Research project: Directive Warning Sounds for Electric Vehicles

Increasing demand from both consumers and legislators for more fuel-efficient vehicles has increased the production of both hybrid and electric vehicles. When driving in electric mode at speeds below around 20 mph, where the level of road-tyre noise is low, these vehicles are significantly quieter than the alternative internal combustion engine vehicles. This is a significant benefit for environmental noise pollution in urban areas; however, the noise emitted by internal combustion engine vehicles is also an important warning signal for pedestrians, particularly those who are visually impaired, and cyclists to indicate that a vehicle is approaching. Therefore, there is currently concern that the lack of noise emissions from electric vehicles may cause a safety risk for pedestrians and cyclists and consequently legislation is being introduced to ensure that hybrid and electric vehicles are audible.

To ensure the safe operation of hybrid and electric vehicles at low speeds it may be necessary to generate an external warning sound using a loudspeaker system. By designing the directivity of the sound field reproduced by the warning system, it may be possible to reach a compromise between providing sufficient audible queues to pedestrians and cyclists and avoiding unnecessary noise pollution. This project, which was part of an MEng Acoustical Engineering Group Design Project, investigated methods of implementing a directional warning sound system for electric and hybrid vehicles.

A directional sound field can be readily produced using an array of loudspeakers; however, for the automotive application the cost and weight of such a system are too high. Therefore, a low-cost directional sound radiator was investigated which uses a single loudspeaker unit coupled with a passive radiating unit which consists of a pipe in which a line of holes is drilled. The individual holes act as sound sources and the system thus produces a directional response similarly to a loudspeaker array. The directivity of this system can be tuned by varying the size and spacing of the holes and to determine the best performance for the warning sound application a number of systems were tested in the ISVR anechoic chamber. The final design was then implemented and tested on a vehicle. The change in sound quality with direction can give a useful cue to pedestrians about the location of the vehicle.