What is ‘NVH’ and why does it matter?

NVH stands for noise, vibration and harshness. When you get back to basics, NVH is all about managing waves.

What kind of waves are we managing? A six-footer at North Gong with an offshore breeze? No. In NVH we speak in terms of airborne and structure-borne waves.

Airborne is sound, structure-born is vibration.

The two can cross over. Structure-borne can become airborne – like a speaker cone vibrating to produce sound. And airborne can become structure-borne – your eardrum converts airborne pressure pulses into movement of the tiny bones in your ear.

Why does NVH matter? Because a noisy, vibrating and harsh car isn’t a nice place to be.

Noise – the N in NVH, is simply unpleasant sound. Sound we don’t like.

It might be your neighbour’s yappy little dog wishing it had better owners at 1am in the morning; it could be your mother-in-law telling you how to cook a steak and get a job; or it might be your best buddy singing Dolly Parton’s ‘Jolene’ after a few too many shandies.

Noise, unlike Enya, is never pleasant. Cars have lots of ways of generating unpleasant noises and they get worse with age – a lot like people when you think about it.

A lot of effort goes into hiding things like brake squeal, booming noises in a hatch, or rattles from a plastic wiring loom connector under your dash.

Vibration – the V in NVH, is the structure-borne noise. It’s something you feel rather than hear.

It’s the feeling in your feet while watching the 6am freight come through on the Moss Vale train platform; it’s the horrible buzzing you feel in your hands when someone bangs the other end of the metal sheet you are holding with a hammer.

Vibration, like noise, is never something you want to feel in a car – unless you are dressing it up to feel like a Le Mans special.

It could be a nasty vibration in your hands through the steering, a vibration in the seat of your pants through the chassis, or a vibration in your ankles through the floor pan.

Harshness – the H in NVH, is a mixture of both sound and vibration. Harshness feels like something is loose and banging around, with an impact and an aftershake.

Noise and vibration are annoying, even irritating. Harshness, on the other hand, makes you think that something is definitely not right.

If noise is career advice from your mother-in-law, harshness is being cancelled by your woke sister.

In car parlance, harshness is things like a flogged-out suspension bush that floats around and then crashes rather than snubbing out smoothly when you hit a bump. It’s the feeling when the dampers overheat and the fluid becomes foam, making the car feel as choppy as an axe murder film.

It’s un-progressive and impactful, like a Donald Trump tweet – there may be a good point but it’s lost in the delivery.

Given noise, vibration and harshness all travel as waves, it begs the question: what exactly is a wave?

Waves are incredible. Almost magical. Have you ever dropped a pebble into a dam? The ripples travel outwards in concentric circles. The water isn’t really moving with a current, but the wave is passing through the water.

It’s expressing the energy of the pebble impact in three dimensions. The water molecules are pushing and pulling each other rhythmically, not flowing.

Another ripple emerges when the displaced water rushes back in and claps together, sending another pulse and shooting a drop of water into the air. This drop later comes crash-landing, creating a further ripple in the surface.

The wave pulses hit the floor of the dam and reflect, like a tennis ball against a brick wall, heading off in a mirrored direction. Like the tennis ball, some absorption takes place when the waves hits the dam floor.

As the wave hits shallow water, more cool things happen, namely refraction. The wave slows down when it hits shallow water.

Finally, waves hit a duck floating in the water. Some of the wave bends around the floating duck – a phenomenon known as diffraction. This, along with refraction, is why waves curve around a point break, always trying to face the shore.

It also explains how a muffler encourages exhaust waves to ‘open up’ and dissipate. Some of the wave can reflect off the duck and head back to the pebble source, depending on the distance between ripples.

This is how sonar finds submarines and how bats find moths.

The wave effects:

The mechanics of this last one is nuts. The sound waves resonate the natural frequency of the boundary material. This energy is converted to heat instead of bouncing off as reflection. This even happens on a molecular level, where light waves resonate electrons of different materials at different frequencies, absorbing those frequencies and reflecting the rest to give us colours.

This also explains why black cars always get hottest in the sun.

