Sound design studios for setting noise requirements on new rolling stock- a future scenario

Transcription

1 Sound design studios for setting noise requirements on new rolling stock- a future scenario S. Leth, A Frid, A Mirza Bombardier Transportation, CoC Acoustics Specialist Engineering Östra Ringvägen 2, , Västerås, Sweden Siv.Leth@se.transport.bombardier.com 1 Abstract The target for railway rolling stock industry is and will be to provide vehicles that cause minimal annoyance to wayside residents and provide a pleasant and comfortable environment for the passengers on board, i.e. the keyword is sound quality. In a future scenario outlined in this paper customer requirements will not be formulated in the traditional way based on legislation and noise values on historical rolling stock. Instead customer and manufacturer are sitting together in sound design studios to define the customer requirements after listening to the sound of different design options. We have in this future situation thus moved away from the blunt and one-dimensional db(a) level and are looking at how to achieve an attractive blend of different noise sources and to adjust their characteristics. This can be done without having to resort to the various complicated sound quality metrics that exist today. In spite of their complexity, the metrics risk to be very limited in comparison with the full multidimensional sound experience in the human brain. With the use of sound synthesis tools it will be possible to auralise different design options in the sound studio and let the human ear and brain decide. Furthermore, aspects like privacy and conversation intelligibility between neighbor passengers could also be demonstrated and fine tuned in the sound studio. As a basic rule to achieve high sound quality tonality should usually be avoided, the sound must not include rattling noise and it should give a safe and relaxing impression. Sources with this type of broad band character could be emphasized in the blend and, potentially, also artificial sound could be added to create masking effects or change the character. In the studio the different scenarios can be listened to and also the cost impact and constraints for different design solutions can be indicated. A demonstration case of a sound studio prototype displayed at the Innotrans fair 2010 is described in the paper. 2 Introduction Silence is an increasingly important design parameter for development of new trains. This is true both for reducing annoyance to the exterior environment and for assuring a high interior acoustic comfort for the passenger. While exterior noise is often subject to legal requirements derived from environmental protection the interior noise is defined in a contractual agreement between the purchaser and the supplier of the rolling stock. Traditionally, interior acoustic requirements are set in terms of A-weighted sound pressure levels. Particularly for onboard interior noise the db(a) metric has for long been considered too blunt and one-dimensional but due to a lack of alternatives it has prevailed as the dominant metric in contractual specifications. With the emerging tools for sound synthesis and reproduction in combination with the train-builder s detailed acoustic know-how of the source characters and transmission paths there are new possibilities opening up. The effect of design options can not only be calculated but also listened to before the vehicle is built in a realistic environment. A cost effective combination of design options can be chosen by the customer based on their influence on the resulting sound character and not the db(a). The paper is structured the following way. It starts off with a review of current customer noise requirements and recent trends. Section 4 gives a summary of past work in the field of sound quality in railway vehicles. Section 5 presents examples of how technical design solutions are used to influence the character of the sound and how pitfalls are avoided. Section 6 introduces the sound

