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EXPERIENCES WITH VARIOUS CONFIGURATIONS OF MICROPHONE ARRAYS USED TO LOCATE SOUND SOURCES ON RAILWAY TRAINS OPERATED BY THE DB AG
Journal of Sound and Vibration (1996) 193(1), 283–293
EXPERIENCES WITH VARIOUS CONFIGURATIONS OF MICROPHONE ARRAYS USED TO LOCATE SOUND SOURCES ON RAILWAY TRAINS OPERATED BY THE DB AG B. B Ingenieurbu¨ro akustik-data, Kirchblick 9, D-14129 Berlin, Germany (Received in final form 20 November 1995) This paper gives an overview of the results and conclusions from measurements made with various configurations of microphone arrays designed to locate sound sources on trains operated by the Deutsche Bahn AG at speeds up to 280 km/h. These arrays comprised one- and two-dimensional arrangements of up to 29 microphones. The characteristics peculiar to microphone arrays will be mentioned only briefly here, because the theoretical background of array measurements is available in other publications. Within the context of the present paper, the sound sources located are those generated mechanically, principally by wheel/rail interactions, and those produced by aerodynamic fluctuations. In addition, noise radiation from cooling systems was also measured during passages of the trains. Examples are given of results obtained with different kinds of arrays during pass-bys of dedicated ICEs and goods trains. These examples demonstrate the usefulness of array measurements for producing detailed information pertaining to the characteristics of individual sound sources. With respect to these individual sound sources, particularly on moving vehicles, array technology provides a much more efficient and capable tool for investigating these sources than does the more conventional technique employing single microphones. 7 1996 Academic Press Limited
1. INTRODUCTION
Various configurations of microphone arrays have proved their usefulness in locating individual sound sources on a variety of different types of railway trains of the Deutsche Bahn AG (DB AG). By appropriately tailoring the arrangement of microphones, i.e., by selecting a line, cross, or X-array of suitable length, the resulting highly directional beam pattern can resolve neighbouring sources of radiated noise on a passing vehicle. Since the basic operating principle of microphone array technology has been described in a number of publications (e.g., references [1, 2]), we shall not repeat it here. Rather, the present contribution is devoted to reporting the results of several types of array measurements. 2. DATA ACQUISITION SYSTEM AND SET-UP OF THE ARRAYS
The akustik-data Engineering Office has a 32 channel personal-computer-based data acquisition system capable of storing information from 31 microphones during a measuring time of approximately 40 s at a sampling frequency of 25 kHz per channel. In 283 0022–460X/96/210283 + 11 $18.00/0
7 1996 Academic Press Limited
284
.
addition, a signal from a light barrier used to identify the position of the train is stored simultaneously. In recent years we have gained experience with various configurations of microphone arrays during several measurement programs on conventional and high speed passenger trains (IC and ICE trains) and on goods trains. Horizontal and vertical line arrays (some with interspersed sub-arrays) have enabled us to locate sound sources in one dimension in the direction parallel to the line of the microphones, i.e., for the horizontal array in the horizontal direction and for the vertical array in the vertical direction. The interspersed line arrays used thus far have consisted of three sub-arrays with 15 microphones each and spacing of 8, 16 and 32 cm, or 12, 24 and 48 cm, which gave a total of 29 microphones, since some of the sensors are shared. Because the total length of an array and the microphone spacing determine the spatial resolution at a given frequency, interspersed line arrays provide larger frequency ranges over which the array can operate. The cross-array is composed of two perpendicular linear line arrays of 15 microphones each, where the central one is shared; this array enables us to locate sound sources in two dimensions. The X-array, which is a cross-array rotated by 45 degrees, was used during some of our measurements with various microphone spacings. With microphone spacings ranging from 48 down to 8 cm, a working frequency range from about 250 to 4500 Hz can be achieved for all array configurations when the required resolution of two neighbouring sound sources is, for example, 1·5 m at the low frequency of 250 Hz. Since the ability of a microphone array to separate neighbouring sound sources is frequency dependent, its resolution increases with increasing sound frequency.
