Sources of noise and vibration (NVH) in automotive

Have you ever been in a car and heard a strange popping sound somewhere? Or felt a slight vibration in the steering wheel at 80 km/h for no apparent reason?

During vehicle development and evaluation, noise and vibration are indicators directly related to operational quality and user experience. Abnormal vibration or noise can originate from various sources and be transmitted to the passenger compartment via different pathways.

Therefore, accurately identifying the source of excitation, frequency characteristics, and transmission path is fundamental to assessing the cause and developing appropriate solutions. For each NVH phenomenon, the conditions under which it occurs, such as vehicle speed, engine speed, load, road surface, or operating mode, must all be considered during testing.

This article focuses on the main sources of vehicle-borne traffic (VBT), the transmission path from the source to the user's perception point, and the factors to consider when measuring and analyzing VBT.

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. Identifying the source of noise and vibration can help improve NVH (Non-Vibration Humidity).

Sources of vehicle-borne vehicle emissions are divided into four main groups: dynamic sources, vehicle-road interaction sources, aerodynamic sources, and structural sources.

1.1. Group 1: Power source from the power system

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 generates cyclical forces during the cylinder's operation. As the engine speed changes, the vibrating components also change accordingly.

Common occurrences:

- Secondary vibration: Low-frequency vibration synchronized with engine speed, noticeable at certain speed ranges, and can be felt through the seat, floor, or steering wheel.
- Engine knocking: Occurs when the fuel 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:

- Gearbox noise: A continuous, high-frequency buzzing sound caused by imperfect gear engagement.
- Tapping noise when shifting gears: Usually due to large clearance between gears or couplings.
- Cardan shaft and universal joint vibration: Causes cyclical vibration, most noticeable when the vehicle accelerates/decelerates.

Electrical system on electric vehicles

Electric vehicles reduce many sources of mechanical noise but also highlight new sources such as:

- Electric motor whining: This is a characteristic sound with a frequency of 1,000 – 10,000 Hz, often clearly audible when the vehicle accelerates. In gasoline cars, the loud engine noise masks this sound, but it is completely noticeable when the user is using an electric vehicle.
- Noise from power converters and controllers: These emit electromagnetic frequencies, typically around 8-16 kHz, sometimes exceeding the hearing range of adults but extremely irritating to children and sensitive individuals.
- Noise from the heat pump and battery cooling system: Particularly noticeable when the vehicle is stationary or at low speeds.

1.2. Group 2: Sources from Vehicle-Road Interaction

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

Tires

Tires are the point of direct contact with the road surface, so they have a significant impact on vibration and noise.

- Rolling noise: Caused by the tire treads constantly colliding with the road surface, the frequency ranges from 50–500 Hz, perceived as a continuous "whooshing" sound in the cabin.
- Tire tread noise: Larger treads are generally noisier than touring tires; high-performance tires are often optimized with tread patterns to reduce NVH.
- Tire resonance vibration: 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 knocking sound: 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 usually caused by unbalanced tires, incorrect wheel alignment, or a faulty ball joint.
- Stabilizer bar knocking sound: Worn stabilizer bars cause a knocking sound when the vehicle leans in corners.

Braking system

- Brake squeal: Occurs at a frequency of 1–16 kHz when the brake pads and brake discs resonate with each other. Although this squeal is not technically dangerous, it is extremely annoying.
- Brake vibration: Strong vibration on the brake pedal or steering wheel when braking hard - often caused by thermal deformation or uneven installation of the brake discs.
- Regenerative whine in electric vehicles: An electronic sound produced when the energy recovery system is active.

1.3. Group 3: Aerodynamic 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 its surfaces, creating vortices – and these vortices generate sound pressure:

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

Ventilation system noise

- Fan noise: Frequency varies with fan speed - a whistling sound at high speed indicates the fan blades are unbalanced or obstructed.
- 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: Structural and Interior Noise

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

- Frame creaking: When the vehicle body twists slightly over uneven surfaces, the joints make a grinding noise.
- Plate resonance: Flat metal plates such as the floor, roof, and doors resonate at frequencies of 60-120 Hz, causing a "rumbling" sound in the cabin.

Interior noise

- Creaking sound: Caused by two hard surfaces rubbing against each other - commonly found in plastic control panels, trim gaps, and assembly points.
- Rattling noise: 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 (electric vehicles)	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:

- Structural transmission path: Vibrations from the engine travel through the engine mounts, into the chassis, to 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 out along this path.
- Airflow path: Sound from the source travels through the air, passing through the soundproofing materials of the vehicle body to enter the cabin. The vibration-absorbing padding, sound-absorbing materials, and the degree of tightness of the vehicle body structure are the determining factors in the effectiveness of soundproofing.

4. Why do manufacturers need to control each source?

- Detecting NVH sources at the design stage using simulation software results in virtually zero modification costs.
- Detection at the sample testing stage : costs can range from a few thousand to tens of thousands of dollars.
- Discovered after mass production: the cost of recalls and line repairs can amount to tens of millions of dollars for a single vehicle.

This is why major automakers 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.

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.

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