Automotive Audio Assessment and Quality Verification
This case study demonstrates the effectiveness of 3D sound intensity measurements for characterizing the sound field inside a vehicle. Results reveal sound radiation and propagation phenomena in a cabin interior. Multiple measurements of the driver's sweet spot were captured for several vehicles of the same model. The proposed measurement methodology is intended to be more comprehensive than traditional point by point microphone tests, and also effective in practice, quick to perform and appropriate for end of line testing and diagnostics environments. The method could be extended for quality verification testing, able to use a single measurement system in order to objectively fingerprint and verify the audio system of a car.
- High spatial repeatability
- Fast measurement solution suitable for in-situ testing
- Characterize the driver's sweet spot
- Compare the performance of several vehicles
Sound visualization: Scan&Paint 3D
The sound field was measured with a Scan&Paint 3D system. Before measuring the acoustic field it is required to have a 3D model of the testing object. The model can be either imported from a CAD file or obtained with a structural scanner. Results presented here were acquired using a structure sensor which was used to scan the car interior for about 3 minutes. The resulting model is used as a visual reference as well as to automatically position the 3D tracker into the measuring environment.
The acoustic data acquisition process starts by manually moving a 3D sound intensity probe whilst a stereo camera is used to extract the instantaneous position of the sensor in the 3D space. The recorded signals are split into multiple segments and assigned to their corresponding locations using a spatial discretization algorithm. The spatial resolution is defined during the post-processing stage and thus can be adjusted depending on the available data. The maximum feasible resolution is determined by the accuracy of the 3D tracker, in this case down to 3 millimeters. Sound pressure, particle velocity or sound intensity across the sound field can then be computed to provide a visual representation of the sound distribution.
Loudspeakers in Automotive Doors
The audio system performance highly depends upon the acoustics of the car doors. The metal inner and outer door panels in combination with the trim layers act as loudspeaker's cabinet. If a direct path exists between the cabin interior and the door cavity (such as door trim gaps and watershield holes), a coupling of modes can results in door cavity issues, decreasing the audio performance or inducing unwanted noises.
Door panels are often actively engaged in the vibro-acoustic response triggered by the loudspeaker signals. The resulting pressure field will come not only from the moving loudspeaker diaphragms but also from the vibrating trim panels. The influence of the latter can contaminate the quality of the reproduced signal. It is therefore key to identify the areas that could significantly contribute to sound radiation besides the loudspeaker drivers.
The results presented here verify that for this vehicle most of the radiation occurs due to the woofer (50 Hz to 250 Hz) and mid-high driver (250 Hz to 1kHz) when the audio is excited with pink noise. However, a source of rattling noise appears at the top right corner of the trim, revealing an excessive gap or a faulty attachment of the panel.
Sound propagation in a car interior
Any acoustic field can be described by the local sound pressure and particle velocity spatial variations. In this case the combination of both quantities, i.e. 3D sound intensity, was used to understand how the acoustic energy is transferred from the speaker drivers to the vehicle cabin. By selecting different frequency bands is possible to identify the woofer (bottom door), mid-high driver (door handle) and tweeter (A-pillar). Any potential issues derived from interaction between loudspeaker drivers in the cross-over frequencies could be detected and troubleshooted effectively by means of 3D sound visualization.
Visualizing Cabin Modes
Complex interference effects between direct and reflected waves from the various surfaces contribute largely to the perceived sound. Numerical simulations are often used to characterize the cabin response in the early stages of the design cycle, however, substantial differences are often found when results are compared to full vehicle experiments. Geometrical discrepancies, coupling mismatches or lack of representative material properties could lead to large errors. Visualizing the acoustic field inside a cabin can be very useful to understand unexpected issues as well as to improve numerical simulations. Results displayed here show two frequency bands where the acoustic field has significant variations close to the headrest, yielding large level differences around the right and left ear of the driver. Changes in the loudspeaker placement, adjustments of the seat or trim package could be implemented to mitigate these problems.
The sound field produced inside several vehicles of the same model was compared in order to understand the degree of repeatability of the acoustic field. The analysis was focused on a small volume around the driver’s head. The use of the same geometrical model allows for only selecting the data within the coordinates of interest. 3D sound intensity spatial variations were compared on a horizontal slice at the the center of the cube. The resulting colormaps and power spectra demonstrate that highly repeatable results can be achieved between different vehicles, but significant differences appear specially in the low frequency range. Having not only a quantitative measure but also spatial information on how sound is distributed can help to identify and troubleshoot any potential problems. If a larger dataset would be measured, a "reference vehicle" pattern which captures the averaged audio fingerprint could be established and quality procedure be implemented based on frequency dependent tolerances.
A scan-based sound visualization technique has been demonstrated that acquires high resolution 3D sound intensity maps in a matter of minutes. Where these tools have been proven effective in the R&D environment, the measurements hereby presented demonstrate their potential for performance verification and diagnostics in end-of-line applications. Further reference data and investigation is proposed to explore the audio fingerprinting application in terms of repeatability and detecting real-world audio system faults. Furthermore, door panels acoustic radiating and squeak and rattle problems were studied using the same tool, revealing problematic mounting points of the trim panels.
Paik, Soonkwon, Manu De Geest, and Koen Vansant. "Interior acoustic simulation for in-car audio design." Sound & Vibration 47, no. 1 (2013): 10-17.
Accardo, Giampiero et al. "Experimental acoustic modal analysis of an automotive cabin." In Experimental Techniques, Rotating Machinery, and Acoustics, Volume 8, pp. 33-58. Springer, Cham, 2015.