Noise, Vibration and Harshness (NVH). Description and Operation

Noise, Vibration and Harshness (NVH) Acceptable Levels

Noise is any unwanted sound, usually unpleasant in nature. Vibration is any motion, shaking or trembling that can be felt or seen when an object moves back and forth or up and down. Harshness is a ride quality concern in which the vehicle's response to the road is transmitted abruptly to the customer. Harshness usually describes a firmer than normal response from the suspension system. NVH is the term used to describe these conditions, which cause varying degrees of customer dissatisfaction. Although a certain level of NVH caused by road and environmental conditions is normal, this section is intended to assist in the diagnosis, testing and repair of NVH symptoms. All internal combustion engines and transmissions produce some noise and vibration; real-world operation adds noise that cannot be controlled. Vibration isolators, mufflers and dampers reduce them to an acceptable level. An unfamiliar driver of the vehicle may think that some sounds are abnormal when in fact the sound is normal for that type of vehicle. As a technician, it is very important to be familiar with the characteristics of the vehicle and to know how they relate to NVH symptoms and their diagnosis. For example, if the vehicle is equipped with an automatic overdrive transmission, it is important to road test the vehicle both inside and outside of the overdrive mode.

Glossary of Terms

Amplitude:

Amount or level of energy produced by a vibrating component (overload). Extreme vibration has a large amplitude. Mild vibration has a low amplitude. Refer to Intensity.

Boom:

Low-frequency or low-pitched noise often accompanied by vibration. Also refer to Drumming.

Buffeting:

Strong noise fluctuations caused by wind gusts. An example is wind gusts at a side window.

Buzz:

Low-pitched sound like a bee. Often a metallic or hard plastic buzzing sound. Also describes high-frequency vibration. The vibration is similar to an electric shaver.

Chatter:

Pronounced series of rapidly repeating rattling or clicking sounds.

Chirp:

Brief high-pitched noise associated with drive belt slip.

Clunk:

Repetitive low-pitched sound. A loud clunk is usually described as a knock.

Click:

Sharp, short, non-resonant sound similar to clicking a ballpoint pen.

Clonk:

Hydraulic knocking sound. The sound is caused by air pockets in the hydraulic system. Also described as hammering.

Driveline/Transmission Clunk:

Heavy or dull, short, low-frequency sound. Occurs mainly on a vehicle that accelerates or decelerates sharply. Also described as a bump.

Conductor:

Components that carry (transmit) vibration frequency from the source to the reactor.

Cycles per second:

Cycles per second. Same as Hertz (Hz).

Crack:

Mid-frequency sound associated with squeak. The sound changes depending on temperature conditions.

Creak:

Metallic creaking noise.

Cycle:

Process of a vibrating component performing its full range of motion and returning to the starting point.

Decibel (dB):

Unit of measurement of sound pressure level, abbreviated dB.

Drone:

Low-frequency steady sound, like a refrigerator compressor. Also described as a moan.

Drumming:

Cyclical low-frequency rhythmic noise, often accompanied by a feeling of pressure on the eardrums. Also described as low rumble, boom, or thunder.

Flutter:

Intermittent medium to high-pitched sound caused by airflow. Like a flag fluttering in the wind.

Frequency:

Rate at which a cycle occurs within a given time.

G-force:

Additional load or weight occurring on an object during acceleration. When measuring vibration level or amplitude without sound, the unit G is added to relate the vibration force to gravitational force. This is similar to measuring the weight of an object, which is also a function of gravity.

Gravel feel:

Scraping or growling in a component, similar to the feeling of driving over gravel.

Grind:

Abrasive sound similar to using a grinding wheel or rubbing sandpaper on wood.

Hz (Hertz):

Unit of measurement used to describe noise and vibration problems, expressed in cycles per second.

Hiss:

Steady high-frequency noise. Sound of a vacuum leak.

Hum:

Steady low-frequency tone that sounds as if you are blowing across the top of a long-neck bottle.

Howl:

Mid-frequency noise between drumming and whine. Also described as a hum.

Buzz:

Mid-frequency steady sound, like a small fan. Also described as a whine.

Intensity:

Physical quality of sound related to the strength of sound or vibration (measured in decibels). The higher the amplitude of sound, the higher the intensity, and vice versa. Refer to Amplitude.

Knock:

Heavy, loud, repetitive sound similar to knocking on a door.

Moan:

Constant low-frequency tone. Also described as a hum.

Ping:

Brief high-frequency sound with slight echo.

Pitch:

Physical quality of sound related to its frequency. Pitch increases as frequency increases, and vice versa.

Pumping feel:

Slow, pulsing movement.

Rattle:

Random and instantaneous or brief noise.

Reactor:

Component or part that receives vibration from the source and conductor and responds to the vibration with movement.

Roughness:

Mid-frequency vibration. Slightly higher frequency than shake. This type of vibration is usually associated with driveline components.

Rustle:

Intermittent sound of varying frequency, similar to rustling leaves.

Shake:

Low-frequency vibration, usually with visible movement of a component. Usually refers to tires, wheels, brake drums, or brake discs if sensitive to vehicle speed, or the engine if sensitive to engine speed. Also called shimmy or wobble.

Shimmy:

Abnormal vibration or side-to-side oscillation felt through the steering wheel while the driveshaft rotates. Also described as wobble.

Shudder:

Low-frequency vibration felt through the steering wheel or seat during light braking.

Thump:

Resonance from flat surfaces, such as seat belt webbing or door panels.

Chirp:

A high-pitched transient sound, similar to the sound of fingers rubbing across a clean window.

Squeal:

A sustained high-frequency noise.

Tick:

A light, rhythmic or intermittent hammer-like tapping, similar to tapping a pencil on the edge of a table.

Knock:

A dull impact caused by two objects striking each other.

Click:

A rhythmic tapping sound, similar to the noise of a clock.

Tip-in Moan:

A light moaning noise heard when accelerating a passenger car, usually at speeds from 40 km/h to 105 km/h.

Transient:

A noise or vibration that is short-lived.

Vibration:

Any movement, shaking or trembling that can be felt or seen when an object moves back and forth or up and down.

Howl:

A constant, piercing noise. Also described as a squeal.

Whistle:

A high-frequency noise with a very narrow frequency band. Examples of whistling noises are a turbocharger or airflow around an antenna.

Wind Noise:

Any noise caused by air movement inside, outside or around the vehicle.