# Waves & Sound — ACT Science
Wave-related passages on the ACT often involve experiments with sound, light, or vibrating strings. Understanding basic wave properties will help you interpret frequency, wavelength, and amplitude data.
Key Concepts
Wave Properties
- Wavelength (): distance between successive crests.
- Frequency (): number of waves per second (Hz).
- Amplitude: height of the wave; relates to loudness (sound) or brightness (light).
- Wave speed: .
Sound Waves
- Sound is a longitudinal (compression) wave.
- Travels faster in solids than liquids, and faster in liquids than gases.
- Speed in air ≈ 340 m/s.
- Higher frequency → higher pitch.
- Greater amplitude → louder.
The Doppler Effect
- Source moving toward you: pitch increases (higher frequency).
- Source moving away: pitch decreases (lower frequency).
Resonance
- Objects vibrate most strongly at their natural frequency.
- Standing waves form at resonant frequencies.
Worked Example
Passage summary: Students measured the speed of sound in different materials.
| Material | Speed (m/s) |
|---|---|
| Air | 343 |
| Water | 1480 |
| Steel | 5960 |
Q: In which medium does sound travel fastest, and why?
A: Steel. Sound travels fastest in solids because particles are closer together and transmit vibrations more efficiently.
Practice Questions
1. A wave has frequency 500 Hz and wavelength 0.68 m. What is its speed?
.
2. An ambulance siren sounds higher-pitched as it approaches. What phenomenon explains this?
The Doppler effect.
3. If the amplitude of a sound wave increases but the frequency stays the same, what changes?
The sound gets louder, but the pitch stays the same.
Want to check your answers and get step-by-step solutions?
Summary
- is the key wave equation.
- Sound travels fastest in solids, slowest in gases.
- Higher frequency = higher pitch; greater amplitude = louder.
- The Doppler effect explains frequency shifts from moving sources.