Electric vehicles and the sound of silence

20 de enero de 2010 Automotiveworld.com

The potential threat of noiseless electric and hybrid cars to pedestrian safety is high on the agenda of many of the world's legislators and Lotus, partnered by car audio specialist, Harman International has developed an electronic sound synthesis system (ESS) - called HALOsonic - which is capable of emulating a vehicle engine note. The sound is transmitted inside the car by the entertainment system's speakers and forward from the front of the car by an external speaker. It can also signal a separate reversing warning.

The collaboration is timely as worldwide legislation to ensure EVs make sufficient noise is imminent. Groups most at risk are the blind, children and cyclists and the National Federation of the Blind has been especially active in the US in trying to address this question before it becomes a problem. A research project at the University of California has established that while a conventional engine can be heard 30 feet away, an electric vehicle will close to 11 feet before being audible.

A question was raised in the UK House of Lords on November 10th which revealed that a nine month government-backed research project into the extent of any problem will start in 2010. Legislation is expected in Japan by 2012, with the United States following soon after, so demand for a solution could soon become substantial. Drive-by tests are already being formulated by the SAE to accommodate the new technology and both Lotus and Harman sit on the committee charged with establishing new standards.

A research project at the University of California has established that while a conventional engine can be heard 30 feet away, an electric vehicle will close to 11 feet before being audible.

While it's too early to put a figure on the size of the market, Harman sales director, Tony Harberman, thinks it could run into "tens of millions of dollars". Apart from giving a voice to EVs there are potential fringe benefits for sound synthesis systems too. There has been interest from OEMs in relation to downsizing where they may want a V8 engine-note replicated for a V6 or even a four-cylinder turbo. OEMs are also interested in the concept of "branded sound", as exclusive to one model of car as visual styling. At the same time, the question of authenticity is under consideration and there is some concern that customers might somehow consider themselves cheated.

However, it's just as possible that manufactured sound may provide considerable scope for extra revenue for manufacturers in much the same way as ring tones for mobile phones. Feedback from customer OEM clinics also suggests that in some circumstances of lifting off an EV or hybrid accelerator, some drivers feel a lack of control, as if coasting. Neither Lotus nor Harman have many doubts that now is a good time to develop the right system. "Manufacturers are not waiting for legislation - they are already motivated by product liability and PR considerations," says Lotus NVH engineer, Andy Mackay.

The HALOsonic ESS uses the same digital sound processor found in an audio system to create the sound - it is not recorded in any way. The pitch of the sound changes with speed. Externally, it is directional and projected in front of the car to minimise noise in the environment. There's an idle sound on starting to warn pedestrians that a car is about to move off and noise is also transmitted inside the car for the benefit of the occupants.

It is here that Harman's experience counts in creating the correct acoustics for the occupants so that the sound appears to be coming from a real engine rather than a virtual one. This experience is important when it comes to developing synthesised sound for automobiles. "We can measure air pressure with a microphone but that doesn't tell us how the brain is responding," says Mackay. The system is being demonstrated in a Toyota Prius and drivers can select from a range of different engine sounds from a conventional four-cylinder to a V8.

Another important aspect of the HALOsonic system is its potential for noise cancellation inside the car. For "noise" read NVH, tiresome low frequency sound caused by engine order vibration and road noise. For years NVH was dealt with by adding heavy sound deadening to the car.

There's an idle sound on starting to warn pedestrians that a car is about to move off and noise is also transmitted inside the car for the benefit of the occupants.

These features, engine order cancellation (EOC) and road noise cancellation (RNC), were demonstrated by Lotus in the early 1990s in a project led by the head of NVH at the time, Malcolm McDonald. The system was installed in a Citroen AX, chosen because it was one of the lightest, cheapest cars available. The AX was equipped with a minimum of sound damping material, which comprised an engine sensor to measure engine order vibration, eight cabin microphones to measure road generated noise and four speakers. The system would detect the onset of the low frequency sound waves responsible for the irritating "booming" inside the cabin and generate sound at exactly the same frequency to cancel it out. Highly effective, it later went on to be used by Nissan in the 1990s, but subsequently disappeared from the radar. Lotus also went on to develop active engine mounts which performed a similar function, insulating the bodyshell from predictable engine vibrations.

The new technology is incorporated into the cars audio system as before, but this time it benefits from more complex algorithms running on a much faster digital processor at a lower cost. It can be combined with the noise generation system if necessary and is adaptive. "You need an adaptive system," explains Mackay, "because the cars are not steady state. Acoustics are affected by the number of passengers, luggage and the age of the car."

NVH caused by engine order vibration is associated with the harmonics of engine rotation. In a four cylinder engine, this usually peaks at around 4,000rpm. "Order" means the number of disturbances which occur on each revolution. A four-cylinder four-stroke engine has two, so the system must deal with second order vibration but it can deal with up to four orders of vibration at the same time.

In what's known as a "feed forward" process, the engine speed is picked up by a sensor and by the time sound has reached the ear, the processor has received a signal from the engine sensor, processed it and transmitted a cancellation signal via the amplifier to the speakers. The frequency levels are low and at 7,000rpm, second order frequency is 233Hz. "This coincides with the region where passive measures are effective, such as structural enhancements or the use of bitumen pads," says Mackay. However, these tend to add weight and cost to the vehicle.

Active methods are more efficient and more effective with error microphones picking up any residual noise and feeding it back into the system, allowing the noise filters within the software to be continually optimised. Algorithms are designed to be very robust, so in the event of sensor failures, the system doesn't inject unwanted noise back into the cabin.

The use of such a system not only does away with the need for the use of structural or damping methods to reduce vibration, but also the need for balancer shafts in four cylinder engines. There's a CO2 implication here because apart from reducing cost, this would also reduce the weight and package size of the engine. Mackay says balancer shafts also consume as much as 1kW of power at peak revs, whereas EOC only consumes a few Watts.

An alternative to using speakers would be the use of a seismic absorber - a similar idea to an active engine mount but mounted anywhere on the body to literally shake it in sympathy with the engine order vibration

An alternative to using speakers would be the use of a seismic absorber - a similar idea to an active engine mount but mounted anywhere on the body to literally shake it in sympathy with the engine order vibration, cancelling it out at source. Such a device need only be around twice the size of a matchbox to be effective and would be an improvement on the active engine mount which only works best vertically.

Road noise is also low frequency, generally below 250Hz. "It's broadband and random", says Mackay, "caused by the vehicle’s interaction with the road surface which excited the structure." The wavelength corresponds to the size of the cabin, with higher frequencies being harder to control because there are more of them.

The effect is likely to be worse in vehicles with class leading handling dynamics. This is where the system is particularly effective, making such cars a match for those which are class leaders in refinement and comfort. The system has been so effective in trials it has been possible to reduce unwanted noise in the cabin by a factor that would be impossible to achieve using conventional methods.

Lotus says it is receiving considerable interest from OEMs on both the synthesis and the noise cancellation system. The latter is due to go into production in 2010 with a major manufacturer and, given the need to reduce weight while increasing refinement, could receive a heartier welcome in the marketplace than it did almost 20 years ago.