2026 Syllabus Objectives
Core:
- Describe the production of sound by vibrating sources
- Describe the longitudinal nature of sound waves
- State the approximate range of frequencies audible to humans as 20Hz to 20000Hz
- Know that a medium is needed to transmit sound waves
- Know that the speed of sound in air is approximately 330–350m/s
- Describe a method involving a measurement of distance and time for determining the speed of sound in air
- Describe how changes in amplitude and frequency affect the loudness and pitch of sound waves
- Describe an echo as the reflection of sound waves
- Define ultrasound as sound with a frequency higher than 20kHz
Supplement:
10. Describe compression and rarefaction
11. Know that, in general, sound travels faster in solids than in liquids and faster in liquids than in gases
12. Describe the uses of ultrasound in non-destructive testing of materials, medical scanning of soft tissue and sonar including calculation of depth or distance from time and wave speed
1. How Sound is Produced
Sound is produced by vibrating sources.
When an object vibrates (moves back and forth rapidly), it creates sound. Here are some examples:
- When you pluck a guitar string, the string vibrates and produces sound
- When you hit a drum, the drum skin vibrates and creates sound
- When you speak, your vocal cords vibrate in your throat
- A tuning fork vibrates when struck, producing a clear musical note
The vibrating object pushes and pulls on the air particles around it, creating a wave that travels through the air to your ears.
What "longitudinal" means:
In a longitudinal wave, the particles (like air molecules) vibrate parallel to the direction the wave travels. Imagine a slinky spring: if you push and pull it along its length, you create a longitudinal wave.
When sound travels through air:
- The air molecules vibrate back and forth in the same direction that the sound is moving
- They don't move up and down (that would be a transverse wave, like a water wave)
Sound is also a mechanical wave:
This means sound needs a medium (a material substance) to travel through. Sound can travel through:
- Gases (like air)
- Liquids (like water)
- Solids (like metal or wood)
Sound cannot travel through a vacuum (empty space with no molecules). This is because there are no particles to vibrate and pass the energy along.
The Bell Jar Experiment proves this:
- Place an electric bell inside a sealed glass jar
- Connect the jar to a vacuum pump
- When the bell rings with air inside, you can hear it clearly
- As the pump removes the air from the jar, the sound gets quieter
- When all the air is removed (creating a vacuum), you can see the bell ringing but hear nothing
This happens because the sound waves need air molecules to travel. No air = no sound transmission.