Properties of Sound Waves
Section: Physics | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
What is Sound?
- Sound is a form of energy produced by vibrating objects that travels as waves through a medium (solid, liquid, or gas) – everything that makes sound is vibrating
- when you speak, your vocal cords vibrate
- a guitar string vibrates when plucked
- a drum skin vibrates when hit
- a phone speaker vibrates to produce sound
- a tuning fork vibrates when struck
- Touch your throat while speaking – those vibrations you feel are your vocal cords vibrating to create sound
How Sound Travels
Sound travels by making particles vibrate in a chain reaction. The particles themselves don't travel far – they just vibrate back and forth in place. It's the energy and vibration pattern that travels through the medium.
- Source vibrates: a vibrating object (like a speaker) moves back and forth
- Pushes nearby particles: when moving forward, it pushes nearby air particles closer together
- Creates compressions: areas where particles are squashed together (high pressure)
- Creates rarefactions: when moving backward, it creates areas where particles are spread apart (low pressure)
- Chain reaction: each particle pushes the next one, passing the vibration along
- Wave travels: the pattern of compressions and rarefactions travels through the medium
- Think of a "Mexican Wave" at a stadium – people stand up and sit down in sequence, the wave travels around the stadium, but each person stays in their seat. Sound works the same way: the wave travels, but particles stay roughly in place
Sound Needs a Medium
- A medium is a substance through which sound can travel. Sound must have particles to vibrate – it cannot travel through empty space (a vacuum)
Sound Through Different Media
| Medium | Examples | Speed of Sound | Reason |
|---|---|---|---|
| Solids | Wood, metal, concrete, glass | ~5000-6000 m/s | Particles close together and tightly connected |
| Liquids | Water, oil, milk | ~1500 m/s | Particles close but can move more freely |
| Gases | Air, helium, carbon dioxide | ~340 m/s | Particles far apart and move freely |
| Vacuum | Space (no particles) | 0 m/s | No particles to vibrate |
- Bell in a vacuum: if you place a ringing bell inside a jar and pump out all the air (creating a vacuum), you can see the bell moving but hear nothing – this proves sound needs a medium to travel
- Space movies often show explosions with loud sounds, but this is scientifically incorrect. In the vacuum of space there's no air (no medium), so explosions would be completely silent. Astronauts communicate using radio waves, not sound waves
- Real-world examples:
- Train tracks: you can hear a train coming from much farther away by putting your ear to the metal tracks, because sound travels faster through the solid metal
- Underwater communication: sound travels well underwater – whales can communicate over vast distances
- Knocking on walls: you can hear through walls because sound travels through the solid material
- Space silence: space is silent because there's no medium for sound to travel through
Loudness and Amplitude
- Loudness is how loud or quiet a sound is – what we perceive with our ears, whether a sound is soft (quiet) or strong (loud)
- Amplitude is the maximum displacement of particles from their rest position. On a wave diagram, it's the distance from the center line to the peak (or trough)
- Peak: the highest point of the wave (maximum displacement upward)
- Trough: the lowest point of the wave (maximum displacement downward)
- Rest position: the center line where particles would be if not vibrating
- Amplitude: distance from rest position to peak (or rest position to trough)
- Physical meaning: large amplitude means particles vibrate with large movements back and forth; small amplitude means particles vibrate with small movements back and forth. Amplitude represents the amount of energy in the wave
Relationship Between Amplitude and Loudness
- Larger amplitude → louder sound. Particles vibrate more violently → more energy → louder sound
- Smaller amplitude → quieter sound. Particles vibrate gently → less energy → quieter sound
| Sound | Amplitude | Loudness | Energy |
|---|---|---|---|
| Whisper | Very small | Very quiet | Low energy |
| Normal speech | Small | Moderate | Medium energy |
| Shouting | Large | Loud | High energy |
| Rock concert | Very large | Very loud | Very high energy |
Loudness and Distance from the Source
As you move further away from a sound source, the sound becomes quieter.
- The loudness (amplitude) of the sound decreases as the distance from the sound source increases
- Smaller amplitude means the sound carries less energy, so it is heard as quieter
- On an oscilloscope trace, waveforms recorded further from the source have a smaller amplitude (shorter height), even though the frequency (and therefore pitch) stays the same
Exam Tip
- Sounds above 85 decibels (dB) can damage your hearing if you're exposed for too long. Always protect your ears around very loud sounds! Permanent hearing damage is irreversible
Pitch and Frequency
- Pitch is how high or low a sound is – what we perceive with our ears, whether a sound is squeaky (high pitch) or rumbling (low pitch)
- Frequency is the number of complete vibrations (or waves) that pass a point every second – frequency is the physical property that determines pitch
Examples of Pitch
| Low Pitch | Medium Pitch | High Pitch |
|---|---|---|
| Thunder rumble | Normal speaking voice | Whistle |
| Bass guitar | Piano middle keys | Piccolo flute |
| Lion's roar | Dog bark | Bird chirp |
| Truck engine | Car horn | Smoke alarm |
Unit: Hertz (Hz)
- 1 Hz = 1 vibration per second
- 50 Hz: 50 vibrations per second (low pitch – bass note)
- 256 Hz: 256 vibrations per second (middle C on piano)
- 1000 Hz (1 kHz): 1000 vibrations per second (high pitch)
- 10,000 Hz (10 kHz): 10,000 vibrations per second (very high pitch)
Human Hearing Range
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