Sources of noise and vibration (NVH) in automotive

Have you ever been in a car and heard a strange "pop" sound coming from somewhere without knowing where it was? Or felt a slight vibration in the steering wheel at 80 km/h for no apparent reason? It's not random – all noises and vibrations have a source, a mechanism, and are controllable. This article will decode them all.

1. Why is it important to know the source of noise?

Each source of noise and vibration has its own frequency characteristics, transmission path, and impact on the occupant's perception. Understanding the source is the first—and most important—step in the entire NVH testing and improvement process.

Sources of vehicle-borne vehicle emissions are divided into four main groups: sources from dynamics (powertrain), sources from vehicle-road interaction, sources from aerodynamics, and sources from structure.

1.1. Group 1: Powertrain NVH

This is the most complex group and accounts for the largest proportion of the vehicle's total vibrational energy.

Internal combustion engine (ICE)

The engine is the "heart" of the car—but also its biggest source of vibration. Each piston explosion creates a pulse, and when thousands of those pulses occur every minute, the result is a complex vibrational field. Common problems:

Secondary vibration (engine order vibration): Low-frequency vibration synchronized with engine speed, creating a "floating" sensation at certain speeds.
Knock/ping sound: Occurs when the fuel-air mixture ignites at the wrong time - especially dangerous and requires early detection.
Mechanical noise from valves, timing chain, oil pump: High-frequency metallic sounds, often a sign of wear or poor lubrication.

Gearbox and drivetrain

The gearbox is where energy is transformed and transmitted—and it's also where vibrations are amplified if not carefully designed:

Gear whine: A continuous, high-frequency buzzing sound caused by imperfect gear engagement.
Clunking sound when shifting gears: Usually caused by a large gap between the gears or couplings.
Cardan shaft and universal joint vibration: Causes cyclical vibration, most noticeable when the vehicle accelerates/decelerates.

Electrical systems in EVs

Electric vehicles have no internal combustion engines, but they offer entirely new sources of cultural enrichment:

Electric motor whine: A characteristic sound at a frequency of 1,000 – 10,000 Hz, often clearly audible when the vehicle accelerates. In ICE vehicles, the engine noise masks this sound – but in EVs, it is completely noticeable.
Inverter and controller noise: These are emitted at electromagnetic frequencies, typically around 8-16 kHz, sometimes exceeding the hearing range of adults but extremely irritating to children and sensitive individuals.
The noise of the heat pump and battery cooling system is particularly noticeable when the vehicle is stationary or at low speeds.

1.2. Group 2: Sources from Vehicle-Road Interaction (Road & Chassis NVH)

This is the group that the driver perceives most directly through the car's body, seats, and steering wheel.

Tires – the first "filter" and also the biggest source of noise.

The tires are the only part of the car that comes into contact with the road – and everything on them is "reported" back into the car:

Road noise/rolling noise: Caused by the tire treads constantly rubbing against the road surface. Frequency ranges from 50–500 Hz, perceived as a continuous "whooshing" sound in the cabin.
Tire tread pattern noise: Larger treads are generally noisier than touring tires; high-performance tires are often optimized with patterns to reduce NVH.
Tire resonance: Occurs when the excitation frequency from the road matches the natural frequency of the tire – usually 80-120 Hz, causing noticeable vibration in the vehicle floor.

Suspension system

The suspension system is designed for filtering and absorption – but when malfunctioning, it becomes a source of NVH (Non-Volatile Organic Compounds):

Shock absorber knock: When the rubber bushings wear down, metal touches metal, creating a "clunking" sound when going over potholes.
Spring squeaking: Due to insufficient lubrication of the friction surface.
Shimmy vibration (steering wheel vibrates at high speeds): This is often caused by unbalanced tires, incorrect camber angle, or a faulty ball joint.
Anti-roll bar knock: Worn bushings cause a knocking sound when the vehicle leans while cornering.

Braking system

Brake squeal: Occurs at a frequency of 1–16 kHz when the brake pads and brake discs resonate with each other. While not technically dangerous, it is extremely annoying.
Brake judder: Strong vibrations in the brake pedals or steering wheel when braking hard - often caused by thermal deformation or uneven installation of brake discs.
Regenerative braking whine in EVs: The electronic sound produced when the energy recovery system is active.

1.3. Group 3: Aerodynamic NVH Sources

This group only appears at high speeds - but when they do appear, they are very difficult to conceal.

