The distance from the top or bottom of the waveform to its baseline shows the amplitude or volume of the sound. The height of the waveform is roughly proportional to how loud the sound is. A flat line indicated silence. A period is the distance between any point in the wave form and the next time that same point appears. The number of periods per second determines the frequency of the sound, Hertz.
Sound frequency is measured in hertz.
One hertz is so low a frequency that human can't hear. Hz to 20 kHz. For example, when you listen to music recorded on a compact disc, the CD-Driver converts the digital audio file back into an analog signal. As shown in Figure 2-6. Now that video can be stored on computer's hard disk, video from a VCR can be played in a window on system's display, and video can be put into the work, presentations, and programs.