Sound: Vibration, Propagation and Hearing

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1Summary 重點摘要

No vibration, no sound; no medium, no travel — how sound is made and heard.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: Sound comes from vibrationSound comes from vibration, provable with three small experiments: pluck a guitar and the string blurs into a band because it is racing back and forth; touch a sounding speaker cone and it tingles under your fingers; press your throat while singing and the vocal cords are felt vibrating. The conclusion: a sounding body always vibrates, and when vibration stops the sound stops at once — the starting point for explaining every sounding phenomenon and the foundation of this lesson.
2. Step 2: A medium is requiredSound needs a medium to travel: in the classic experiment an alarm clock rings inside an evacuated bell jar, and the more air is pumped out, the fainter the ringing, until a near-vacuum leaves it almost silent — proof that a vacuum cannot carry sound. That is why astronauts on a spacewalk cannot hear each other even face to face and must use radios, because radio waves do travel through vacuum. On Earth air is everywhere, which is why we can talk at any time.
3. Step 3: The speed raceSound travels at different speeds in different media — fastest in solids, slower in liquids, slowest in gases: press your ear to a long rail and a train's sound arrives through the rail before it comes through the air, proving solid conduction is faster. In air sound moves at about 340 metres per second, which is why lightning is seen before thunder is heard: multiply the count of seconds by 340 to estimate the storm's distance and decide whether to take shelter.
4. Step 4: Pitch — a frequency gamePitch is decided by frequency: the faster the vibration, the higher the frequency and the higher the pitch. Flick a plastic ruler overhanging a desk edge — the shorter the overhang, the faster the vibration and the higher the note; a guitarist fretting higher shortens the string, quickens the vibration and raises the pitch. Human hearing runs from about 20 to 20,000 vibrations per second; beyond the top lies ultrasound, below the bottom infrasound, and neither can be heard by us.
5. Step 5: Loudness and timbreSound has two further properties to distinguish: loudness follows amplitude — tap a drum gently for a soft sound and strike hard for a loud one, the bigger the amplitude the louder the sound; timbre follows the source — the same song on piano, violin and in a human voice feels different each time. In answers the pairings must not be swapped: pitch with frequency, loudness with amplitude, timbre with the nature of the source. Conductors rely on timbre to pick out each instrument.
6. Step 6: ApplicationsAcoustics has become a toolbox of practical technology: ultrasound scans let doctors see foetuses and internal organs without surgery; sonar sends sound to the seabed and reads the echoes for depth and fish shoals; bats and dolphins are born echolocators, finding their way in the dark; ultrasonic cleaners shake grime off glasses and jewellery with high-frequency vibration. Science turning into useful tools is exactly why we study it.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• Vibration makes sound; stop vibrating, stop sounding.
• Sound needs a medium — vacuums are silent.
• Speed: solid > liquid > gas; about 340 m/s in air.
• Pitch–frequency, loudness–amplitude, timbre–source.
• Ultrasound: scans, sonar, echolocation.