High Society Kustom Garage

Noise, vibration, and harshness

Noise, vibration, and harshness, referred to as NVH, are factors that greatly influence the comfort and quality of a vehicle.  NVH refers to the sound, vibration, and harshness that a driver and passengers experience while driving a car.  It refers to the entire range of vibration perception, from feeling to hearing.  It is caused by the car’s mechanical systems, electrical systems, and the car’s interaction with road surfaces.  It’s also caused by the turbulence created by its passage through the air.  The goal of automotive manufacturers is to design vehicles with a smooth and quiet ride with minimal vibration and harshness.  In this blog, we will explore the importance of NVH and how it affects the automotive industry.

By definition, Noise is defined as a non-harmonious or discordant group of sounds.  Vibration is the oscillating, reciprocating, or other periodic motion that is typically felt rather than heard.  Harshness is used to describe the unpleasant feeling associated with unwanted sound and/or vibration, especially from short-duration actions.


Noise is perhaps the most obvious aspect of NVH, as it is the most easily perceivable.  In the automotive world, noise can be broken down into three categories: powertrain, road, and wind.

Powertrain noise refers to the sound generated by the engine and transmission.  This can include everything from the low growl of a V8 to the whine of a turbocharger.  We love the roar of a strong engine note and lumpy exhaust burble.  But undesirable mechanical sounds and vibrations can emerge from the engine bay.  The whirring of a thermo fan, or the clicking of lifters that need adjusting.

Road noise, as the name suggests, is the sound generated by the interaction between the tires and the road surface.  This can include everything from the thump of a pothole to the hum of tire tread.  Excessive road noise can be tiring and distracting like powertrain noise, especially on rough roads.  Tyres, especially low profile, are a major source of noise.  This is because they have less area in which to absorb even small impacts.  Wide tyres are more susceptible because of their larger contact patch.  Off-road tyres are the worst offenders.

Wind noise is the sound generated by the movement of air over the exterior of the vehicle.  This includes everything from the whistle of a poorly sealed window to the roar of air passing over the roof.  While wind noise is less fatiguing than powertrain or road noise, it’s still a distraction, especially at higher speeds.  Interrupted aiirflow over the body is noisy, especially at higher speeds and in cars with the aerodynamics of a house!  The result is howling, whistling, or buzzing noises in the cabin.


While noise is the most easily perceivable aspect of NVH, vibration is often the most uncomfortable.  Vibration can be caused by lots of factors, including engine and transmission mounts, suspension components, and even the tires themselves.


The final aspect of NVH is harshness.  This refers to the overall discomfort experienced by the driver and passengers of a vehicle.  Harshness can be caused by a variety of factors, including poor cabin acoustics, uncomfortable seats, and rough roads.

Cabin acoustics play a major role in the overall comfort of a vehicle. A poorly designed cabin can result in excessive noise and vibration, making the driving experience uncomfortable and fatiguing.  A well-designed cabin can minimize the impact of noise and vibration, resulting in a more comfortable and enjoyable driving experience.

The importance of NVH in the automotive industry can’t be overstated.  A vehicle with excessive noise, vibration, or harshness lessens the driving experience.  This leads to driver fatigue, discomfort, and even injury.  It can also reduce the resale value of the vehicle and damage the manufacturer’s reputation.  Therefore, automotive manufacturers invest significant resources into reducing NVH to improve the overall quality of their vehicles.  “Designers are getting more conscious of it, because of the importance of comfort in the car” says Ivan Mini, global market manager for automotive interiors and safety components at Dow Corning.

Manufacturers spend tens of millions of dollars nowadays engineering out as many of these unpleasantries as possible.  But how do we treat our older classics and muscle cars?  We’ll attempt to cover that in detail here.


One of the challenges in managing NVH is balancing comfort and quiet with the need for performance and efficiency.  Adding sound-deadening materials and vibration-damping materials can add weight to the vehicle, which can reduce performance and fuel efficiency.  However, reducing NVH can also improve the vehicle’s overall quality and perceived value.  It’s up to you to decide how much you want to eliminate on your ride.

Cancelling out NVH comes down to three basic principles.  Reduce it at the source, isolate it from the main structure, or absorb as much as possible.


