Part one of two
A host of various noise and vibration issues normally exist during the operation of any vehicle. However, when levels of these factors become noticeable to the customer, he or she may perceive any annoying noises and/or vibrations as problems. NVH (noise, vibration and harshness) is the commonly used term used when discussing these conditions.
In some cases, any noise or harshness complaints may stem from the vehicle owner’s perception, or their expectations of how the vehicle should behave.
The NVH condition that is a concern does not need to be the strongest vibration or the loudest noise. It could be one that has recently developed and was not previously present.
As an example, tire tread noise from a vehicle equipped with large tread block tires might be acceptable to the owner of a 4x4 who recognizes this as a natural by-product, while others may find this as totally unacceptable. A complaint on the same vehicle could be much more subtle, caused by a driveline problem.
Because we sense vibration and sound using different senses, we tend to discuss them separately. However, vibration and sound are essentially identical.
A sound is a vibration (pressure fluctuation) of the air. Vibrations and sounds are both expressed as waves per second called Hertz (Hz).
Vibrations that are felt are under 200 Hz. Vibrations between 20 Hz – 20,000 Hz are audible by humans. Vibrations over 20,000 Hz are ultrasonic and are not audible by humans.
Vehicles offer three major sources of potential vibration. These include the engine and engine accessories, driveline and wheels and tires. Each of these sources usually rotate at different speeds or frequencies. A component generating a vibration can be associated with one of the source groups if the frequency of the vibration can be determined.