Wind noise and noises in general are dealt with under the label Noise, Vibration, Harshness, or NVH in short.
Due to the continuous reduction in drive noises, wind noise has come to the fore in the vehicle and is perceived to a greater extent by the customer.
There are various causes of wind noise. They can be due to the design of the vehicle, or they can occur after a repair. They are mostly caused by poorly mounted components, which must be located and installed in the correct position.
In order to carry out targeted diagnosis, it is important to know the basics of noise formation and sound transmission.
Normal air flow noises are caused by air blowing against even, flat vehicle surfaces, such as the roof, doors and side windows. When the vehicle is moving fast, air films (turbulence) form, which cause variations in air pressure. These variations in air pressure spread in the form of sound waves and are transferred to the vehicle interior via the side windows and seals.
If air flows over an edge on a vehicle, the air flow cannot follow the shape of the surface, but separates at the edge. Eddies are formed, which collapse again after a certain time or distance. The associated variations in air pressure create a corresponding sound wave.
Streaming noise occurs if there are leaks in the sealing system to the vehicle passenger compartment. The noise is caused when stationary air mixes with moving air. As a result, the noise increases as the streaming speed increases.
Cavity noises are noises that occur when the air column is caused to oscillate.
Workshop diagnosis
Before carrying out repair work, a visual inspection of the vehicle must be carried out. The gaps in the doors, the sunroof and at all other body parts must be checked in particular.
When the doors are adjusted to fit exactly, development of wind noise can often be eliminated at high speeds (lifting of doors off the seals).
The following points are also to be checked:
- Check that windows are completely closed.
- Check air ducts and vents for correct installation.
- Check protruding trims or plastic parts.
- Check that all blanking plugs are present.
Road tests
Wind noise can usually only be located through road tests.
NOTE:There should always be two people present during test drives to find noises. A driver who reconstructs the situation causing the noise, and a person to carry out the checks.
The following points should be taken into account for such test drives:
- Check that the tire pressure is correct.
- Remove non-standard ancillary components from the vehicle.
- Choose a dry, flat road with as little traffic as possible.
- Carry out the road test in all speed ranges. Use a high gear so that the engine noise is low.
If it is difficult to detect the noise sources, the search can be made easier by masking potential areas.
Sequence
A basic prerequisite for a problem description with subsequent diagnosis is the performance of a test drive with the customer..
Only once the customer's problem description is clear should the service technician begin with the diagnosis of the problem.
The service technician should carry out specific road tests to achieve further containment of the problem.
Sequence (schematic):
1. Customer concern
- What is the customer concern and what details can he supply about the wind noise?
- Under which conditions does the wind noise appear?
2a. Diagnosis and corrective measures Sequence A: The diagnosis is possible based on the information supplied by the customer.
- Carry out corrective measures to remedy the wind noise.
- Road test the vehicle to check that the concern is resolved. The vehicle must be driven in exactly same way as when the wind noise was produced earlier.
- The corrective measure performed based on the information supplied by the customer was not successful. Further fault finding must now be carried out in the workshop (see Sequence B).
2b. Diagnosis and corrective measures Sequence B: The diagnosis is not possible based on the information supplied by the customer.
- Test for faults, referring to any TSB (Technical Service Bulletin) which may be relevant.
- Visually check external seals, check gaps.
- Visually check the vehicle for traces of accident repair and retrospectively attached ancillary components.
- Perform a diagnosis based on the road test.
- Carry out corrective measures based on the diagnosis.
- Perform another road test. The vehicle must be driven in exactly same way as when the wind noise was produced earlier.
- If this road test does not show that the work has been successful, additional techniques such as powder testing, stethoscope testing or ultrasonic detection must be employed.
The vehicle acoustics do not always make it possible to draw up a clear diagnosis. It is therefore all the more necessary to use all methods of detecting and suppressing NVH problems.
3. Comparison of vehicles constructed in the same way.
If no clear diagnosis is possible based on a customer concern, a comparison test drive should be carried out on a vehicle constructed in the same way.
Test equipment
Diagnosis of wind noise requires good hearing, basic knowledge of acoustics and experience. Tools can provide assistance for the diagnosis and reduce the fault finding times.
Stethoscope
Here, the tightness of the vehicle passenger compartment is checked. The ventilator blower is set to the highest setting and the doors and windows are closed. There is now a corresponding overpressure in the passenger compartment.
The stethoscope is now used to listen to the door and window seals. The sound of the streaming air can be heard at the leaks.
Smoke pipe
The preparatory work is the same as for the stethoscope test. By scanning past the seal areas with the smoke pipe, a leak can be visually detected based on the changed smoke path.
Ultrasonic detector
The ultrasonic detector is a further method of finding leaks in the vehicle passenger compartment. Here, an ultrasonic noise generator is placed in the passenger compartment. The closed vehicle is then inspected from outside with the corresponding detector. The detector will show any leaks present.
Powder
Door seals that do not make close contact can be detected by coating the contact surface of the door seal with white powder. To do this, the door is carefully closed and re-opened. In this way, the door seals which do not touch will be visible.
