📚 GCSE WJEC Science: Sound – Key Points | GCSE WJEC 科学：声 考点精讲
Sound is a form of energy produced by vibrating objects. It travels as longitudinal waves and can be reflected, refracted, and absorbed. Understanding the properties of sound waves is essential for the GCSE WJEC Science specification, covering topics from pitch and loudness to ultrasound and seismic waves.
声音是由振动体产生的一种能量形式。它以纵波的形式传播，能够被反射、折射和吸收。理解声波的特性对 GCSE WJEC 科学考试至关重要，内容涵盖音调、响度、超声波和地震波等。
1. Production of Sound | 声音的产生
Sound is produced when an object vibrates, causing the surrounding particles to oscillate back and forth. These vibrations create compressions and rarefactions in a medium such as air, water, or solids.
声音是由物体振动产生的，振动使周围的粒子来回振荡。这些振动在空气、水或固体等介质中形成压缩和稀疏。
A vibrating tuning fork, for example, moves forwards to squeeze air particles together (compression) and backwards to spread them apart (rarefaction). Sound cannot travel through a vacuum because there are no particles to transmit the vibrations.
例如，一个振动的音叉向前移动时挤压空气粒子（压缩），向后移动时拉开粒子（稀疏）。声音不能在真空中传播，因为没有粒子来传递振动。
2. Longitudinal Waves | 纵波
Sound waves are longitudinal, meaning the particle oscillations are parallel to the direction of energy transfer. This is different from transverse waves, where oscillations are perpendicular.
声波是纵波，意味着粒子的振动方向与能量传递方向平行。这不同于横波，横波的振动方向垂直于传递方向。
In a longitudinal wave, regions of high pressure (compressions) and low pressure (rarefactions) move through the medium. A graph of displacement against distance for a sound wave shows a series of peaks and troughs, but physically the particles move back and forth along the same axis.
在纵波中，高压区（压缩）和低压区（稀疏）在介质中移动。声波的位移-距离图显示一系列波峰和波谷，但物理上粒子沿同一轴线来回移动。
3. Frequency and Pitch | 频率与音调
Frequency is the number of complete vibrations per second, measured in hertz (Hz). A higher frequency corresponds to a higher pitch. The human ear can typically detect frequencies from about 20 Hz to 20,000 Hz.
频率是每秒完整振动的次数，单位为赫兹（Hz）。频率越高，音调越高。人耳通常能检测到大约20 Hz到20,000 Hz的频率。
When a guitar string is tightened, it vibrates faster, increasing the frequency and producing a higher note. On an oscilloscope trace, a higher pitch appears as more waves packed into the same time interval.
当吉他弦拉紧时，它振动得更快，频率增加，发出更高的音调。在示波器轨迹上，更高的音调表现为在相同时间间隔内更多的波形。
4. Amplitude and Loudness | 振幅与响度
Amplitude is the maximum displacement of particles from their rest position. In a sound wave, a larger amplitude means more energy is carried, resulting in a louder sound.
振幅是粒子偏离其平衡位置的最大位移。在声波中，振幅越大，携带的能量越多，声音越响。
Loudness is measured in decibels (dB). A sound of 0 dB is the threshold of human hearing, while prolonged exposure to sounds above 85 dB can damage hearing. On an oscilloscope, a taller wave indicates a greater amplitude and thus a louder sound.
响度以分贝（dB）为单位。0 dB是人耳听觉的门槛，长时间暴露在85 dB以上的声音会损害听力。在示波器上，更高的波形表示更大的振幅，从而声音更响。
5. Wave Speed, Wavelength & the Wave Equation | 波速、波长与波动方程
The speed of sound in air is approximately 330 m/s at 0°C and increases with temperature. Sound travels faster in solids and liquids due to closer particle spacing.
空气中声速在0°C时约为330 m/s，并随温度升高而加快。声音在固体和液体中传播更快，因为粒子间距更紧密。
The relationship between speed (v), frequency (f), and wavelength (λ) is given by:
速度（v）、频率（f）和波长（λ）之间的关系为：
v = f × λ
where v is in m/s, f in Hz, and λ in metres. For a sound wave with frequency 500 Hz and wavelength 0.66 m, the speed is 500 × 0.66 = 330 m/s.
其中v的单位是m/s，f的单位是Hz，λ的单位是米。对于频率为500 Hz、波长为0.66 m的声波，速度为500 × 0.66 = 330 m/s。
6. Echoes and Reflection of Sound | 回声与声音的反射
Sound waves can be reflected off hard surfaces, producing echoes. An echo is heard when the reflected sound reaches the listener more than 0.1 seconds after the original sound, allowing the brain to distinguish the two signals.
声波遇到坚硬表面会反射，产生回声。当反射声在原始声音发出后超过0.1秒到达听者时，大脑便能区分两个信号，从而听到回声。
Echo sounding (SONAR) uses the time taken for a sound pulse to travel to the seabed and back to calculate depth. Using the formula distance = speed × time, and dividing by 2 to account for the round trip, one can determine depth accurately.