To lessen NVH, start by reducing the number and intensity of noises, vibrations, and impacts generated initially.  A racing-oriented chassis and suspension, though very rigid, will inevitably be a bit rough.  But a cruiser can compromise some agility for the sake of comfort.  Softer suspension, rubber instead of neoprene bushes, and slightly more side wall on the tyres.  These will work together to reduce the amount of shock being transferred from the road surface to occupants.

Choose engine and transmission mounts to isolate the engine and transmission from the rest of the vehicle.  This will reduce the amount of vibration transmitted to the cabin.  Worn or damaged mounts can allow excessive vibration to enter the cabin, resulting in an uncomfortable and fatiguing driving experience.

Likewise, suspension components, such as shocks, struts, and bushings, can also contribute to vehicle vibration.  Suspension components that need replacement or repair can allow excessive vibration to be transmitted to the cabin.

Tire vibration is another factor that can impact the overall comfort of a vehicle.  Tires that are out of balance or improperly inflated can generate excessive vibration, resulting in a jarring and uncomfortable ride.


To reduce NVH, automotive manufacturers use a variety of techniques and technologies.  One of the most important techniques used to reduce NVH is the use of sound-deadening materials.  These materials are designed to absorb sound waves, reducing the amount of noise that enters the cabin. Sound-deadening materials can be found in many areas of the vehicle, including the doors, floor, roof, and engine bay.  Treatments like Dynamat, Car Builders and similar constrained layer damping (CLD) mats are well known, but are often misrepresented.  Without knowing how they work, you may not be getting the results you were expecting.  The function of CLD’s is to add mass to thin, light sheet metal, limiting any droning sounds created.  This is known as deadening.

When a CLD is applied to a vibrating structure, the treatment converts vibration to negligible heat.  Remember, you can’t destroy energy, only convert it. These are often added to the floor and transmission tunnel.  But due to the stamped shaping in these panels (to add strength), you aren’t getting great bang for your buck.  While adding density here does help, it is more beneficial to deaden the larger, flatter panels. Adding a deadener to just the roof and outer door skins will get the biggest results as far as deadening goes.  Just 60% coverage will get results.  Adding mass to the entire interior surface via CLD’s does reduce resonance, but it starts to have diminishing returns.  A second layer in larger panels is beneficial, but less noticeable.  So, starting with the areas most prone to vibration is advisable, especially where weight is of concern.


The next area to address is airborne noise.  Having door and window rubbers and seals in good condition can make a huge difference, especially with the windows up.  Having things properly sealed will prevent whistling and wind noise from entering the cabin.  It will also help your air conditioning to work more efficiently.  However, the main source of unwanted sounds in most cars isn’t airborne sources.  It’s usually the body and chassis vibrating to externally induced sources.  That is where, once all the leaks are closed up, real progress will be made. These externally sourced noises are a result of energy transfer from a vibrating object to another object that can be vibrated.  That vibration comes through as road noise.


What CLDs won’t do though is combat road noise. By design, they are stuck directly onto the surface without any form of decoupling.  Road noise is an airborne sound issue and so requires a different type of treatment.  Acoustic treatments are used to reduce the transmission of sound waves and vibrations through the air.  These treatments include the use of foam or other absorbent materials in the vehicle’s headliner, door panels, and floor mats.

Acoustic treatments can also include the use of sound-absorbing materials in the engine compartment, exhaust system, and suspension system.  This is done after dampening by using a mass-loaded vinyl or similarly dense material suspended on a layer of foam. The foam acts as an air spring.  This air spring prevents sound from penetrating the dense barrier layer by allowing high-energy sound waves to be deflected.  Sometimes these products also have another layer of foam on the top surface, leaving the barrier layer completely independent and free to perform its job.

Adding a low expansion foam between braces and bodywork will stop them from rattling together. Just be sure not to use too much as an excess of this can warp body panels.  As the cured foam is hygroscopic (absorbs moisture from the air), it’s a good idea to seal and paint any exposed foam.  This is to prevent rust from forming. Likewise, on hanging panels such as front guards, a thin layer of neoprene where the parts come into contact with one another will prevent any noise as the car twists and moves. It’s important that this barrier isn’t affected by heat or oil, and is water resistant, so you don’t introduce rust.


At this point, if all treatments have been properly conducted, you will have successfully reduced NVH to the point that the weak link will be the windows, of which I am unaware of any coating or product that can be applied.  Congratulate yourself on a job well done by enjoying a little quiet time on the road.

NVH Testing
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