| Summary: | 博士 === 國立交通大學 === 機械工程系所 === 96 === The goal of this thesis is to establish the analysis, design and evaluation of electro-acoustical transducers. In recent years, microspeakers are key components of many 3C products especially for portable devices. Due to size limitation, microspeakers suffer from the problem of low output level and nonlinear distortion. To address the issue, an optimization technique is presented for design of microspeaker. The optimization procedure is based on an electro-mechano-acoustic (EMA) model and electro-mechanical parameters. Characteristics including voice-coil impedance, frequency response and harmonic distortion are evaluated. The results show that significant improvement in output performance and excursion limitation has been gained by using the optimal design. On the other hand, the diaphragm serves as not only a sound radiator but also the suspension. Thus, the pattern design of the diaphragm is crucial to the overall response and performance of a microspeaker. Traditional approach for modeling microspeakers using lumped-parameter models is generally incapable of modeling flexural modes in high frequencies. In this thesis, a hybrid approach that combines finite element analysis (FEA) and EMA analogous circuit is presented to provide a more accurate model than the conventional approaches. The mechanical impedance obtained using FEA is incorporated into the lumped parameter model. On the basis of this simulation model, the pattern design and optimal number of the diaphragm is optimized using the Taguchi method and sensitivity analysis.
The combination of FEA and EMA analogous circuit technique is also can be used in the piezoelectric buzzer design and electret loudspeaker simulation. The analysis starts with modeling the diaphragm structure by using FEA model. The FEA model is then converted into electro-mechanical two-ports to fit into the EMA analogous circuit. Electrical impedance of the piezoelectric diaphragm is simulated using the model. An ‘added-mass’ method is developed to identify the lumped parameters of the piezoelectric diaphragm. Electrical impedance and on-axis sound pressure level (SPL) of a piezoelectric buzzer (containing the diaphragm and case) can be simulated by solving the loop equations of the analogous circuits. On the other hand, the loudspeaker made of thin, light and flexible electret material lends itself well to the space-concerned applications. Electrical impedance measurement reveals that the coupling between the electrical system and the mechanical system is weak, which renders conventional parameter identification and electroacoustic modeling procedures used in voice-coil loudspeakers impractical. To predict the loudspeaker’s dynamic response, FEA is conducted on the basis of a simple model and a full model. In the simple model, FEA is applied to model the electret membrane, leaving the rest of system as rigid parts. In the full model, FEA is applied to model the entire membrane-spacer-back plate assembly. The mechanical impedance is calculated with the FEA harmonic analysis. The mechanical impedance is combined with the acoustical impedance due to the back cavity and pores as well as the radiation loading at the front side to predict the volume velocity of the membrane and the resulting on-axis SPL. The response data predicted by the simulation compare very well with experimental measurements.
The constrained optimization technique also can be applied piezoelectric buzzers and vented box system design. This technique aims at finding the optimal parameters of the cavity and vented box configuration of electro-acoustical transducers using a systematic design procedure based on vibration absorber theory, where the system is viewed as two coupled serial and parallel oscillators. The characteristic equation of the mechanical and acoustical system can be derived form the vibration absorber theory. Based on the EMA simulation platform and characteristic of the mechano-acoustical system, a design chart is devised to determine the parameters such that the system delivers the maximal output at the low-frequency end. In addition, constrained optimization is also applied to best reconcile the acoustic output and the housing constraints. Experiments were conducted to justify the optimal design. The results showed that the performance was significantly improved using the optimal design over the original design.
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