Airborne and structure-borne noise operate with the same principles that these water waves do.

A good way think of waves is the difference between surfing and rafting. Because the water is essentially staying still in a wave (until it breaks), the surfboard quickly picks up speed relative to the water which helps it plane.

A canoe going down rapids, on the other hand, is moving with the water, so it needs a lot more buoyancy.

Think of a wave as a rolling pin moving under a carpet. The carpet isn’t moving, just conforming to the energy of the rolling pin.

So the surfer has the speed of the roller pin, plus any speed they get from ‘sending it’ down the line. The canoe, relative to the water, is sitting still.

There are myriad ways that cars generate waves. Given the energy they are expending, they are basically a wave creation machine.

When you are driving, the waves come from the road, from the air and from the car itself. Noises are carried through the air into the cabin, and vibrations get passed through the chassis until they reach your body.

It’s the ‘thunk’ you hear as you drive over the railway level crossing in Thirlmere after visiting the railway museum. It’s the wail from a Ferrari engine as you hear a showy yet deeply insecure driver ‘give it some jandal’ on the Bondi boulevard.

It’s the beating noise you hear after your four-year-old drops the back window while you are sitting at 110 on the Hume on the way to Tumut to visit your aunty.

The problem with your car’s cabin is that, broadly speaking, it’s a drum. It’s a big, hollow box that gets hit by the road, the air and the car’s chassis and powertrain itself – simultaneously.

The car’s cabin acts as an amplifier for every little bit of vibration that the car body picks up on, and directs it at you and your friends, the occupants.

Big, hollow boxes come with many advantages, like safety and weather resistance, but they make the life of NVH engineers very difficult.

I was once in Boston about to go into a baseball game and came across some street buskers playing on plastic buckets with their lids on. They were incredible drummers and put on a fantastic show.

But those plastic buckets, when hit hard with a drumstick, made a lot of noise. Imagine being inside that bucket. It would be deafening. The pressure pulses would be enough to pop your eardrums – being outside the bucket was loud enough.

Being inside sealed cavities is so bad for your ears that all cars have discretely hidden one-way air vents hidden away at the back of the cabin. I once closed a door on a prototype car with these blocked off. Not a pleasant experience.

Engineering out NVH problems takes some trickery, but it fundamentally comes down to those wave characteristics we covered earlier: reflection, refraction, diffraction and absorption.

A range of different materials and designs are used to generate a combination of these four effects, depending on the problem you need to fix. They can be used to soften or enhance the sound or vibration depending on what you are looking for.

Here are five key ingredients:

NVH engineers have some pretty cool technology at their disposal to make your ride a comfortable place to be.

Vibration is measured with accelerometers and noise is measured with… you guessed it, microphones.

But these aren’t your generic, run-of-the-mill kit. Piezoelectric materials generate a tiny amount of electric charge (think a few electrons) when a mechanical stress is applied – by either the microphone drum, or a mass attached to the accelerometer.

The tiny amount of electric charge travels along a special lead cable before being amplified, then measured with a high-frequency data logger.

NVH engineers install these accelerometers in all sorts of places using the strangest ingredients. They use dental filling paste to mount them to suspension arms to ensure that there is no damping effect. They use water-cooled mounts and install them on red-hot turbochargers.

They can find the natural frequency and resonances of a part by attaching an accelerometer, hanging the part (like a wheel or suspension arm) in the air and donging it with a hammer (which also has a piezo sensor installed in it).

They can do modal surveys, creating a visualisation of how each part is vibrating or resonating at different input frequencies.

They use complex mathematics like Fourier transforms to break down the spectral components of a noise – in other words, to see how much of each frequency a noise has.

They can calculate transfer functions to understand how a vibration passes from component to component.

Quantifying those good and bad vibes needs some jargon, so memorise these for your next pub quiz.

The height of a wave is called the amplitude. The distance from peak to peak is its wavelength, and the gap between waves measured in time is its frequency (measured in Hertz, which means waves per second).

So, there you have it. Waves 101, or the wonders of NVH. Something to think about on your next drive down to the beach for a surf.