Inside Saab’s Noise and Vibration Harshness Lab

On November 21st I was invited to NEVS in Trollhättan and met with:

• Patrik Torehov (Senior Analysis Engineer)
• Eskil Lindberg (Technical Specialist)
• Göran Fredriksson (Manager Public and Customer Relations)
• Mikael Östlund (Public Relations Officer)

After introducing the 900, Saab decided it was time to take aim for the premium market with the 9000. An important aspect of a premium car is the feeling of comfort inside. To aid in this area a NVH testing lab was constructed in 1984.

“The NVH lab influences all interfaces between chassis/drivetrain and the body, interior insulation, exterior design (wind noise and resonants), tires, interior information sounds and even engine noise characteristics such as idle, engine torque, shift points and more.”

The lab is divided into several rooms. A couple of rooms are semi-anechoic chambers suitable for testing heavier equipment (solid floor), then there is one reverberation room adjacent to an anechoic chamber. These two rooms are separated by a thick wall with a one square meter big hatch that can be opened between them. Once that hatch is open, all sound entering the anechoic chamber can only originate from the reverberation room.

Tests typically focus on individual parts, material or an entire car.

“To generalize, we focus on all dynamics (sound and vibration) above 20 Hz. The human ear doesn’t detect sounds below 20 Hz. On the other hand, it is in the area between 0 and 20 Hz that handling is determined, which is outside the NVH lab’s domain. But a very important component for both acoustic and handling is rubber bushings. It is designed to create a soft transition between stiff metal components. In the car industry the NVH engineers want the softest bushing possible, whereas the engineer responsible for handling desire rather stiff bushings (yielding a sportier handling).”

Currently the lab is busy preparing for the start of production of the 9-3N to ensure that it complies with current regulations in all the major markets. Compliance testing generally consists of pass-by tests, but some interior tests are also carried out.

The lab is utilized throughout the life-time of any given Saab model. They utilize a shaker to simulate years of wear and tear, and then subject the car to additional listening tests to see if it still performs as expected.

“Another example where the NVH lab yields a direct influence on existing models is tuning of the exhaust system. We put a lot of effort into making sure our cars has the right sound.”

Patrik demonstrates as a 9-3N is put through one of the benchmarks. The door proves a little challenging to close with a near-dozen cables sticking out. “The cables do not last long here, but there are only a few ways of getting the data out” says Eskil.

Placed on huge rollers, the 9-3N’s engine spins up and the monitors start showing engine information and noise measurements. The rollers are two meters in diameter and both FWD and AWD configurations are supported. AWD support became necessary when Saab added XWD as an option. Patrik mentions that “It was a lot of work installing the extra set of rollers in the basement”. The whole room is supported by spring coils in order to dampen vibrations from the outside. “Small earthquakes, big trucks, it does not matter — you will not notice them inside here”.

(“No comment” was the answer when asked the obvious question concerning the 9-3N and possibility of AWD)

At the end of the test run, advanced computer software is used to analyze the output. Problem areas are identified and often the NVH engineers will make specific suggestions on how to improve the product. Patrik explains that within Saab’s relatively small organization, it is easy to locate the engineer responsible for the initial design. The NVH engineers work closely together with the designers on all phases of development, from the specification phase all the way until the model gets retired from the product portfolio.

Loudness is not the only characteristic of sound observed. Some sounds are just plain annoying. Other sounds are very subjective. In earlier models, such as the 99, the turbo noise was a selling point. Now other characteristics are more important. Sometimes the NVH engineers have to go with their gut feeling when deciding if a particular sound is pleasant or annoying.

“From an acoustical point of view, every car from the production line is a separate entity because at high frequencies the wave lengths of sounds and vibrations are so tiny that two seemingly identical components (as observed by the naked eye) can result in different acoustical responses. Thus it takes a continuous effort to ensure that every car from the production keeps the same level of acoustical characteristics. E.g. squeaks and rattles where the solution might be a piece of soft tape between two panels, or adding insulation material. We in the NVH lab conduct many inquiries both short and long term so that you, the customer, will enjoy a good acoustical environment with the proper Saab feeling.”

The thought of revving a stationary car to 200 kph while standing still made me a bit uncomfortable so I asked Patrik if any nervousness was felt when he first performed this operation. “Not so much. The only issue thus far, is to remind myself not to step out of my own personal car when the speedometer is still showing 100 kph.”

Finally, the last piece of the NVH Lab facilities is a short test track outside. Pass-by noise is measured as the car accelerates on a standardized asphalted surface. What we car enthusiasts find endearing can apparently be quite annoying to certain elements of civilized society.