Sounds play only when you press the button and stop by themselves; keep your volume low. The 3D motion is slowed down millions of times and the particles move much farther than real air does, so you can see the pattern. Red = particles squeezed together (compression), blue = spread out (rarefaction).
A microphone turns sound into an electrical signal and an oscilloscope draws it. More waves on the same screen = higher frequency = higher pitch. Taller waves = bigger amplitude = louder.
Humans hear from about 20 Hz to 20,000 Hz (less at the top as we get older). Sounds too low for us are infrasound; too high are ultrasound. Ranges are approximate.
Sound is a longitudinal wave: the particles move back and forth along the direction the wave travels. The graph underneath shows the same wave as changes in air pressure. Pick Basic (7 labels) or Advanced (12), then try the blank diagram, the label quiz and match the jobs. On a phone, swipe the diagram sideways.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back.
Each line describes the job of one part. Tap a part name, then tap the box next to the job it does. Tap a filled box to take the word back.
Tap a card, then tap the box it belongs in. Press Check when all 12 are placed. Tap the empty card area to put a selected card back.
A printable worksheet with an answer key: label the sound wave, pitch or loudness, read four oscilloscope traces, calculations (wavelength, echo, speed in steel, thunder), true or false, and two explain questions. The label part uses the level chosen in the diagram above.
The same physics with numbers. Change any value; the answer updates.
Sound level β = 10 log₁₀(I / I₀) with I₀ = 10⁻¹² W/m²: ten times the intensity adds 10 dB, twice the intensity adds about 3 dB. Intensity I = ΔP²ₘₐₓ / (2ρv) grows with the square of the pressure amplitude and falls as 1/r² from a point source. The pressure wave is 90° out of step with the particle displacement (pressure is highest where particles are most bunched, not where they have moved farthest): ΔPₘₐₓ = ρvωsₘₐₓ.
Pluck a string, hit a drum or hum and you can feel something vibrating. A vibrating object pushes on the particles of air next to it, those push their neighbors, and the vibration is passed along as a sound wave. When it reaches your ear it makes your eardrum vibrate too, and your brain hears sound.
Sound is a longitudinal wave: the particles move back and forth in the same direction the wave travels, making regions squeezed together (compressions) and spread apart (rarefactions). The particles themselves stay roughly in place; only the energy moves on. Because it needs particles, sound needs a medium (solid, liquid or gas) and cannot cross a vacuum. It travels fastest in solids, where particles are close together and strongly linked.
The frequency (vibrations per second, in hertz) sets the pitch; the amplitude (how big the vibrations are) sets the loudness. The distance from one compression to the next is the wavelength λ, and speed = frequency × wavelength (v = fλ). In one medium the speed is fixed, so a higher note has a shorter wavelength. Loudness is measured in decibels, a scale based on ratios of intensity.
Hard surfaces reflect sound as echoes: bats and dolphins find their prey by echolocation, ships map the sea floor with sonar, and doctors make ultrasound scans of babies. Soft, uneven materials absorb sound, which is why recording studios are lined with foam. Ear defenders protect workers from noise louder than about 85 dB.
Set the fork to 440 Hz and switch from air to water and steel: the speed and wavelength jump while the pitch stays the same. Choose a vacuum and press Hear it. Pump the air out of the bell jar. In Echo, set the cliff to 170 m and time the echo (about 1 s). Then play the oscilloscope game.
| Medium (about 20 °C) | Speed of sound | Time to travel 1 km |
|---|---|---|
| Air | 343 m/s | 2.9 s |
| Water | 1,480 m/s | 0.68 s |
| Steel | about 5,960 m/s | 0.17 s |
| Vacuum (space) | no sound | — |
| In air the speed rises about 0.6 m/s for every 1 °C warmer. | ||
| Sound level | Example |
|---|---|
| 0 dB | Quietest sound a healthy young ear can hear |
| 30 dB | Whisper |
| 60 dB | Normal conversation |
| 85 dB | Heavy traffic: hearing damage with long exposure |
| 110 dB | Rock concert |
| 120–130 dB | Threshold of pain |
| 140 dB | Jet engine at 30 m |
Sound is made when something vibrates: a guitar string, a drum skin, a loudspeaker cone or your vocal cords. The vibrating object pushes and pulls on the particles of the material around it, and each particle passes the push on to its neighbors. This travels outward as a sound wave of compressions and rarefactions until it reaches an ear.
Key takeaway: no vibration, no sound.In a longitudinal wave the particles vibrate back and forth along the same direction the wave travels. In air, sound squeezes particles together (compressions) and spreads them apart (rarefactions) along its path. In a transverse wave, such as a wave on a rope or light, the vibration is across the direction of travel.
Key takeaway: sound: back and forth along the path.Pitch is how high or low a sound is and depends on the frequency: more vibrations per second sound higher. Loudness is how strong a sound is and depends on the amplitude, the size of the vibrations, which sets how much energy the wave carries. A soft high note and a loud low note show the difference.
Key takeaway: frequency sets pitch; amplitude sets loudness.No. Sound needs particles to pass the vibration along, and space is almost a perfect vacuum. Astronauts talk by radio, which uses electromagnetic waves that can cross empty space. In the bell-jar experiment the ringing fades as the air is pumped out, even though the bell keeps vibrating.
Key takeaway: sound needs a medium.How fast sound travels depends on how stiff the material is and how dense it is. In solids the particles are packed closely and held together by strong forces, so a push is passed on very quickly. Gases are much less stiff, so sound is slower: about 343 m/s in air, 1,480 m/s in water and around 6,000 m/s in steel.
Key takeaway: stiffer materials carry sound faster.An echo is sound reflected back from a hard surface. Because the sound travels there and back, distance = speed × time ÷ 2. If an echo from a cliff returns after 1 second in 20 °C air, the cliff is about 343 × 1 ÷ 2 ≈ 170 m away. Bats, dolphins, sonar and ultrasound scanners all use echoes.
Key takeaway: distance = speed × time ÷ 2.Frequency is how many waves pass a point each second, measured in hertz (Hz). Wavelength is the distance from one compression to the next, in meters. Amplitude is the biggest change from the undisturbed state: for sound, the biggest change in air pressure. Speed, frequency and wavelength are linked by v = fλ.
Key takeaway: v = f × λ.Ultrasound is sound with a frequency above about 20,000 Hz, too high for humans to hear; bats, dolphins and medical scanners use it. Infrasound is below about 20 Hz, too low to hear; elephants, whales, earthquakes and volcanoes produce it.
Key takeaway: outside 20 Hz–20 kHz we cannot hear it.Sound level is measured in decibels (dB). Normal talking is about 60 dB. Long or repeated exposure above about 85 dB, such as heavy traffic, power tools or loud headphones, can permanently damage the hair cells in the inner ear; around 120–130 dB sound becomes painful. Turning headphones down and using ear protection prevents damage.
Key takeaway: protect your hearing above about 85 dB.The physics is real: the speeds, wavelengths, echo times, the vacuum and the oscilloscope traces follow the true formulas. But the animation is slowed down enormously and the particle movement is exaggerated: in real air the particles move far less than a millimeter, and you could never see a 440 Hz wave move. Very short or very long wavelengths are drawn closer to the middle size.
Key takeaway: real numbers, slowed-down and exaggerated pictures.