These terms are grouped together under the label Noise, Vibration, Harshness, or NVH in short.
The task of vehicle development and production is to ensure that noises caused by the vehicle do not disturb the driver and passengers. Moreover, the the external noises emitted by the vehicle must not exceed the thresholds set by law.
The following section gives an overview of how noise, vibration and harshness can occur in the vehicle and what remedial action is possible.
Low to middle tones are considered to be unpleasant. They are palpable and noticeable as oscillations and vibrations throughout the body. Loud howling and whistling is painful to the ears.
A noise usually consists of a superimposition of different tones which spread as oscillations.
Each of these oscillations has a specific oscillating time and can be measured in frequencies. The frequency describes the number of oscillations per second. The frequency unit is specified in Hertz (Hz).
The human ear can perceive frequencies between 20 and 20000 Hz.
Noises can already be contained where they occur or, if this is not possible, can be confined with suitable measures. The basic procedures are the damping of oscillating parts, the insulation of components or the absorption of the noises through appropriate materials.
If a damper is installed next to an oscillating mass, the characteristic of the damper will reduce the movement of this mass accordingly (e.g. bumper on chassis).
Damping affects the resonance of an object or system.
In oscillation technology, the term isolation means decoupling (separation) of components and systems.
An engine is mounted in sprung elements, so that as little oscillation as possible is passed to the vehicle.
In automotive technology, the isolation technique used is nearly always rubber mounting. The elasticity of the rubber acts like a spring.
Sound waves are reflected from hard surfaces Through the use of absorption material, sound waves hit soft surfaces and are absorbed by them. The composition and thickness of the material used plays an important role here.
A soft surface, depending on its composition, absorbs the sound waves and reduces their energy.
NVH elements are installed to prevent airborne sound transfers to the passenger compartment in different body cavities.
For the exact installation position of an NVH element, please refer to the vehicle-specific repair instructions.
If an NVH element is to be reused, the bonding on the body panel must be detached. To do this, the body panel must be heated in the area around the NVH element. The bonding can be detached at approx. 170° C. The damaged panel part can now be carefully dismantled.
Before installing the new panel part, PU adhesive must be applied to the contact areas between the panel and the NVH element.
Test techniques, measuring devices
The shortest route to an accurate diagnosis results from:
- general information on the problem vehicle and a comparison test with a vehicle of the same construction, without NVH problems.
- vehicle history, including repair history and usage patterns.
- condition history, especially any relationship to repairs or sudden change.
- knowledge of probable causes.
- application of diagnosis procedures in which the vehicle is split into corresponding areas.
The diagnosis and correction of noise, vibration and harshness concerns requires:
- a road or system test to determine the exact nature of the concern.
- analysis of probable causes.
- checking of the cause and elimination of the faults found.
- a road test or system test to make sure the concern has been corrected or brought back to within an acceptable range.
It is often very difficult to locate noises that are audible in the passenger compartment based on the problem description provided by the customer and the road tests performed. The direction of the noise can be detected subjectively, but the source of the noise cannot be found.
NOTE:For a selection of simple test tools, see the wind noises section.
Stethoscope
Using the stethoscope, you can listen to the entire vehicle passenger compartment to locate noise sources more easily. This test procedure can be carried out either while the vehicle is moving, or with the engine running and the vehicle stationary, depending on the concern. The noise source can be assumed to be where the stethoscope identifies the highest noise radiation.
NOTE:For safety reasons, only the passenger should carry out the stethoscope testing while the vehicle is moving.
Application examples:
- For very frequent noises in the passenger compartment.
- For engine noises that penetrate through the dashboard into the passenger compartment.
- Wind noise.
- Noise outside the vehicle that is routed inside, such as roadway, tire or water spray noises.
NOTE:With the stethoscope it is possible to locate medium and high frequency noise paths (caused by leaks) while the vehicle is moving. The stethoscope is not suitable for diagnosis of low frequency droning problems.
Ultrasonic measuring device
The ultrasonic detector is a good and reliable test method for acoustic problems. It is used in a similar way to the stethoscope. In principle, it is suitable for all high frequency interior noises and for leaks in the body seals.
The device consists of an ultrasonic transmitter and a receiver. During use, the transmitter sends an ultrasonic signal which is received at the problem zones by the receiver.
Electronic NVH tester
The measuring device described below is used for diagnosis of the solid-borne sound and solid-borne sound transmission paths. The device is particularly suitable for medium and high frequency noise analyses. In order to obtain a positive diagnosis of droning problems (low frequency noises) and their sources, you must have sufficient experience of how to use this measuring device.
NOTE:In the NVH area, diagnosis of droning problems is one of the most difficult tasks and sets high requirements of the service technicians.
The device works according to the following operating principle: Accelerometers (transmitters) are fitted on various vehicle components or body areas. The signals recorded here can be listened to one after the other on headphones or speakers via the different channels. Simultaneous illustration of several or all measuring channels (for comparison) is only possible visually on the display of the measuring device.
NOTE:Before using the NVH tester in the service, the service technician should take part in an NVH training course to ensure effective use of this device during the road test. A description of the function and application of the NVH tester is enclosed with the device.
Layout and operation:
- The test device has six different channels for noise diagnosis.
- Each channel is marked in color on the terminal, cable and test device.
- The solid-borne sound recorded is transmitted to the test device or the headphones by the magnetic accelerometers (transmitters).
- There is an amplifier on the test device with which the signal strength and the corresponding channel can be set.
- Only the noises from a transmitter are transferred to the headphones.
- All connected cables can be visually illustrated individually or simultaneously on the display.
- The test device saves the recorded data.
- The recorded data can be imported to a PC and evaluated.
