📚 Waves and Sound | 波与声
Waves are everywhere around us, from the ripples on a pond to the music we hear. In IGCSE Physics, understanding waves and sound is essential for explaining how energy travels through different materials and how we perceive the world. This article provides a clear, revision-focused guide to the key ideas, definitions, formulas, and applications you need to master.
波无处不在，从池塘中的涟漪到我们听到的音乐。在IGCSE物理中，理解波和声是解释能量如何通过不同材料传播以及我们如何感知世界的关键。本文提供一份清晰、紧扣考点的复习指南，涵盖核心概念、定义、公式和应用。
1. What is a Wave? | 什么是波
A wave is a disturbance that transfers energy from one place to another without transferring matter. The particles of the medium vibrate about their fixed positions, but they do not move along with the wave.
波是一种扰动，它把能量从一个地方传递到另一个地方，而不传递物质。介质的粒子在其固定位置附近振动，但不会随波一起移动。
Energy transfer: waves carry energy, not matter.
能量传递：波携带能量，而非物质。
Medium: some waves need a medium (sound, water waves); others do not (light, radio waves).
介质：有些波需要介质（声波、水波），有些则不需要（光、无线电波）。
Particle motion: particles vibrate, but their average position stays the same.
粒子运动：粒子振动，但平均位置保持不变。
2. Transverse and Longitudinal Waves | 横波与纵波
Waves are classified into two main types based on the direction of particle vibration relative to the direction of wave travel.
根据粒子振动方向与波传播方向的关系，波分为两大类。
Transverse wave: particles vibrate perpendicular to the direction of energy transfer. Example: light waves, ripples on water.
横波：粒子振动方向垂直于能量传播方向。例如：光波、水面波纹。
Longitudinal wave: particles vibrate parallel to the direction of energy transfer. Example: sound waves in air.
纵波：粒子振动方向平行于能量传播方向。例如：空气中的声波。
A longitudinal wave consists of compressions (regions where particles are close together) and rarefactions (regions where particles are spread apart).
纵波由疏密相间部分组成：密部（粒子密集的区域）和疏部（粒子稀疏的区域）。
3. Key Wave Measurements | 波的关键物理量
To describe a wave fully, we use several measurements. You must know their definitions, symbols, and units.
要完整描述一个波，我们需要几个物理量。你必须知道它们的定义、符号和单位。
| Quantity | Definition | Unit |
| Wavelength (λ) | Distance between two consecutive identical points on a wave (e.g. crest to crest) | metres (m) |
| Frequency (f) | Number of complete waves passing a point per second | hertz (Hz) |
| Amplitude (A) | Maximum displacement from the equilibrium position | metres (m) |
| Period (T) | Time taken for one complete wave to pass a point | seconds (s) |
The period and frequency are related by: T = 1 ÷ f, and f = 1 ÷ T.
周期与频率的关系：T = 1 ÷ f，f = 1 ÷ T。
4. The Wave Equation | 波速公式
is the distance it travels per second. For any wave, wave speed = frequency × wavelength.
波速是波每秒传播的距离。对于任何波：波速 = 频率 × 波长。
v = f × λ
v = wave speed in metres per second (m/s)
v = 波速，单位米每秒（m/s）
f = frequency in hertz (Hz)
f = 频率，单位赫兹（Hz）
λ = wavelength in metres (m)
λ = 波长，单位米（m）
f 440 Hz and a wavelength of 0.75 m in air. Calculate its speed: v = 440 × 0.75 = 330 m/s.
例题：一声波的频率为440 Hz，在空气中的波长为0.75 m。求波速：v = 440 × 0.75 = 330 m/s。
5. Reflection, Refraction and Diffraction | 反射、折射与衍射
Waves can change direction and spread out when they encounter obstacles or move into different media. These behaviours are fundamental to many applications.
波在遇到障碍物或进入不同介质时会发生方向改变或扩展。这些行为是许多应用的基础。
Reflection: when a wave bounces off a surface. The angle of incidence equals the angle of reflection.
反射：波遇到表面后弹回。入射角等于反射角。
Refraction: when a wave changes speed as it enters a different medium, causing it to change direction.
折射：波进入不同介质时速度改变，从而方向改变。
Diffraction: when a wave spreads out after passing through a gap or around an obstacle.
衍射：波通过狭缝或绕过障碍物后扩展。
Diffraction is most noticeable when the gap is similar in size to the wavelength. Sound waves have long wavelengths, so they diffract easily around corners — this is why you can hear someone speaking from around a wall.
当狭缝大小与波长相近时，衍射最明显。声波波长较长，因此容易绕射过拐角——这就是你能听到墙后有人说话的原因。
6. Sound Waves | 声波
Sound is a longitudinal wave produced by vibrating sources. It travels through solids, liquids and gases, but not through a vacuum.
