Sound Waves
What is sound, really? Change the pitch and loudness and watch the air molecules squeeze and stretch, find out which animals can hear it, and try sending sound through space.
Class 8 · Class 9 SoundSound plays only while you choose to hear it.
Try this
0 of 5 doneTune to 440 Hz, the note musicians tune their instruments to.Hint: press 🎵 A note, or slide carefully
Go higher than any human can hear. Who can still hear it?Hint: above 20,000 Hz
Go lower than humans can hear, where elephants talk.Hint: below 20 Hz
Make a sound wave exactly 1 metre long in air.Hint: λ = v ÷ f, and sound in air goes 343 m/s
Try to send sound through empty space.Hint: pick Space (vacuum)
What's happening
Sound is a squeeze-and-stretch wave
A vibrating object, like a speaker cone or your vocal cords, pushes on the air next to it. The air molecules bunch up (compressions) and spread out (rarefactions), and pass the push along. The molecules themselves only jiggle back and forth; it's the pattern that travels. That makes sound a longitudinal wave.
Pitch and loudness
Frequency is how many vibrations happen each second, in hertz (Hz). More vibrations mean a higher pitch. Amplitude is how big each vibration is: bigger squeezes sound louder, measured in decibels (dB). Humans hear from about 20 Hz to 20,000 Hz; below that is infrasound and above it ultrasound.
Speed, frequency and wavelength
Wavelength λ is the distance from one compression to the next. Sound's speed depends on what it travels through: about 343 m/s in air, 4 times faster in water and over 17 times faster in steel. With no molecules at all, in a vacuum, sound can't travel.
Who figured this out?
Marin Mersenne
1636, FranceWorked out how the pitch of a string depends on its length, tension and weight, and made early measurements of the speed of sound.
Robert Boyle
About 1660, EnglandPut a ticking or ringing device in a jar and pumped the air out: the sound faded away, showing that sound needs air to travel.
Alexander Graham Bell
1876, USAPatented the telephone, turning sound into electric signals and back. The decibel, the unit of loudness, is named after him.
Learn more
- Sound · Wikipedia
- Hearing range · Wikipedia
Check yourself
1. A sound has frequency 686 Hz in air (speed 343 m/s). What is its wavelength?
2. What happens to the pitch of a sound if its frequency increases?
3. Why can't astronauts talk to each other directly in space without radios?
4. Through which does sound travel fastest?
Common questions
What are the characteristics of a sound wave?
A sound wave is described by its frequency (vibrations per second, in hertz), amplitude (size of the vibration), wavelength (distance between two compressions), time period (time for one vibration) and speed. We hear frequency as pitch, amplitude as loudness, and the mix of frequencies as the quality or timbre of the sound.
What is the relation between frequency and wavelength?
Speed = frequency × wavelength, v = fλ. For sound in air at about 343 m/s, a 343 Hz sound has a wavelength of 1 m. Doubling the frequency halves the wavelength, because the speed stays the same in a given material.
What is the audible range of sound for humans?
Humans can hear sounds from about 20 Hz to 20,000 Hz (20 kHz). Sounds below 20 Hz are called infrasound; elephants and whales use it. Sounds above 20 kHz are ultrasound; bats, dogs and dolphins can hear it. The upper limit falls as people get older.
Why can't sound travel through a vacuum?
Sound is a mechanical wave: it needs particles of a medium (solid, liquid or gas) to pass the vibration along. A vacuum has no particles, so sound cannot travel through it. Robert Boyle showed this around 1660: a ringing bell in a jar went quiet as the air was pumped out.
What is the speed of sound in air, water and steel?
At room temperature, about 343 m/s in air, about 1480 m/s in water and about 5960 m/s in steel. Sound travels faster in solids and liquids than in gases, and faster in warm air than in cold air.
What are the uses of ultrasound?
Ultrasound is used in sonography to see babies and organs inside the body, in SONAR to find the depth of the sea and locate submarines and fish, to detect cracks in metal parts, and to clean small objects like jewellery. Bats and dolphins use it to find their way by echolocation.
Speeds are typical values at room temperature. Loudness in decibels is a rough guide; the picture is slowed down enormously so you can see the motion.
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