Noise vibration harshness (NVH) is a field of engineering which designs the vibro-acoustic properties of different machines for maximum comfort and reliability. NVH engineers work on a great variety of products, ranging from household appliances like fridges to different means of transport like airplanes, trains and passenger cars.
Let’s focus on the automotive industry. Vehicle manufacturers of today are much concerned with sound and vibration engineering. Besides mechanical performance (think of speed, acceleration and handling) and interior comfort, the customer has developed precise expectations of the acoustic performance. The definition of NVH holds a slightly pessimistic connotation and might suggest that it only concentrates on reducing the undesired sounds. On the contrary, NVH engineering has nowadays become the area of sound design, stretching from the tiniest nuances in engine roar to the muffled boom of a shutting door. Altogether they contribute to the customer perception of premiumness or sportiness of a vehicle, hence the value on the market.
However, it is not the only goal of an NVH engineer to make a machine as silent as possible. In fact, it is quite easy to get any noisy and vibrating machine silent, by simply introducing heavier counter masses or more insulation material. For passenger cars, weight is an important design criterion, so simply adding mass for vibration isolation might not be successful strategy. Alternatively, if one can prove that there is a clever design that can achieve the same NVH performance with no extra cost, one can gain a lot of new friends within the company.
An intelligent design could utilize the mass of existing components for vibration isolation, e.g., the 12V battery. Or one could attach the vibration source to parts of the vehicle that are already very stiff, e.g., for crash reasons. One could also utilize the masking effects that other noise sources produce (e.g., only start the compressor after the fan already started blowing). So, this is really the art of NVH engineering: designing comfort with minimal resources.
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