Picturing a sound wave
We can’t see sound, but scientists can picture it. A sound wave is a series of tiny pressure changes travelling through a medium like air.
A longitudinal wave
Sound is a longitudinal wave: the particles of air vibrate back and forth in the same direction the wave travels.
Imagine a slinky stretched on a table. Push one end in and out. You’ll see bunched-up coils travelling along, followed by stretched-out sections.
| Part | What it is | In air |
|---|---|---|
| Compression | Particles pushed close together | Slightly higher air pressure |
| Rarefaction | Particles spread apart | Slightly lower air pressure |
As a speaker cone pushes forward, it creates a compression. As it pulls back, it creates a rarefaction. The cone moves back and forth hundreds or thousands of times each second, sending out a stream of compressions and rarefactions.
The pressure changes are tiny. Even a loud sound changes air pressure by less than 0.1%.
Measuring a sound wave
| Property | For a sound wave | What we hear |
|---|---|---|
| Wavelength | Distance from one compression to the next | Related to pitch |
| Frequency | Number of compressions passing each second (Hz) | Pitch |
| Amplitude | How much the pressure changes | Loudness |
| Speed | How fast compressions travel | Depends on the medium |
Wave equation
speed = frequency × wavelength
In air at 20 °C, sound travels at about 343 m/s.
| Sound | Frequency | Wavelength in air |
|---|---|---|
| Very low bass | 20 Hz | About 17 m |
| A note (A4) | 440 Hz | About 0.78 m |
| High whistle | 5000 Hz | About 7 cm |
| Highest human hearing | 20 000 Hz | About 1.7 cm |
Low sounds have wavelengths longer than a school bus. That’s why bass from a car stereo can travel through walls and around corners more easily than high notes.
Drawing sound as a waveform
Because compressions and rarefactions are hard to draw, scientists often show sound as a waveform: a graph of air pressure over time. It looks like a transverse wave, but it’s really just a picture of a longitudinal wave.
- Peaks = compressions (high pressure)
- Valleys = rarefactions (low pressure)
- Taller peaks = louder sound
- Closer peaks = higher pitch
An oscilloscope or a sound-recording app shows waveforms on a screen.
| Waveform shape | Sound |
|---|---|
| Smooth, regular wave (sine wave) | A pure tone, like a tuning fork |
| Regular but complex shape | A musical instrument or voice |
| Jagged, irregular | Noise, like static or a crash |
Why instruments sound different
When a flute and a violin play the same note, they have the same main frequency, called the fundamental. But each instrument also produces fainter higher frequencies called overtones (or harmonics). The mix of overtones gives each instrument its unique sound, called timbre. That’s why waveforms of the same note look different on different instruments.
How speed changes
| Condition | Effect on speed of sound |
|---|---|
| Warmer air | Faster (about 0.6 m/s faster for each °C) |
| Solids | Much faster than air |
| Liquids | Faster than air |
| Vacuum | No sound at all |
Breaking the sound barrier
When a jet travels faster than sound, it outruns its own compressions. They pile up into a single strong shock wave, heard on the ground as a sonic boom. In 1947, Chuck Yeager became the first pilot to officially fly faster than sound. A cracking whip also makes a tiny sonic boom: its tip moves faster than sound!
Using sound waves
- Ultrasound imaging uses high-frequency sound echoes to see inside the body.
- Sonar maps the sea floor and finds submarines.
- Seismic surveys use sound-like waves to find oil and study Earth’s interior.
- Noise-cancelling headphones create waves that cancel incoming sound.
Common mistakes
- Thinking sound is a transverse wave because waveforms look wavy.
- Thinking air moves from the speaker to you. Air particles only vibrate in place.
- Thinking all sounds of the same pitch look the same as waveforms.
Try it yourself
- Push and pull a slinky. Identify compressions and rarefactions.
- Use a free oscilloscope app to view your voice saying "ooo" and "sss." Compare the waveforms.
- Calculate the wavelength of a 343 Hz note in air.
(Answer to 3: 1 m)
Quick recap
- Sound is a longitudinal wave of compressions and rarefactions.
- Frequency gives pitch; amplitude gives loudness.
- Waveforms graph pressure changes over time.
- Overtones give each instrument its timbre.
- Faster than sound, waves pile up into a sonic boom.