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Modeling of Rocking Modes in Electroacoustic Transducers

Rocking motion of the radiator is a severe problem in headphones, micro-speakers, and other kinds of loudspeakers. This causes voice coil rubbing, which limits the maximum acoustical output at low frequencies. Causes of this problem are small imbalances in the distribution of the stiffness, mass, and magnetic field in the gap. Modal analysis and lumped parameter modeling are used in this paper to explain the generation of rocking modes. This theory is the basis for a new measurement technique using laser vibrometry to quantify the rocking behavior and to identify the dominant root cause in the design or manufacturing process. Rocking modes radiate less sound than the piston mode because the tilting of the radiator generates positive and negative contributions to the total volume velocity. A new model describes the generation of the fundamental mode and the first two rocking modes. This model requires only three state variables (displacement and two tilting angles) and a minimum number of lumped parameters to describe the excitation of the three modes. Rocking behavior becomes very critical in transducers that do not have a spider, such as headphones or micro-speakers.


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