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Challenge B: An environmentely friendly railway 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 , 721 73, Västerås, Sweden email: [email protected] 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 Challenge B: An environmentely friendly railway studio concept and describes how it was first realised by Bombardier at the Innotrans fair 2010. 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 Challenge B: An environmentely friendly railway 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.

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