How Sound Is Made and Travels
FoundationSound comes from vibration and travels through solids, liquids or gases — never a vacuum. Volume is how loud, pitch is how high; protect your hearing by keeping away from too-loud noise.
- Sound comes from vibration; when vibration stops, sound stops.
- Sound travels through media — gases, liquids and solids — but never a vacuum.
- Volume is loudness in decibels; pitch is highness set by frequency.
文體小知識：聲學實驗說明文用身邊的小實驗解釋聲音的秘密，振動、傳播和音調高低都能親自試驗。
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1Every sound we hear begins with vibration. The vocal cords in the throat vibrate when we speak; a guitar string swings back and forth when plucked; a drum skin trembles up and down when struck. Tap a drum sprinkled with beans and the beans jump — visible proof of vibration. When the vibration stops, the sound stops too.
2Sound needs a medium to travel: solids, liquids and gases all carry it. Air brings sounds to our ears; press an ear to a desk while a friend taps the far end and the sound comes through the solid more clearly; a voice through water while fishing scares the fish away. The medium cannot be lacking: put a ringing alarm clock inside an air-pumped vacuum jar and the sound almost vanishes — a vacuum carries no sound.
3Sound has volume and pitch. Volume is how loud a sound is, measured in decibels: quiet talk is about forty decibels, a busy street about eighty. Strike the drum harder, the skin vibrates more widely, and the volume rises. Pitch is how high or low a sound is: the faster the vibration (the higher the frequency), the higher the pitch, like a bird's thin chirp; the slower the vibration, the lower the pitch, like a cello's deep note.
4Noise is sound that disturbs us, and long exposure to high volume damages hearing. Protect your ears: keep headphone volume below sixty per cent and use them under sixty minutes at a time; stay away from loud places such as construction sites, or wear ear defenders; when a huge noise comes, open the mouth or cover the ears to guard the eardrums. Hearing is precious — protect it from childhood.
1Summary 重點摘要
Sound comes from vibration and travels through solids, liquids or gases — never a vacuum. Volume is how loud, pitch is how high; protect your hearing by keeping away from too-loud noise.
2Vocabulary cards Vocabulary
An object swinging rapidly back and forth, the cause of all sound.
The thin folds in the throat that vibrate to make the voice for speaking and singing.
The substance that carries sound, such as air, water or wood.
A space with no matter at all, where sound cannot travel.
How loud a sound is, measured in decibels.
How high or low a sound is, decided by how fast something vibrates.
How many times something vibrates each second; the higher the frequency, the higher the pitch.
The unit for measuring volume.
The thin film in the ear that receives sound vibrations; too-loud sound can damage it.
3Step by step Steps
- Step 1: Find the source of sound — vibrationAll sound is made by vibration; the vibrating object is the source of the sound. For example, press your hand on the throat while singing and feel the vocal cords vibrate; flick a ruler held against the desk edge and it sounds as it swings up and down. The drumhead vibrates when a drum is struck, and a guitar string vibrates when plucked. Find the object that is vibrating, and you have found the source of the sound — the first step in studying sound.
- Step 2: Prove sound stops when vibration stopsVibration and sound are closely linked: when the vibration stops, the sound stops too. For example, strike a gong and press it with a hand; the vibration stops and the sound vanishes at once. This simple experiment tells us that the key to making sound is the object's constant vibration; while the vibration continues, the sound continues. Our own vocal cords work the same way when we speak, which is why the voice turns hoarse after too much talking — the cords need rest.
- Step 3: Test sound through solidsSound needs a medium to travel, and solids, liquids and gases can all act as the medium. For example, one pupil taps one end of a long wooden desk softly while another presses an ear to the other end; the sound through the solid is heard more clearly than through air. Ancient soldiers at night put an ear to the ground to hear distant hooves and judge how far the enemy was — using exactly the principle of sound through solids.
- Step 4: Test sound through liquidsLiquids also carry sound, and we can prove it with an experiment. For example, seal a ringing phone in a plastic bag and sink it in a water tank; the ring is still heard outside, proving water carries sound. Fishermen keep quiet on the bank because voices travel through air and water and scare the fish away; under the water of a swimming pool you can still hear clapping from the bank — liquids carry sound very well.
- Step 5: Prove a vacuum carries no soundSound cannot travel through a vacuum, because a vacuum has no medium at all. For example, put a ringing clock in a glass jar and pump out the air slowly; the bell grows fainter and fainter, and returns when air flows back in. The experiment also explains the Moon: with no air on the surface, astronauts must talk by radio even when they are close together, so the sound can reach the other's ears.
- Step 6: Tell volume from pitchThe strength of a sound is its volume, and the highness or lowness is its pitch — two different things. For example, tap the same drum lightly and hard; the skin vibrates with different widths, so the volume differs. Flick a ruler with a shorter overhang and it vibrates faster, so the pitch rises. The wider the vibration, the louder the sound; the faster the vibration, the higher the pitch. Knowing the difference lets us hear more detail in music.
- Step 7: Practise protecting hearingOur ears are delicate, and long exposure to loud sounds damages hearing, so we must protect them in the right way. For example, keep headphone volume under sixty per cent and use them under an hour at a time; wear ear defenders near noisy places such as construction sites; when a huge noise comes, open the mouth or cover the ears to guard the eardrums. Damaged hearing hardly ever recovers, so protecting our ears from childhood lets us hear clearly for life.
4Diagrams Diagrams
5Worked example Example
Example: Yan puts a ringing clock inside a glass jar and slowly pumps the air out. Describe what she hears and explain why.
- At first the bell rings clearly, because plenty of air inside the jar carries the sound.
- As the pump runs, the air grows thinner, the medium for sound lessens, and the bell grows fainter.
- When the air is nearly gone the bell is almost silent, yet the hammer is still seen moving through the glass — the vibration is there, but the sound cannot travel out.
- Conclusion: sound needs a medium; with the jar close to a vacuum, no sound can travel.
- Answer: The ringing faded from loud to almost silent because the air inside the jar was slowly removed; this proves sound needs a medium and a vacuum carries none — the same reason astronauts on the airless Moon must talk by radio.
6Common mistakes Mistakes
7Quick check Quick check
Q1How is sound produced?
Q2Can a vacuum carry sound? Why?
Q3What is pitch, and what decides it?
Q4With your ear on the desk, through what does the tapping sound travel?
Q5What rules should we follow with headphones?
8Recap Recap
- Sound comes from vibration; when vibration stops, sound stops.
- Sound travels through media — gases, liquids and solids — but never a vacuum.
- Volume is loudness in decibels; pitch is highness set by frequency.
- Higher frequency means higher pitch: a shorter ruler vibrates faster and sounds sharper.
- Protect hearing: headphones under sixty per cent volume and an hour at a time, away from loud places.