Wind Noise

As the vehicle moves, air strikes the vehicle's surfaces, creating vortices – and these vortices generate sound pressure:

A-pillar whistle: The shape of the A-pillar creates a point where airflow separates – if poorly designed, this forms a vortex that produces a noticeable whining sound at speeds above 100 km/h.
Door seal leak: Over time, the rubber door seals deteriorate, creating small gaps that allow air to enter – causing a constant "hissing" sound.
Rearview mirror and antenna noise: Aerodynamic shape creates vibrations at high speeds.
Roof and panoramic roof noise: A NVH (Noise, Vibration, and Harshness) weakness of many modern SUVs due to insufficient structural rigidity.

Ventilation system noise (HVAC Noise)

Fan noise: Frequency varies with fan speed - a whistling sound at high speed indicates an unbalanced fan or obstruction.
Air duct noise: When the airflow speed is too high, it creates aerodynamic noise in the duct system.
Air diffuser vibration: Causes resonant vibration in the cabin at certain fan speeds.

1.4. Group 4: Structure-Born & Interior Noise (Structure-Born & Interior NVH)

This is a group that is often overlooked in design – but it's the one that end users complain about the most.

Body structure noise

Body groan: When the vehicle body twists slightly over uneven terrain, the joints make a scraping sound.
Panel boom: Flat metal panels such as the floor, roof, and doors resonate at frequencies of 60–120 Hz, causing a "rumbling" sound in the cabin.

Interior noise (Squeak & Rattle)

This is the number one enemy of perceived quality:

Squeaking sound: Caused by two hard surfaces rubbing against each other - commonly found in plastic control panels, trim gaps, and assembly points.
Rattle sound: Caused by loose parts or gaps that vibrate due to vehicle vibrations - glove compartment lid, door handles, screws not tightened to the correct torque.
Seat creaking: The seat frame, adjustment sliders, and upholstery can all be sources of noise if not properly installed.

2. Compilation of Cultural Resources Based on Location on the Vehicle

Location	Main source of cultural activities	Typical frequency
Engine room	Engine, gearbox, cooling fan	20 – 500 Hz
Wheels and suspension	Tires, shock absorbers, stabilizer bar	50 – 300 Hz
Brake	Brake pads, brake discs	1.000 – 16.000 Hz
Body and frame	Resonant steel plate, joint	60 – 200 Hz
Interior	Plastic, chair, drawer	100 – 2.000 Hz
Aerodynamics	Wind, door seals, mirrors	500 – 5.000 Hz
HVAC system	Fans, pipes	200 – 4.000 Hz
Electric motors (EVs)	Inverter, motor, regenerative braking	1.000 – 10.000 Hz

3. NVH Transmission Path: From Source to Driver's Ear

Knowing the source isn't enough—it's equally important to understand how vibrations and sound travel from the point of origin to the vehicle occupants:

Structure-borne path: Vibrations from the engine travel through the engine mounts, into the chassis, across the floor, through the seats, and to the driver's body. The quality of the engine mounts and suspension bushings determines how much vibrational energy is filtered along this path.
Air-borne path: Sound from the source travels through the air, passing through the sound insulation materials of the vehicle body to enter the cabin. The vibration-absorbing padding, sound-absorbing materials, and the degree of airtightness of the vehicle body structure are the determining factors in the effectiveness of soundproofing.

4. Why Do Manufacturers Need to Control Each Source?

The modern automotive industry operates on the principle of "early detection, low-cost repair":

Discovering NVH sources at the design stage using simulation software: modification costs are almost zero.
Discoveries at the prototype stage : costs range from a few thousand to tens of thousands of dollars.
Discovered after mass production : recall and line repair costs can amount to tens of millions of dollars for a single vehicle.

This is why major automakers—from Toyota to VinFast—integrate NVH measurement and analysis into every stage of product development, from concept design to pre-production vehicle testing. Every noise source is checked, every vibration path is analyzed, every resonance is eliminated—before the first vehicle reaches the customer.

A car is not just a collection of mechanical components. It is a complex acoustic system—where dozens of noise and vibration sources coexist, interact, and influence each other.

Controlling NVH means controlling the perceived quality of the entire vehicle. And to control it, you need to know exactly what you're dealing with – from the inverter whining in an electric vehicle to the rattling of the glove compartment lid in an SUV. No source is too small to ignore if it can affect the driver's experience.

GPower Vietnam offers a full range of NVH testing equipment – from accelerometers and acoustic microphones to comprehensive vibration analysis systems. Contact us for advice on the right solution for your project.