📚 Sound and Hearing: How Vibrations Become What We Hear | 声音与听觉：振动如何变成我们听到的声音
Sound is one of the most familiar forms of energy in everyday life, but its behaviour is often misunderstood. In Cambridge Lower Secondary Science, you need to explain how sound is produced by vibrations, how it travels as a longitudinal wave, and how frequency and amplitude are linked to pitch and loudness.
声音是日常生活中最常见的能量形式之一，但它的行为常常被误解。在剑桥初中科学课程中，你需要解释声音如何由振动产生、如何以纵波形式传播，以及频率和振幅如何与音调和响度相关。
1. Sources of Sound: Every Sound Starts with a Vibration | 声源：一切声音都始于振动
Every sound begins with a vibrating object. A guitar string moves back and forth, a loudspeaker cone pushes on the air, and the vocal cords in your larynx open and close rapidly. These vibrations disturb the particles around them.
每一个声音都始于振动的物体。吉他弦来回运动、扬声器音盆推动空气、喉部的声带快速开合。这些振动会扰动周围的粒子。
- Plucking a string makes it vibrate at a certain rate. | 拨动琴弦会使它以一定频率振动。
- Striking a drum skin pushes air particles together and then pulls them apart. | 敲击鼓面会把空气粒子推到一起，然后再拉开。
- A loudspeaker cone moves in and out to create compressions and rarefactions in the air. | 扬声器音盆内外运动，在空气中形成密部和疏部。
2. Sound Cannot Travel Through a Vacuum | 声音不能在真空中传播
Sound needs a medium to travel through because particles must pass on the vibration from one place to another. In a vacuum there are no particles, so sound cannot be transmitted. This is why explosions in space would be silent.
声音需要介质才能传播，因为必须有粒子把振动从一个地方传到另一个地方。在真空中没有粒子，因此声音无法传播。这就是为什么太空中的爆炸会是无声的。
The classic bell jar experiment demonstrates this idea. As air is pumped out of the jar, the ringing bell becomes quieter even though it can still be seen moving. When air is let back in, the sound returns.
经典的钟罩实验证明了这一观点。当空气被抽出钟罩时，铃声会变得很轻，尽管仍能看到铃在振动。当空气重新进入时，声音又回来了。
3. Longitudinal Waves: Compressions and Rarefactions | 纵波：密部与疏部
Sound travels as a longitudinal wave. The particles of the medium vibrate back and forth in the same direction as the wave travels, rather than at right angles to it. This creates alternating regions of high and low pressure.
声音以纵波的形式传播。介质中的粒子沿着波传播的方向来回振动，而不是垂直于波的方向。这样就会形成高压区和低压区交替出现的现象。
- A compression is a region where particles are pushed close together. | 密部是粒子被推得很近的区域。
- A rarefaction is a region where particles are spread further apart. | 疏部是粒子分布更稀疏的区域。
- The energy travels through the medium, but the particles only vibrate around a fixed point. | 能量通过介质传播，但粒子只在固定位置附近振动。
This is different from a transverse wave, such as a water wave or a light wave, where particle displacement is perpendicular to the direction of energy transfer.
这与横波不同，如水波或光波，在横波中粒子的位移方向与能量传递方向垂直。
4. The Speed of Sound in Different Materials | 声音在不同材料中的速度
Sound travels at different speeds in different materials. In general, it travels fastest in solids, slower in liquids, and slowest in gases. This is because the particles in solids are closer together, so they pass on vibrations more quickly.
声音在不同材料中的传播速度不同。一般来说，它在固体中传播最快，在液体中较慢，在气体中最慢。这是因为固体中的粒子靠得更近，因此能更快地传递振动。
| Material | 材料 | Approximate speed of sound | 声音速度近似值 |
|---|---|
| Steel | 钢 | 5000 - 6000 m/s |
| Water | 水 | 1490 m/s |
| Air at 20 °C | 20 °C 的空气 | 343 m/s |
When particles are tightly packed, the disturbance is transferred more rapidly from particle to particle. This is why you can hear a train approaching by placing your ear on the rail before you hear it through the air.
当粒子排列紧密时，扰动可以在粒子之间更快地传递。这就是为什么把耳朵贴在铁轨上会比通过空气更早听到火车驶来。
5. Frequency and Pitch | 频率与音调
Frequency is the number of complete vibrations or waves produced per second. It is measured in hertz, Hz. The frequency of a sound wave determines its pitch.
频率是每秒产生的完整振动或波的次数。它以赫兹（Hz）为单位。声波的频率决定了音调。
- High frequency means a high-pitched sound, such as a whistle. | 高频率意味着高音调的声音，例如哨声。
- Low frequency means a low-pitched sound, such as a drum or thunder. | 低频率意味着低音调的声音，例如鼓声或雷声。
For a wave maker or oscilloscope reading, frequency can be calculated from the period T:
对于波源或示波器读数，频率可以通过周期 T 来计算：
f = 1 / T
where f is frequency in hertz and T is the period in seconds. Humans can normally hear frequencies in the range of about 20 Hz
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