📚 Sound and Hearing Revision Guide | 声音与听力备考专项
This revision guide covers the essential concepts of sound waves and hearing for Year 10 CAIE Physics. You will learn how sound is produced and transmitted, the relationship between frequency and pitch, how the human ear detects sound, the properties of ultrasound, and common applications such as sonar. Each section is designed to help you recall key facts, apply formulas, and tackle exam-style questions with confidence.
本备考指南涵盖十年级 CAIE 物理中声波与听力的核心概念。你将学习声音如何产生与传播，频率与音高的关系，人耳如何检测声音，超声波的特性以及声呐等常见应用。每个部分都旨在帮助你回忆关键知识点，运用公式，并自信地应对考试风格的题目。
1. What Are Sound Waves? | 什么是声波？
Sound waves are longitudinal mechanical waves that transfer energy through a medium by causing particles to vibrate parallel to the direction of wave travel. They consist of compressions (regions of high pressure) and rarefactions (regions of low pressure). Unlike electromagnetic waves, sound cannot travel through a vacuum because there are no particles to vibrate.
声波是纵波性质的机械波，通过使介质粒子沿波传播方向来回振动来传递能量。声波由密部（高压区域）和疏部（低压区域）组成。与电磁波不同，声音不能在真空中传播，因为没有粒子可以振动。
- Sound is a longitudinal wave.
- 声音是纵波。
- It requires a medium (solid, liquid or gas).
- 它需要介质（固体、液体或气体）。
- Compressions: particles are close together; Rarefactions: particles are spread apart.
- 密部：粒子聚集；疏部：粒子分散。
2. Production of Sound | 声音的产生
Sound is produced by vibrating objects. For example, a tuning fork vibrates back and forth, pushing air molecules to create compressions and rarefactions. In musical instruments, strings, air columns or membranes vibrate. The frequency of vibration determines the pitch of the sound, while the amplitude affects the loudness.
声音由振动着的物体产生。例如，音叉来回振动，推动空气分子形成密部和疏部。在乐器中，弦、空气柱或薄膜振动。振动频率决定声音的音高，而振幅影响响度。
- Vibrating source → disturbance in medium → sound wave.
- 振动源 → 介质扰动 → 声波。
- Greater amplitude of vibration produces a louder sound.
- 振动幅度越大，声音越响。
3. How Sound Travels | 声音如何传播
Sound travels fastest in solids, slower in liquids, and slowest in gases. This is because particles in solids are packed closely together, allowing vibrations to be passed on more rapidly. In air at room temperature, the speed of sound is approximately 330–340 m/s. The speed increases with temperature and stiffness of the medium.
声音在固体中传播最快，在液体中较慢，在气体中最慢。这是因为固体中的粒子紧密排列，使振动能更快地传递下去。在室温空气中，声速约为 330–340 m/s。声速随温度和介质刚度的升高而增加。
- Speed in solids > liquids > gases (generally).
- 固体中的速度 > 液体中的速度 > 气体中的速度（通常）。
- Sound cannot travel through a vacuum.
- 声音不能在真空中传播。
4. Speed of Sound Formula | 声速公式
The speed of a sound wave can be calculated if you know the distance travelled and the time taken. In echo problems, you will often need to double the distance if the sound travels to a surface and back.
如果知道声音传播的距离和时间，就可以计算声速。在回声问题中，如果声音传到表面再返回，通常需要将距离乘以二。
speed = distance / time
v = d / t
When using the wave equation for sound, you can also relate speed, frequency and wavelength:
当使用声波的波动方程时，也可以联系速度、频率和波长：
v = f × λ
- v = speed (m/s), f = frequency (Hz), λ = wavelength (m)
- v = 速度（米/秒），f = 频率（赫兹），λ = 波长（米）
5. Frequency, Pitch, and Loudness | 频率、音高与响度
Frequency is the number of complete vibrations per second, measured in hertz (Hz). A higher frequency corresponds to a higher pitch. Loudness is related to the amplitude of the wave: a larger amplitude means a louder sound. The ear is more sensitive to some frequencies than others, and loudness is also affected by distance from the source.
频率是每秒完整振动的次数，单位为赫兹（Hz）。频率越高，音高越高。响度与波的振幅有关：振幅越大，声音越响。人耳对某些频率比对其他频率更敏感，响度也受声源距离的影响。
- High frequency → high pitch (e.g., a whistle).
- 高频 → 高音（如哨声）。
- Low frequency → low pitch (e.g., a drum).
- 低频 → 低音（如鼓声）。
- Loudness depends on amplitude and distance; unit: decibel (dB).
- 响度取决于振幅和距离；单位：分贝（dB）。
6. The Human Ear and Hearing Mechanism | 人耳与听觉机制
The ear converts sound waves into electrical signals that the brain interprets. Sound enters through the outer ear and travels down the ear canal, causing the eardrum to vibrate. These vibrations are passed through the ossicles (small bones) in the middle ear to the cochlea in the inner ear. The cochlea contains fluid and tiny hair cells that convert vibrations into nerve impulses.
耳朵将声波转化为大脑能够解读的电信号。声音通过外耳进入，沿耳道传播，使鼓膜振动。这些振动通过中耳的听小骨（小骨头）传递到内耳的耳蜗。耳蜗含有液体和微小的毛细胞，将振动转化为神经冲动。
- Outer ear: pinna collects sound.
- 外耳：耳廓收集声音。
- Middle ear: eardrum and ossicles amplify vibrations.
- 中耳：鼓膜和听小骨放大振动。
- Inner ear: cochlea converts vibrations to electrical signals.
- 内耳：耳蜗将振动转化为电信号。
- Auditory nerve sends signals to the brain.