3. SELECTED RESULTS MEASURED WITH HORIZONTAL LINE ARRAYS POSITIONED PARALLEL TO THE TRACK
We measured sound generated by various types of wheels and investigated the effectiveness of several types of wheel-noise absorbers during a number of measurement programs involving ICE test trains. These tasks provide typical examples of the use of what we call the wayside horizontal (WH) array. This type of line array is positioned horizontally, with its line of microphones parallel to the track. By virtue of its ability to resolve sound sources lying at different longitudinal locations along the train, the total sound generated by a single wheel as well as the corresponding narrow-band spectrum can be determined. By sweeping (steering) the array beam to follow the source, the effects of source convection can be eliminated. Since the measurements were of interest mainly in the frequency range 1000–4500 Hz, i.e., the frequency range in which wheel noise is important, a ‘‘simple’’ array comprising 15 microphones positioned at intervals of 8 cm was sufficient for the task at hand (WH08 array, see Figure 1). The WH08 array was positioned at a lateral distance of 5·7 m from the centreline of the track. On the basis of our experience, we have found that at this distance, the boundary layer of the passing train does not interact with the microphones, regardless of the length of the train. Furthermore, at this distance of 5·7 m, neighbouring sound sources separated by about 1·5 m can be resolved in the focal plane (here at the near rail) at the 1000 Hz frequency. The measurements were made at pass-by speeds between 100 and 280 km/h. As an example of a result measured with the WH08 array, in Figure 2 is shown the time history of the A-weighted sound-pressure level (SPL) generated by the ICE test train as it passed by at a speed of 200 km/h. Although the horizontal array does not give us any information as to the vertical distribution of sound sources, we know from complementary results measured with vertical arrays that the peaks in the time history are associated with
285
Figure 1. The WH08 array as an ICE test train passed by; the light barrier can also be seen on the near rail.
sound radiated by the wheels. Also, by virtue of their locations relative to the train co-ordinate, it is clear that the maximum levels are associated with the wheels and represent SPLs at the measuring station due to noise radiated principally in the axial direction by the wheels. The different peak SPLs in Figure 2 are a consequence of the fact that the test train was equipped with several different kinds of wheels, including acoustically optimized
Figure 2. The time history of the A-weighted sound-pressure level during a pass-by of an ICE test train at 200 km/h, measured with the WH08 array at 5·7 m distance from the centreline of the track (after reference [3]).
286
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wheels, rubber-sprung single-ring wheels of type Bo 84, wheels of type Ba 14 with noise absorbers of type VSG and type MAN/GHH and, for comparison, wheels of type Ba 14 having no noise absorbers. Without going into detail, we shall simply say that by making a number of measurements over a wide range of train speeds, we were able to establish statistically supported results for the sound emission of each type of wheel as well as for the dependence of the SPLs on speed. In particular, the rubber-sprung single-ring wheels proved to be an extremely advantageous wheel type from an acoustical standpoint. With the exception of the rubber-sprung wheels, all other types of wheels, with or without noise absorbers, were found to have a speed exponent of about 3. The rubber-sprung wheels had an exponent of 2·2, which is a distinct advantage for the high speed trains. An example of results of measurements designed to locate sound sources on a dedicated goods-wagon test train is shown in Figure 3. Although these results were also measured with a WH array, it was not the ‘‘simple’’ type discussed above but, rather, a WH interspersed array (WH08/16/32 array). This array enabled us to distinguish individual sound sources along the train (i.e., the wheels) at frequencies down to about 250 Hz. A photograph of this array can be found in reference [4]. The goods train for these test runs comprised two wagons of type Hbbillns 305 (type Ba 02 wheels with tread brakes), two wagons of type Shimmns 708 (type Ba 02 wheels with tread brakes) and two wagons of type Sgss-y 703 (type Ba 06 wheels with disc brakes). These wagons are identified in Figure 3 in the respective sequence with ‘‘G1’’, ‘‘G2’’ and ‘‘G3’’. The other cars on the test train were passenger coaches the wheels of which had tread brakes and, between wagon-pairs G1 and G2, a laboratory car the wheels of which were equipped with disc brakes. As before, the time histories show that the local peak SPLs occur at the positions corresponding to the wheels. As can be seen, the peak SPLs generated by the two wagon types equipped with tread brakes (G1 and G2) are more or less similar, whereas the peak
Figure 3. Time histories of A-weighted sound-pressure levels measured with the WH08/16/32 array positioned at a distance of 5·7 m from the centreline of the track during pass-bys of the dedicated goods train at speeds of about 60 (- - - -) and 100 km/h (——); frequency range here is 250–4500 Hz (after reference [5]).
