Waves and Sound
Grade 8 · Science · Study Guide
Explore how waves carry energy through matter and how sound travels through different media.
Waves are disturbances that transfer energy from one place to another without transferring matter. Transverse waves move perpendicular to the wave's direction of travel, while longitudinal waves (like sound) compress and expand the medium parallel to the direction of travel.
Sound is a longitudinal mechanical wave that requires a medium to travel. It moves faster through solids than liquids, and faster through liquids than gases, because the particles are more tightly packed and can transfer energy more quickly.
Key wave properties include wavelength (distance between two identical points), frequency (number of waves per second, measured in Hz), amplitude (height of the wave, related to energy), and wave speed (speed = frequency × wavelength).
| Medium | Speed of Sound (m/s) |
|---|---|
| Air (20°C) | 343 |
| Water | 1,480 |
| Steel | 5,960 |
| Property | Transverse Wave | Longitudinal Wave |
|---|---|---|
| Particle motion | Perpendicular to wave | Parallel to wave |
| Example | Light, water waves | Sound waves |
| Parts | Crests and troughs | Compressions and rarefactions |
Key terms
- Frequency
- The number of complete wave cycles per second, measured in hertz (Hz).
- Wavelength
- The distance between two consecutive crests or troughs of a wave.
- Amplitude
- The maximum displacement of a wave from its rest position; related to energy.
- Longitudinal Wave
- A wave where particles move parallel to the direction of wave travel, like sound.
Practice questions
Sound travels fastest through which medium?
- Air
- Water
- Steel
- A vacuum
Show answer
Sound travels fastest through solids because the tightly packed particles transfer energy more efficiently.
What property of a sound wave determines its loudness?
- Frequency
- Wavelength
- Amplitude
- Wave speed
Show answer
Amplitude determines the amount of energy in the wave, which we perceive as loudness.
Sound is a transverse wave.
Show answer
Sound is a longitudinal wave — particles compress and expand parallel to the direction of travel.
The number of wave cycles per second is called ___.
Show answer
Frequency is measured in hertz (Hz) and determines the pitch of a sound.
Why can't sound travel through outer space?
Show answer
This distinguishes mechanical waves (need a medium) from electromagnetic waves (do not need a medium).
Fun facts
- Sound cannot travel through outer space — there is no medium for it to move through, which is why space is completely silent.
- Bats use echolocation, emitting ultrasonic sound waves above 20,000 Hz that bounce back to help them navigate in the dark.
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