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. The innovation uses a new patented mechanism for suspending and guiding the moving magnetic mass to reduce acoustic distortion of audio signal.
The principle is based on the assembly of two stages of antagonistic springs, allowing a strict oscillating movement guided orthogonally with respect to the radiating surface, while reducing very strongly the transverse movements, such as pitch and roll, largely responsible for the loss of quality and distortion of the reproduced audio signal.
The resonant frequency related to the impedance of the transducer can also be modified by changing the material and geometry of the suspension springs. Two digital prototypes using this suspension configuration were developed and their performance compared in terms of frequency bandwidth, transmitted acoustic power and radiation directivity pattern. The calculated transfer functions of the transducers were then convolved with time signals to assess their ability to reproduce the audio signal with accuracy.
The effectiveness of this new generation of transducers has been demonstrated by numerically simulating the acoustic radiation of a vehicle semi-structural trim, equipped with two transducers integrated into the structure.
Several prototypes have been realized and show an excellent coincidence between experimental and calculated results. The performance and audio reproduction quality of inertial transducers has been demonstrated by the implementation of a multi-point audio system in a real car where the transducers signals are processed and equalized to manage and control the sound quality inside the vehicle.