Measurement of loudspeakers with a Doppler laser vibrometer and the exponential sine sweep excitation technique

This paper presents an experimental study of vibrations of the cone of loudspeakers. Measurements of the axial acceleration have been performed in hundreds of points on the surface of the cone, employing a Laser Doppler Vibrometer and using the exponential sine sweep excitation signal. The recorded signal has been transformed into an impulse response by convolution with the matched inverse sweep signal. From the knowledge of the acceleration in each point of the radiating surface, the free field sound pressure in axis at 1m distance has been computed. The research focuses on obtaining results comparable to FEM simulations, based only on the linear mechanical behaviour of the loudspeaker cone and avoiding any acoustic interference. Moreover, comparing the laser measurements of many samples, the authors seek to evaluate the influence of known variations in the loudspeaker components or production process on the final performances of the device. Post- processing of experimental results is carried out employing a Matlab script, which computes also operational deflection shapes of the loudspeaker cone.