There are a few important elements that automakers must consider if they want to make comfortable cars. Aside from high-quality materials for the cabin, ergonomic design, and driving comfort, cabin silence would be the most important factor.
There are various kinds of noise coming from a driving car, such as engine noise from the powertrain, drag and wind noise, or road noise. Among them, the road noise is the loudest and the most common.
The human hearing range is commonly given as 20 to 20,000 Hz, and the noise from a driving car has a range of 20 to 10,000 Hz. Booming (20~150 Hz), tire cavity (150~250 Hz), and rumble (250~500 Hz) are low-frequency (under 500 Hz) noise, and they are observed on the roads. On the other hand, the high frequency (above 500 Hz) pattern noise is measured from air conditioners, ventilated seats, and wind noise.
Road sound is the noise due to the interaction of tire and pavement. Within the speed range from 60 to 80 km/h, when the road noise becomes a dominant noise source in the vehicle, changes in the noise level inside the cabin can make passengers stressful. Sometimes passengers even face a panic attack.
Because of various noise sources, the minimization of interior cabin noise has been a key topic of research in the automobile industry. This problem was first approached through Passive Noise Controls, where physical treatments such as structural damping and acoustic absorption were used under the hood or in the engine room.
Passive noise control is sound reduction by noise-isolating materials such as insulation, sound-absorbing tiles, or a muffler rather than a power source. A stronger body structure also makes the cabin quieter, and so do double-pane windows.
Although these methods were a success, the mass reduction required to lower CO2 emission and fuel consumption can lead to considerable augmented noise levels, which should be adequately compensated by integrated, light noise abatement means.
Another problem is that low-frequency noise is hard to minimize and costs a lot to handle. Because of this, some vehicles are specifically designed to reduce air resistance.
As a result, the ANC(Active Noise Control) started to be used in the Hyundai Palisade and Kia K9. Active noise canceling is best suited for low frequencies (65~125 Hz).
Other companies such as headphone manufacturers are already using the ANC technology commonly, but it is much easier to apply. In noise-canceling headphones, the relatively small space between the headphone speaker and the eardrum is one reason that ANC can be made to work so well in these applications.
Besides, due to the limitations of noise measurement and analysis technology, the existing ANC for vehicles was only able to be utilized when noise was constant and the occurrence of the noise predictable. The current ANC technology has been most commonly used to counteract constant engine noise.
Hyundai Motor Group’s RANC(Road-noise Active Noise Control), however, eliminates road noise by huge.
Because of various noise sources, the minimization of interior cabin noise has been a key topic of research in the automobile industry. This problem was first approached through Passive Noise Controls, where physical treatments such as structural damping and acoustic absorption were used under the hood or in the engine room.
The RANC system uses velocity sensors, amplifiers, microphones, along with Digital Signal processor (DSP), to cancel the noise. It also uses the audio already built in the vehicle to make the system as simple as possible.
Using an acceleration sensor, RANC calculates the vibration from the road to the car and the control computer analyzes road noise. Finding the optimal locations for the sensors is one of the most important factors to trace the sources of vibration. The Hyundai research team received a patent for its technology.
Given that it only takes about 0.009 seconds for road or engine noise to reach the passenger, the whole process should take less than that. As its computation and signal transfer speeds are optimized, it only takes 0.002 seconds to analyze the noise and produce an inverted soundwave, generated by the DSP (Digital Signal Processor).
The microphone constantly monitors the road noise cancelation status and sends the information to the DSP. The RANC is able to conduct accurate noise analysis and rapid computation to combat road noise for the driver’s seat, the passenger seat and rear seats separately.
This is possible because the RANC system uses light signals, which travel 300,000 kilometers per second. With the right algorithm and technology, the system can control any noise regardless of the distance. The RANC also minimized all the time needed for computations and measurements.
Based on tests evaluating road surface, vehicle speed, and different seating positions, RANC was able to reduce in-cabin noise by 3dB. That 3dB level is roughly half the noise level as compared without RANC.
Thanks to this achievement, the Hyundai Motor Group can potentially decrease the amount of unsprung weight in a vehicle, utilizing fewer sound-insulating parts and dampers as compared with before. The RANC system decreases low-frequency noise emitted especially by elevated viaducts or rough roads significantly.
The NVH Research Lab in Hyundai Motor Group (Head Researcher Kang-duck Lee) went through years of development for the mass production of RANC. Especially, the pre-production phase involved an open innovation between industry and academia, with participation by Korea Advanced Institute of Science and Technology (Prof, Young-Jin Park), WeAcom, ARE, BunYoung and more. Furthermore, the mass production phase was carried out with a global car audio company, Harman to increase the degree of competition.
“At the beginning of the development, the analog sensors we used cost over 20,000 USD each, so we had to switch to digital sensors. That meant that 2 years of research had to go down the drain because we have been depending solely on the analog sensors. We had to make everything again – the velocity sensors, microphones, the controllers and all,”, said Kang-duck Lee, Head Researcher.
Hyundai Motor Group went through six years of research and development for the production of the RANC, to overcome various obstacles.
The application of the RANC system can be endless. The advantage of the system is that it is lightweight. The weight of the velocity sensor, microphone, and the controller combines reaches only 1kilogram.
Looking to the future, in-cabin quietness becomes more important as internal combustion vehicles are phased out. Vehicle interior noise primarily comes from three sources: vehicle powertrain noise, road noise, and wind noise. There is almost no powertrain noise from electric and fuel cell electric vehicles, so quelling road and wind noise becomes even more imperative (passengers can hear it from 100 km/h in conventional vehicles, but 80 km/h in electric vehicles.).
Utilizing RANC can significantly reduce road noise and create a serene cabin for future electric and fuel cell electric vehicles. Hyundai is working on improving the system to cover the frequency range from 500 to 5,000 Hz, and it will be first used in the upcoming Genesis.
HMG Journal Operation Team
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