287
levels associated with the two wagons fitted with disc brakes are significantly lower. It is also apparent in Figure 3 that the peak SPLs corresponding to the wheels on the four ‘‘inner’’ axles of the two G3 wagons are significantly lower than the peak levels due to the wheels on the ‘‘outer’’ axles of this pair of wagons. The reason for these differences is that the outer wheels are subjected to stronger excitations by the rails than are the inner wheels, because the wagons contiguous to the G3 wagons have wheels equipped with tread brakes. Corrugations on the treads of these wheels excite the rails and thereby increase the SPL of the outer wheels on the G3 wagons. Therefore, only the peak SPLs generated by the inner wheels on the G3 wagons can be taken as being representative for this wagon type. As a statistical average over a number of pass-bys, these wheels generate peak SPLs that are about 9 dB lower than the corresponding SPLs associated with the G1 wagons, and 7 dB lower than the peak SPLs associated with the G2 wagons. The speed exponents for all wheels on all wagons were found to lie between 3·0 and 3·3. Not only can the noise level radiated by a particular wheel be measured by focusing the beam of the array on it or, rather, by sweeping the beam through a certain angle so as to track the wheel, but the noise spectrum, excluding convective effects, can also be determined. Spectra for the wheels on goods wagons G1 to G3 are shown in Figure 4. The spectra were averaged over a number of pass-bys at speeds of about 100 km/h. These results are taken from those measured with the three sub-arrays: i.e., from their individual ranges of working frequencies, and patched together. Of the various spectral peaks visible in Figure 4, those at frequencies above about 1500 Hz are a consequence of the eigenfrequencies of the wheels. The distinct peak levels in the two spectra of noise radiated by the wheels of type Ba 02 show essentially good agreement with one another. Because the third spectrum is for noise radiated by wheels of type Ba 06, which have different eigenfrequencies from those of the Ba 02 wheels, there is only limited agreement between this third spectrum and those for the Ba 02 wheels.
Figure 4. Averaged sound-pressure spectra measured with the WH08/16/32 array in the wheel regions of wagons G1 (. . . . , type Ba 02 wheels), G2 (- - - -, type Ba 02 wheels) and G3 (——, type Ba 06 wheels) at a speed of approximately 100 km/h, Df = 23·3 Hz (after reference [5]).
288
. 4. SELECTED RESULTS MEASURED WITH LINE ARRAYS POSITIONED VERTICALLY WITH RESPECT TO THE TRACK
An interspersed array (the WV08/16/32 array) positioned vertically with respect to the track is shown in Figure 5. We have used this array to measure, among other things, the vertical distribution of sound sources on goods trains. A result of these investigations is given in reference [4]. Another WV interspersed array was used to study radiated noise produced by the DSA 350 S pantograph on the ICE. Since we suspected that relatively low frequency components of aerodynamic noise would be generated by the pantograph, and we wanted to ensure that the array was capable of locating these sound sources, we selected an array configuration with a large total length for this task. Sub-arrays having microphone spacings of 12, 24 and 48 cm were thus interspersed to form the WV12/24/48 array (see Figure 6). The sub-arrays were arranged in such a way that the centre microphone of each sub-array was positioned at a height of almost 4·6 m above the surface of the rails: i.e., at the height corresponding to the middle of the raised pantograph. For two frequency ranges, in Figure 7 are shown sound-source distributions measured with the WV array depicted in Figure 6. The distributions in Figure 7 are for the region encompassing the rear ICE power car at a pass-by speed of 264 km/h. The result in the frequency range 586–781 Hz was measured with the WV48 sub-array, whereas the WV24 sub-array was used to obtain the result in the frequency range 1172–1367 Hz. Although
Figure 5. The WV08/16/32 array.
289
Figure 6. The WV12/24/48 array.
a WV array is theoretically only capable of resolving sound sources lying at different vertical positions, there are two reasons why the results have a quasi-two-dimensional character. First, the SPL due to the sound sources decreases progressively with increasing distance from them. Second, the localized sources very probably have strongly directional radiation patterns with their maxima directed towards the wayside. As can be seen in the Figures 7(a) and (c), the source distributions, represented here in shades of grey taken from original coloured representations, do have a quasitwo-dimensional character. These results clearly show the sound sources due to the pantograph (as well as other sound sources that we cannot discuss here), whereby these pantograph noise sources are essentially concentrated in its head and foot region. This spatial decomposition of the source region becomes even clearer when we make a cut through the two-dimensional sound-source distributions at the co-ordinate of the pantograph. The results of this procedure, which are shown in Figures 7(b) and (d), are the unweighted and A-weighted SPL distributions in the vertical direction above the rail. These last two sub-figures show a local maximum SPL lying between 5·2 and 5·4 m above the rail. Obviously, these results represent sound emission from the head of the pantograph. In the higher of the two frequency ranges, there is a strong peak SPL associated with the region lying between 4·0 and 4·2 m above the rail: i.e., within the foot region of the pantograph. In each case, the radiated noise sources are generated by flow interactions with the respective components of the pantograph.
