Audio synthesis and noise reduction in modern vehicles

Rafael A. Barkas, Gerhard Pfaffinger, Harman -March 29, 2013

Audio optimization has become a mainstream issue for today’s auto designers. From compensating for the absence of engine noise to keeping sound levels down in the vehicle interior, audio synthesis and noise suppression solutions can help.

In seeking ways to counter rising fuel prices and lessen environmental impacts, the auto industry is staking out two key trajectories. One is the industry's commitment to lightweight design. The new VW Golf, for instance, weighs in at over 100 kg lighter than its predecessor. The other is the trend toward hybrid and electric vehicles that can complete at least part of a journey solely on electric power.

What both developments have in common is the challenge they present for vehicle noise optimization. The lower the vehicle’s target weight is, the less material can be installed to reduce noise, meaning that noise penetrates the vehicle interior (and passenger ears) unmitigated.

Electric motor-powered vehicles, on the other hand, by their very nature emit little or no noise at all. As a result, pedestrians are unable to hear an electric vehicle until it is virtually upon them – or worse still, they fail to hear the car at all. Moreover, the lack of engine sound exacerbates the impact of other vehicle noise, meaning that road or wind noise, or wind may be perceived as relatively loud in the vehicle interior.

To counteract these effects, auto manufacturers need effective systems which can synthesize sound in electric vehicles to warn pedestrians, and that can also suppress interior noise successfully. A solution that excels at both, external sound synthesis and internal noise suppression, is HALOsonic, a joint development of HARMAN and the British developer Lotus Engineering.

Figure 1: Complete audio synthesis and noice cancellation system in a car

Protection through sound
The acoustic pedestrian safety feature in HALOsonic works according to a relatively simple principle. A loudspeaker is mounted to the front of the vehicle. This has the effect of projecting sound forward to the front of the car. The audio source is supplied with signals that correspond to the driving situation, ensuring that the synthesized sound is consistent with the vehicle's speed and acceleration.

One challenge is to design and position the speaker and its electronics such that they will not be damaged by the extreme conditions in the engine compartment. In addition to the intense heat that can occur here – temperatures of 120 °C are common – the highly sensitive components are exposed to a multitude of environmental impacts. When the vehicle is travelling at speed, wind, rain, dust, small rocks – all pose risk to the loudspeaker which has to be designed to withstand these elements. The electronics themselves are easier to protect. Since the electronic componentry does not have to be in a particular location, it can easily be mounted in the interior of the vehicle.

The sound produced in external sound synthesis is optimized for audibility. With frequencies between 50 Hz and 3 to 4 Kilohertz, it is right in the middle of the human auditory field, which ranges from about 16 Hertz to – in infants - 20,000 Hertz.

Sound as a brand signature
While safety is the primary interest, external sound synthesis also creates opportunities for automakers to reinforce their brand by creating unique audio signatures for their vehicles. Cars with internal combustion engines already routinely make use of sound engineering to “soundscape” the brand.

A Ferrari, for instance, sounds different to a BMW, and each has an audio signature which is in turn distinct from that of a Renault. This differentiation is now also possible with electric and hybrid vehicles. A Toyota Prius, for example, and an Opel Ampere each have their own particular audio signatures.

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