回声探测（声纳）利用声脉冲传到海底并返回的时间来计算深度。使用公式距离 = 速度 × 时间，并除以2以考虑往返行程，便可精确确定深度。
7. The Human Ear and Hearing Range | 人耳与听觉范围
The ear converts sound vibrations into electrical signals sent to the brain. The outer ear collects sound, the eardrum vibrates, the middle ear bones amplify the motion, and the cochlea in the inner ear converts vibrations into nerve impulses.
耳朵将声音振动转换成电信号传送给大脑。外耳收集声音，鼓膜振动，中耳听小骨放大运动，内耳的耳蜗将振动转化为神经冲动。
Hearing range decreases with age and exposure to loud noise. Infrasound (below 20 Hz) and ultrasound (above 20 kHz) are not audible to humans, though some animals can detect them.
听觉范围随年龄增长和暴露于巨大噪音而减小。次声波（20 Hz以下）和超声波（20 kHz以上）人类听不到，但一些动物可以检测到。
8. Ultrasound | 超声波
Ultrasound refers to sound waves with frequencies above 20,000 Hz. In medicine, ultrasound imaging uses the partial reflection of sound at boundaries between different tissues to create images of internal organs and foetuses.
超声波指频率高于20,000 Hz的声波。在医学上，超声波成像利用声波在不同组织界面处的部分反射来创建内脏和胎儿的图像。
Ultrasound is also used in industrial cleaning, welding plastics, and SONAR. Unlike X-rays, ultrasound is non-ionising and considered safer for monitoring foetal development.
超声波还用于工业清洗、塑料焊接和声纳。与X射线不同，超声波是非电离的，被认为对胎儿发育监测更安全。
9. Seismic Waves: P-waves and S-waves | 地震波：P波和S波
Seismic waves are generated by earthquakes. P-waves (primary waves) are longitudinal and can travel through both solids and liquids. S-waves (secondary waves) are transverse and travel only through solids.
地震波由地震产生。P波（纵波）是纵波，能穿过固体和液体。S波（横波）是横波，只能在固体中传播。
The detection of P-waves and S-waves on seismic stations provides evidence for the structure of the Earth’s interior, including the liquid outer core, because S-waves cannot pass through it.
地震台站检测到的P波和S波为地球内部结构（包括液态外核）提供了证据，因为S波无法穿过液体外核。
10. Factors Affecting Speed of Sound | 影响声速的因素
The speed of sound depends on the medium. In general, vsolids > vliquids > vgases. Stiffness and density of the material affect speed: sound travels faster through stiffer media and slower through denser gases at constant temperature.
声速取决于介质。一般来说，v固体 > v液体 > v气体。材料的刚度和密度会影响速度：声音在刚度较大的介质中传播更快，在恒温下密度较大的气体中传播较慢。
In air, temperature is a key factor. For every 1°C rise, the speed increases by about 0.6 m/s. This is why sound can travel farther at night when air near the ground is cooler.
在空气中，温度是关键因素。每升高1°C，声速增加约0.6 m/s。这就是为何夜间近地面空气较冷时声音可以传播更远。
11. Applications of Sound Waves | 声波的应用
Sound finds use in communication (speech, music), navigation (SONAR), medical diagnostics (ultrasound scans), and material testing (flaw detection). Musical instruments rely on standing waves formed by vibrations of strings or air columns.
声音应用于通信（语言、音乐）、导航（声纳）、医学诊断（超声波扫描）和材料检测（缺陷探测）。乐器依赖弦或空气柱振动形成的驻波。
Noise pollution is managed using sound insulation and barriers that absorb or reflect sound energy. Understanding sound properties enables the design of better concert halls and quieter machinery.
通过使用吸音或反射声能的隔音材料和屏障来管理噪音污染。理解声音特性有助于设计更好的音乐厅和更安静的机械。
12. Required Practical: Oscilloscope & Wave Measurements | 必做实验：示波器与波形测量
In the WJEC specification, pupils may investigate the relationship between frequency and pitch, or amplitude and loudness, by connecting a microphone to an oscilloscope. The number of waves per second gives frequency; the height of the trace indicates amplitude.
在WJEC规范中，学生可以通过将麦克风连接到示波器来研究频率与音调、振幅与响度的关系。每秒波的个数给出频率；轨迹的高度表示振幅。
To measure the speed of sound in air, a common method uses two microphones at a known distance apart connected to a timer. By measuring the time delay for a sound pulse to travel between them, speed = distance / time can be calculated.
为了测量空气中的声速，常用方法是将两个麦克风隔开已知距离，连接到计时器。通过测量声脉冲在它们之间传播的时间延迟，可以计算速度 = 距离 / 时间。
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