NVH (noise, vibration, harshness)

What is NVH?

NVH 'noise, vibration, harshness' plays a role in many different areas, from washing machines and lawnmowers to motor vehicles, aircraft, wind turbines, etc. Every object that moves or is put into motion can cause potentially critical noise and vibration problems. In combination with motor vehicles, NVH is an important criterion for driving comfort. This includes the sum of all audible and perceptible vibrations that affect this comfort. Together, car body, power train, chassis, exhaust system, ventilation/air conditioning and other subsystems produce a complex combination of acoustic noises and vibrations.

The lower the noise and vibration level of a vehicle, the higher its driving comfort is rated. The human ear detects noises in a range from 20–20,000 Hz. The NVH analysis and NVH optimization of the complete vehicle and its components are of great importance, especially in the early stages of vehicle development, particularly in the powertrain. 

In addition to the NVH analysis of classic power trains, the NVH analysis of electrified powertrains is growing in importance due to government emission regulations. With the elimination of the dominant source of noise – the combustion engine – as well as the lightweight design, other vibration sources and noise become apparent. The vibration response of an electrified engine leads to other effects that are different from those of powertrains with classic combustion engines. This places new challenges on development.

What does 'noise' mean in the context of NVH?

Noise phenomena primarily include all noises that originate in the powertrain. In addition, rolling noise that occurs as a result of the contact between tires and the road as well as the airflow noises that arise from the aerodynamics during travel. At speeds of up to 50 km/h, noises that originate in the powertrain are dominant; at speeds between 50 km/h and 100 km/h, rolling noise dominates. The intensity of airflow noise increases with speed. 

What does 'vibration' mean in the context of NVH?

Vibration refers to any type of mechanical oscillation. The vibration that is transferred to the drive via the car seat can be felt. Its origin may be, e.g., the oscillation energy that is transferred from the engine to the engine mounts and from there to the structure and ultimately to the seat. Vibration energy originating from the same source could also become a noise as a result of amplification in the cabin. 

What does 'harshness' mean in the context of NVH?

Harshness refers to the subjective assessment of noise and vibration phenomena.

How is NVH measured and analyzed?

Vibration has a number of causes and many consequences. Because the phenomena often overlap, they must be examined individually and isolated from one another. As a rule, the measuring chain consists of a sensor (accelerometer, microphone, intensity probe, laser vibrometer), a data acquisition system and a data analysis system. The analysis of NVH data generally requires complex algorithms and special software.

What does 'vehicle acoustics' mean in the context of NVH?

In acoustics, a distinction is made between two types of sound – structure-borne sound and airborne sound. Vehicle acoustics are always a combination of both. Structure-borne sound is produced by oscillating components or the complete vehicle. Airborne sound is caused by the spread of pressure waves in the air and components. Thus, engine noises and vibrations reach the ear of the driver in two ways. Structure-borne sound is transferred through the brackets, the car body and the ground. Airborne sound is formed in the combustion chamber and wanders through the engine block, the firewall and the dashboard.