PHYSICS · Stage 7 · Chapter 3: Sound
Physics 3.1 Sound waves and how they travel — Vibrations, Compressions and Mediums
5 min read 6 parts Lesson 1 of 4
Lesson summary
3.1 Sound waves and how they travel — Vibrations, Compressions and Mediums — Alhussan International Academy — Physics (Stages 7–9) notes: Describe how sound waves are produced and travel, why sound cannot travel through a vacuum, and how its speed differs between solids, liquids …
- Through the air
- Rarefaction
- Slinky model
- Longitudinal wave
- Transverse wave
- Can travel through
Part 1 of 6Sound comes from vibrations
- Vibration = a repeated backwards and forwards movement, and sound is produced by vibrating objects.
- Sources — a vocal cord, a loudspeaker, a drum skin, a guitar string, a tuning fork or a bell.
- Polystyrene beads on a loudspeaker cone bounce up and down, which shows the cone vibrates.
- Energy — sound transfers energy from a source, such as a drum, to a detector, such as your ear.
Part 2 of 6How a sound wave travels
- Medium = the material through which a wave travels, such as air, water or walls.
- Through the air — the vibrating cone makes the layer of air next to it vibrate, which makes the next layer vibrate, and so on.
- The sound wave moves through the air to your ear, and you hear a sound.
- The air itself does not travel away from the cone, because the particles only vibrate backwards and forwards.
- Compression — a region of a sound wave where the particles are close together.
- Rarefaction — a region where the particles are further apart.
- Slinky model — moving the end of a Slinky spring forwards and backwards makes the coils move together and then apart as the wave moves along.
- The individual coils do not travel to the end, but the wave does.
- Transfer — vibrations are transferred by particles vibrating.
Part 3 of 6Longitudinal and transverse waves
1Wave
— a wave transfers energy without transferring matter.
2Longitudinal wave1 point
— the particles vibrate to and fro in the same direction as the wave travels.
- Sound waves and some earthquake waves are longitudinal, and a Slinky pushed in and out shows one.
3Transverse wave1 point
— the particles move at right angles (90°) to the direction of travel.
- A rope moved up and down shows one, and so does a stadium (Mexican) wave in which fans move up and down as the wave moves along.
Part 4 of 6What sound can and cannot travel through
1Can travel through2 points
— air, water and solid materials such as walls.
- You can hear sounds underwater, and dolphins and whales communicate over very large distances by making and hearing sounds.
- Someone talking loudly in the next room can be heard because sound travels through walls.
2Vacuum
= a space that is completely empty of everything, including air.
3No sound in a vacuum1 point
— sound cannot travel through a vacuum because there are no particles to transfer the vibration from the source to the detector.
- There is a vacuum between the stars, so sound cannot travel through space.
Part 5 of 6Speed of sound
- Order — sound travels fastest in solids and slowest in gases.
- Reason — the particles in a solid are closer together than in a gas, so the vibration is passed on more quickly.
| Material | Speed of sound (m/s) |
|---|---|
| Air at sea level | 330 |
| Water | 1500 |
| Metal | 5000 |
- Measuring — stand a known distance from a wall, make a sharp sound and time the echo.
- Speed = distance ÷ time, using the total distance there and back, which gives about 330 m/s in air.
- Repeat and find a mean, and use a long distance so human error in the stopwatch is small compared with the total time.
Part 6 of 6Pitch and loudness
1Frequency1 point
— it is the number of vibrations per second, measured in hertz (Hz).
- A faster vibration (higher frequency) makes a higher-pitched sound.
2Amplitude1 point
— a bigger vibration makes a louder sound.
- A drum hit harder vibrates with a larger amplitude and sounds louder but has the same pitch.
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