What makes a car sound good?

Noise, vibration and harshness (NVH) measures are all aimed at stopping a car from sounding bad, but it’s also about making a car sound good. And who knows a thing or two about making a car sound good?

Ferrari, that’s who.

More specifically, Ferrari NVH engineers.

After the McLaren 12C, the British supercar marque poached a Ferrari NVH engineer to make their engines sound less like a soup blender and more like a symphony.

It worked. The cars started sounding better in short order.

Which brings me to the point. Ferrari NVH engineers know about orders and orders are what make a car sound good.

Orders is just a mathematical term. It means multiplied by. First order means multiplied by one. Second order means multiplied by two. Fourth order means multiplied by four.

What are we multiplying? The engine speed, or rpm.

Given that all of the sound and vibration used in the order comes from a cylinder firing, the number of cylinders you have dictates which orders you want to enhance and which you want to dampen.

In a V8 engine the main focus would be on the second and fourth orders with a little bit of eighth for good measure. A V6 engine would be the third and sixth order. A V12 engine would be the third, sixth and some 12th order.

The NVH maestro would conduct his symphony to accentuate different parts of his orchestra at different parts of the rev range.

How did the NVH maestro conduct that orchestra? Pipes. Or more specifically, pipe lengths.

Given that sound travels at the speed of sound (who would have thought), supercar NVH engineers tune the length of the exhaust pipes so that special things happen when sound waves meet up with other sound waves.

Where did these sound waves come from? The ignition stroke of the engine is where the music is made.

Like a guitarist plucking a string, an Italian supercar engine creates an explosion when a cylinder fires. But a guitar string vibrating without a guitar doesn’t make music and neither does an explosion without a Ferrari. Or more specifically, a Ferrari engine and exhaust.

This is where the magic of superposition comes in.

When two waves meet each other they average each other out. The peaks and troughs, that is. If a peak meets a peak, they add to each other. If a trough meets a trough, the same thing happens in reverse.

If a peak meets a trough, they cancel each other out and there is no sound at all.

All engineers love a spreadsheet and supercar NVH engineers are no exception. They calculate the speed the wave will be travelling and how far apart the peaks and troughs are, depending on the engine speed.

From this they can figure out what length to have the sections of exhaust in order to enhance certain frequencies.

From there they get artistic, considering the lengths of the exhaust pipes coming from one of the 12 cylinders and where that should intersect with the pipe coming from another cylinder.

Where the pipes coming from this intersection should meet, and where, or if the pipes coming from the right-hand six cylinders should meet the left-hand bank of six cylinders.

The conductor of this eight- or 12-piece orchestra has to time these pulses to perfection, depending on the rpm.

The layout of the exhaust pipes and mufflers create resonances which accentuate the engine’s voice.

On top of that, careful consideration is given to how the pressure pulses in the exhaust will behave as waves.

The four main ways that waves interact (discussed in ‘What is ‘NVH’ and why does it matter?’) apply in the exhaust.

They refract after joining together, slowing down; they diffract after reaching the edge of a muffler, bending around a corner; they reflect when they reach the end of the muffler, rebounding and heading back towards the engine, creating lots of opportunities for constructive and destructive interference; and they are absorbed by the muffler lining material and the metal of the exhaust pipe itself.

Anyone who has played with a two-stroke go-kart engine will know that you can tune the power band of the engine by shortening and lengthening the exhaust pipe. It is designed for easy adjustments.

The pressure pulses from the explosive combustion reflect back when they reach the end of the muffler and actually help the intake and exhaust flow more efficiently at a different point in the rev range.

The same thing goes with sound. You can tweak the exhaust lengths to make beautiful music at different points in the rev range and the same thing works on the intake side.

At this point, a tip of the hat to Ferrari powertrain engineers is in order. A drum doesn’t make much noise without a drummer (and a good clean drum strike).

Cam timing and overlap greatly contributes to the nice noises you hear from a sports car engine.

As best demonstrated by Ace Ventura, when the intake or exhaust ports are open, nice noises in the form of pressure waves come surging out of the cylinder.

As luck or the beauty of creation would have it, nice noises often go hand in hand with a finely tuned, optimally efficient engine.

In other words, an engine punching above its weight really sings, and the singing actually contributes to the performance of the engine, which makes it sing better (love a good virtuous cycle).

Raining on our parade, along come turbochargers.

While capturing waste energy from exhaust gas seems like a good idea on the face of it, turbines that chop up the beautiful sound waves on the intake and exhaust side don’t do a lot for the symphony.

Imagine a wind turbine between you and the Melbourne Symphony Orchestra. On the face of it, good for the environment, but terrible for the sound.

Supercar NVH engineers actually route some of the unadulterated intake and exhaust pulses to little resonators inside the cabin to make it sound like you are driving a supercar, not a vacuum cleaner.

Worse, some companies (Japanese and German, allegedly) cheat by playing backing vocals on your sound system depending on the rpm and how heavily planted your right foot is.

This is the NVH equivalent of Milli Vanilli.

Milli Vanilli was on to something as he prophetically didn’t sing in ‘Girl I'm Gonna Miss You’. The days of the naturally-aspirated Ferrari V12 seem to be numbered.

Electric motors are replacing Ferrari V12s in engine bays around the world at a breakneck pace. Heck, they aren’t even in the engine bay.

Electric motors, while providing instant and close to maximum torque at any rpm don’t make loud dinosaur noises, but rightly or wrongly the dinosaurs died out.

Like most potentially lethal objects, any new car that hits the market needs to pass a wide range of certification tests before it can be sold to the public.

One of these tests is the drive by (or pass by) noise test. The regulation is in place to make sure that cars don’t deafen pedestrians.

A sound level meter is placed on the side of the road; the car approaches at 50km/h, then boots it as it drives past. The car isn’t allowed to be noisier than a certain number of decibels depending on the power and segment.

NVH engineers discovered a loophole. About 10 years ago, exhaust valves started to hit the market. Butterfly valves are incorporated into the exhaust system which reroute the exhaust gas flow through extra chambers in the muffler.

This is handy for a sports car with different powertrain modes. Nice and quiet for daily tootling and loud and brash for showing off at Surfers Paradise.

As luck would have it, these come in handy for ‘drive by’ tests…

Apart from exhaust lengths, and wedging foam into every nook and cranny before the bosses take a drive, the main thing I noticed NVH engineers changing at McLaren was engine mounts.

Soft or hard, this is the question.

As you will have guessed, soft engine mounts mean most of the engine’s vibrations are soaked up by the rubber bushes.

On the other hand, hard engine mounts make the car feel like the engine has been bolted to the car’s chassis – like a Le Mans racer. They also make you feel like doing a spanner check.

With the advent of electric cars, the symphonic science of engine orders and exhaust runner lengths will no longer be a thing.

NVH engineers will cease to care about soft or hard engine mounts – an electric motor doesn’t feel racy when it’s hard-mounted.

They may well be left with sticking foam to every surface they can find in a bid to keep their jobs.

Like many who have come before them, supercar NVH engineers will figure out a way to survive.

Perhaps by taking a renewed interest in tyre tread patterns, or by irritating vehicle dynamics engineers with requests to soften suspension bushes without considering the steering trade-offs.

I shouldn’t be pessimistic. NVH is one of the most complex disciplines in automotive engineering. There will always be deeper to dig down that rabbit hole.

Paul Batten is a race and rally driver and a former senior vehicle dynamics engineer for McLaren Automotive and, prior to that, Prodrive/Ford Performance Vehicles.