|The Safe & Sound’ Hybrid system. Click to enlarge.|
Lotus Engineering has developed technologies to synthesize external sound on electric and hybrid vehicles to counteract the growing concern these ‘quiet’ vehicles pose to pedestrians and cyclists. Lotus applied the technology to a standard Toyota Prius to create a ‘Safe & Sound’ Hybrid technology demonstrator vehicle.
The solution that Lotus has devised is a novel reapplication and development of its Sound Synthesis technology. Sound Synthesis is one element in the Lotus suite of patented active noise technologies, comprising three main systems each of which can be used individually or in combination: Active Road Noise Cancellation, Engine Order Cancellation and Sound Synthesis.
To synthesize the engine sound, a road speed signal is taken from the vehicle and a waterproof loudspeaker system is positioned adjacent to the radiator allowing the sound to emanate from the front of the vehicle. When the car is operating on the electric motor only, throttle and speed dependent synthesized sound projects a realistic engine sound in front of the vehicle. Once the vehicle has passed, the sound is not heard.
The technology was designed around the behavior of a conventional engine, using an existing engine sound which makes it instantly recognisable with the pitch and frequency helping to identify vehicle distance and speed. If the hybrid’s engine starts operating, at higher speeds or throttle demands or lower battery levels, the control system automatically stops the external synthesis.
When the powertrain control system switches the car back to running on the electric motor only, the synthesis controller instantaneously sets the system running again. It is all completely automatic and the driver hears almost none of the additional sound.
In order to generate the engine sound, recordings of a suitable donor engine were made and analyzed to establish the characteristic frequencies at different engine speeds. These frequencies are then entered into the synthesis controller in the form of a ‘voice’ which outputs the sound through an amplifier and out through the loudspeakers.
Lotus Active Noise Technologies. Active Road Noise Cancellation (ARNC) and Engine Order Cancellation (EOC) look to reduce noise levels in the cabin, particularly at frequencies that are audibly unpleasant. In the case of road noise, the system reduces broadband noise levels at frequencies below 250Hz whereas EOC tackles harmonic frequencies generated by ignition events in the engine.
Input signals from the engine (for EOC) or sensors mounted to the suspension system (for ARNC) are fed into the electronic controller, as are sound signals, measured by microphones located in the cabin. The software algorithms of the controller then calculate what sound is needed to provide cancellation and the speakers of the in-car entertainment system are used to put this into the cabin. All this occurs in the space of a few hundredths of a second and repeats and adapts constantly through the control system, seamlessly and instantaneously adapting to changes in speed or road condition. The cancellation system operates on the input signals so other noise in the vehicle such as the audio system and speech are not interfered with or cancelled. The result is a quieter, more pleasant cabin.
The third system, Sound Synthesis, enhances the sound in the cabin. The control system uses engine speed signal, a throttle position sensor and the in-car entertainment system to add sound. In this way a car could be made to sound sportier or be given the sound characters of, say, a flat 6 or V8 engine. Coupled with EOC and ARNC, the interior sound in the cabin can be tuned to enhance the driving experience and match the brand attributes of the vehicle.