Research On The Theory And Key Technologies Of Audio Directional System

The audio directional system is the first sound source that can generate an audible sound with high directivity. The parametric acoustic array theory provides the theoretical basis for utilizing the nonlinear propagation effect of the sound in the air to generate an audible sound beam with directivity. This thesis explores the basic theory and the key system implementation technologies for using the parametric acoustic array as an audible sound source to produce an audible sound with high directivity, which is significant for promoting the fundamental research and the application of the audio directional system. The main research contents and achievements are summarized as follows:The basic working principle of the audio directional system is explained according to Helmholtz's nonlinear acoustic theory and the parametric acoustic array theory. Furthermore, the analytic solution of the KZK(khokhlov-zabolotskaya-kuznetsov) formula has been derived in this dissertation to obtain a more accurate theoretical model for the parametric acoustic array.A graphical method for three-dimensional directivity is proposed. By the method, it is verified theoretically that only the plane array of the transducer can provide the required three-dimensional directivity, and a higher directivity can be obtained by increasing the number of the array units to suppress the sidelobe level. Based on such findings, the design methods for a polyvinylidene fluoride(PVDF) film transducer with an adjustable resonant frequency and a integral type PVDF film transducer are presented. It is demonstrated by the testing result that the integral type PVDF film transducer can generate an audible sound with very high directivity.The signal processing methods, including the DSB method, the SSB method and the square rooting method, are compared according to the self-demodulation theory. The theoretical formulas for calculating their sound pressure and harmonic distortion when a single-frequency signal or a broadband signal is inputted are presented by establishing their theoretical self-demodulation models. The breakthrough points for the crucial technologies of the audio directional system are clarified by confirming the key parameters affecting the sound pressure of the self-demodulated signal and obtaining their corresponding influence mechanism.A distortionless squared envelope condition for developing the signal processing method of the audio directional system is proposed, and two new signal processing methods, the improved AM method and the N^th-order approximate square rooting method, have been developed. It is verified by experiments that the improved AM method can reduce the harmonic distortion of the DSB method effectively, and the N^th-order approximate square rooting method is useful to decrease the harmonic distortion of the square rooting method.The signal process platform for the audio directional system, which is based on the fixed-point DSB technology, has been designed and fabricated. Then, the signal processing algorithms have been carried out after the exploratory research on their real-time realizing technologies. Moreover, the simulation models of the related signal processing algorithms have been established by MATLAB/Simulink.The aforementioned theory and key technologies employed, a prototype of the audio directional system has been developed successfully. It can be applied in experimental research. It is found in the experiments for testing the self-demodulated signal of the DSB method that the theoretical results predicted by the self-demodulation theory and "Berktay's Far-Field Solution" are inconsistent with the measured result to a certain extent. The influencing factors of the self-demodulated signal and their influence mechanism are also clarified by testing the self-demodulated signal of the square rooting method.