2 studio concept and describes how it was first realised by Bombardier at the Innotrans fair The paper concludes with an outlook and vision for the future. 3 Customer noise requirements Every contract for new railway vehicles includes customer requirements on external as well as internal noise. For intercity and high speed trains a high acoustic comfort for the passengers is expected. Traditionally this is typically translated to a db(a)-value below 70 db(a) at cruising speed. Recent years has seen a stabilisation around 65 db(a) for trains at conventional speeds. A new challenge is that the noise requirements for high speed and very high speed trains tend to be pushed towards the requirements for conventional speeds despite the contradiction by the law of physics of increased noise source strength with increasing speed. It is also a fact that the fast developing new markets in Asia and Africa are not satisfied with lesser performance of the vehicles than the mature European markets. In some cases, the new markets even have more stringent interior noise requirements. In addition to the db(a)-value STI or RASTI is sometimes used as an indicator for conversation intelligibility and privacy. The underlying ideas are described in [1]. A recent example is the call for tender from the Stockholm transport authority (SL) for new trams and metro vehicles. Many specifications also have limits on tonality in order to avoid the presence of disturbing pure tones. SNCF has recently proposed loudness as an alternative metric for interior noise [2]. Investigations have shown that it correlates better with passenger annoyance than the A-weighted sound pressure level for stationary noise. It is also stated in [2] that loudness is more appropriate than the today commonly used criteria for tonality and transient noise events. 4 Sound quality in railway vehicles The last 15 years has seen a growing interest to include sound quality in acoustic requirement setting. In the early years, metrics such as loudness, sharpness, fluctuation strength, etc (and combinations of these) were compared against jury tests in order to find a relevant indicator to replace the db(a) [3], [4]. There were, however, no conclusive results and it was evident that the situation is too complex and multi-faceted to describe with a single value sound quality metric [5] In recent years, the focus has more shifted towards subjective listening experience, similar to the acoustic branding in automotive industry. Sound aspects are more and more becoming an integrated element in interior design. The goal is to take into consideration the need for both private and social areas within the train, the need for conversations with adjacent passengers and audibility of loudspeaker announcements, in addition to suppression of single, disturbing tones. For example, an extremely low background sound level can aggravate the effects of unwanted sound disturbances. A db(a)-value taken in isolation does not contain any information on the character of the sound, which strongly affects the sound quality experienced. Concepts from architectural and urban acoustics can also be transferred to railway applications [6]. The term soundscape means the deliberate and subtle manipulation of the original sound by adding artificially created elements to enrich the experience and change the atmosphere. This concept has successfully been implemented in urban contexts like parks, shopping malls and airport terminals [7]. The similar soundwalk concept incorporates the spatial variation of the sound field for instance experienced by a pedestrian in a city or a passenger on board a train heading for the bistro car. 5 Influence of train design solutions Rolling noise is the part of the noise from the railways that has attracted by far the most attention in the past and this is still the case today. This focus may change while moving on to analyzing not only the level but also the quality and the character of noise. Rolling noise has a broad band waterfall-like character which is normally not very disturbing, if it is not too loud. The optimum for the future may not be to work on additional costly efforts to minimize rolling

3 noise at running speed. Design solutions to further reduce rolling noise to the exterior such as full bogie skirts on all bogies are developed but are not always implemented due to tradition and special constraints from the infrastructure. The rolling noise contribution to the interior should necessarily not be kept to an absolute minimum, but rather at an appropriate leave to provide a good balance between privacy and speech intelligibility, in the passenger compartment. If the rolling noise is reduced too much, situations may occur; when requests are made on adding the type of broad band noise associated with rolling noise in order to increase privacy through a masking background noise. It has actually been suggested to do that using speakers or the air conditioning system. Instead, efforts can in the future be more directed towards the reductions of clearly annoying noises. There are a number of clear cases where efforts can be made directly without more extensive annoyance studies to precede. Any pure tones that are prevailing must be reduced and also intermittent and impulsive noises that can be disturbing need attention if they are present. This means that for example fans from HVAC units and circulation fans are installed assuring no pure tones in the resulting sound spectra are present. This requires, not only assuring noise requirements are tested and predicted on system levels but also that the entire system as installed with ducts, outlets and so on is considered. Traction equipment is another area where tones are to be fully eliminated. Rotor stator interaction can be optimized to avoid tonality form motors during acceleration, transformer setting can be adjusted to avoid tonality during standstill, converters with inductors should have proper insulation to avoid tones, fans are temperature controlled for minimum noise annoyance and so on. Very annoying tonal noise sources often heard in urban areas and in railway stations are curve squeal and brake screech. Brake screech is often heard and widely propagated in the large central railway stations. Brake squeal is an intermittent phenomena and its occurrence depends on a number of parameters. Statistical methods and extensive testing is required to evaluate any methods implemented to solve the problem. Solutions must be found that are fully compliant with the very strict requirements of brake capacity on modern rolling stock, especially at high speed trains. One is extended implementation of electric braking. The only use of traditional brake systems could be for emergency situations. For curve squeal a combination of lubrication, dampers and radial steering can solve a large majority of the problems if only systematically implemented. 6 Sound studio exhibition at Innotrans 2010 Innotrans is the world s largest railway fair arranged in Berlin, Germany every second year. The last edition of Innotrans in 2010 attracted more than visitors from all over the world. Some of these took the opportunity to experience the sound design studio installed inside a Bombardier ITINO train displayed at the fair. The installation consisted of 13 loudspeakers, 4 subwoofers, 4 window actuators, 2 floor shakers and 2 projectors; all controlled by a graphical touch screen interface. The basis of the Innotrans sound exhibition consistsed of multi-channel recordings on a Bombardier REGINA train. In order to capture a realistic spatial distribution of the sound field, near field recordings are made along the walls, the ceiling and the floor. In this way the characteristics of the noise entering various parts of the saloon is established. Recordings are made during one complete running cycle from standstill through acceleration to cruising and back to standstill through a deceleration phase. Figure 1 illustrates a part of the grid of microphones and accelerometers used for the measurements.

