チャープ信号によるパラメトリック超音波のパルス圧縮

摘要

The directivity of parametric difference frequency sound is narrower than that of linear sound with same frequency radiated from a sound source with same aperture size. In addition, the difference frequency sound can propagate for long distance, because the energy dissipation of sound in a medium at low frequency is less than those of usual ultrasounds for measurements and medical imaging. However, for applications of difference frequency sound on measurements and imaging, that has the disadvantage of low spatial resolution because of low frequency. In this report, we proposed the application of pulse compression technique to difference frequency sound to solve the problem of low spatial resolution. The realization of compressed parametric difference frequency sound, especially relationship between compressed pulse width and sweep frequency width and window function (envelope function) of chirp signal, were investigated using computer simulations. The results indicated the realization of pulse compressed parametric difference frequency sound with properties comparable to that of linear sound compression using a suitable window function.%パラメトリック差音は，同一開口音源から放射される同一周波数の線形超音波に比べ，指向性が鋭い特徴を持つ.また，高周波超音波に比べ周波数が低いため，遠方まで伝搬が可能である.ただし，パラメトリック差音を超音波計測や，映像化で利用するには，低周波であるため距離分解能が低いといった問題点がある.本報告において，我々は低分解能という問題点を改善するために，チャープ信号によるパラメトリック差音の圧縮化の提案を行う.シミュレーションにより，パルス圧縮パラメトリック差音実現化の検討，チャープ信号の掃引周波数幅や窓関数(包絡関数)と得られる圧縮パルス幅の検討を行った.その結果，適切な窓関数を選択することで，線形超音波と同程度の圧縮がパラメトリック差音においても実現できることが示された.