Waves and Sound
Grade 10 · Science · Study Guide
Understand the properties of waves, how sound travels, and why we hear what we hear.
A wave is a disturbance that transfers energy through matter or space without permanently displacing the medium. Waves are broadly classified as mechanical (require a medium) or electromagnetic (can travel through a vacuum). Sound is a mechanical longitudinal wave — the particles in the medium vibrate parallel to the direction of wave travel, creating alternating compressions and rarefactions.
Waves are characterized by amplitude (height of the wave; related to energy), wavelength (distance between identical points on consecutive waves), frequency (number of wave cycles per second, measured in Hertz), and wave speed. These are related by the wave equation: v = fλ, where v is speed, f is frequency, and λ (lambda) is wavelength. The speed of sound in air at room temperature is approximately 343 m/s, but it travels faster through liquids and solids because the particles are closer together.
The pitch of a sound corresponds to its frequency — higher frequency = higher pitch. Loudness corresponds to amplitude. The Doppler effect describes how perceived frequency changes when the source or observer is moving: a siren sounds higher-pitched as an ambulance approaches and lower as it passes. Resonance occurs when an object vibrates at its natural frequency due to an external vibration at the same frequency.
| Medium | Speed of Sound (m/s) |
|---|---|
| Air (20°C) | 343 |
| Water (25°C) | 1,480 |
| Steel | 5,960 |
| Vacuum | 0 (cannot travel) |
Key terms
- Wavelength (λ)
- The distance between two consecutive crests (or troughs) of a wave, measured in meters.
- Frequency (f)
- The number of complete wave cycles per second, measured in Hertz (Hz).
- Amplitude
- The maximum displacement of a wave from its rest position; related to the energy of the wave.
- Longitudinal wave
- A wave where particles vibrate parallel to the direction of wave travel (e.g., sound waves).
- Doppler effect
- The change in observed frequency of a wave when the source and observer are moving relative to each other.
- Resonance
- When an object vibrates at its natural frequency because it is driven by an external source at that same frequency.
Flashcards
- Transverse wave
- A wave where particles vibrate perpendicular to the direction of travel (e.g., light, water surface waves).
- Compression
- Region of a longitudinal wave where particles are pushed close together (high pressure zone).
- Rarefaction
- Region of a longitudinal wave where particles are spread apart (low pressure zone).
- Hertz (Hz)
- Unit of frequency equal to one cycle per second.
- Wave speed equation
- v = fλ; wave speed equals frequency multiplied by wavelength.
- Pitch
- The perceived highness or lowness of a sound, determined by its frequency.
Practice questions
A sound wave has a frequency of 440 Hz and travels at 340 m/s. What is its wavelength?
- 0.77 m
- 1.3 m
- 440 m
- 149,600 m
Show answer
Answer: 0.77 m
Using v = fλ: λ = v/f = 340/440 ≈ 0.77 m. Higher frequency means shorter wavelength.
Sound can travel through a vacuum.
Show answer
Answer: False
Sound is a mechanical wave — it requires a medium (solid, liquid, or gas) to travel through. It cannot travel through the vacuum of space.
The number of wave cycles per second is called the ___.
Show answer
Answer: frequency
Frequency is measured in Hertz (Hz) and equals the number of complete wave cycles passing a point per second.
Which property of a sound wave determines how loud it sounds?
- Frequency
- Wavelength
- Speed
- Amplitude
Show answer
Answer: Amplitude
Amplitude is the energy of a wave — a larger amplitude sound wave carries more energy and is perceived as louder.
The Doppler effect causes a moving sound source to seem higher-pitched as it approaches you.
Show answer
Answer: True
As the source moves toward you, each wave is compressed into a shorter distance, increasing the frequency (and pitch) you hear. As it moves away, waves stretch out and pitch drops.
Fun facts
- The loudest sound ever recorded was the 1883 Krakatoa eruption — it was heard 4,800 km away and circled the Earth four times as a pressure wave.
- Bats use echolocation to navigate in complete darkness, emitting ultrasonic pulses above 20,000 Hz that humans cannot hear.
- The 'sonic boom' an aircraft makes breaking the sound barrier is a continuous shockwave, not a single explosion.
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