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The Panel Noise Contribution Analysis (PNCA) is a well-known methodology for an airborne Transfer Path Analysis (TPA) in car interior. Pressure contribution from the individual panels at a reference point can be very accurately calculated. Acoustic Trim package treatment can therefore be optimized in terms of frequency and panel area which saves money and time. The method uses only one type of sensors so called particle velocity probes for measuring source strength as well as transfer function (with a reciprocal measurement). Traditionally the PNCA makes use of a big amount of probes at fixed points (about 50) hence non-stationary conditions can be measured as well. Typically the measurement is performed in 3 sessions resulting in 150 individual panels. Because of the low spatial resolution the method can only be used at mid-low frequency range.

Automotive suppliers provide multi-layer trims mainly made of porous materials. They have a real expertise on the characterization and the modeling of poro-elastic materials. A dozen parameters are used to characterize the acoustical and elastical behavior of such materials. The recent vibro-acoustic simulation tools enable to take into account this type of material but require the Biot parameters as input. Several characterization methods exist and the question of reproducibility and confidence in the parameters arises. A Round Robin test was conducted on three poro-elastic material with four laboratories. Compared to other Round Robin test on the characterization of acoustical and elastical parameters of porous material, this one is more specific since the four laboratories are familiar with automotive applications. Methods and results are compared and discussed in this work.

The challenge of a NVH development is to define a link between the target of the OEMs expressed in terms of acoustic performance, weight and cost and the design of the optimized acoustic package reaching this target. The “Vehicle Acoustic Synthesis Method” (VASM) has been developed in order to create this link. The VASM method, which is an energy based hybrid simulation technique, calculates the Sound Pressure Level at ear location from the combination of sound power measurements and acoustic frequency response functions (FRF) panel/ear, either measured or simulated with Ray-Tracing Methods.

A novel measurement tool is developed that is capable to measure the reflection coefficient of acoustic materials in situ and thus in real live situations such as a car. The measurement tool is a combination of two novel methods, the surface impedance method [1], [2], and the mirror source method [3]. The surface impedance method measures the acoustic impedance close to the surface of an acoustic absorbing material. The method is very sensitive for highly reflective surfaces [1], [2], [6]. The mirror source method uses a miniature monopole sound source that is placed close by the acoustic reflecting material. A particle velocity microphone (a Microflown [4], [5]) is placed close to the monopole source in such way that its sensitive direction is aiming at the acoustic reflecting material and its non sensitive direction is aiming at the source. This way it is only measuring the ‘mirror source’: the reflected image of the monopole sound source.

The particle velocity field close to a source almost matches the surface vibration whereas the sound pressure field is mainly caused by the background noise. Here a new method is proposed that is to simply listen to the particle velocity field to find sources. The method shows to give a very fast first impression of the acoustic problem at hand.

Monopole sound sources (i.e. omni directional sound sources with a known volume velocity) are essential for reciprocal measurements used in vehicle interior panel noise contribution analysis. Until recently, these monopole sound sources use a sound pressure transducer sensor as a reference sensor. A novel monopole sound source principle is demonstrated that uses a Microflown (acoustic particle velocity) sensor as a reference sensor. As compared to a sound source that uses a sound pressure transducer as a reference sensor, the new sound sources demonstrated are relatively easy to calibrate, not sensitive to changes in ambient temperatures, and suitable to use in all sorts of acoustic environments.

The purpose of the SONVERT project is to create a link between the acoustical sources of a car and the environment in terms of traffic and architecture. Based on well validated approaches, it introduces the notion of a “macro-source” which integrates the major acoustic sources: engine, tires and exhaust, taking into account the low and high frequency aspects, from measurements made on real vehicles. The macro-source is then integrated into an original approach dealing with outdoor propagation. The proposed method can consequently be seen as a first step toward a global approach for the study of traffic noise in real conditions.

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.

