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What should electric cars sound like?
14 Dec 2011
Simulation can solve the surprising danger from quiet vehicles - as pedestrians no longer receive the vital aural cues they are used to. So will future electric cars emit a V8 rumble?
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Automotive | Technology
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 | | The modern drive towards efficiency has led to the primacy of the battery, rendering leading electric car technology the key to competitive advantage. But with the vital sound cues of conventionally powered cars set to disappear, what artificial sounds should be created to fill the sound gap? |
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As pedestrians we quickly decide from the sound a car makes what its basic driving parameters are, such as acceleration, speed, direction, even the attitude of the driver – and we mostly do this subconsciously. As almost-silent hybrid and other quiet vehicles gain in numbers alongside their Internal Combustion Engine (ICE) counterparts, pedestrians are no longer receiving the vital aural cues they are used to, and this may compromise their safety.
Research by the National Highway Traffic Safety Administration (NHTSA) in the US has identified that quiet vehicles are more likely to have accidents with people than normal ICE vehicles, so pedestrian safety is an increasing consideration for quiet vehicle manufacturers. The most obvious dangers occur where people and vehicles exist in close proximity – in town centres, for example – where wind and tyre noise at speeds below 30 km/h are quiet and background noise is enough to mask motor and powertrain sound.
With legislative proposals on two continents already, it appears likely that global legislation within the next decade will require manufacturers to implement external warning systems. So the question arises: how should a quiet vehicle sound?
 | | Enlarge image | | The ESS module simulates the sounds made by vehicles moving through a virtual world as experienced by a pedestrian - complete with common urban sound objects |
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There are three basic groups in the debate: environmentalists, who want a quiet environment with greatly reduced traffic noise; pedestrians and the partially sighted, who want sounds that clearly identify moving vehicles; and motor manufacturers, who want to project a brand image. Although these three groups are not necessarily totally opposed, finding the right compromise is not a simple task. To address this issue, Brüel & Kjær now offers the PULSE Exterior Sound Simulator (ESS), a module that can be added into the proven Noise, Vibration and Harshness, (NVH) simulator platform.
The ESS module is an equally valuable tool for traditional ICE vehicle development, and it can help design discrete sources, such as intake and exhaust systems, or create distinctive exterior sounds, like a pleasing exhaust burble on a sports saloon. Whatever the application, differentiation is the keyword, both in terms of accurately discerning individual vehicles and creating a sound that is ’on-brand’ and enhances a vehicle’s image.
 | | Enlarge image | | The environment includes a variety of commonly experienced situations such as carparks and roundabouts to create a complete soundscape |
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The ESS module was created to accurately simulate the exterior sounds of any vehicle moving through a virtual environment. In the design phase of a project, these sounds can be modified in real-time by the user, and the sounds of specific vehicles can be assessed quickly and easily in a variety of scenarios and background noise conditions. Using the same simulation in the evaluation phase, selected sounds can be tested and experienced by lots of people quickly in the lab, negating the need to stand on a variety of street corners as test vehicles are driven by at different times of day. One great benefit of working in a simulated environment is that the scenario is controlled, meaning that multiple evaluations can be made with comparable results in compressed time. A simulated environment can also account for future scenarios where electric cars - and their artificial sounds - are commonplace, by bringing multiple vehicles into the simulation, all with or without the designed exterior sound.
 | | Enlarge image | | Using the simulator for evaluation, selected sounds can be very quickly experienced by the same people on the same route, and then modified to display the difference |
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The ESS module includes a virtual urban environment, complete with roundabouts, junctions, traffic lights, controllable traffic, car parks and urban furniture. Combining ambient and specific sound objects like the bleeps from pedestrian crossings, the ESS module provides a complete soundscape in which to assess exterior sound accurately. Realistic but controllable ambient and traffic noise is vital to ensure effective benchmarking, as are distance attenuation, pan effects, Doppler shift, source attenuation and directivity, all of which feature in the synthesised sound just as they would in the real world.
The PULSE™ Exterior Sound Simulator is equipped with channel mapping and various 3D rendering methods (5.1, 8.1, Wave Field Synthesis, etc.) that allow it to directly drive multi-speaker systems. Each sound object in the model can be directly assigned to different speakers or positioned in space by the software to create a spatialised sound field. This takes into account factors such as the relative speed and heading of the source and the observer and the directivity of the sources.
 | | Enlarge image | | The ESS module is equipped with channel mapping and 3D rendering methods that drive multispeaker systems |
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The ESS software contains sound objects that are essentially collections of sound data and the rules needed to allow them to represent normal automotive functions such as rolling noise or engine noise. One of these sound objects, the pitchable loop, is typically used in quiet vehicle sounder systems and allows a repeating sound to vary according to vehicle parameters like speed and accelerator pedal position.
The sound objects assigned to a virtual vehicle can be anything from a full ICE engine (which is more complex than it seems) to alien, sonar-like whirring bloops, and all can be modified in real-time. Once a final set of sounds has been designed, evaluated and selected in the ESS module, the sounds can be downloaded to a SimSound module that is then plugged into a real car for a final on-road validation trial.
Sound design is already a central concern for automotive developers because it has a significant impact on customer perception. Sound design will continue to be a major aspect of the identity of future cars, an aspect that looks set to become instantly recognisable for different models – possibly as much as the name on the back or the look of the grille. Designing that sound to work with the requirements of both the environment and pedestrian lobby groups is the key challenge.
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