| Description and Operation General Vehicle noises when driving cannot be avoided. The aim of all development efforts is to reduce wind noise. The quieter that noise producing components such as the engine, drive train, tyres etc. become, the more the wind noise at high speeds will be heard. Apart from general wind noise, some noises can arise which are heard as an irritating hissing or whistling. The causes of these noises may be partly caused by production or repair factors. They are mostly caused by poorly mounted components, which must be located and installed in the correct position. Most noise experienced in the vehicle interior is wind noise. As speed increases, the level of sound in the vehicle interior increases with it. Normal air flow noises At high speed, as air flows past uniform flat surfaces such as the roof, side windows, etc., turbulent air layers form, in which air pressure variations occur. Part of this pressure variation spreads in the form of sound waves and is transferred to the vehicle interior by the side windows and seals. Here the noise is particularly loud because of the closeness of the side window to the driver or passenger. The noise experienced is mostly a low roaring, which becomes louder as speed increases. Noises caused partly by changes in the direction of air flow and by air flow around separate components If air flows over an edge on a vehicle, the air flow cannot fully the contour of the surface, and it breaks up at the edge. Eddies are formed, which collapse again after a certain time or distance. Change in the direction of air flow The build-up and decay of turbulence and the associated pressure changes lead to creation of sound waves. A similar effect can be observed at the A-pillar on a vehicle. Here the area where eddies are usually formed is directly above the side window. This makes it easy for the noise (roaring) to be passed to the vehicle interior. Eddy current zone Eddy currents are also formed underneath the vehicle, and so associated noise is also produced. When air flows past smaller components or through smaller gaps (such as the radiator grill), the roaring noise becomes a whistling, which increases in pitch and becomes louder as speed increases. Noise caused by vibrating seals Seals which are not making firm contact in the window and door areas can also lead to vibrations caused by pressure variations on the outside of the vehicle which then means transfer of noise into the vehicle interior. The vibrating part of the seal operates here in the same way as loudspeaker membrane. Origin of vibrations (diagram) 2 - Radiation into the vehicle interior Escaping noises Small leaks in the vehicle interior sealing system allow small currents of air to enter. This leads to the creation of "free stream noise" (a stream of air mixes with still air and in doing so causes turbulence). The loudness of free stream noise increases strongly with increasing escape speed and can be heard as a hissing noise (for example, letting the air out of a tyre). Cavity noises If a bodywork cavity is open to the outside of the vehicle, this can cause the air volume in the cavity to resonate. The note produced depends on the volume of the cavity and the size of the opening. (For example, making a note by blowing across the top of a bottle). Wind noise with the sliding roof open Problem: - When the sliding roof is open at about 60 km/h a low frequency noise is produced (drumming).
Cause: - When the sliding roof is fully open, perceptible air pressure variations occur inside the vehicle because of the air current over the opening.
Corrective action: - At speeds above about 60km/h, close the sliding roof slightly until the drumming noise is no longer heard.
Wind noises from the door seals - Because of the air currents passing by the sides of the bodywork and over the roof, the pressure outside is slightly lower. Because of the heating and ventilation system the pressure inside the vehicle is slightly higher. This pressure difference causes, for example, the doors to lift away from the seals by several millimetres at certain speeds. If there is also a side wind affecting the moving vehicle, this effect is increased.
- The resulting differences in gap are compensated for by the use of flexible door seals (when the door seals are intact).
