Although the closing sound of a car door is objectively not related to the intrinsic qualities of the vehicle, it is an important subjective parameter for vehicle evaluation. This closing sound can emphasize the impression of a solid, rockproof car body or can draw the attention to a rather cheap, flimsy vehicle. Several other adjectives can be used to describe the closing sound, which is certainly, often unconsciously, an element that improves or lowers the global image of the vehicle. However, this sound is hardly related to more obvious criteria of a vehicle’s quality. Nowadays, there are several low cost, low class vehicles that do have an equal or even better closing sound than more expensive upper class vehicles. Indeed, it is by simple and cheap measures that this closing sound can be shaped and engineered. During the previous years LMS Engineering carried out several projects that improved the quality image of vehicles.
In a first step, boundary conditions are established which allow a repetitive and accurate measurement of the interior and exterior noise when closing a car’s door. Also the acoustical effect of these external boundary conditions of the door closing process are described. These conditions are amongst others : position of microphones, door closing force and speed, force-input location,… In a second step the closing sound of typically 40 identical vehicles is acquired at the end of the production line in the car factory. As could be expected, the statistical variation of the closing sound is rather high. Therefore out of the large experimental database a virtual reference sound is synthesised for further comparison. A benchmarking test consists out of the measurement of the closing sound of 10 different competitive vehicles. These closing sounds originate from mid and upper vehicles and are considered "best of class". As an example, a detailed comparison of these competitive closing sounds with the virtual reference sound of a mid class passenger car indicated that the objective difference was situated at the higher frequencies. Above 1 kHz the competitor noise was significantly less. (see figure)
The most tedious part of the project consisted in correlating the mechanics of the lock and the dynamic content of the noise time signals.
Although the closing sound has only a limited duration, in this 1,8-second time period 15 mechanical events have been identified. These events are all related to the motion of the internal parts of the lock and the contacting of the door to the rubber seals. The importance of these 15 events has been investigated by means of wavelet and Sound Quality analysis. The wavelet analysis enabled the quantification of the frequency content of the individual events. By means of replaying and editing the measured sound signals the subjective importance of individual events has been established. Event number 8, the rebound of the release cam to the lock cover proved to have the highest high frequency content. The recorded time signal has then been edited and the event number 8 has been removed from the original time series. Replay of this modified signal by the sound quality system allowed a subjective evaluation and a significant improvement was observed.
Once the physics behind the annoying part of the closing sound are known, the definition of countermeasures is rather straightforward. As proof of concept, in the picture below, the effect of the addition of 3 small rubber pieces at strategic locations within the lock is presented. In this case study the effect of 3 events has been surpressed by small and cheap measures. The impact noise of event number 8 completely disappears if a small piece of rubber is attached to the lock cover. An alternative solution consists of the redesign of the lock cover in order to avoid impact.
