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What is NVH?

NVH is a measure of how much unpleasant aural and tactile feedback the car delivers as you drive.
Stephen Corby
Contributing Journalist
CarsGuide

20 Oct 2015 • 6 min read

What does noise, vibration and harshness mean and what techniques used by carmakers to reduce it?

Acronyms are part of the patois of today's obviously exhausted youth. Actually saying all the words in "You Only Live Once", "Not Suitable For Work" or "Rolling On Floor Laughing" would apparently leave them spent, so they have cleverly shortened their speaking time by acronymising common terms because, like, YOLO.

Even these clever clogs can't keep up with the world champions of acronym production, however, the car companies.

Manufacturers like to stuff as many of these as they can into every car they build, from the exciting sounding ESP through EBD, DOHC, AEB, BLIS and plenty more, but there is one in particular that they spend millions trying to do without – NVH.

Exposure to noise, vibration and harshness is best kept to a minimum

It stands for Noise, Vibration and Harshness, and is basically a measure of how much unpleasant aural and tactile feedback the car delivers as you drive.

It's pretty well known that, much like cholesterol levels, stab wounds and K-Pop "music", exposure to noise, vibration and harshness is best kept to a minimum.

So what does NVH actually mean?

Put simply, the noise is what you can hear, the vibration is what you can feel and the harshness is how much of an effect thumps, bumps, noise and vibration have on the cabin and its occupants.

Removing and isolating passengers from NVH is a field that's seen monumental advances over the past few decades. Originally the preserve of the most luxurious marques - and an acronym that was only even discussed by premium brands - NVH reduction is a global push, spurred on by consumers who want increasing insulation from the environment they're driving through. Perhaps to avoid the sound of other humans ROFLing.

Today, we even expect sporty convertibles to be NVH minimal.

This presents a quandary for engineers. Traditional sound and vibration-deadening material adds weight, which can spoil acceleration, handling and fuel economy, so they've had to come up with lighter and more effective methods than the traditional bit of tar paper.

In some cases, though, technology that offers a better drive can also offer a quieter ride. Unfortunately, it doesn't come cheaply.

What causes NVH?

Noise, vibration and harshness are caused by the car's mechanical and electrical systems, as well as the car's interaction with road surfaces and its passage through the air.

The main sources of NVH in a car are its engine, drivetrain and tyres, as well as the sound of airflow along its body.

Tyres are a huge source of noise

While we love a good engine note or exhaust burble, undesirable mechanical sounds and vibrations can emanate from a car's oily bits, such as a howling radiator fan, a shuddering when idling at traffic lights or a harsh, wince-inducing thump from a pothole.

Tyres are a huge source of noise, due to vibrations created by their rotation, which then travel along suspension mounts and into the cabin.

Low-profile tyres, found on sportier cars, tend to transmit more vibration through the suspension because the tighter, stiffer rubber can't absorb as much. Wide tyres are more susceptible due to a larger contact patch, but the soft, wide tread patterns of off-road tyres are the worst offenders.

Airflow over the car's body causes quite a bit of noise, especially at higher speeds and in large cars with the aerodynamic properties of a shipping container. Any interruptions the air faces in its journey across the car – such as aerials, roof racks and door mirrors – are rewarded with howling, whipping or buzzing noises in the cabin.

So, how do you reduce NVH?

Cancelling out NVH comes down to three basic principles – reducing it at the source, isolating it from the car's main structure and absorbing as much as possible before it enters the cabin.

The first – if not the easiest – way to reduce NVH is to curb the number and volume of the sounds, vibrations and shocks produced in the first place.

Aerodynamics, for instance, play a huge part in the amount of noise. As air flows over the car body, any disturbances in its path create vibrations of the air particles, which is how sound is transmitted.

Car makers strive to make precise and well-balanced engine and drivetrain components to curb vibration

The weirdly shaped headlights on the Nissan Leaf are a good example of aerodynamics at work. According to Nissan, the headlight covers are shaped to redirect airflow past the wing mirrors; without some subtle air bending, the wind noise over the mirror housing would be conspicuously audible over the golf cart hum of the electric motor.

Car makers strive to make precise and well-balanced engine and drivetrain components to curb vibration; similarly, tyre manufacturers vary compounds and tread patterns to reduce unwanted harshness.

The second and perhaps equally complex way to reduce NVH is to isolate the car's main structure from the source. Complex and expensive suspension tech such as independent, multi-link and hydro-pneumatic systems isolate the chassis and steering column from road surfaces, removing vibration and shocks.

To further isolate the sources of noise, engine, suspension and exhaust components are mounted on rubber and polyurethane bushes rather than solid metal; the softer compounds work against the transmission of vibrations.

The last resort in the war against NVH is also the most conventional: absorbing as much as possible. Mechanically speaking, this tends to refer to steering dampers, which absorb the worst of the shocks introduced by poor road surfaces.

In the cabin, passive sound absorption can extend from a smattering of rubber-like sound deadening material fixed to various points along the chassis, all the way up to foam-filled pillars and double-glazed windows.

Double-pane glass and sound insulation tends to be pretty heavy, so the extent of sound deadening usually relates to the car's price and purpose. Rolls-Royces, for instance, are built like recording studios.

The latest advance in reducing NVH only focuses on noise, so it doesn't make the holy triumvirate of NVH reduction, but is worth mentioning nonetheless. Active noise cancellation, as found in $400 headphones, is making its way into high-spec passenger cars. It operates by sampling the amplitude and frequency of noise in the cabin and pumping a sound wave out of the speakers at the exact opposite phase of the original noise. When the two waves find each other, they cancel out, resulting in an eerily silent cruise.

They work in tandem with the stereo, so, even while you're listening to music, it'll pump out an extra note to counter the road noise, leaving you to enjoy K-Pop in peace.

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