third octave bands)

Work is now in progress to improve the characterization of moving sources using the antenna. The main drawbacks of the existing methods are: - the estimation of the sound pressure level of the source is carried out during the tracking of the source. The directivity pattern of the antenna is evolving during the tracking, with a poor selectivity on both sides and a better one in the front. Error is then introduced in the assessment of the sound pressure level of the source. - several antenna configurations the must be used to cover the whole frequency band from 100 Hz to 8 kHz to avoid aliasing, which leads to different directivity patterns according to the frequency band. Then, noise maps can not be compared from one configuration to another one.

Source power quantification

Identification of the source power, in order both to look for reduction potential and to feed models of external noise propagation was attempted as soon as array measurements were available. As an example, the acoustic array measurements carried out in DEUFRAKO K project were further analyzed by INRETS, where a first estimation of the level of the sources was carried out, by summing the acoustical energy received on a zone surrounding the assumed position for the source.

Level at 5m 100 kph 200 kph 300 kph 350 kph dB(A) Wheels, coach 82.4 89.7 97.2 98.1 Wheels, forward 82.9 89.4 102.5 100.4 power car Wheels, rear 81.7 89.6 102.7 98 power car Pantograph 82.1 91.3 103.5 104.3 Cooling fan, 78.9 87.9 101.3 104 front Cooling fan, 78.3 79.6 102.1 100.5 rear Front window / 77.9 88.5 102.2 104.9 Roof Intercar gap 81.7 87.1 92 98.1 Bogie (aero) 76.8 78.6 90.1 93.3

Table 3: Estimation of the level of the sources of a TGV A (DEUFRAKO project) [2]

Later in DEUFRAKO K2 [3] some monopole fitting to the array results was attempted. Unfortunately, in order to prove the method, a loudspeaker was put onboard the train. It appeared that the loudspeaker source was not retrieved by the method. Then, the exploitation of the method on other sources less precisely located on the train could not be reliably done. For processing the TGV Duplex results at 350 kph, a different method was developed [4], where sources contents in third octave bands could be estimated. A summary of the results is given in Figure 9.

Figure 9:Source power estimation on a TGV Duplex at different speeds, from [4]

The analysis of different estimations throughout different processing was found relatively coherent for the various sources identified on trains. The results obtained can be summarized by the following conclusions: - At 200 kph, the rolling noise is an important source but the noise radiated by the area located around the first bogie (bogie and windscreen) is of the same order, - At 300 kph, the area around the first bogie is the main source but the noise radiated by the cooling and the pantograph can not be neglected, - At 350 kph, the area located around the first bogie and the pantograph radiates much more. These data are currently used as input of a pass-by noise simulation software like MAT2S [5] and VAMPASS [6] to assess the contribution of each source to global pass-by noise. The most radiating source is not always the main source in terms of contribution to the pass-by noise due to the number of sources, their location, their spectra, which is described hereafter.

Implementation in source models

Short review of source models When looking at models for the calculation of external noise, based on physical source knowledge two types can be considered: -Mmodels for the calculation of noise in the dwellings: for this kind of model, ground absorption has to be considered and meteorological effects if distances bigger than 300-500 m are to be considered. However, only “simple” source models are sufficient to describe the sources for the modeling purposes generally 1 to 5 levels are considered depending on the type of train modeled. Examples of such models are the RMR model, which is the default model recommended by EU for the application of the Environmental Noise Directive, NMPB [9] model in use in France , which was incorporated into CADNAA in use in France, and Schall 03 or its new version in use in Germany [10]. The source description in these models is still global and generally adjusted from pass-by level measurements. -Models for the near field assessment of the pass-by noise e.g. at 7.5 m or 25m for high speed. For these models, in order to correctly represent the train pass-by, a finer description of sources is needed, which nowadays implies using data from acoustic array measurements. Examples of such models are PROHV[2], MAT2S [3], and more recently the “new generation” VAMPASS model [6] which enables sound synthesis, that is listening to the train pass-by noise, as well as the pass-by level simulation. This VAMPASS model, developed within the SILENCE project also features possible time variation of the sources characteristics, such as starting or braking cycles , variable speed… As an example, simulations of the pass-by of a TGV at 320km/h was carried out. The pass-by level is indicated in Figure 10, the levels of different sources in third octave bands are indicated on Figure 11.