声波是由振动源产生的纵波。它能在固体、液体和气体中传播，但不能在真空中传播。
Sound propagates through the compression and rarefaction of particles.
声音通过粒子的疏密变化传播。
In solids, particles are closer together, so sound travels fastest; in gases, sound travels slowest.
在固体中粒子间距小，声速最快；在气体中声速最慢。
Approximate speed of sound in air: 330–340 m/s (at room temperature).
空气中声速约为330–340 m/s（室温下）。
7. Loudness, Pitch and Quality | 响度、音调与音色
Three terms describe what we perceive when we hear a sound. They are linked to the physical properties of the wave.
三个术语描述我们听到声音时的主观感受，它们与波的物理性质相关。
| Perceived sound | Wave property |
| Loudness (响度) | Amplitude (振幅) |
| Pitch (音调) | Frequency (频率) |
| Quality / timbre (音色) | Waveform shape (波形形状) |
A louder sound has a larger amplitude; a higher pitch has a higher frequency. The quality of a sound depends on the number and relative strength of overtones, which gives different instruments their unique sound.
声音越响，振幅越大；音调越高，频率越大。音色取决于泛音的数量和相对强度，这也是不同乐器具有独特声音的原因。
8. Echoes and Echo Sounding | 回声与回声测深
When sound reflects off a hard surface, an echo is produced. The time between the original sound and the echo can be used to measure distance.
当声音遇到坚硬表面反射时，会产生回声。根据原声与回声之间的时间差可以测量距离。
distance = speed × time / 2
The factor 2 is included because the sound travels to the surface and back.
除以2是因为声音走了往返路程。
Applications: echo sounding in ships to measure ocean depth, sonar systems.
应用：轮船测深、声呐系统。
returns after 0.8 s. Speed of sound in water is 1500 m/s. Depth = 1500 × 0.8 ÷ 2 = 600 m.
例题：船向水下发出声脉冲，0.8 s后收到回声。水中声速为1500 m/s。水深 = 1500 × 0.8 ÷ 2 = 600 m。
9. The Human Ear and Frequency Range | 人耳与听觉频率范围
The human ear can detect sound waves in a limited frequency range. Understanding this range is important in health, physics, and society.
人耳能够听到的频率范围有限。理解这个范围对健康、物理和社会都很重要。
Normal human hearing range: approximately 20 Hz to 20,000 Hz (20 kHz).
正常人耳听觉范围：约20 Hz至20,000 Hz（20 kHz）。
Sounds below 20 Hz are called infrasound; sounds above 20 kHz are called ultrasound.
低于20 Hz的声音称为次声波；高于20 kHz的声音称为超声波。
As people age, the upper limit of hearing often decreases.
随着年龄增长，听觉上限通常会下降。
10. Ultrasound and Its Uses | 超声波及其应用
Ultrasound is sound with a frequency higher than 20 kHz. It can be used to form images because it reflects off boundaries between different materials.
超声波是频率高于20 kHz的声波。因为它能在不同材料边界处反射，可用于成像。
Medical scanning: ultrasound is used to view unborn babies; it is non-invasive and safer than X-rays.
医学扫描：超声波用于观察胎儿；无创且比X射线更安全。
Cleaning: ultrasound vibrations remove dirt from delicate objects.
清洗：超声波振动可清除精密物件上的污垢。
Industrial testing: ultrasound detects cracks inside metal structures without damaging them.
工业检测：超声波检测金属内部裂纹而不损坏结构。
11. Comparing Sound and Light Waves | 声波与光波的比较
Sound and light share some wave properties but differ in important ways. This comparison often appears in exams.
声波和光波具有一些共同的波动性质，但在重要方面存在差异。这类对比常常出现在考试中。
| Property | Sound | Light |
| Type of wave | Longitudinal | Transverse |
| Medium required | Yes | No |
| Speed in vacuum | Cannot travel | 3 × 10⁸ m/s |
| Speed in air | ≈ 330 m/s | ≈ 3 × 10⁸ m/s |
12. Exam Tips and Common Mistakes | 考试技巧与常见错误
Here are some practical tips to avoid losing marks on waves and sound questions.
以下是一些实用技巧，帮助你在波与声的题目中避免丢分。
Always state the unit: frequency in Hz, wavelength in m, speed in m/s.
务必写单位：频率用Hz，波长用m，速度用m/s。
Use the formula triangle for v = f × λ to rearrange correctly.
使用公式三角形来正确变形 v = f × λ。
When calculating distances with echoes, remember to divide the total distance by 2.
用回声计算距离时，记得把总路程除以2。
Do not confuse pitch with loudness: pitch relates to frequency, loudness relates to amplitude.
不要混淆音调与响度：音调对应频率，响度对应振幅。
For wave diagrams, label the amplitude, wavelength, and if it is a longitudinal wave, mark compressions and rarefactions.
画波形图时，标注振幅、波长；若是纵波，标出密部和疏部。
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