📚 IGCSE WJEC Science: Sound – Key Points Revision | IGCSE WJEC 科学：声 考点精讲
Sound is a form of energy that travels as longitudinal waves. In the IGCSE WJEC Science specification, you need to understand how sound is produced, how it travels through different media, what determines its pitch and loudness, and how echoes and ultrasound are used in everyday life. This article covers every major test point with bilingual explanations to help you revise efficiently.
声音是一种以纵波形式传播的能量。在 IGCSE WJEC 科学大纲中，你需要理解声音是如何产生的、如何在不同介质中传播、什么决定音调和响度，以及回声和超声波在日常生活中的应用。本文用中英双语解释每一个主要考点，帮助你高效复习。
1. Nature of Sound Waves | 声波的本质
Sound waves are longitudinal waves made up of compressions and rarefactions. In a compression, particles are pushed close together, while in a rarefaction they are spread apart. The vibrations of particles are parallel to the direction the wave travels.
声波是由压缩和稀疏组成的纵波。在压缩区，粒子被推挤到一起；在稀疏区，粒子分散开。粒子的振动方向与波的传播方向平行。
2. Production of Sound | 声音的产生
Sound is produced when an object vibrates. The vibrating object causes the surrounding medium (such as air) to vibrate, creating a series of compressions and rarefactions that move outwards. For example, a tuning fork produces sound because its prongs vibrate back and forth rapidly.
声音是由物体振动产生的。振动的物体使周围介质（如空气）振动，形成一系列向外传播的压缩和稀疏。例如，音叉产生声音是因为它的叉臂快速来回振动。
3. Transmission Through Media | 声音的传播介质
Sound needs a medium to travel; it cannot travel through a vacuum. It travels fastest in solids, slower in liquids, and slowest in gases. This is because particles in solids are closer together and can pass on vibrations more efficiently. In a vacuum, there are no particles, so sound cannot propagate.
声音需要介质才能传播；它不能在真空中传播。声音在固体中最快，液体中较慢，气体中最慢。这是因为固体中的粒子排列更紧密，能更高效地传递振动。真空中没有粒子，所以声音无法传播。
4. Speed of Sound | 声速
The speed of sound in air is approximately 340 m/s at room temperature. It increases with temperature because warmer air particles move faster and transmit vibrations more quickly. The relationship between speed (v), frequency (f) and wavelength (λ) is given by the wave equation:
v = f × λ
You must be able to calculate any one variable if the other two are given. For instance, a sound wave of frequency 500 Hz and wavelength 0.68 m has a speed of 500 × 0.68 = 340 m/s.
声速在室温空气中约为 340 m/s。声速随温度升高而增加，因为更热的空气粒子运动更快，能更快地传递振动。速度（v）、频率（f）和波长（λ）之间的关系由波动方程给出：
v = f × λ
你必须能够在已知两个变量的情况下计算另一个变量。例如，一个频率 500 Hz、波长 0.68 m 的声波，其速度为 500 × 0.68 = 340 m/s。
5. Pitch and Frequency | 音调与频率
Pitch is how high or low a sound seems to a listener. It depends on the frequency of the sound wave. A higher frequency gives a higher pitch. Frequency is measured in hertz (Hz). For example, a piccolo produces a high pitch because it vibrates at a high frequency, while a bass drum produces a low pitch due to a low frequency.
音调是听者感觉到的声音的高低。它取决于声波的频率。频率越高，音调越高。频率以赫兹（Hz）为单位。例如，短笛发出高音调是因为它以高频率振动，而低音鼓因振动频率低而发出低音调。
6. Loudness and Amplitude | 响度与振幅
Loudness is the measure of how strong a sound sensation is. It depends on the amplitude of the sound wave. A larger amplitude means a louder sound, because more energy is carried by the wave. Loudness is usually measured in decibels (dB). If you pluck a guitar string harder, the amplitude of vibration increases and the sound becomes louder.
