Mapping 3D Sound Intensity Streamlines in a Car Interior

Abstract

Sound source localization techniques in a car interior are hampered by the fact that the cavity usually is governed by a high number of (in)coherent sources and reflections. In the acoustic near field, particle velocity based intensity probes have been demonstrated to be not susceptible to these reflections allowing the individual panel contributions of these (in)coherent sources to be accurately determined. In the acoustic far field (spherical) beam forming techniques have been used outdoors in the free field, which analyze the directional resolution of a sound field incident on the array. Recently these techniques have also been applied inside cars, assuming that sound travels in a straight path from the source to the receivers. However, there is quite some evidence that sound waves do not travel in a straight line. The Maritime Institute of Stetting in Poland made numerous 3 D sound intensity measurements demonstrating an erratic pattern of sound intensity streamlines [1], [2] His approach was transferred from a lab to an actual car cabin upon request of a larger European car manufacturer. At 900 positions inside the car the 3D intensity is measured with a 3 D sound intensity probes using three particle velocity sensors. Such a probe is not susceptible to the pressure-intensity index. Several speakers that are driven in sequence are used as controlled sound sources. The results demonstrate that even with a single sound source, the 3D intensity streamlines are strongly bending, suggesting that far field techniques do not point towards the sound source. INTRODUCTION A number of different types of experimental approaches exist to localize and quantify sound sources in cabin interiors, all having their specific strengths and weaknesses. The aim is always to detect the acoustically weak part of the vehicles in order to define appropriate counter measures. In the near field several techniques are used such as window-based methods [3], [4], intensity measurements [5], laser scanning vibrometry measurements [6], and holographic technologies [7]. In the far field, spherical beam forming [8] has been introduced. The concept assumes that sound waves travel in a straight line from the sound source to the receiver. This assumption was verified in this study. For that purpose, a four channel three dimensional sound intensity probe is used to measure the intensity levels inside a vehicle at a large number of measurement positions. Several loudspeakers are placed inside the car and powered in sequence to study the influence of different the locations of the source. To get a good impression of the location of the measurement points, the inner geometry of the car is digitized with a specially designed tool. As it can be difficult to display 3D intensity information in a 3D environment, an attempt is made to make the results more understandable using various visualization techniques.