How It Works: Active Noise Control

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Unless it’s a flat-out and gnarly sports car, most people like their vehicles quiet inside. Bank-vault quiet has long been associated with luxury, and mainstream automakers are now trying to turn down noise to make their vehicles feel more upscale, too.

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Automakers put sound-deadening barriers in their vehicles, including in the floor and cowl, but even with modern lightweight materials, they can only add so much.

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How It Works: Active Noise Control Back to video

Piling it in more adds weight, and that in turn affects fuel consumption. Many companies are now also using an electronic method, active noise control (ANC), which cancels out the noise rather than just muffling it.

ANC becomes even more important as car companies adopt fuel-saving engine technologies. Engines with cylinder deactivation can generate unpleasant, low-frequency noise when they’re running on three or four cylinders, as can engines running at lower revolutions when their multi-speed transmissions move into their higher gears as quickly as possible to save fuel.

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This is in addition to the sounds created in all vehicles, including road noise from tires and suspension vibration, and wind noise.

How ANC works

The technology works on the same principle as noise-cancelling headphones. Microphones in the vehicle’s cabin pick up sounds and relay them to the system’s computer, which determines the sound waves. Then, for each wave of unwanted noise, it comes up with a wave that’s completely the opposite. It issues it as a sound out of the stereo speakers, whether the stereo’s on or not.

Known as “destructive interference,” it’s virtually imperceptible to the occupants, but it’s enough to cancel out the problem. The peak-and-valley air pressure waves of the ANC sound, 180 degrees opposite to the high and low waves from the engine or road noise, essentially combine both sets of waves into an almost flat line, and very little – if any – of the unwanted noise reaches the occupants’ ears.

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Other noise-cancelling methods

There are numerous ways to quiet a vehicle’s cabin, beyond just adding sound-deadening material. Acoustic glass can be used in the windshield and side windows. All windshields are made of two pieces of glass with vinyl sandwiched in between, to keep it together if it breaks.

While that helps to cut down noise on its own, acoustic glass has an extra layer of noise-reducing vinyl. An acoustic windshield can make a considerable difference in cabin noise, but it is pricier than a regular one.

Automakers also add seam sealant between body panels for noise reduction as well as for waterproofing, and may use hydraulic body mounts to reduce vibration.

Hyundai’s new RANC

Hyundai recently announced a new spin on existing ANC technology, which it calls Road Noise Active Noise Control, or RANC . The company says RANC can make a vehicle even quieter because it overcomes the limitations of active noise control – namely, that ANC only works when the unwanted noise is constant and predictable, such as that produced when an engine is running.

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Road noise, on the other hand, can be erratic and take just a split-second to make its presence known to occupants, and ANC can’t react quickly enough.

In addition to interior microphones and sensors used for ANC, the RANC system also uses an acceleration sensor to calculate vibration from the road and sends it to a control computer for analysis.

The signal speed and calculation are very fast, taking 0.002 seconds to analyze the noise and then send an inverted soundwave through the stereo speakers to counteract it. Hyundai says it takes about 0.009 seconds for road noise to reach the vehicle’s occupants otherwise, so the sound waves would be flatlined before anyone could hear them.

In tests, RANC reduced noise in the cabin by three decibels, roughly half the noise of a car without it. It has the potential to reduce the amount of sound-deadening material needed in vehicles, which in turn reduces overall weight. The system also seems custom-made for electric vehicles, where road and wind noise can be even more noticeable to occupants since there’s no engine noise alongside.

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Active Noise Addition

But while a quiet interior might give a vehicle a premium feel, not all cars are meant to be silent, and that can be an issue in a performance car. It’s not the same if you can’t hear the engine growl, and that might not come through loud and clear if the cabin’s been insulated and its noise-controlled.

To get around this, some automakers play a different type of music through the stereo’s speakers: a recording of the engine, cued up so the sound matches what the real engine’s actually doing. Drivers will still hear what’s under the hood, but the recording beefs up the sound.

Much of a performance car’s rumble is the exhaust note, and some include a switch that opens a valve for a louder and sportier sound.

But when it’s too much, Ford includes “Quiet Time” mode on its Mustang GT, which lets you cut the exhaust noise when starting or driving. Apparently one of Mustang’s engineers annoyed his neighbour with his loud exhaust first thing in the morning. The neighbour called the cops, and that prompted the invention of this ultimate in noise cancellation.