Sound principles – Saab Noise & Vibration Center in Trollhättan

Dr Per-Olof Sturesson is the manager of Saab’s impressive Noise & Vibration Center in Trollhättan, Sweden. It’s a place brimming with technical equipment all designed to help every Saab car incorporate just the right amount of sound and at just the right level. Jonathan Arnold went to meet him.

NOISE,VIBRATIONS, RATTLES, squeaks… every car has them. And every car manufacturer tries to refine or eliminate them so that we only hear what we want or need to hear in the best possible way. It’s a complex business where science and technology work hand in hand to ensure that the sounds that you hear when you are in your car have exactly the right character.

Dr Per-Olof Sturesson, the manager of the Noise & Vibration Center at Saab, together with a team of dedicated boffins, have the unenviable task of analysing all of these noises, vibrations, squeaks and rattles. A visit to the Noise & Vibration Center reveals a series of rooms full of strange shapes and weird equipment that investigate all of these sound characteristics in minute detail, with the ultimate aim of making your Saab driving experience that much more enjoyable.

Sturesson says: “There are fundamental vibration modes of any structure, whether it is a violin string or a car. We measure the attachment points of, say, a car body to a chassis and what the wave propagation in a structure is. We also examine structural vibration, for example a bumpy road and its impact on the car. But sound is also an information carrier, such as the air conditioning unit or the demister – these are sounds you need to hear functioning correctly at the right levels and type of sound. Complete silence is not good; you cannot have it.”

At times it is tricky to keep up with Sturesson’s monologue on the detailed scientific work that he and his team conduct in the Noise & Vibration Center at Saab. He obviously gets an enormous amount of satisfaction from assessing, tweaking and refining the sounds that impact on, or within, every Saab and he wants to share that knowledge. Along with 15 engineers and three simulation engineers – some of whom have PhDs and most of whom have MAs – the Saab Noise & Vibration team can spend days investigating one tiny aspect of a Saab’s sound characteristics. Sturesson says: “A customer never buys a car because of ‘good’ road noise, but won’t buy one because of ‘bad’ road noise.”

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There are some fundamental aspects to the work that Sturesson and his colleagues do.This might be summed up by stressing that certain sounds can be impressive in that they create an emotional attachment with the customer (the right engine noise, for example, or the way in which the air conditioning unit works); there are sounds that need to be informative, that tell you something about the function or build quality, or communicate a message to the customer; and then there are those plain irritating noises which simply need to be eliminated. “You need a balance to get the right sound treatment,” adds Sturesson.

Sturesson steps into the first of the ‘sound’ rooms, called the reverberation room.“It’s a room with almost zero acoustic damping or absorption and measures ‘decay’ time as amplitude reduces. It tells us how good a particular material is at absorption, for example a fleece material that might be put between the body roof and the roof liner.There are always challenges such as weight reduction or environmental factors, or temperature stability if it’s in the engine compartment.”

The reverberation room is decked out with Plexiglas boards on the ceiling to create a diffuse sound field so that the sound pressure is the same all around. There’s also a rotating microphone to measure the sound pressure levels.

Next door is the anechoic chamber. The reverse of the reverberation room, its purpose is to create an environment where there is no noise, or at least a heavily reduced noise level. Here areas such as rear seat ventilators can be measured to a minute level. At times a Saab car will be wedged in between the reverberation room and the adjoining anechoic chamber, with one half in each location.“By doing this,” says Sturesson, “we can measure the ‘hot spots’, the weak spots in the acoustic isolation system of the car. It’s very important to have a quiet interior for diesel cars.You don’t want to have the perception that a diesel engine is sitting on your knees!”

Another much more imposing room has a new Saab 9-5 running constantly. It looks like a patient in a very sophisticated hospital emergency room, wired up to a host of different monitors and computer equipment. Of course, there’s nothing wrong with this car. Sturesson and his colleagues are simply monitoring its performance in a bid to further refine and improve.The new 9-5 is hooked up to a chassis all-wheel-drive dynamometer with a range of sensors under the seat to pick up vibrations from all directions.

Sturesson adds: “With the equipment in this room we can conduct a deeper analysis of driveline-induced noise and vibrations, such as speed variation on the driving wheels, the seat rail or steering wheel vibrations and the state of the vehicle using the vehicle electrical interface, so you know what state is applied. We can conduct wide-open throttle sweeps or ‘part load’ sweeps – say 20 per cent of the full throttle. All of the noises and vibrations gathered by this process are acquired via a data acquisition unit, which costs more than £85,000, followed by analyses in state-of the-art computer software.”

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It takes three to four days to assess the noise and vibration information provided by the different gear shifting positions on the new 9-5 with automatic transmission. But it’s all for a good cause.“A great deal of engineering effort and skills are required to keep refining and improving,” says Sturesson.

For the future, the Noise & Vibration Center will have an entirely new set of problems to contend with. Says Sturesson: “With the advent of electrical propulsion you lose the ‘masking’ effect of the combustion engine. As a result of this lowering of the noise level from the engine compartment, other sounds become more audible. It’s all about high-frequency noises. The combustion engine generates noise and vibration from 20 to 2,000kHz, and the electric engine up to 5-10kHz.”

While Sturesson and his team look to the future, the new Saab 9-5, says Sturesson, is “one of the quietest cars we have ever built”, with its three sealing systems on the doors to isolate the driver from the noise produced by wind and rain.“At cruising speed the driver feels ‘remote’ from noise sources. We aim to ensure the customer feels hermetically sealed in the cockpit.”

So the work continues. It is no coincidence that many of the Noise & Vibration engineers are also musicians. “Our psychoacoustics engineer plays the violin and bagpipes,” says Sturesson. “Others play instruments and some have recording studios. It goes hand-in-hand with people working on acoustics.”

Sturesson and his team of dedicated engineers are doing vital work to ensure your Saab driving experience is as good as it can possibly be. Their work may be lower-profile than other areas, but without their dedication your time in the cockpit of your Saab would be an altogether noisier affair.

Source: Saab Magazine 2011-01

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