Routes by which external sound is transferred to the inside of a vehicle Wind noises Overview | | Type of wind noise | Where do they arise? | | Normal | Normal air flow noises | Roof, side windows | | | Noises caused partly by changes in the direction of air flow and by air flow around separate components | A-pillars, side mirrors, antennas, windscreen wipers | | Serious | Noises caused by vibrations seals | Door gaps too wide, door/window seals not seated firmly | | | Air escape and air passage noises | Leaks in the bodywork sealing system | | | Cavity noises | Unsealed bodywork cavities | | The factors listed under "serious" require corrective action. | | | Summary The creation and intensity of wind noise on a vehicle depends on the fundamental shape and structure of the different components. There are two subjective evaluation and service diagnosis methods in which sound levels can be reduced and sound nature improved in a vehicle interior: - Reduction in the intensity of the noise sources
- Reduction of the transmission routes
In order to achieve a general improvement in the wind noise situation in the vehicle, both factors must be taken into account. The sealing systems of the doors, windows and tailgate have a major influence on the acoustic comfort in a vehicle. Problem sources in practice Seal on driver and passenger door - Diagnosis:
- Wind noise from the door seal at the top - Cause:
- The alignment of the door frame to the roof is incorrect - Corrective action:
- Re-align the door with the side window open - Check the door seal to sealing surface contact area. The width of the sealing area must be at least 5mm. The correct gap between the door and A-pillar or door and roof Adjustment of the tailgate - Diagnosis:
- Wind noise coming from where the roof meets the top edge of the tailgate - Cause:
- Tailgate is set too high - Corrective action:
- Loosen the tailgate where it is attached at the hinges and move the tailgate further down. When doing so, pay attention to the gaps with adjacent components. - Reset the lock if necessary. Tailgate too high Sliding roof adjustment - Diagnosis:
- Wind noise from the sliding roof - Cause:
- Sliding roof incorrectly adjusted - Corrective action:
- Adjust the sliding roof according to the following requirements: - There must be an equal gap all round the sliding roof. - The sliding roof must be adjusted to suit the roof contour. - The front edge of the sliding roof should close flush with the top face of the roof itself, or slightly lower. - The seal around the sliding roof must touch it evenly all round. Correct installation of the sliding roof Adjustment of the right-hand windscreen wiper arm - Diagnosis:
- Wind noise from the area of the right-hand windscreen wiper - Cause:
- Windscreen wiper arm is set too high - Corrective action:
- Free the wiper arm from its shaft and adjust it so that it lies below the air turbulence which exists at the wind guide plate when it is in its rest position. Adjustment of the wiper arm Sequence for performing a wind noise diagnosis (diagram) - 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?
- Sequence A: A diagnosis is possible based on the details supplied by the customer.
2. Road test the vehicle to check that the concern is resolved. The vehicle must be driven in the exactly same way as when the wind noise was produced earlier. 3. If the corrective measures based on the information supplied by the customer were not successful, further testing must be carried out in the workshop (see sequence B). - Sequence B: A diagnosis based on details supplied by the customer is not possible
1. Test for faults in the workshop, referring to any TSB (Technical Service Bulletin) which may be relevant. 2. Visually check external seals, check gaps. 3. Visual check: Have any components been fitted to the vehicle after manufacture or has the vehicle been repaired after an accident? 6. Perform another road test. The vehicle must be driven in the exactly same way as when the wind noise was produced earlier. 7. 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. Workshop diagnosis Before repair work is carried out, a visual check of the vehicle must be performed, paying special attention to the fit of the doors. 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). Stethoscope testing - The stethoscope is a simple and appropriate tool for finding bodywork air leaks. If the blower is turned on at the highest speed and all the doors, windows and other openings are closed, the air pressure inside the vehicle can be made higher than that outside. The outside of the vehicle is then checked for streams of escaping air.
Powder test - By coating the contact surface of the door seal with white powder, door seals that do not make close contact can be found. To do this, the door is carefully closed and re-opened. In this way, the sealing surfaces which do not touch will be visible.
Ultrasonic detector - It is also possible to detect leaks using an ultrasonic detector and a tone generator. To do this, the ultrasonic tone generator is put inside the vehicle and the outside of the vehicle is tested using the detector tuned to the generator. The detector will quite easily show any leaks present.
Road tests - The causes and origins of wind noises can often be located by a road test. Attention should be paid to the following points:
- Choose a dry, flat and straight road with as little traffic as possible. - Make sure that the tyres are at the correct pressure, to avoid tyre noise. Driving technique - Perform the road test at all speed ranges, using the highest gears (fourth or fifth).
- The road test should be performed by two persons, and different seating positions should be tested.
- Prepare the vehicle as necessary (use adhesive tape to blank off areas).
- Pay special attention to door openings and door seals during the road test.
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