- 听觉神经将信号传送至大脑。
7. Hearing Range and the Effect of Age | 听力范围与年龄影响
The typical human hearing range is from about 20 Hz to 20 000 Hz (20 kHz). Sounds below 20 Hz are called infrasound, and those above 20 kHz are ultrasound. As people age, the upper frequency limit tends to decrease because hair cells in the cochlea become damaged or less responsive, especially to high frequencies.
人耳的典型听力范围大约在 20 Hz 到 20 000 Hz（20 kHz）之间。频率低于 20 Hz 的声音称为次声波，高于 20 kHz 的称为超声波。随着年龄增长，听力上限通常会降低，因为耳蜗中的毛细胞会受损或反应减弱，尤其对高频声音。
- Young person: up to 20 kHz.
- 年轻人：可达 20 kHz。
- Older person: upper limit may drop to 12–15 kHz or lower.
- 年长者：上限可能降至 12–15 kHz 或更低。
- Infrasound: used by some animals (elephants) for communication.
- 次声波：某些动物（大象）用于交流。
8. Ultrasound and Its Applications | 超声波及其应用
Ultrasound refers to sound waves with frequencies above 20 kHz, beyond human hearing. Ultrasound is used in medicine for scanning (e.g., monitoring a foetus during pregnancy) because it can reflect off different tissue boundaries. It is also used in sonar for measuring water depth or detecting fish, and in industrial cleaning and flaw detection.
超声波是指频率高于 20 kHz、超出人耳听力范围的声波。超声波在医学上用于扫描（例如，孕期胎儿监测），因为它能在不同组织界面反射。它还用于声呐来测量水深或探测鱼群，以及工业清洗和缺陷检测。
- Medical imaging: ultrasound echoes create images.
- 医学成像：超声回声形成图像。
- Sonar: sound navigation and ranging; uses echo time to calculate distance.
- 声呐：声音导航与测距；利用回声时间计算距离。
- Non-destructive testing: detecting cracks in metal.
- 无损检测：检测金属中的裂缝。
9. Echoes and Sonar Calculations | 回声与声呐计算
An echo is a reflection of sound that arrives at the listener after a delay. The minimum distance for a distinct echo is about 17 m in air, assuming the sound takes at least 0.1 s to travel to the surface and back. In sonar, a pulse of ultrasound is sent out, and the time for the echo to return is measured. The distance can be found using the speed formula, remembering that the travel time is for the round trip.
回声是声音经反射后延迟到达听者耳朵的现象。在空气中，产生清晰回声的最小距离约为 17 m，假设声音往返至少需要 0.1 s。在声呐中，发射一个超声波脉冲，并测量回声返回的时间。可利用速度公式计算距离，注意传播时间是往返时间。
distance = (speed × time) / 2
- Use half the total time because the sound travels to the object and back.
- 使用总时间的一半，因为声音往返了一次。
- mple: if echo returns after 0.5 s in air (v ≈ 340 m/s), distance = (340 × 0.5) / 2 = 85 m.
- 举例：若回声在 0.5 s 后返回，空气中 v ≈ 340 m/s，距离 = (340 × 0.5) / 2 = 85 m。
10. Noise Pollution and Hearing Protection | 噪声污染与听力保护
Excessive noise can cause temporary or permanent hearing damage. Noise pollution is often measured in decibels (dB). Prolonged exposure to sounds above 85 dB can be harmful. Ear protectors, such as earplugs or earmuffs, reduce the intensity of sound reaching the eardrum. Limits on industrial noise and use of sound barriers help reduce noise pollution.
过量的噪声会导致暂时或永久性的听力损伤。噪声污染通常以分贝（dB）来衡量。长时间暴露在 85 dB 以上的声音中可能有害。耳塞或耳罩等听力保护器能降低到达鼓膜的声强。对工业噪声加以限制并使用声屏障有助于减少噪声污染。
- Threshold of pain: about 120 dB.
- 痛阈：约 120 dB。
- Ear protection reduces amplitude of vibrations reaching the inner ear.
- 听力保护器能减小到达内耳的振动幅度。
11. Summary of Key Formulas | 关键公式总结
Memorise these relationships and be ready to rearrange them for any variable. Always check units: speed in m/s, frequency in Hz, wavelength in m, distance in m, time in s.
记住这些关系式，并能针对任一变量进行公式变形。务必核对单位：速度用 m/s，频率用 Hz，波长用 m，距离用 m，时间用 s。
| Formula / 公式 | Explanation / 说明 |
|---|---|
| v = f × λ | Wave speed equation / 波速公式 |
| v = d / t | Speed, distance, time / 速度、距离、时间 |
| d = (v × t) / 2 | Distance using echo (round trip) / 回声测距（往返） |
12. Exam Tips for Sound and Hearing | 声音与听力考试小贴士
In the CAIE Physics exam, you may be asked to describe an experiment to measure the speed of sound, to explain how ultrasound imaging works, or to compare the hearing range of different age groups. Always use scientific vocabulary such as longitudinal, compression, rarefaction, amplitude, and frequency. When solving echo problems, draw a simple diagram to visualise the path of sound.
在 CAIE 物理考试中，你可能会被要求描述一个测量声速的实验，解释超声成像的工作原理，或比较不同年龄段的听力范围。务必使用纵波、密部、疏部、振幅、频率等科学词汇。解决回声问题时，可以画一张简图来直观展示声音的传播路径。
- Stick to correct units and show all working in calculations.
- 使用正确的单位，并写出计算过程。
- Learn key values: speed of sound in air ≈ 340 m/s, human hearing range 20 Hz – 20 kHz.
- 记住关键数据：空气中声速 ≈ 340 m/s，人耳听力范围 20 Hz – 20 kHz。
- Be able to interpret oscilloscope traces: relate time base to frequency.
- 能够解释示波器波形：将时基与频率相联系。
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