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Root Cause Analysis of Rocking Modes

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Most micro-speakers, headphones, and some cone loudspeakers exhibit undesired rotational vibration patterns called rocking modes. These are caused by inhomogeneous distribution of mass, stiffness, and force factor shifting the center of gravity, stiffness, and electrodynamic excitation away from the pivot point, which is the cross point of the nodal lines of the two rocking modes. This paper focuses on practical measurements of rocking modes using laser vibrometry, parameter identification, and root cause analysis. New characteristics are presented that simplify the interpretation of the identified parameters. Due to the high quality factor of the rocking resonators, only a very small asymmetrical force is required (which is usually a few percent of the transversal force) to generate a critical rocking behavior having more energy than the desired piston mode. Assessing the relative rocking level and identifying the imbalances is a convenient way to keep voice coil rubbing under control and to avoid impulsive distortion impairing the quality of the reproduced sound. A new technique was validated by numerical simulations and systematic modifications of a real transducer. The diagnostic value of the new measurement technique is illustrated on a transducer used in headphones.

JAES Volume 64 Issue 12 pp. 969-977; December 2016
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AES - Audio Engineering Society