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 traveling 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.