290
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Figure 7. Sound-source distributions measured with the WV48 sub-array in the frequency range 586–781 Hz (a,b) and with the WV24 sub-array in the frequency range 1172–1367 Hz (c,d) during a pass-by of an ICE test train at 264 km/h. ——, Unweighted results; - - - -, A-weighted results (after reference [6])
By analyzing a number of such A-weighted SPL distributions in the frequency range from about 40 to 1600 Hz, measured during many pass-bys of the ICE, we were able to determine accurately the total wayside noise generated by the DSA 350 S pantographs on both the front and rear power cars. We also investigated the radiated noise produced by flow interactions with the retracted pantographs. Results of these measurements are particularly pertinent to the case in which an ICE power car is equipped with two pantographs, one directly behind the other, and one always being retracted (the ICE version used for border crossing operations into Switzerland).
291
5. ONE RESULT MEASURED WITH AN X-ARRAY
Due to space limitations, only one result from measurements with X-shaped arrays can be given here. Such an array with a microphone spacing of 48 cm (WX48 array) is shown in Figure 8. As before, this array configuration was used for locating sound sources on the ICE pantograph DSA 350 S in the frequency range from about 200 to 800 Hz. A measured sound-source distribution in the region of the rear ICE power car and the contiguous middle coach is shown in Figure 9. As in Figure 7, the representation reproduced here in shades of grey was taken from a coloured representation that shows sound sources much more distinctly. The figure indicates that in the frequency range under investigation, the horns are the primary sources of wayside noise in the head region of the pantograph (see also result of the WV48 sub-array in Figure 7). A sound-source distribution in the frequency range from 1500 to 4500 Hz measured with the WX08 array on two goods wagons of type Hbbillns 305 is depicted in reference [4]. The figure given there clearly demonstrates that, for these wagon types and the above-mentioned frequency range, wheel/rail interactions are the dominant sound sources of wayside noise. Even a cursory examination of the examples of measured results presented here indicates that array technology is capable of resolving single sound sources, also at high train speeds, and can thereby determine the acoustical characteristics peculiar to each source. Array technology represents a significant improvement over a single microphone because, in general, the latter is only able to measure the superposition of the acoustic fields produced by all sound sources in the vicinity. Nevertheless, it must be recognized that the higher density of information available through array technology, as compared with that associated with a single microphone, involves greater expense in both equipment and time. Experience has shown, however, that the gains accrued by our understanding of the acoustical processes are fully worth this higher expense.
Figure 8. The WX48 array.
Figure 9. The sound-source distribution measured with the WX48 array in the frequency range 400–680 Hz during a pass-by of an ICE test train at 271 km/h (after reference [6]).
292 .
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ACKNOWLEDGMENTS
Financial support and other assistance for these investigations by the Deutsche Bahn AG is gratefully acknowledged. In particular, we would like to thank the staff of Department ZTQ 15 and its head, Dipl.-Ing. G. Ho¨lzl, for their help and co-operation. REFERENCES 1. B. B, W. F. K III and E. P 1987 Journal of Sound and Vibration 118, 99–122. Wheel/rail noise generated by a high-speed train investigated with a line array of microphones. 2. C. O. P and B. B 1994 Proceedings of the World Congress on Railway Research (WCRR ’94), 371–376. The microphone array: a tool on the path towards reducing railway noise. 3. B. M¨ and B. B 1993 Ingenieurbu¨ro akustik-data, Report No. 93/2. Schallemission von ICE-Ra¨dern mit und ohne Regelabsorbern, von schalloptimierten Ra¨dern und von gummigefederten Einringra¨dern Bo 84. 4. G. H¨ 1995 The 5th International Workshop on Railway and Tracked Transit System Noise. Low-noise goods wagons. 5. B. B 1994 Ingenieurbu¨ro akustik-data, Report No. 94/1. Schallquellenlokalisierung an Gu¨terwagen Hbbillns 305, Shimmns 708 und Sgss-y 703. 6. B. B, M. H and M. S¨ 1994 Ingenieurbu¨ro akustik-data, Report No. 94/6. Bestimmung und Analyse des Gera¨uschs des ICE-Stromabnehmers DSA 350 S.
