What Is NVH & How It Affects Passenger Comfort?

What is NVH? NVH stands for Noise, Vibrations & Harshness – particularly in the context of automobiles, especially passenger vehicles. Occupants of a moving vehicle often experience noise, vibrations & harshness, or NVH. NVH is …

NVH

What is NVH?

NVH stands for Noise, Vibrations & Harshness – particularly in the context of automobiles, especially passenger vehicles. Occupants of a moving vehicle often experience noise, vibrations & harshness, or NVH. NVH is very unpleasant at varying motor speeds and also causes fatigue on both the vehicle and occupants. Hence, it is necessary to have a proper NVH design of an automobile as it is critical for the safety and comfort of the occupants.

Some buyers prefer a silent/quiet passenger vehicle, while others like a particular/unique exhaust note of a car or a bike. In some cases, manufacturers ask the NVH engineer to change the sound quality by adding or subtracting particular harmonics rather than making the vehicle quieter. You can readily measure the noise and vibrations, while harshness is a subjective quality. It is either measured via jury assessments or with the help of analytical tools. These tools provide results that reflect human subjective impressions. These analytical tools belong to the branch of physics called psychoacoustics.

Noise, Vibrations & Harshness: Definition

Noise means the sound emanated from a moving object. Vibrations mean the rapid swinging of an elastic solid or fluid. Harshness is the severity of the sound or noise emitted by the vehicle or its components. Just like aerodynamics, NVH is an advanced subject of automotive engineering that needs specialized testing facilities. These days, NVH engineering is a major part of the overall care package. Automotive manufacturers use special hemi-anechoic chambers and extremely sophisticated listening devices to analyze and tune the sound characteristics of new vehicles.

Anechoic Chamber (Courtesy: http://acoustafoam.com)

NVH is typically an engineering field. An interior NVH assessment considers the noise and vibration experienced by the occupants of the cabin. However, exterior NVH mainly deals with the noise emitted by the vehicle, which includes the drive-by noise. Most often, objective measurements of NVH do not correlate well with the subjective assessment of human observation.

NVT: Types of Noises & Vibrations in An Automobile

The sources of noise in a vehicle can be classified as

Aerodynamic – e.g. wind noise

Mechanical – e.g. engine, drive-line, tire contact patch and road surface, brakes etc.

Electrical – e.g., electromagnetically induced acoustic noise and vibration coming from electrical actuators, alternator, or traction motor in electric cars.

A vehicle has many sources of noise, viz. the engine, drive-line, brakes, tire’s contact patch with the road surface, and the wind. Noises from cooling fans or other engine accessories are quite common. Many problems could arise as the noise or vibrations are transmitted through various paths and then emitted into the cabin. Engineers call these structure-borne noises. Air-borne noises and vibrations are produced acoustically and travel through airborne paths.

Watch How To Diagnose NVH Issues In A Car Here:

Manufacturers try to reduce structure-borne noises by isolation and airborne noises by absorbing them, or through barrier materials. Furthermore, you can sense the vibrations at the seat, floor, pedals, armrests, or steering wheel. You can visually sense some of the problems occurring from vibrations, such as vibrating of the gear-lever or rear-view mirror. Carmakers also employ advanced technologies such as active noise cancellation in cars to reduce noise levels.

Noise Level Rating

Noise Level rating or N in NVH (Noise, Vibrations, and Harshness) is the measurement or amount of noise generated by a car. Hence, it is directly proportional to the car’s overall level of comfort. The government’s certification agencies define the law and regulate the maximum level of external noise of a vehicle.

There are three main methods of improving NVH:

  1. Reducing the source strength, such as making a noise source quieter with a muffler or improving the balance of a rotating mechanism
  2. Interrupting the noise or vibration path, with barriers (for noise) or isolators (for vibration)
  3. Absorption of the noise or vibration energy by using foam noise absorbers, or tuned vibration dampers

NVH engineers face many challenges, such as deciding which of the above or what combination to use for solving a particular problem.

Manufacturers use the following methods for improving NVH:

  • Use of tuned mass dampers, sub-frames,
  • Balancing/modifying the stiffness or mass of structures,
  • Retuning exhausts and intakes,
  • Modifying the characteristics of elastomeric isolators,
  • Adding sound-deadening or absorbing materials, or
  • Using active noise control.

However, in some cases, engineers must make substantial changes in vehicle architecture to resolve the problems cost-effectively. Some not-for-profit organizations such as ASHRAE and VISCMA provide specifications, standards, and requirements of NVH testing that cover a wide range of industries, including automotive.

Watch How NVH Analysis Is Done Here:

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