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Sound design: why motors sound the way they sound

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I The concert hall is dead silence. Then the first note. Loud and impressive, as if the singer wanted to report back after a long abstinence. A deep growl like that of a lion opening its mouth and whose sound is connected to an amplifier. The concert hall bears the number 911. Porsche 911.

The sound design is very important at Porsche

Dr. Bernhard Pfäfflin sits in this concert every day. He is head of the department for acoustics and vibration technology at the Porsche development center in Weissach. The man who, with his team, gives every single Porsche its voice. In addition to creating noises such as doors slamming or the clicking of individual buttons, the engine's sound design is of particular importance in his work. “Everything that has to do with emotionality is very important for Porsche,” he says. 'In addition to the design, the sound contributes to the emotional appeal.'

The same applies to the Italian sports car manufacturer Lamborghini. 'In a car like this, where the engine and the design play such a big role, people have to immediately recognize by the sound that it is a Lamborghini,' explains Maurizio Reggiani, Head of Technical Development. Because sound is not just sound. Even if the same engine sits under the sheet metal cladding, they are not identical twins, just siblings. Every car has its own characteristics. “Our supercharged 3.0-liter straight-six is ​​in the BMW 135i and 335i. The exhaust system and the intake are different, ”says Robert Mirlach, Head of Engine Acoustics at BMW. A major difference. After all, the exhaust system and intake system are like different instruments in an orchestra. Together with the noises of the engine mechanics, they form a harmony.

The computer plays a major role in sound design

Most car manufacturers or exhaust system developers develop this on the computer. After the engineers have eliminated all disturbing noises on the car, they consider which sound suits the character of the car. “The Porsche GT3 should sound different than the Panamera,” says Pfäfflin. The same goals are pursued at BMW. While the V8 engine in the X family expresses its strength with a low-frequency bubbling, the driver in the Z models should perceive the V8 as powerful and eager for revs.

At Porsche, where the sound is completely self-developed becomes,the second step is the calculation of the many components using a special program. 'We calculate every volume, air filter, pipe, silencer, every part that is involved in the gas exchange system,' says Pfäfflin. Four to five of the umpteen variations are shortlisted. The components are built as prototypes and evaluated in the recording studio. The sound designers modify them until the desired sound is created. 'We don't just listen to it and say 'sounds good' or not,' says Pfäfflin. 'We graphically display on the computer what tone and sound look like and how the sounds are composed.'

The sound designers work tooth in tooth with the engine developers

Use these graphics to evaluate the specialists develop the sound and decide which screws to turn. The whole thing is like a big puzzle. With his Bavarian colleagues, sound designer Robert Mirlach uses the previous model to decide which parts of the sound he would like to remove, emphasize, add to or change. Then he sits in the car and tests the sound. 'If I then feel the acceleration while driving and I perceive the contact with the road, then it happens again and again that I want to perceive the sound more intensely than when listening to the synthetically generated sound in the laboratory.'

The sound designers are working closely with the engine developers. Lamborghini does not have a dedicated sound design department. The engine development engineers also take on this issue. In the first step, the Italians generate a basic sound on the computer, which results from the number of cylinders, the ignition sequence and the geometric shape of the engine. Then they work on the details. These details are numerous. When the driver steps on the accelerator, he not only wants to feel the acceleration, but also hear it. That's why Porsche, BMW and Lamborghini make sure that the customer hears the noise in the interior and therefore edit the intake system.

Every manufacturer has its own methods, but as is so often the case: All roads lead to Rome. At BMW, in addition to the intake on the kidneys, a second opening is used that goes towards the front wall, but is closed there. No air is sucked in this way, but a sound is generated. Porsche developed an opening in the air filter housing that is supposed to let the noise into the interior. The closer the intake is to the driver, the better the sound in the interior. In the Boxster, the intake is located behind the driver.

Characteristics of the engine are decisive for the sound

The characteristics of the engine are also fundamental to the sound. Whether it only has four cylinders or even twelve is about as big a difference as the beeping of a mouse and the roar of a bear. In order to deal with the characteristics of the engine, the sound specialists work with so-called engine orders, which are ever differentdifferentiate according to the number of cylinders. In a six-cylinder engine, each cylinder fires every other revolution of the crankshaft. So there are three ignition processes per revolution. Therefore the engine has a strong third order. With the eight-cylinder it would be a fourth order.

