声音是一种由物体振动引发的物理现象.ppt

Sound is a physical phenomenon produced by the vibration of matter, such as a violin string, or a block of wood. As the matter vibrates, pressure variations are created in the air surrounding it. This alteration of high and low pressure is propagated through the air in a wave-like motion. When a wave reaches the human ear, a sound is heard.,声音是一种由物体振动引发的物理现象，如小提琴的弦声等。物体的振动使其四周空气的压强产生变化，这种忽强忽弱变化以波的形式向四周传播，当被人耳所接收时，我们就听见了声音。,Sound / Audio,声音和音频技术涉及的主要是声波(声学信号)的处理。编码、录音机或数字音频带的存储、音乐及语音处理是这一领域的主要课题。 Sound methodology and audio techniques engage in processing these sound waves (acoustic signals). Important topics in this area are coding, storage on recorders or digital audio tapes, music and speed processing.,声音是由物体的振动产生的，这种振动引起了周围空气压强的振荡，我们称这种振荡的函数表现形式为波形. Sound is produced by the vibration of matter. During the vibration, pressure variations are created in the air surrounding it. The pattern of the oscillation is called a waveform,Basic sound Concepts,Figure 3.1:Oscillation of an air pressure wave.,Amplitude,Time,Air Pressure,One Period,+,如果每隔一定时间波形就重复相同的形状，那么就称这个时间为周期。一般然声音的波形既不会十分光滑也不会有很强的周期性。与非周期性的声音相比，具有周期性的声音听起来要悦耳得多。乐器的演奏、元音、鸟鸣及风声都是周期性的声音，而打击乐器声、咳嗽声、喷嚏声以及流水声则是非周期性的。 The waveform repeats the same shape at regular intervals and this portion is called a period. Since sound waves occur naturally, they are never perfectly smooth or uniformly periodic. However, sounds that display a recognizable periodicity tend to be more musical than those that are nonperiodic. Examples of periodic sound sources are musical instruments, vowel sounds, the whistling wind and bird songs. Nonperiodic sound sources include unpitched percussion instruments, coughs and sneezes and rushing water.,声音的频率是周期的倒数，它表示的是声音在1秒钟内的周期数，单位是赫兹(Hz)。千赫(kHz),即1000Hz,表示每秒振动1000次Boo87。声音按频率可作如下划分: The frequency of a sound is the reciprocal value of the period; it represents the number periods in a second and is measured in hertz (Hz) or cycles per second (cps). A convenient abbreviation, kHz (kilohertz), is used to indicate thousands of oscillation per second: 1 kHz equals 1000 Hz Boo87. The frequency range is divided into:,Frequency,次声 020Hz 人耳能听见的声音 20Hz20KHz 超声 20KHz1GHz 特超声 1GHz10THz Infra-sound from 0 to 20 Hz Human hearing frequency range from 20Hz to 20kHz Ultrasound from 20kHz to 1GHz Hypersound from 1GHz to 10 THz,多媒体系统中使用的声音一般只限于人耳能听到的范围，我们把人耳所能听见的声音为音频(audio),将这个范围内的声波称为声学信号(acoustic signal)。例如语音是由人发出的声学信号，乐器的频率范围在20Hz之间。我们将语音和音乐之外的音频信号统称为噪音。 Multimedia systems typically make use of sound only within the frequency range of human hearing. We will call sound within the human hearing range audio and the waves in this frequency range acoustic signals Boo87. For example, speech is an acoustic signal produced by humans; music signals have a frequency range between 20Hz and 20khz. Besides speech and music, we denote any other audio signal as noise.,声音有振幅，振幅的主观感觉是声音的大小。声音的振幅大小取决于空气压力波距平均值(也称平衡态)的最大偏移量。 A sound also has an amplitude, a property subjectively heard as loudness. The amplitude of a sound is the measure of the displacement of the air pressure wave from its mean, or quiescent state.,Amplitude,计算机并不直接使用连续平滑的波形来表示声音，它是每隔固定的时间对波形的幅值进行采样，用得到的一系列数字量来表示声音。图3.2是经过数字采样的波形示意图。 The smooth, continuous curve of a sound waveform is not directly represented in a computer. A computer measures the amplitude of the waveform at regular time inter