HALOsonic can also influence sound inside the vehicle by optimizing the audio signals that passengers hear. As cars become lighter, insulation material becomes thinner, and considerably more unwanted noise gets through to the passengers' ears. The HARMAN solution counteracts this with two systems: Engine Order Cancellation (EOC) and Road Noise Cancellation (RNC). Both work based on the same principle that employs anti-noise to counteract engine or road noise.
Figure 2: External sound synthesis allows for more focused sound emission and less noise pollution
EOC helps to control engine noise. This type of noise is called narrowband because at a given time, it occurs over a narrow frequency range whose value depends on the engine's rotational speed (RPM). HALOsonic analyzes the sound signature of the engine throughout the relevant RPM range. Thus, the HALOsonic system “knows” what sound waves an engine will produce at a certain RPM. These sounds include the primary sounds created by combustion and the motion of the major engine components, and also the harmonics of those frequencies. EOC is capable of achieving a peak noise reduction of up to 20 dB.
Cancellation of the engine sounds is not perfect, largely because the engine and the speakers are in different physical locations. In order to help correct for this imperfection, microphones are positioned above the seating locations. These microphones pick up any residual engine sound that is not cancelled. The HALOsonic processor then creates an inverted version of the residual engine noise and sends it out through the audio system, which increases the overall efficacy of the noise cancellation.
Figure 3: Thanks to EOC, a reduction of up to 20 dB is possible over a wide rpm range. For higher resolution click here.
RNC works similarly to engine noise cancellation and ensures that unwanted noise caused by the road surface and tire resonance is less audible in the interior. It senses vibration coming from the road, and then creates an inverse sound wave through the car's audio system to cancel out the noise caused by the road-induced vibration. To implement RNC, each wheel assembly is typically fitted with a pair of accelerometers. By comparing the output of the two accelerometers, the HALOsonic processor can measure the activity of the suspension at each wheel, and thus knows when, where, and at what intensity road vibration – and thus, road noise – is occurring in the vehicle. The HALOsonic processor then creates an inverse sound wave through the car’s audio system to cancel out the noise caused by the road-induced vibration. However, the nature of this noise is different in that, unlike engine noise, road noise is very broadband, extending over a wide range of frequencies. Therefore, the system is able to reduce the peaks of this noise by 10dB and 4-5dB across the target frequency range. This level of performance significantly improves the perceived quality and comfort of the car’s interior.
As with EOC, RNC does not perfectly cancel road noise because the road noise and the cancelling sound wave from the audio system come from different physical locations. To enhance the efficacy of RNC, the same type of small overhead microphones used for EOC are used to provide feedback to the HALOsonic processor. The microphones pick up any remaining road noise, then the HALOsonic processor produces additional sound waves to cancel out the residual road noise.
The power of sound
At present, sound synthesis and noise suppression technology is still in its infancy (so far there are only a few manufacturers developing the systems, let alone installing them in vehicles). Yet development is moving ahead at a brisk pace. The more vehicles are equipped with quiet hybrid or electric motors, the greater need there is for solutions to make the engines more audible to pedestrians. And interior noise reduction has been an industry focus for decades already. So we can expect to hear more – or hear less, depending on the objective – from these two areas in the future.
About the authors
. Rafael A. Barkas is Director of Business Development, Harman Lifestyle Division – Automotive Audio SBU.
. Gerhard Pfaffinger is Senior Manager, Harman Lifestyle Division – Automotive Audio SBU.
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