EXPERIENCES WITH VARIOUS CONFIGURATIONS OF MICROPHONE ARRAYS USED TO LOCATE SOUND SOURCES ON RAILWAY TRAINS OPERATED BY THE DB AG B. B Ingenieurbu¨ro akustik-data, Kirchblick 9, D-14129 Berlin, Germany (Received in final form 20 November 1995) This paper gives an overview of the results and conclusions from measurements made with various configurations of microphone arrays designed to locate sound sources on trains operated by the Deutsche Bahn AG at speeds up to 280 km/h. These arrays comprised one- and two-dimensional arrangements of up to 29 microphones. The characteristics peculiar to microphone arrays will be mentioned only briefly here, because the theoretical background of array measurements is available in other publications. Within the context of the present paper, the sound sources located are those generated mechanically, principally by wheel/rail interactions, and those produced by aerodynamic fluctuations. In addition, noise radiation from cooling systems was also measured during passages of the trains. Examples are given of results obtained with different kinds of arrays during pass-bys of dedicated ICEs and goods trains. These examples demonstrate the usefulness of array measurements for producing detailed information pertaining to the characteristics of individual sound sources. With respect to these individual sound sources, particularly on moving vehicles, array technology provides a much more efficient and capable tool for investigating these sources than does the more conventional technique employing single microphones. 7 1996 Academic Press Limited
1. INTRODUCTION
Various configurations of microphone arrays have proved their usefulness in locating individual sound sources on a variety of different types of railway trains of the Deutsche Bahn AG (DB AG). By appropriately tailoring the arrangement of microphones, i.e., by selecting a line, cross, or X-array of suitable length, the resulting highly directional beam pattern can resolve neighbouring sources of radiated noise on a passing vehicle. Since the basic operating principle of microphone array technology has been described in a number of publications (e.g., references [1, 2]), we shall not repeat it here. Rather, the present contribution is devoted to reporting the results of several types of array measurements. 2. DATA ACQUISITION SYSTEM AND SET-UP OF THE ARRAYS
The akustik-data Engineering Office has a 32 channel personal-computer-based data acquisition system capable of storing information from 31 microphones during a measuring time of approximately 40 s at a sampling frequency of 25 kHz per channel. In 283 0022–460X/96/210283 + 11 $18.00/0
7 1996 Academic Press Limited
284
.
addition, a signal from a light barrier used to identify the position of the train is stored simultaneously. In recent years we have gained experience with various configurations of microphone arrays during several measurement programs on conventional and high speed passenger trains (IC and ICE trains) and on goods trains. Horizontal and vertical line arrays (some with interspersed sub-arrays) have enabled us to locate sound sources in one dimension in the direction parallel to the line of the microphones, i.e., for the horizontal array in the horizontal direction and for the vertical array in the vertical direction. The interspersed line arrays used thus far have consisted of three sub-arrays with 15 microphones each and spacing of 8, 16 and 32 cm, or 12, 24 and 48 cm, which gave a total of 29 microphones, since some of the sensors are shared. Because the total length of an array and the microphone spacing determine the spatial resolution at a given frequency, interspersed line arrays provide larger frequency ranges over which the array can operate. The cross-array is composed of two perpendicular linear line arrays of 15 microphones each, where the central one is shared; this array enables us to locate sound sources in two dimensions. The X-array, which is a cross-array rotated by 45 degrees, was used during some of our measurements with various microphone spacings. With microphone spacings ranging from 48 down to 8 cm, a working frequency range from about 250 to 4500 Hz can be achieved for all array configurations when the required resolution of two neighbouring sound sources is, for example, 1·5 m at the low frequency of 250 Hz. Since the ability of a microphone array to separate neighbouring sound sources is frequency dependent, its resolution increases with increasing sound frequency.