Figure 10:Pass-by level of a TGV at 320 km/h simulated with the VAMPASS model

Figure 11:Contribution of different sources to the pass-by level and 1/3 octave band spectrum of a TGV at 320 km/h at 25m, simulated with VAMPASS: dark blue: total, white: rolling, red: aero bogies, light blue panto cavity (front) and rear panto and cavity: black, green : motor coaches cooling fans

Simulation of source reduction to total noise As an example of the capabilities of simulation models to predict global noise reduction obtained by reduction of one or several sources, simulation of the reduction of rolling noise by implementation of wheel noise absorbers, track noise absorbers ,or their combination was carried out. The efficiencies of absorbers, as experimentally measured in other projects was introduced in the rolling noise sources. The simulated efficiency of the implementation of different type of absorbers or their combination is reported in Table 4

TGV Réseau CASE 1 CASE 2 CASE 3 Pass- by simulated Reference Rail and wheel Rail absorbers Wheel noise level with absorbers absorbers VAMPASS V320 – 25m 93.3 91.3 92.2 92.3

Table 4: Simulated Pass-by level of a TGV at 320 km/h with rolling noise reduction

The simulated signature and third-octave spectrum of the Case 1 (wheel and track absorbers are shown on Figure 12 .

Figure 12:Contribution of different sources to the pass-by level and 1/3 octave band spectrum of a TGV at 320 km/h at 25m, simulated with VAMPASS with reduction of rolling noise with both wheels and track absorbers

The individual contributions of single source are simulated separately from the global one which may entail a 1 to 2 dB variation in some third octave bands. This explains the contribution of aerodynamic noise exceeding the global one at 100 Hz. Despite what was announced above on the importance of rolling noise, its reduction with both wheels and rail absorbers, which may vary from 5 to 11 dB in some third octave bands, make the contribution of aerodynamic noise in bogie cavities slightly higher than that of the rolling noise. The simulated case 2 (track absorbers only) is shown on Figure 13.

Figure 13:Contribution of different sources to the pass-by level and 1/3 octave band spectrum of a TGV at 320 km/h at 25m, simulated with VAMPASS with reduction of rolling noise with track absorbers only.

As up to now wheel absorbers were implemented only on a commercial TGV Duplex trainset, results obtained and reported in [11] cannot be directly compared. However, the predicted trend (higher efficiency of both measures, relatively low efficiency of track absorbers only because of the higher contribution of the wheel to rolling noise at higher speed is in line with the measurements) The capacity of VAMPASS to efficiently predict reduction on global noise obtained by action on a single source was shown on a significant example, which shows the same trends as experimental results;

Conclusion

External noise from high speed trains was measured both globally and using acoustic arrays in now classical test situations. The various series of high speed trains from different countries (TGV, ICE, ETR…) show very close values on the same measurement site and the differences on various TSI compliant sites is up to about 2 dB The regression law of the global sound pressure level according to the train speed, provides a regression coefficient 30 log(Vtrain) which is valid up to 380 kph .This confirms that the contribution of the rolling noise remains important for TGV running at commercial speeds (300 kph or 320 kph). For antenna measurement, the recent improvement of the measurement devices must be accompanied by an improvement of the array processing itself to develop an accurate and robust method to characterize the sources, which is still to be built . The sources characteristics becomes more and more important data as they are used as input of pass-by simulation software as one’s developed in [6]. These software are needed to carry out parametric studies to define the most relevant combination of noise reduction solutions to reduce the noise of existing and future trains. The latest generation of such prediction models such as VAMPASS show a good potential to simulate the global noise reduction that can be expected by acting on individual sources.

References

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