Achieving Low-Latency Active Noise Cancellation in Automotive Settings

Driven by rising customer expectations, over the last decade there has been a significant shift of priorities within vehicle design. A far greater proportion of automotive engineers’ focus is now placed on enhancing the driving experience, to make the time that people spend in their cars as enjoyable as possible. This includes—among other initiatives—keeping the background noise in vehicle cabins to an absolute minimum. 

There are various sources that can generate significant levels of noise, from the vehicle’s internal mechanics to the surrounding outdoor environment – all of which can have negative impact on the journey for its occupants. Among the most prominent noise sources are adverse weather conditions (such as wind and rain), tires making contact on uneven road surfaces, chassis vibrations, the movement of the vehicle’s suspension, its climate control system, etc. Managing cabin noise is not all about comfort though. There are also safety concerns that need to be factored in, with some studies showing that the ongoing presence of low-frequency sounds can contribute to driver fatigue and potential safety concerns.   

With sales of electric vehicles (EVs) rising, the need to combat unwanted road noise in ultra-quiet EVs is an issue that was never recognized in traditional cars because internal combustion engines (ICE) largely masked road noise. EV models are intrinsically quieter than their ICE-based counterparts, as their electric motors do not produce enough noise to drown out noise from the elements.

One solution is to insulate vehicle interiors with sound-absorbing materials. Unfortunately, this takes up valuable room in a highly space-constrained application setting and is an expensive installation that would create added costs likely deemed unnecessary by automobile manufacturers. Furthermore, passive noise-reducing materials of this kind can add somewhere in the neighborhood of 30-45kg to the vehicle’s total weight - which is clearly problematic. As a result, automobile manufacturers are keen to find an alternative solution that applies active noise cancellation technology.

Bringing active cancellation to the automotive industry

The principles behind active noise cancellation functionality are relatively simple to follow. Sensor devices detect the unwanted sound. Processing resources are then used to examine the waveform, and an equivalent inverted signal is generated to counterbalance it. Such mechanisms have been used in audio-visual applications, such as high-end headphones, for some time now.

There are certain differences that need to be considered when implementing this in vehicles. The industry needs a compact, lightweight and cost-effective solution capable of high performance. As with any component or subsystem situated in an automotive environment, the capacity to operate reliably under challenging conditions is essential. The constituent sensor technology must be robust enough to cope with extreme temperatures, vibrations, shocks, liquid ingress, etc.  

The complexity of the cabling which connects all of the sensors distributed throughout the vehicle chassis to their accompanying processing units, must also be efficiently configured and ensure no additional excessive bill-of-materials (BoM) costs. Moreover, cable harnessing is one of the foremost contributors to a vehicle’s overall weight. Keeping weight down is particularly important in EV designs so that the range between recharges is not curtailed. The lighter the vehicle the longer the range!

Maximizing the performance of noise cancellation implementations

There are two key performance parameters to consider when measuring the effectiveness of noise cancellation systems. The first is responsiveness. The system must receive and analyze the captured signal within the quickest possible timeframe, and then generate the corresponding inverted signal to compensate, in order to achieve acceptable results. This is why the shorter the latency period, the more effective the noise cancellation process will be. Secondly, the sensors’ ability to detect the largest possible proportion of the noise signals is paramount. Sensors will fail to pick up signals under a certain threshold (and these will thereby be ignored by the noise cancellation system). Therefore, automotive engineers need to specify sensors that exhibit a very low ‘noise floor’. 

A fully automotive optimized solution

Through collaboration with semiconductor vendor Analog Devices and acoustic software provider Silentium, Molex has introduced a solution for dealing with vehicle noise issues. Not only is it effective, but it also adheres to the specific requirements set by automotive manufacturers (in terms of cabling weight, BoM costs, continued operational reliability, etc.). The Molex road noise cancelling (RNC) sensors feature advanced MEMS-based accelerometer technology. Additionally, they possess a low latency figure that is below 150μs (typical) –   a standout attribute unmet by competing solutions.

The RNC sensors utilise Automotive Audio Bus (A2B) proprietary interface technology developed by Analog Devices. This enables high-fidelity audio to be delivered, while minimizing cabling - so that the cost and weight do not become prohibitive.

Typically, each sensor would be connected directly to the signal processing unit via individual cables, but this is very inefficient from both a weight and space utilisation perspective. However, A2B interface technology allows an increased number of signals per chain to be processed and streamlines the cabling required. The data transfer rates that the A2B network supports allows RNC sensors to transmit noise signals to their assigned processing unit in under 2ms. 

The A2B audio bus technology connects all the sensors in a daisy-chain arrangement. This streamlines network topology by reducing weight, space and material costs. Up to nine sensors can be connected together over a 30m cable length resulting in a 30% reduction in cabling weight.

To ensure that the largest possible proportion of noise is eliminated, Molex RNC sensors deliver heightened sensitivity. The upshot of this is an ultra-low noise floor, which means more of the original waveform can be detected and subsequently addressed. Figure 2 illustrates a sensor device with a noise floor that is way above 100µg/√Hz compared to a Molex RNC sensor (which has a noise floor below 100µg/√Hz) assimilating the captured noise signal. The outcome is that the Molex RNC Sensor can eliminate 90% of all unwanted noise. This can be achieved over an extensive range of different frequencies – all the way from 20Hz to 1kHz. Even noise at slower vehicle speeds can be mitigated.

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Figure 2: The benefits of a lower noise floor - a) shows the original noise signal b) the noise floor of a standard sensor c) the noise floor of a Molex RNC

Based on the popular Molex Mini50 family, the connectors incorporated into this active noise cancellation solution enable 50% space savings to be derived over standard USCAR 0.64 connectors. The sensors are housed in heavy-duty enclosures (with ones that are IP6K9K rated being available if required) allowing the sensors to be positioned closer to where the noise is emanating (e.g. the road itself) and accelerating the whole noise cancellation process.

Conclusion

As cars become lighter and increasingly electrified, reducing road noise will emerge as a significant imperative.  Fortunately, Molex RNC sensors provide today’s automakers a head-start on designing and deploying a lighter, higher-performing, more flexible, and more efficient solution for reducing unwanted noise.

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