3. SELECTED RESULTS MEASURED WITH HORIZONTAL LINE ARRAYS POSITIONED PARALLEL TO THE TRACK
We measured sound generated by various types of wheels and investigated the effectiveness of several types of wheel-noise absorbers during a number of measurement programs involving ICE test trains. These tasks provide typical examples of the use of what we call the wayside horizontal (WH) array. This type of line array is positioned horizontally, with its line of microphones parallel to the track. By virtue of its ability to resolve sound sources lying at different longitudinal locations along the train, the total sound generated by a single wheel as well as the corresponding narrow-band spectrum can be determined. By sweeping (steering) the array beam to follow the source, the effects of source convection can be eliminated. Since the measurements were of interest mainly in the frequency range 1000–4500 Hz, i.e., the frequency range in which wheel noise is important, a ‘‘simple’’ array comprising 15 microphones positioned at intervals of 8 cm was sufficient for the task at hand (WH08 array, see Figure 1). The WH08 array was positioned at a lateral distance of 5·7 m from the centreline of the track. On the basis of our experience, we have found that at this distance, the boundary layer of the passing train does not interact with the microphones, regardless of the length of the train. Furthermore, at this distance of 5·7 m, neighbouring sound sources separated by about 1·5 m can be resolved in the focal plane (here at the near rail) at the 1000 Hz frequency. The measurements were made at pass-by speeds between 100 and 280 km/h. As an example of a result measured with the WH08 array, in Figure 2 is shown the time history of the A-weighted sound-pressure level (SPL) generated by the ICE test train as it passed by at a speed of 200 km/h. Although the horizontal array does not give us any information as to the vertical distribution of sound sources, we know from complementary results measured with vertical arrays that the peaks in the time history are associated with
285
Figure 1. The WH08 array as an ICE test train passed by; the light barrier can also be seen on the near rail.
sound radiated by the wheels. Also, by virtue of their locations relative to the train co-ordinate, it is clear that the maximum levels are associated with the wheels and represent SPLs at the measuring station due to noise radiated principally in the axial direction by the wheels. The different peak SPLs in Figure 2 are a consequence of the fact that the test train was equipped with several different kinds of wheels, including acoustically optimized
Figure 2. The time history of the A-weighted sound-pressure level during a pass-by of an ICE test train at 200 km/h, measured with the WH08 array at 5·7 m distance from the centreline of the track (after reference [3]).
286
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wheels, rubber-sprung single-ring wheels of type Bo 84, wheels of type Ba 14 with noise absorbers of type VSG and type MAN/GHH and, for comparison, wheels of type Ba 14 having no noise absorbers. Without going into detail, we shall simply say that by making a number of measurements over a wide range of train speeds, we were able to establish statistically supported results for the sound emission of each type of wheel as well as for the dependence of the SPLs on speed. In particular, the rubber-sprung single-ring wheels proved to be an extremely advantageous wheel type from an acoustical standpoint. With the exception of the rubber-sprung wheels, all other types of wheels, with or without noise absorbers, were found to have a speed exponent of about 3. The rubber-sprung wheels had an exponent of 2·2, which is a distinct advantage for the high speed trains. An example of results of measurements designed to locate sound sources on a dedicated goods-wagon test train is shown in Figure 3. Although these results were also measured with a WH array, it was not the ‘‘simple’’ type discussed above but, rather, a WH interspersed array (WH08/16/32 array). This array enabled us to distinguish individual sound sources along the train (i.e., the wheels) at frequencies down to about 250 Hz. A photograph of this array can be found in reference [4]. The goods train for these test runs comprised two wagons of type Hbbillns 305 (type Ba 02 wheels with tread brakes), two wagons of type Shimmns 708 (type Ba 02 wheels with tread brakes) and two wagons of type Sgss-y 703 (type Ba 06 wheels with disc brakes). These wagons are identified in Figure 3 in the respective sequence with ‘‘G1’’, ‘‘G2’’ and ‘‘G3’’. The other cars on the test train were passenger coaches the wheels of which had tread brakes and, between wagon-pairs G1 and G2, a laboratory car the wheels of which were equipped with disc brakes. As before, the time histories show that the local peak SPLs occur at the positions corresponding to the wheels. As can be seen, the peak SPLs generated by the two wagon types equipped with tread brakes (G1 and G2) are more or less similar, whereas the peak
Figure 3. Time histories of A-weighted sound-pressure levels measured with the WH08/16/32 array positioned at a distance of 5·7 m from the centreline of the track during pass-bys of the dedicated goods train at speeds of about 60 (- - - -) and 100 km/h (——); frequency range here is 250–4500 Hz (after reference [5]).
