第三章针对超声波检测信号特点，给出超声频谱分析的基本原理与方法。
Chapter 3 provides primary principles and approaches of ultrasonic spectrum analysis in accordance with features of ultrasonic testing signals.
一声频定向换能器，以定向发射超声波给接听者，其中该超声频段部分经空气自解调而还原为原音频信号。
Because the ultrasonic wave has the directivity, the audible audio frequency demodulated from the interior of the ultrasonic wave also has the directivity and forms a directional audible sound.
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