Acoustic particle velocity sensors can be an alternative sensor category for end of line testing next to microphones, accelerometers or scanning laser vibrometers. As any other category of transducers, particle velocity sensors have their specific features. The acoustic particle velocity field is far less susceptible to background noise than the sound pressure field, allowing acoustic testing to be carried out in a manufacturing environment with significant background noise levels [2]. Close to a vibrating surface, acoustic particle velocity is a good estimate of the normal structural velocity, allowing non contact vibration measurements [4]. The results of some case studies will be summarized.

With PU probes the sound pressure and acoustic particle velocity can be measured directly. Over recent years, the in situ surface impedance method, making use of such a probe, has proven to be an alternative to Kundt's tube measurements for product development type of work. The in situ method can also be used for the purpose of quality control on the acoustic material, be it during manufacturing or assembly, ensuring the best possible way to monitor the practical effectiveness of the acoustic package designed earlier on. In order to assess the variance of the acoustic package material leaving the assembly line, a relevant number of samples were taken over time. The quality of both the headliners, and the passenger seats were measured, of 25 cars of the same type. The robustness of the measurement method will be discussed, and the results will be presented.

The weight reduction challenge has taken a new shape in the past two years due to high pressure on CO2 emissions in the automotive industry. The new question is: what level of acoustic performance can you get with an insulator weighting less than 2500 g/m2? The existing solutions at this weight being mainly dissipative (absorption) concepts give a satisfactory performance only if the pass-throughs are poor and present critical leakages. Respecting the less than 2500 g/m2 weight target, we have developed a wide range of new or optimized concepts switching from extremely absorbing to highly insulating noise treatments playing with multi-layers insulators (typically three to four layers), in combination or not with tunable absorbers on the other side of the metal sheet (in the engine compartment for example).

The NVH optimization process of a power train often consists in a target setting for the acoustic power radiation of the engine in free field working conditions (in an anechoic or semi-anechoic room). This method requires the engine to be dismounted from the car and to be measured in an anechoic or semi-anechoic room which is costly and time consuming. Moreover the free-field characterization is not a good predictor of the acoustic behavior of the power train when it is mounted in the engine bay of a car (very reactive field). This paper presents a number of existing methods to determine the acoustic power radiation pattern of the engine mounted in a car using an intensity probe which is based on a pressure sensor and a particle velocity sensor. For the lower frequencies the velocity probe is used, for the higher frequencies both pressure and velocity is used to measure intensity. A new method for the mid-low frequency range is presented.

The lightweighting research on noise treatments since years tends to prove the efficiency of the combination of good insulation with steep insulation slopes with broadband absorption, even in the context of bad passthroughs management implying strong leakages. The real issue lies more in the industrial capacity to adapt the barrier mass per unit area to the acoustic target from low to high segment or from low petrol to high diesel sources, while remaining easy to manipulate. The hybrid stiff insulator family can realize this easily with hard felts barriers backfoamed weighting from 800 g/m2 to 2000 g/m2 typically with compressions below 10 mm. Above these equivalent barrier weights and traditional compressions of 7 mm for example, the high density of the felts begins to destroy the open porosity and thus the absorption properties (insulation works anyway here, whenever vibration modes do not appear due to too high stiffness…).

Leakage ranking of vehicle cabin interiors is an important quality index for a car. Noise transmission through weak areas has an important role in the interior noise of a car. Nowadays the acoustic leakage inside a cabin can be measured with different techniques: Microphone array-based holography, Trasmission loss measurement, Beamforming analysis, Sound intensity P-P measurements and ultrasound waves measurements. Some advantages and limits of those measurement approaches for quantifying the acoustic performance of a car are discussed in the first part of this paper. In the second part a new method for fast leakage detection and stationary noise mapping is presented using the Microflown PU probe. This method is called Scan & Paint. The Microflown sensor can measure directly the particle velocity which in the near field is much less affected by background noise and reflection compared with normal microphones.