287
levels associated with the two wagons fitted with disc brakes are significantly lower. It is also apparent in Figure 3 that the peak SPLs corresponding to the wheels on the four ‘‘inner’’ axles of the two G3 wagons are significantly lower than the peak levels due to the wheels on the ‘‘outer’’ axles of this pair of wagons. The reason for these differences is that the outer wheels are subjected to stronger excitations by the rails than are the inner wheels, because the wagons contiguous to the G3 wagons have wheels equipped with tread brakes. Corrugations on the treads of these wheels excite the rails and thereby increase the SPL of the outer wheels on the G3 wagons. Therefore, only the peak SPLs generated by the inner wheels on the G3 wagons can be taken as being representative for this wagon type. As a statistical average over a number of pass-bys, these wheels generate peak SPLs that are about 9 dB lower than the corresponding SPLs associated with the G1 wagons, and 7 dB lower than the peak SPLs associated with the G2 wagons. The speed exponents for all wheels on all wagons were found to lie between 3·0 and 3·3. Not only can the noise level radiated by a particular wheel be measured by focusing the beam of the array on it or, rather, by sweeping the beam through a certain angle so as to track the wheel, but the noise spectrum, excluding convective effects, can also be determined. Spectra for the wheels on goods wagons G1 to G3 are shown in Figure 4. The spectra were averaged over a number of pass-bys at speeds of about 100 km/h. These results are taken from those measured with the three sub-arrays: i.e., from their individual ranges of working frequencies, and patched together. Of the various spectral peaks visible in Figure 4, those at frequencies above about 1500 Hz are a consequence of the eigenfrequencies of the wheels. The distinct peak levels in the two spectra of noise radiated by the wheels of type Ba 02 show essentially good agreement with one another. Because the third spectrum is for noise radiated by wheels of type Ba 06, which have different eigenfrequencies from those of the Ba 02 wheels, there is only limited agreement between this third spectrum and those for the Ba 02 wheels.
Figure 4. Averaged sound-pressure spectra measured with the WH08/16/32 array in the wheel regions of wagons G1 (. . . . , type Ba 02 wheels), G2 (- - - -, type Ba 02 wheels) and G3 (——, type Ba 06 wheels) at a speed of approximately 100 km/h, Df = 23·3 Hz (after reference [5]).
288
. 4. SELECTED RESULTS MEASURED WITH LINE ARRAYS POSITIONED VERTICALLY WITH RESPECT TO THE TRACK
An interspersed array (the WV08/16/32 array) positioned vertically with respect to the track is shown in Figure 5. We have used this array to measure, among other things, the vertical distribution of sound sources on goods trains. A result of these investigations is given in reference [4]. Another WV interspersed array was used to study radiated noise produced by the DSA 350 S pantograph on the ICE. Since we suspected that relatively low frequency components of aerodynamic noise would be generated by the pantograph, and we wanted to ensure that the array was capable of locating these sound sources, we selected an array configuration with a large total length for this task. Sub-arrays having microphone spacings of 12, 24 and 48 cm were thus interspersed to form the WV12/24/48 array (see Figure 6). The sub-arrays were arranged in such a way that the centre microphone of each sub-array was positioned at a height of almost 4·6 m above the surface of the rails: i.e., at the height corresponding to the middle of the raised pantograph. For two frequency ranges, in Figure 7 are shown sound-source distributions measured with the WV array depicted in Figure 6. The distributions in Figure 7 are for the region encompassing the rear ICE power car at a pass-by speed of 264 km/h. The result in the frequency range 586–781 Hz was measured with the WV48 sub-array, whereas the WV24 sub-array was used to obtain the result in the frequency range 1172–1367 Hz. Although
Figure 5. The WV08/16/32 array.
289
Figure 6. The WV12/24/48 array.
a WV array is theoretically only capable of resolving sound sources lying at different vertical positions, there are two reasons why the results have a quasi-two-dimensional character. First, the SPL due to the sound sources decreases progressively with increasing distance from them. Second, the localized sources very probably have strongly directional radiation patterns with their maxima directed towards the wayside. As can be seen in the Figures 7(a) and (c), the source distributions, represented here in shades of grey taken from original coloured representations, do have a quasitwo-dimensional character. These results clearly show the sound sources due to the pantograph (as well as other sound sources that we cannot discuss here), whereby these pantograph noise sources are essentially concentrated in its head and foot region. This spatial decomposition of the source region becomes even clearer when we make a cut through the two-dimensional sound-source distributions at the co-ordinate of the pantograph. The results of this procedure, which are shown in Figures 7(b) and (d), are the unweighted and A-weighted SPL distributions in the vertical direction above the rail. These last two sub-figures show a local maximum SPL lying between 5·2 and 5·4 m above the rail. Obviously, these results represent sound emission from the head of the pantograph. In the higher of the two frequency ranges, there is a strong peak SPL associated with the region lying between 4·0 and 4·2 m above the rail: i.e., within the foot region of the pantograph. In each case, the radiated noise sources are generated by flow interactions with the respective components of the pantograph.
