Consumers are now increasingly accustomed to great audio in the home – in part, thanks to the proliferation of surround sound home cinema – and already have high expectations on availability of good quality audio in the car cabin. This demand has led to more advanced in-car audio systems, especially with factory-fit systems now offering improved clarity and resolution. In fact, so good is the current in-cabin audio quality offering that a recent survey conducted by infotainment supplier Harman showed 88% of respondents recognising that music in the car makes driving a more pleasurable experience. Of particular note is the rise among younger age groups enjoying music on the move – over 70% of 18-24 year olds now prefer listening to music in the car over anywhere else. Coupled with the opportunities to upsell premium audio, unlocking further revenue streams, it is not surprising that nearly all OEMs offer a branded infotainment system.
Yet, there remains one sound that can completely destroy this experience and ruin the best designer’s efforts to create an appealing in car environment: road noise. The continuous low frequency rumble and drone caused by vibrations transmitted from the road surface into the vehicle cabin can sometimes make even basic conversation difficult, let alone enjoying quality audio. Exacerbated by design trends, notably increase in wheel size and the usage of light weight structures, in-cabin noise intrusions appear to be set to stay for the long haul. Counteracting this increase in noise by increasing the music volume offers little solace and only adds fuel to fire in the form of increased distraction and stress.
However, the answer could well be to fight noise with noise – by applying “anti-noise” to one of the harshest and most challenging of environments – namely the vehicle’s in-cabin space. Anti-noise is created by generating a sound wave that is opposite in phase to the unwanted noise, effectively cancelling it out; the process is similar to that used in noise cancelling headphones.
Lighter vehicles, wider tyres: noisier vehicle interiors?
Trends in vehicle design and manufacturing have a significant impact on noise levels in vehicle cabins. Multi-link suspensions create multiple noise paths into the vehicle. While stiff bushings are often the best choice when trying to design a vehicle with great handling and cornering, they tend to allow more unwanted noise into the cabin. When combined with the universal trend toward wider, low-profile tires, the result is more road noise being transmitted through the structure of the vehicle, creating a ‘droning’ low frequency noise. Moreover, in their pursuit to meet regulations around fuel economy and CO2 emissions, OEMs worldwide place considerable emphasis on weight reduction. The latest Peugeot 208, for example, is 100kg lighter than the previous model, and the new BMW M4 tips the scales at 80kg less than its predecessor. To pare down the weight, OEMs often use light weight structural materials, which in turn negate the chassis capability to damp-out noises coming from the road surfaces, causing in-cabin noise intrusions.
With car manufacturers becoming more weight conscious, any solution to mitigate road noise inside the car must work without putting back in the weight that OEMs are trying to save. Traditional NVH (noise, vibration, and harshness) methods typically involve adding heavy damping material to tackle the low frequency noise. The added weight increase increases fuel consumption and emissions and can lead to degradation in vehicle handling and overall performance.
Make some noise
It’s here where supplier innovation might be able to provide a novel solution – and from unexpected quarters. At International CES 2015 in Las Vegas, Harman’s booth featured a demo vehicle equipped with an electronic solution that uses the car’s standard audio system, to cancel low frequency road noise transmitted into the car-cabin – a solution that does not induce the weight gains normally associated with traditional NVH damping measures.
To develop such a solution, Harman partnered with automotive development specialist Lotus Engineering to create the HALOsonic suite of noise management solutions, built on patented Active Noise Control Technologies. The suite included Road Noise Cancellation (RNC), a first-of-its-kind technology for the automotive industry, which works by generating anti-noise to counteract the road noise. By minimising the low frequency road noise in the cabin, RNC aims to enable OEMs to use lighter materials and improve fuel economy without compromising in-cabin noise levels.
To implement RNC, accelerometers are placed along the suspension and chassis of the vehicle, to provide a reference for the cancellation signal thereby enabling the system to measure correlation of vibration coming from the road and the resulting noise inside the cabin. Subsequently, the proprietary algorithm in the controller creates inverse sound waves that are delivered through the car’s standard audio system, cancelling out the noise caused by the road-induced vibrations. Additionally, error microphones in the roof of the car provide adaptive feedback on the cabin noise level in order to fine-tune the cancellation, helping occupants enjoy an enhanced driving experience. As a reminder of its audio experience from the music and recording industry, Harman has included in its RNC system offering its patented True Audio technology; this technology ensures that only unwanted road noise is cancelled, while preserving music signals in a similar audio range.
The supplier says an uncompromised sound experience is more important than ever to today’s drivers. A quieter vehicle interior means drivers and passengers are less likely to become tired or distracted, particularly over long journeys. By taking the stress-inducing low frequency road noise out of the mix, RNC technology provides OEMs with greater opportunities to use lightweight materials and improve fuel economy without the risk of compromising quality and harmony in the car cabin. Expect to be hearing much more about such technologies in the future…