4 Figure 1 Microphone and accelerometer grid used for near field recordings on a Bombardier REGINA train. The straight forward way of implementing the recordings in a sound studio would be to replace every microphone in Figure 1 with a loudspeaker and every accelerometer with a shaker to reconstruct the sound field inside the saloon. After some experimenting it was however discovered that some channels were redundant and hence they could be omitted without affecting the perceived sound. In total there were 10 channels used in the sound studio installation. This was necessary and sufficient in order to separate the sound field vertically, horizontally and longitudinally. To demonstrate the effect of design changes the recorded signal is filtered in order to remove parts of the signal corresponding to the design improvement in question. To do so it is necessary to have a good understanding of the different noise sources in the train and their most important transmission paths. Figure 2 illustrates the translation of some important design improvements into their corresponding effects on the sound spectra. HVAC Absorption Track quality Figure 2. Translation of design improvements into schematic filters. Solid: baseline, dotted: improved. Increasing the acoustic absorption inside the saloon will mainly affect the high frequency noise and reduce the reverberation time. An acoustically improved HVAC unit will emit a broadband noise without audible tones. The track quality included in Figure 2 is not within the scope of the vehicle manufacturer however it is immensely important for the interior noise and constitutes a good example on how the railway noise is a matter if train and track interaction. The filters corresponding to the different design improvements are applied to the recorded signals in real time inside the sound studio.

5 In this way it is possible to turn a design improvement on and off to emphasize its effect. Furthermore the filters are only applied to the channels relevant for the different design improvements, e.g. the filter representing an improved HVAC unit is only applied to the channels at ceiling level. Figure 3 shows the ITINO and the passenger compartment used for the sound studio. A small saloon is preferable since it enables a better control of the sound field. Figure 3. Bombardier ITINO displayed at Innotrans The passenger compartment right behind the drivers cab was hosting the sound studio exhibition. The recordings on the REGINA are near field recordings and hence the effect of the room is kept at a minimum. However during playback of the signals in the ITINO the sound will be influence by the frequency response of the loudspeaker and the positioning of the loudspeaker. This effect can be reduced by measuring the impulse response of the loudspeaker at its correct position inside the train and compensate for it through an equalizer. Figure 4 illustrates the setup of the sound studio. The saloon is equipped with loudspeakers at floor and ceiling level. Instead of using loudspeakers at window level small actuators are attached to the window panes turning them into vibrating membranes. The actuators are fed with a signal recorded by an accelerometer mounted to the window pane on the REGINA train. This gives a noise injection to the saloon distributed over the entire window surface which particularly gives a realistic impression when seated close to a window. Subwoofers are hidden underneath the seats and shakers are squeezed in between the seats and the floor to add vibration. Dynamic projections on the window panes changes characteristics with the sound inside the saloon to give an impression of motion. As an extra feature a fake passenger is installed by placing a loudspeaker in the far end of the saloon. This one is used to playback pieces of a phone conversation in order to illustrate the masking effect of different background noises. The heart of the sound studio consists of a touch screen, displaying a

6 graphical user interface which enables the visitors to select the operation mode of the vehicle and turn different design changes on and off. Furthermore the screen gives real time information on the sound level and the sound spectrum inside the saloon. Figure 4. Sound studio setup inside the ITINO train on Innotrans Figure 5 gives a closer look at the control screen interface. The current operating mode is indicated at the bottom of the screen. A spectrogram is drawn in real time and the overall A-weighted level together with the current speed is given in the upper right corner. A full demonstration of the sound studio begins at standstill where mainly the ventilation noise is heard. When moving into the acceleration phase traction noise is first pronounced but as the vehicle is reaching higher speed the rolling noise and structure-borne noise entering through the bogie is getting more dominant. At any stage of the journey the visitor is able to play around with the different design changes. Since the contribution from different sources is changing some design improvements will only be heard at certain operation modes. The ventilation noise for instance constitutes only a minor contribution to the interior noise at cruising and hence an optimized HVAC unit will not have any significant effect. Improving the track quality on the other hand will change the level and the characteristics of the sound completely.