Due to increasing attention paid to the optimization of leakages and passthroughs in general, measurements on cut out modules in large coupled reverberant rooms are often carried out in the middle and high frequency range, in order to optimize the insulation performance of trims installed in their actual environment (Transmission Loss). Using optimal controlled mounting conditions, we have been able to extend the frequency range to the low frequencies in order to validate trim FEM models of a headliner cut out module with structureborne and airborne excitations.

The interior noise of a car is a general quality index for many OEM manufacturers. A reliable method for sound source ranking is often required in order to improve the acoustic performance. The final goal is to reduce the noise at some positions inside the car with the minimum impact on costs and weight. Although different methodologies for sound source localization (like beamforming or p-p sound intensity) are available on the market, those pressure-based measurement methods are not very suitable for such a complex environment. Apart from scientific considerations any methodology should be also “friendly” in term of cost, time and background knowledge required for post-processing. In this paper a novel approach for sound source localization is studied based on the direct measurement of the acoustic particle velocity distribution close to the surface. An airborne transfer path analysis is then performed to rank the sound pressure contribution from each sound source.

In order to reach the new 2020 CO2 emissions regulations, we have developed a wide range of lightweight noise treatment technologies going from pure absorbing to highly insulating ones, depending on the pass-through quality situation. This Generalized Light-Weight Concepts family was first optimized using the 2D Transfer Matrix Method (TMM) combined with quick SEA approaches. Taking into account thickness 3D maps with TMM is an efficient and quick intermediate “2,5D” optimization method, but it is not a real 3D approach. This work presents a new 3D optimization procedure based on poroelastic finite elements including intermediate cavities (like Instrument Panels) for designing these Generalized Light-Weight Concepts. A parallel reflection deals with products and processes in order to check the feasibility of the resulting 3D optimized insulator designs.

It has been shown that the combination of a particle velocity (u) sensor and a sound pressure (p) microphone can successfully be used to determine the reflection coefficient of acoustic materials [1, 2, 3 and 4]. In this paper, a practical measurement technique and a so called pu-probe impedance meter are described that require no Kundt's tube or anechoic room. The procedure allows fast (less than 60 seconds) sound absorption measurements under both normal and oblique angles of incidence on small test samples (less than 30*30cm). The total size of the pu-probe impedance meter is 60 centimeters and weights less than 1kg. The method can be used in combination with a sound card based software package. At first, the pu-probe is calibrated in approximately 20 seconds. The frequency response of the impedance sensor is transformed to an impulse response. This response is time windowed to cancel out the room reflections.

In order to reach OEMs acoustic treatment targets (improving performance while minimizing the weight and cost impact), we have developed an original hybrid approach called “Vehicle Acoustic synthesis method”[1] to simulate - and therefore to optimize - noise treatments for both insulation and absorption, and to calculate the resulting Sound Pressure Level (SPL) at ear points for the middle and high frequency range. To calculate the SPL, we identify equivalent volume velocity sources from intensity measurements, and combine them to acoustic transfer functions (panel/ear) measured or computed with ray tracing codes using the reciprocity principle. Compared to our first approach [1], this paper shows a new measurement technique using pressure-particle velocity probes [2]. This technique allows to reduce acquisition time by a factor four, and makes therefore possible a synthesis method on a complete car within two weeks.

The use of inertial transducers to replace traditional loudspeakers is an innovative way to reproduce a quality audio signal in a vehicle cockpit while significantly reducing on-board mass and overall volume of the audio system. An electrodynamic inertial exciter is an actuator commonly used for the realization of distributed mode loudspeakers or DML (Distributed Mode Loudspeaker) to generate vibrations of a panel radiating an acoustic wave. As for a loudspeaker, an inertial exciter implements a coupling process and is based on the interactions between a current and a magnetic field. The coil is movable and the magnetic mass stationary in the case of the loudspeaker, while the reverse is true for the inertial exciter. This paper presents the development process of a new inertial transducer and its optimization by digital simulation, validated by tests on physical prototypes.