原创 Introduction to Audio Signals(音信的介绍)

  Audio signals are generally referred to as signals that are audible to humans.Audio signals usually come from a sound source which vibrates in the audible frequency range.The vibrations push the air  to form pressure waves that travels at about 340 meters per second.Our inner ears can receive these pressure signals and send them to our brain for further recognition.

  所谓声音信号（Audio Signals）简称音信，泛指由人耳听到的各种声音的信号。一般来说，发音体会产生震动，此震动会对空气产生压缩与伸张的效果，形成声波，以每秒大约340公尺的速度在空气中传播，当此声波传到人耳，耳膜会感到一伸一压的压力信号，内耳神经再将此信号传送到大脑，并由大脑解释与判断，来分析此信号的意义。

  If we consider repeated patterns within audio signals,we can classify them into two categories:

  Quasi-periodic sound:The waveforms consists of similar repeated patterns such that we can perceive the pitch.Examples of such sounds include monophonical playback of most music instruments(such as pianos,violins,guitars,etc)and human's singing/speaking.

  Aperiodic sound:The waveform do not consists of obvious repeated patterns so we cannot perceive a stable pitch.Examples of such sounds include thunder pounding,hand clapping,unvoiced part in a humans's utterance,and so on.

  若以信号的规律性，可以分类如下：

  准周期音：波形具有规律性，可以看出周期的重复性，人耳可以感觉其稳定音高的存在，例如单音弦乐器、人声清唱等。

  非周期音：波形不具规律性，看不出明显的周期，人耳无法感觉出稳定音高的存在，例如打雷声、拍手声、敲锣打鼓声、人声中的气音等。

  In principle,we can divide each short segment(also know as frame,with a length of about 20ms)of human's voice into two type:

  Voiced sound:These are produced by the vibration of vocal cords.Since they are produced by the regular vibration of the vocal cords,you can observe the fundamental periods in a frame.Moreover,due to the existence of the fundamental period,you can also perceive a stable pitch.

  Unvoiced sound:These are not produced by the vibration of vocal cords.Instead,they are produced by the vibration of vocal cords.Instead,they are produced by the rapid flow fo air through the mouse,the nose,or the teeth.Since these sound are produced by noise like rapid air flow,we can not observed the fundamental period and no stable pitch can be perceived.

  以人声而言,我们可以根据其是否具有音高而分为两类,如下:

  Voiced sound:由声带振动所发出的声音，例如一般的母音等。由于声带振动，造成规律性的变化，所以我们可以感觉到音高的存在。

  Unvoiced sound:由嘴唇所发出的气音，并不牵汲声带的震动。由于声波没有规律性，所以我们通常无法感受到稳定音高的存在。

  要分辨这两种声音，其实很简单，你只要在发音时，将手按在喉咙上，若有感到震动，就是voiced sound,如果没有感到震动，那就是unvoiced sound.