Theory And Control Of High Directivity Audio Sound Wave

The sound is the most direct carrier of information. And it's important tool in production and life. Several thousand years ago the human begins to research sound. At the beginning of 20th century the linear acoustics theory had been perfected. Along with the development of nonlinear acoustics in recent decades and Westervelty brought forward parametric array in 1960s, a great breakthrough has made in the linear acoustics. The idea of sound wave of low frequency can't form sharp directivity has been changed. The acoustics parametric array system uses nonlinear effect of the medium to create audible, highly directional sound beam. The reproducing sound with information can be projected and direct to a specific area and specific audience. The theory of acoustics parametric array has been research. Some key technology has been perfected. A framework of acoustics parametric array system is be proposed. The main achievements and work can be shown as followings:(1) The theoretical basis of acoustics parametric array is westervelty's parametric array theory and Berktay's far field solution. But Berktay's far field solution is limited to axial position. In here Berktay's far field resolution has been improved. The responses of on-axis and off-axis both was considered while reproducing secondary acoustic wave equation was deduced. And the attenuation of reproducing sound was considered too. The numerical simulation results were compared with Pompei's experiment. The improved far field solution can express distribution of the sound field more accurately.(2) Due to the nonlinear effect in air, ultrasonic can produce low frequency sound. So the directivity of ultrasonic transducer array is very important to reproduce low frequency sound with highly directivity. Some different kinds of transducer arrays and corresponding acoustic parametric array were detailed researched. The rectangle array composed of circular piston elements was chosen as key point. The directivity of rectangle array was detailed discussed. A new mathematics model of transducers array model has been proposed. The directivity of rectangle array has been improved and the side lobes are almost restrained by this new method. The efficiency of ultrasonic transducer is improved too.(3) The reproducing secondary sound is produced through two steps. The first must do the conversion from audio to ultrasonic. Next the ultrasonic is changed to audio by the self-demodulated in nonlinearly air. The second conversion can't be control because of it depend on air. So the first conversion is very important. And the reproducing secondary sound field is special. So the theory and control technology of nonlinearly sound field was detailed researched. The conclusion shows that there are direct relations between reproducing sound field and envelope shape. The single side band amplitude modulation (SSB) method was proposed. For a single frequency signal the single sideband envelope is equal to ideal envelope. For multi-frequency signal the single sideband envelope can be matched as closely as possible this ideal envelope by perfect frequencies, amplitudes and phases. In the case of low power, narrowband and no preprocessing, the SSB method can reproduce high-fidelity audio too.(4) The acoustic parametric array can be realized by an audio signal processing system. The knowledge involves multidisciplinary sciences. A framework of acoustics parametric array was designed. The audio frequency digital signal processing subsystem is very important in whole acoustic parametric array system. The signal acquisition, signal processing and modulation of signal, etc. are controlled in this subsystem. It is key that generates secondary sound with low distortion. The dynamic range control processor, the distortion reduced preprocessing of sound signal and amplitude modulator, etc. are detailed researched in this subsystem. The final purpose of the research is to design an acoustic parametric array with high fidelity tone quality and flat frequency response.The dissertation makes a systematic research on theory of acoustics parametric array. And the design of the acoustics parametric array system has been performed. Mathematics tools such as MATLAB, etc. were used for numerical calculation during the study period. As the result, the numerical simulation has been performed on two-dimensional space and three-dimensional space. These results are examined by a comparison with the experimental results and existing theory. The algorithms that are proposed in dissertation are better than the previous ones. The main achievement of research work is valuable both in academic study and in practice.