Research On The Theory And Key Technologies For Micro Audio Directional System

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.