响度是衡量声音感觉强度的量。它取决于声波的振幅。振幅越大，声音越响亮，因为波携带的能量更多。响度通常用分贝（dB）度量。如果你用力拨动吉他弦，振动振幅增大，声音就变得更响亮。
7. Echo and Reflection | 回声与反射
An echo is a reflection of sound that arrives at the listener after a short delay. Sound waves bounce off hard, flat surfaces just like light reflects off a mirror. The time delay between sending a sound and hearing its echo can be used to measure distances, for example in sonar. If the time taken is t seconds and speed of sound is v, the distance to the reflecting surface is (v × t) / 2.
回声是经过短暂延迟后到达听者的声音反射。声波遇到坚硬平整的表面会反弹，就像光在镜面上反射一样。发送声音与听到回声之间的时间差可以用来测量距离，例如声呐。如果所用时间为 t 秒，声速为 v，那么到反射面的距离为 (v × t) / 2。
8. Ultrasound and Its Uses | 超声波及其应用
Ultrasound refers to sound waves with frequencies higher than 20 000 Hz, above the human hearing range. It is used in medical imaging (e.g., scanning unborn babies), industrial flaw detection, and cleaning delicate equipment. Ultrasound waves reflect differently from different tissues or materials, allowing images or defect maps to be created without invasive procedures.
超声波指的是频率高于 20 000 Hz、超出人类听觉范围的声波。它被用于医学成像（如扫描胎儿）、工业探伤和清洁精密设备。超声波在不同组织或材料界面上反射特性不同，因此能无创地生成图像或缺陷图谱。
9. The Audible Range | 可听范围
The average human ear can detect sound frequencies from about 20 Hz to 20 000 Hz. Frequencies below 20 Hz are called infrasound, and those above 20 000 Hz are ultrasound. As people age, the upper limit of hearing usually decreases. Animals like dogs can hear ultrasound, which is why dog whistles work at frequencies beyond human hearing.
人耳平均能听到从约 20 Hz 到 20 000 Hz 的声音频率。低于 20 Hz 的称为次声波，高于 20 000 Hz 的为超声波。随着年龄增长，听觉的上限通常会下降。狗等动物能听到超声波，这就是狗哨工作在人类听觉范围之外的原因。
10. Oscilloscope Traces | 示波器波形
An oscilloscope can display sound waves as a voltage-time graph. The horizontal axis represents time, while the vertical axis represents the amplitude. From the trace, you can measure the time period (T) and amplitude. Frequency is calculated by f = 1/T. A trace with more cycles within the same time span indicates a higher frequency; a taller trace indicates greater amplitude and therefore a louder sound.
示波器可以将声波显示为电压-时间图像。横轴代表时间，纵轴代表振幅。从波形上可以测量周期 (T) 和振幅。频率通过 f = 1/T 计算。在相同时间跨度内包含更多周期的波形表示更高的频率；波形越高表示振幅越大，因此声音更响。
11. Sound and Energy Transfer | 声音与能量传递
Sound transfers energy from one place to another without transferring matter. As the wave travels, particles oscillate back and forth about their fixed positions, passing kinetic energy to neighbouring particles. The amount of energy carried depends on the amplitude and frequency. Prolonged exposure to loud sounds can damage hearing because too much energy is delivered to the eardrum and inner ear.
声音将能量从一个地方传递到另一个地方，但不传递物质。波传播时，粒子在固定位置附近来回振动，将动能传给相邻粒子。携带的能量大小取决于振幅和频率。长时间暴露在响亮的声音中会损伤听力，因为过多的能量传递到鼓膜和内耳。
12. Summary Checklist | 复习要点清单
Make sure you can: describe sound as a longitudinal wave; explain how sound is produced and why it requires a medium; recall and use the wave equation v = fλ; distinguish between pitch and loudness with reference to frequency and amplitude; interpret oscilloscope traces; describe uses of ultrasound; calculate distance using echo times. Revise using labelled diagrams, practice calculations, and real-life examples.
确保你能：将声音描述为纵波；解释声音如何产生以及为什么需要介质；记住并运用波动方程 v = fλ；参照频率和振幅区分音调和响度；解读示波器波形；描述超声波的用途；利用回声时间计算距离。结合带标注的图表、计算练习和实际例子进行复习。
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