The potential danger to pedestrians—particularly those who are blind or partially sighted—of near-silent hybrid and EVs (electric vehicles) approaching at low speeds has prompted Lotus Engineering to develop a synthesized internal combustion (IC) engine sound system that can be used to warn of the vehicle's approach. The engine sound is projected via an amplifier and loudspeakers.
Called Safe&Sound and installed for demonstration purposes in a Toyota Prius, the system is an extrapolation and development of Lotus’ Sound Synthesis Technology, which is part of a suite of patented active-noise technologies that also include active-road-noise and engine-order cancellation.
For the hybrid application of synthesizing an engine sound, a road-speed signal is taken from the vehicle and a weatherproof loudspeaker system placed near the car’s radiator. If the car is propelled only by its electric motor, throttle- and speed-dependent synthesized sound projects a "realistic" engine note ahead of the vehicle. When the vehicle has passed a fixed point, the sound is no longer heard.
The technology was designed around the behavior of a conventional four-cylinder Toyota engine. The system uses an engine sound, which makes it instantly recognizable, with pitch and frequency helping identify both the vehicle’s distance from the listener and its speed. Passengers in the vehicle hear almost nothing of the synthesized sound.
If the car’s IC engine starts, the synthesized sound ceases but restarts if only the electric motor is providing motive power.
To create the required sound, recordings of the Toyota engine were made and analyzed to establish the characteristic frequencies at various engine speeds. These frequencies were then inputted to the synthesis controller in the form of a "voice." The resultant sound is broadcast via an amplifier and loudspeakers.
Lotus Managing Director Mike Kimberley said: "We hope that legislators will introduce minimum noise requirements for vehicles to encourage the adoption of technologies such as ours, which will ultimately increase pedestrian safety."