7 Figure 5. Control screen user interface. During cruising the visitor is able to turn the fake passenger on and off to hear the trade-off between speech privacy and speech intelligibility. Another feature which is enabled during cruising is the somewhat more experimental concept of soundscapes as discussed in Section 4. In contradiction to classical noise mitigation this is done by adding sound through loudspeakers in order to complement the already existing train sound. The result is very much dependent on personal preferences but the aim is to design a soundscape which is subtle but effective and appreciated by many. 7 Vision Setting of acoustic requirements for new rolling stock may in the future be done in sound design studios. These studios need to be highly sophisticated to give a real and representative feeling of a passenger compartment in a train. The acoustics of the studio must be possible to fully control with an extensive speaker and shaker system. It must also be perceived as a real compartment since the experience of the sound environment is very closely linked to all other environmental parameters. In this type if more developed studios it would be possible for a manufacturer to sit together with a potential customer for new rolling stock and set goals, for example for the interior sound quality in different parts of the train. In an ideal world, different parts of the desired sound quality can be associated with the use of different noise control measures and the associated costs can be estimated. A cost benefit analysis in a listening scenario can be made directly on line in the studio. Some of the sound quality parameters may be contradictory such as good speech intelligibility together with a substantial privacy and reduced annoyance from mobile phones. One possible solution to this is that the local sound environment or soundscape at a certain seat is adjustable with the use of switching on and off speakers and microphones in a control system, control of the directivity of a speaker in the head rest or pulling down more or less absorption or screening. Another possibility is to create different sound environments in different parts of the trains and the passengers can move depending on what they would like to do, relax, eat, listen to music, sleep, talk with a colleague on the train or on the phone.

8 8 Concluding remarks The first steps are taken to make the move in the railway industry away from the prevailing db(a) towards more cost efficient solutions based on assessing the sound quality or the soundscape for the interior noise as well as for the exterior noise. For the interior sound environment the comfort is key and for the exterior still reduced annoyance is of great importance to eliminate any adverse health effects of noise. Sound design studios may be a tool to move in the direction of targeting an optimal setting of sound quality parameters and have more cost efficient solutions overall for future new rolling stock. The feedback received from the visitors at Innotrans underlines the potential of the sound design studio. The visitors had varying experience within the field of railway vehicles and railway noise, ranging from the complete novice to the railway noise expert. In any case the sound design studio was proven to be an effective tool in communicating the governing parameters behind a good interior train sound. The effect of design improvements was auralised and the trade-off between low noise and other important goals could be demonstrated. Acknowledgements We would like to thank the team of acoustic specialists at Bombardier Transportation who contributed with data for this paper. Furthermore we acknowledge the effort from Konstfack and ÅF Ingemansson in preparing and executing the sound design studio at Innotrans. References [1] Sylwan, O. Optimum choice of internal noise requirements for operated railway vehicles. Proc. 7th International congress on Sound and Vibration, Garmisch-Partenkirchen, Germany, , 2000 [2] Poisson, F and Boullet, I. The loudness : a new indicator to specify the acoustic comfort inside trains, WCRR2008, Seoul. [3] Wahlström, P. Sound quality criteria for railway vehicles. Proc. 6th International Congress on Sound and Vibration, Copenhagen, Denmark, [4] Högström, C. Sound quality of air conditioning systems in trains. Proc. Sound Quality Symposium 2002, Dearborn, Michigan, USA, [5] Högström, C and Frid, A. Sound quality aspects in the design of railway vehicles. Proc. Sound Quality Symposium 2002, Dearborn, Michigan, USA, [6] Billström, N. Sound design of high speed railroad cars. Proc. 16th International Congress on Sound and Vibration, Krakow, Poland, 2009 [7] (in Swedish)