Noise, Vibration & Harshness

When you read car brochures or watch car review videos, you might occasionally come across the acronym NVH. It stands for Noise, Vehicle and Harshness. It usually appears when car reviewers and manufacturers want to explain how refine the car is. NVH is one of the important factors one need to consider before buying a new car because it could affect your driving experience.

SO, WHAT DOES NVH REALLY MEAN?

To simply put, the noise is what you hear, the vibration is what you can feel, and the harshness is how much of effect thumps, bumps, noise and vibration have on the cabin and its occupants. In today’s world, most people prefer comfort above performance. As long their vehicle can move from one point to another point comfortably, then they will buy it. Except some car enthusiastic that prefer performance over comfort. Comfort in cars is achieve when you hear less noise and feel a smooth driving experience. To achieve this level of comfort in cars, automotive engineers and car manufacturer need to consider several factors. But before that, we need to know and identify what affect NVH in cars but for this article we are going to focus with noise and vibration only.

Have you ever, when you are driving on a highway, at high speed but still under the speed limit, you heard wind noise and you instinctively will roll up your window because you thought maybe you did not close the window properly. Or maybe, while driving on a highway and suddenly you feel like somehow your car has been insulated and soundproof for several kilometers, before returning to its original state; not soundproof and noisy. That is the result of high-level NVH in cars.

NVH IN CARS

For the first case, when you drive at high speed, you will create more wind resistance. The air flowing around the car body will be in contact with the geometry of the A-pillar, the mirrors, and the undercarriage. This air resistance causes the noise you heard inside your car. For the second case, you feel like your car is suddenly insulated and soundproof is because of the road condition. When you drive in bad road condition or old tar, the rough surface will transmit its vibration and its’ effect through the wheels, suspension system, chassis, and inside the cabin. This vibration is translated into noises that you heard. The rougher the road surface condition the harder the vibration and louder the noise, and vice versa.

Apart from both cases mention, noises in cars can come from various reasons such as engine vibration, old and hard rubber components, cars’ aerodynamic, and exhaust modification. To cancel out NVH, there are three basic principles which are reducing it at the source, isolating it from the car’s main structure, and absorbing as much as possible before enters the cabin.

REDUCTION AT THE SOURCE

The first way to reduce NVH is to curb the volume and number of sounds and vibrations produce initially. Aerodynamics play a huge part in the amount of noise. When air flows over the car body, any disturbance in its path will create vibrations of the air particle. Nissan leaf is a good example for aerodynamic. According to Nissan, the weird shape headlight is to redirect airflow away from the wing mirror to help minimize unwanted noise. Other than that, engine vibration also will affect NVH of cars. Three cylinders engine vibrates harder than four-or-more cylinder engine. This is because of 3-cylinder engine has unbalance firing system and cause them to rock. The vibration is more visible at low rpm. To solve this issue, manufacturer will add balancing shaft to counter the force and some other solutions that is hard to explain so let’s call it magic made by engineer. BMW seems to have got it right, their 3-cylinder engine produce only minor vibration, and maybe only during starting. Another car, Rolls Royce Ghost Series 2, also used engine that exhibit very minor vibration. The engine is so smooth that during ignition, when a coin no larger than 50 Malaysia cents was put vertically on it, it does not move or topple.

tuning, engine block, tuned

ISOLATION FROM THE CAR CHASSIS

The second way to reduce NVH is to isolate the car’s main structure from the source. The suspension system has a major role in this. Suspension systems such as MacPherson strut, multi-link, and double-wishbone all function to isolate and reduce vibrations coming from road and tire, from the car chassis so people inside the cabin can experience pleasant driving and quite moment. There is another expensive and more complex suspension tech such as the independent and hydro-pneumatic system that isolates chassis and steering column from the road surface, thus removing vibration and shocks. To further isolate the source of noise, engine, suspension, and exhaust components are mounted on rubber and polyurethane bushes rather than solid metal; the softer compounds work against the transmission of vibrations. The idea is being used for our shoes. The soles of our shoes are made of materials such as natural rubber, polyurethane or polyvinyl chloride (PVC) that has soft characteristic so every step we take will be comfortable and quiet

ABSORBTION AS MUCH AS POSSIBLE

The final resort in reducing NVH in cars is actually one of the conventional ways which is to absorb as much as possible. Mechanically speaking, this refers to a steering damper that absorbs the worst shocks produced by the poor road surface. In the cabin area, passive sound absorption can extent to filling pillars with foam and double-glazed windows, all the way to smattering rubber rubber-like sound deadening material fixed to various points along with the chassis. Double-pane glass and sound insulation tend to be quite heavy and that is why most cars do not have them. Sound deadening material is normally used in cars with extremely higher price ranges such as Rolls Royce and Bentley. The experience inside these cars is the same as being inside a recording studio. Basically, one can say expensive cars with sound-deadening materials is a portable anechoic chamber (not to be taken literally).

WHAT SHOULD YOU LISTEN FOR WHEN BUYING YOUR NEW CAR

When buying a new car, and the NVH level is something important for you, then you should consider test driving the car before signing anything. You need to listen out for all these annoyances which will become harder to bear over the course of ownership. For example, when you test drive and you can hear considerably loud wind noise or maybe you found that the engine idling is quite loud and vibrate much, then you need to take that into account, or you will not be satisfied with your buying. For example, with Malaysian’s latest car; the X50, on the paper, the engine used was a 3-cylinder which as we discussed produced a lot of vibrations and noises. But after being reviewed by media and car reviewers, they said inside the cabin was quite well-insulated and has low noise and vibration, even though being with the 3-cylinder engine. This is why test driving your potential new car is important because even though the numbers on the paper are either good or bad, but in the end what important is the driving experience. Everyone has a different taste in buying a car.

Reference

  • Corby, S. (2015). What is NVH? Retrieved from carsguide.com website: https://www.carsguide.com.au/car-advice/
  • Wang, X. (2010). Rationale and history of vehicle noise and vibration refinement. In Vehicle Noise and Vibration Refinement. https://doi.org/10.1533/9781845698041.1.3

 

Written by: Muhammad Syahid Bin Abu Bakar

Posted on December 15, 2020

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