Recently, radar based micro-Doppler signature analysis has been successfully applied in various sectors including defence, biomedical, and automotive. This article presents the novel use of radar micro-Doppler for loudspeaker analysis. The approach offers the potential benefits of characterizing the mechanical motion of a loudspeaker in order to identify defects and design issues. Compared with acoustic based approaches, the use of radar allows reliable measurements in an acoustically noisy end of a production line. In addition, when compared with a laser vibrometric approach, the use of radar micro-Doppler reduces the number of measurements required and provides direct access to the information of the metallic components of the loudspeaker. In the paper experimental results and analysis of the micro-Doppler signatures of loudspeakers using low cost radar systems are presented. Based on Thiele&Small parameters, the voice coil displacement is modelled and micro-Doppler signatures for a single tone and a sine sweep stimulus are presented. Furthermore, in order to characterize the speaker with a single radar measurement, a methodology to measure mechanical frequency response of loudspeakers is also shown. 19 Index Terms-Radar, micro-Doppler, micro-Doppler analysis, loudspeaker analysis, industrial processes. I. INTRODUCTION 20 L OUDSPEAKERS condition monitoring is an important 21 topic in audio manufacturing. Laser based analysis tools 22 have been shown [1] to yield significantly better results 23 compared to traditional acoustic ones. The former approach 24 is more frequently used in advanced markets like automotive 25 audio components and systems, while the latter is widely 26 used in R&D and manufacturing of acoustic transducers and 27 consumer products (e.g. loudspeakers or audio products).

This paper presents the progress of sine-sweep based measurements to gather impulse responses of musical instruments, in particular acoustic guitars. In the attempt of quantifying the accuracy limit of the methodology a fully working instrument has been reworked , and all phases of the process have been measured before and after each modification step. This provided useful data to perform a sensitivity analysis of this technique, and also showed the similarity between tuning a soundboard and tuning a dipole loudspeaker system based on a resonating plate and an exciter. The collected data proved that the measurement method is capable of capturing the influence of the varnish being applied or removed from the soundboard, the presence or absence of structural reinforcing struts (braces), and the presence or absence of further weight reduction applied to the braces of the soundboard. The findings suggest this approach is mature and ready to be used in combination with FEM simulations, computer aided machining, and additional manufacturing in order to achieve a desired frequency response for acoustic soundboards or panel loudspeakers. Furthermore, the responses collected with this method can be used for auralisation and creative purposes, enabling the community of guitar makers and players to progress in the difficult task of correlating quantitative data with players' preference.

In the last decades a strong interest has developed on radar micro-Doppler analysis targeting applications in a number of sectors including defence, bio-medical and automotive. This paper investigates for the first time the use of radar micro-Doppler for loudspeaker analysis. This approach offers the potential benefits to characterize the mechanical motion of a loudspeaker and to identify defects and design issues. Compared with acoustic based approaches, the use of radar information allows reliable measurements in an acoustically impaired environment. In addition, when compared with a laser vibrometric approach, the use of micro-Doppler reduces the number of measurements required and provides direct access to the information of the metallic components of the loudspeaker. In the paper experimental results and analysis of the micro-Doppler signatures of loudspeakers using low cost radar are presented.

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