摘要
微型声频定向系统是一种将声音控制在狭窄区域内高指向性传播的新型扬声器，它以非线性声学参量阵理论为基础，通过微型换能器将调制有可听声成分的超声波发射至空气中，利用高指向性超声波在空气传播过程中的非线性交互效应，自解调出具备高度指向性的可听声。本文主要围绕微型声频定向系统换能器理论和低功耗高保真非线性信号处理理论展开探索性研究，为其在便携式多媒体设备中实现高指向性声音传播提供理论参考和实践依据。
根据Helmhotlz非线性声学理论、Westervelt双频声学参量阵理论和Berktay宽带声学参量阵理论，本文首先阐明了声频定向系统工作原理和系统组成，运用拟线性理论对KZK非线性声波运动方程进行了求解，该方程充分考虑了声波在空气中传播时的非线性、吸收和散射效应，方程拟线性解表明声频定向系统产生的自解调可听声声压与已调制超声信号包络平方对时间的二次导数成正比关系。
在微型声频定向系统的基础上，本文提出了以圆形多层薄膜层叠板为基本结构的微型声频定向换能器力学理论模型，模型以MEMS微机械加工工艺为基础，采用黏合层技术使镀有贵金属电极并沿厚度方向极化的PZT薄膜与绝缘衬底硅结合在一起。在理论模型的基础上，导出了压电多层板运动方程和换能器方程，明确了各薄膜层厚度和半径对换能器中心振幅、谐振频率、有效机电耦合系数的影响机理，探讨了阵元尺寸、阵元个数和阵元间距等参数对指向性的影响。根据理论模型研制出了正方形和六边形两种基于MEMS的微型换能器阵列，作为微型声频定向系统的发声部件。
在单边带全载波调幅算法的基础上，本文提出了基于动态载波控制与n阶失真补偿的低功耗高保真非线性信号处理理论模型。根据KZK方程拟线性解和Berktay远场解理论，探讨了换能器尺寸及工作频率对远近场临界距离的影响、空气吸收衰减系数以及载波频率对声场指向性的影响。从理论上分析了双边带全载波调幅法、平方根法和单边带全载波调幅法等常见声频定向信号处理算法的输出功率和谐波失真，明确了各算法中声源面积、调制度、声波频率等参数对二者的影响机理。
根据以上理论研究成果，本文以正方形MEMS微型换能器阵列作为发声部件，以D类微型功率放大器作为换能器驱动部件，使用DSP技术实现了基于动态载波控制和一阶失真补偿的单边带全载波调幅算法，研制出用于手机的微型声频定向系统样机，搭建了微型声频定向系统声学测试平台，对微型换能器阵列做了阻抗特性、声压级频率响应以及指向性测试，测试结果有效验证了本文提出的换能器理论模型的正确性，对样机做了自解调可听声频率响应、总谐波失真、指向性以及功耗测试，实验结果验证了本文提出的低功耗高保真非线性信号处理理论模型正确可行。
A micro audio directional system is presented as a novel loudspeaker which cangenerates audible sound in a narrow conical beam. Based on the theory of nonlinearacoustic parametric array, it emits an ultrasonic sound modulated by audible signal intothe air through a micro transducer array. Because of the nonlinear interactive effect inthe process of propagation, the ultrasonic sound is demodulated to produce the audiblesound beam. Micro transducer theory and nonlinear signal processing methods with lowpower consumption and high fidelity are explored to provide the theoretical andpractical experience for utilizing the sound propagation with high directivity in portablemultimedia devices as well as in mobile phone.
The thesis begins with reviews of Helmholtz1`s nonlinear acoustics theory,Westervelt`s two-frequency acoustic parametric array theory and Berktay`s widebandparametric array theory. These solutions are used to explain the principle and structureof audio directional system. The general solution of KZK parabolic wave equation,which accounts for nonlinearity, absorption and diffraction, is derived by employingquasilinear theory. The quasilinear solution indicates that the sound pressure level of thedemodulated signal is proportional to the second derivative of the square of themodulated signal`s envelope.
A piezoelectric micromachined ultrasonic transducer is structured as circularmultilayered thin film laminated elastic plate. The electroded PZT film with polarizationin the thickness-direction is adhered to SOI (Silicon-On-Insulator) wafer with an adherelayer. The governing equations and differential extension and bending equations of thetransducer are obtained for clarifying the radius and thickness effects of each layer.Those vibration characteristics effect include resonant frequency, electromechanicalcoupling coefficient and center flexural displacement. The directivity of the transducerarray effects due to element size, element quantity and element spacing distance arediscussed. Two micro transducer arrays, the hexagon and foursquare emitter of themicro directional loudspeaker, based on MEMS technology are designed and fabricated.
A single side band modulation signal processing model is developed with dynamic carrier control and n order distortion compensation.By the quasilinear solution of KZKequation and Berktay`s far-filed solution, the primary ultrasonic sound field andsecondary demodulated audible sound filed are modeled to analyze the directivity of theaudio directional system. The transducer size and working frequency effects ontransition point between near-field and far-field, the effects of attenuation coefficient foratmospheric absorption and the sound frequency effects on the directivity areinvestigated. The output power and harmonic distortion of double side band full carriermethod, square-root method and single side band full carrier method are analyzed andcompared theoretically.
A micro audio directional system prototype is constructed to be applied in mobilephone. The prototype consists of DSP signal processing platform, class D poweramplifier and micro MEMS-based transducer array et al. The single side band amplitudemodulation method with dynamic carrier control and one-order distortion compensationalgorithm is realized in the prototype by DSP technology. A testing platform isconfigured by employing the above theoretical results and research findings. Theimpedance, sound pressure frequency response and directivity of the transducer arrayare measured and analyzed to examine the availability of the theoretical model. It isdemonstrated that the testing results matches with the theoretical results. Thedemodulated audible sound pressure frequency response, total harmonic distortion,directivity and power consumption are tested and the results indicate that the signalprocessing model proposed in the dissertation is correct and meets the requirements ofthe micro audio directional system.
引文
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