'We have these suggestions from combustion, these are the basic orders,' explains Dr. Pfafflin. “So that an engine sounds like a Porsche engine, we add the intermediate rules. This is how we compose and differentiate ourselves. We not only have a third or fourth order in our engines, but also a one and a half order or a second order in the six-cylinder. That is what makes the special Porsche sound. ”

Details are crucial in the sound design

In addition, little things flow into the sound that you would never have expected. In the development process, for example, the engineers ask themselves numerous questions about the surface radiation of the engine: How do I design the cylinder heads or the crankcase walls? With ribs - how many ribs and how much radiation do they allow? Or: How are the noises emitted by the camshaft?

The third important element in addition to the intake system and engine noise is the exhaust system. Here pipe lengths, pipe cross-sections and silencer volumes can be varied and sounds can be changed. The geometry of the exhaust system is also crucial. “If you have a bank of five or six cylinders, a completely different sound can be produced depending on the routing of the exhaust system. For example, you can combine five cylinders in one tube, or three in one or two in one and all of these together in one tube, ”explains Maurizio Reggiani from Lamborghini. Another role is played by the silencer, the internal structure of which can be designed in different ways. “We build a few prototypes of silencers and with the help of a microphone we compare the simulation with the real sound. That means we adapt the geometry of the silencer to the basic sound of the respective engine, ”Reggiani adds.

The exhaust system is the third important component

The BMW V8 engine is different the internal structure in the left and right part of the exhaust system. Adding up these asymmetries results in a different sound than the simple symmetrical arrangement. This is so concise that Robert Mirlach recognizes it blindly. “My vision is that you sit in a café on the street and can already clearly hear the sound when a BMW comes by. With the in-line six-cylinder from BMW I have no problem with that, and the V8 engines also have a very unique sound at BMW due to their asymmetry. '

However, the sound artists have to take into account restrictions in their work - those legally stipulated worldwide Noise regulations. In Germany, the statutory limit of 74 decibels must not be exceeded. To be able to meet these,Some manufacturers use exhaust flaps that only open at higher speeds. With the flaps open, the purring kitten becomes a snarling tiger. For Porsche and BMW, the legal test is no reason to use flaps. It's about the sporty sound in the upper speed range. At Porsche, the flaps open in sport mode, among other things, at BMW the timing is adjusted to each car depending on the engine speed and gear.

How do the exhaust flaps work?

Maurizio Reggiani from Lamborghini explains the function of these flaps: “They can be closed in the area of ​​legal regulations, for example. And only when you get out of this area do they open and make a different sound. We thus comply with the noise regulations and give the customer the emotion when they press the accelerator. ”

The Mini Cooper S, on the other hand, almost challenges you to take your foot off the accelerator. Then he bubbles out of the tailpipe in a rough voice. BMW calls it push bubbling. Before you take it away, the fuel is injected again for a few hundred seconds. However, this feature is reserved for the most powerful engine variant of the Z4 and the Mini Cooper S. “When a colleague from another series came up to me and absolutely wanted a bubbler for his model series,” Robert Mirlach chats, “I tried it out on a sporty limousine, but found that it didn't match the character of the vehicle . “

What fits and what doesn't are decided not by musicians, but usually by engineers. Most of the acoustics and vibration technology department at Porsche are vehicle technology engineers, mechanical engineers or physicists. Some make music in their spare time. 'For each series we have a chief designer who does the sound design for the system,' says Bernhard Pfäfflin. “You need a lot of experience, otherwise you get bogged down. There are so many levers that can be used to influence the sound.

The future of sound design in electric cars?

There are still many levers, but work could be done in the near future the sound designer look a little different. Then it is important to make electric cars sound nice. “That is a special challenge. We want to achieve the same result as with internal combustion engines: an electric vehicle or hybrid vehicle in e-mode should sound characteristic of Porsche. 'Even if this concert may not sound like it does in the 911 concert hall.

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