290
.
Figure 7. Sound-source distributions measured with the WV48 sub-array in the frequency range 586–781 Hz (a,b) and with the WV24 sub-array in the frequency range 1172–1367 Hz (c,d) during a pass-by of an ICE test train at 264 km/h. ——, Unweighted results; - - - -, A-weighted results (after reference [6])
By analyzing a number of such A-weighted SPL distributions in the frequency range from about 40 to 1600 Hz, measured during many pass-bys of the ICE, we were able to determine accurately the total wayside noise generated by the DSA 350 S pantographs on both the front and rear power cars. We also investigated the radiated noise produced by flow interactions with the retracted pantographs. Results of these measurements are particularly pertinent to the case in which an ICE power car is equipped with two pantographs, one directly behind the other, and one always being retracted (the ICE version used for border crossing operations into Switzerland).
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5. ONE RESULT MEASURED WITH AN X-ARRAY
Due to space limitations, only one result from measurements with X-shaped arrays can be given here. Such an array with a microphone spacing of 48 cm (WX48 array) is shown in Figure 8. As before, this array configuration was used for locating sound sources on the ICE pantograph DSA 350 S in the frequency range from about 200 to 800 Hz. A measured sound-source distribution in the region of the rear ICE power car and the contiguous middle coach is shown in Figure 9. As in Figure 7, the representation reproduced here in shades of grey was taken from a coloured representation that shows sound sources much more distinctly. The figure indicates that in the frequency range under investigation, the horns are the primary sources of wayside noise in the head region of the pantograph (see also result of the WV48 sub-array in Figure 7). A sound-source distribution in the frequency range from 1500 to 4500 Hz measured with the WX08 array on two goods wagons of type Hbbillns 305 is depicted in reference [4]. The figure given there clearly demonstrates that, for these wagon types and the above-mentioned frequency range, wheel/rail interactions are the dominant sound sources of wayside noise. Even a cursory examination of the examples of measured results presented here indicates that array technology is capable of resolving single sound sources, also at high train speeds, and can thereby determine the acoustical characteristics peculiar to each source. Array technology represents a significant improvement over a single microphone because, in general, the latter is only able to measure the superposition of the acoustic fields produced by all sound sources in the vicinity. Nevertheless, it must be recognized that the higher density of information available through array technology, as compared with that associated with a single microphone, involves greater expense in both equipment and time. Experience has shown, however, that the gains accrued by our understanding of the acoustical processes are fully worth this higher expense.
Figure 8. The WX48 array.
Figure 9. The sound-source distribution measured with the WX48 array in the frequency range 400–680 Hz during a pass-by of an ICE test train at 271 km/h (after reference [6]).
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ACKNOWLEDGMENTS
Financial support and other assistance for these investigations by the Deutsche Bahn AG is gratefully acknowledged. In particular, we would like to thank the staff of Department ZTQ 15 and its head, Dipl.-Ing. G. Ho¨lzl, for their help and co-operation. REFERENCES 1. B. B, W. F. K III and E. P 1987 Journal of Sound and Vibration 118, 99–122. Wheel/rail noise generated by a high-speed train investigated with a line array of microphones. 2. C. O. P and B. B 1994 Proceedings of the World Congress on Railway Research (WCRR ’94), 371–376. The microphone array: a tool on the path towards reducing railway noise. 3. B. M¨ and B. B 1993 Ingenieurbu¨ro akustik-data, Report No. 93/2. Schallemission von ICE-Ra¨dern mit und ohne Regelabsorbern, von schalloptimierten Ra¨dern und von gummigefederten Einringra¨dern Bo 84. 4. G. H¨ 1995 The 5th International Workshop on Railway and Tracked Transit System Noise. Low-noise goods wagons. 5. B. B 1994 Ingenieurbu¨ro akustik-data, Report No. 94/1. Schallquellenlokalisierung an Gu¨terwagen Hbbillns 305, Shimmns 708 und Sgss-y 703. 6. B. B, M. H and M. S¨ 1994 Ingenieurbu¨ro akustik-data, Report No. 94/6. Bestimmung und Analyse des Gera¨uschs des ICE-Stromabnehmers DSA 350 S.