Every sound you have ever heard was a squeeze of air arriving at your eardrum. A speaker cone pushes forward, air piles up in front of it; the cone pulls back, the air thins out. Squeeze, thin, squeeze, thin — and that pattern sprints across the room at about 340 metres every second. Sound is the everyday face of the longitudinal wave, and this page is about how it works.
📘 What you need to know
Sound waves are longitudinal, so they need a medium and cannot cross a vacuum
An oscillating source changes the density of the medium, creating compressions and rarefactions
Frequency sets the pitch: high f → high pitch
Amplitude sets the volume: large A → loud
The human audible range is 20 Hz – 20 kHz; below is infrasound, above is ultrasound
Sound travels at about 340 m s−1 in air, faster in hotter air, and fastest in solids
How a sound wave is made
Sound starts, like every wave, with an oscillating source — a loudspeaker cone, a guitar string, your vocal cords. As the source vibrates back and forth it keeps changing the density of the air right next to it.
Push forward and you crowd the air molecules together: a compression, a small region of high pressure. Pull back and you leave the molecules more spread out: a rarefaction, a region of low pressure. Those regions then travel outwards, one after another, and the air molecules themselves just shuffle back and forth on the spot.
The speaker cone squeezes and stretches the air. Bunched lines are compressions (high pressure), spread lines are rarefactions (low pressure). One wavelength runs from one compression to the next.
Oscillating source speaker, string
changes air density
Compressions & rarefactions
arrive at
Eardrum vibrates
Because sound is longitudinal, it is always a mechanical wave — it needs particles to pass the squeeze along. Take the air away and there is nothing left to compress. That’s why the vacuum of space is completely silent, no matter how violent the explosion.
The spectrum of sound
Sound waves form a continuous spectrum of frequencies. Our ears, though, only respond to a slice in the middle of it:
The audible range
20 Hz ≤ f ≤ 20 kHz
Anything below 20 Hz is infrasound; anything above 20 kHz is ultrasound. Both are perfectly real sound waves — your ear simply cannot detect them.
Sound below 20 Hz is infrasound; above 20 kHz it is ultrasound. Elephants rumble below our range; bats squeak above it.
Region
Frequency
Can humans hear it?
Example
Infrasound
Below 20 Hz
No
Elephant calls, earthquakes
Audible range
20 Hz – 20 kHz
Yes
Speech, music
Ultrasound
Above 20 kHz
No
Bat echolocation, medical scans
Feel the size: in air, a 20 Hz rumble has a wavelength of 340 ÷ 20 = 17 m — longer than a classroom. A 20 kHz squeak has a wavelength of just 17 mm. Same speed, wildly different λ.
Pitch and volume
Two properties of the wave control what you actually hear, and it’s easy to swap them by accident. Keep them separate:
Frequency → pitch. A high frequency (short wavelength) sounds high-pitched; a low frequency (long wavelength) sounds low-pitched.
Amplitude → volume. A large amplitude sounds loud; a small amplitude sounds quiet.
Top row: same frequency, different amplitude — the volume changes. Bottom row: same amplitude, different frequency — the pitch changes. The red arrows mark the amplitude.
Turning up the volume knob does not make the music higher. It stretches the amplitude, so the speaker pushes the air harder — louder, same notes. Change the frequency instead and you change the note itself. Two knobs, two effects, never mixed up again.
The speed of sound
In air at room temperature, sound travels at roughly 340 m s−1. Two things change that number.
1. Temperature
The hotter the air, the faster the sound. Heating the air raises the average kinetic energy of its particles, so they are moving faster and pass the compression on to their neighbours more quickly.
2. The state of the medium
Sound is a hand-me-down of vibrations from particle to particle, so the closer the particles, the faster the hand-off:
Solids — fastest. Particles are packed tightly and bonded, so a vibration is passed on almost instantly.
Liquids — in between.
Gases — slowest. Particles are far apart and must drift a long way before colliding, which is inefficient.
Medium
State
Typical speed of sound
Air (room temperature)
Gas
~340 m s−1
Water
Liquid
~1500 m s−1
Steel
Solid
~5000 m s−1
Vacuum
—
No sound at all
🎯 When a sound changes medium
The frequency stays the same. It is fixed by the source that made the sound, not by the material.
The speed changes — that’s a property of the new medium.
So the wavelength must change to keep v = fλ true. Faster medium → longer λ.
WE 1
A bat emits a call of frequency 40 kHz. Taking the speed of sound in air as 340 m s⁻¹, calculate the wavelength of the call, and state whether a human could hear it.
Step 1 — convert the frequency to hertz
f = 40 kHz = 40 000 Hz
Step 2 — rearrange the wave equation for λ
λ = v / f
λ = 340 / 40 000 = 0.0085 mλ = 8.5 mmStep 3 — compare with the audible range
40 kHz is above 20 kHz, so this is ultrasound.
No human can hear it. Short wavelengths like this are exactly why bats can pick out small insects.
WE 2
A student stands 85 m from a large cliff and claps once. She hears the echo 0.50 s later. Calculate the speed of sound in the air.
Step 1 — find the distance the sound actually travels
It goes to the cliff and back again.
d = 2 × 85 = 170 mStep 2 — use speed = distance / timev = 170 / 0.50v = 340 m s⁻¹Forget the “and back” and you’d get 170 m s⁻¹ — exactly half. Every echo question hides that factor of 2.
WE 3
A tuning fork of frequency 500 Hz is struck and held against a steel rail. Sound travels at 340 m s⁻¹ in air and 5000 m s⁻¹ in steel. Calculate the wavelength of the note in each material.
Step 1 — the frequency is set by the fork, so it is 500 Hz in bothStep 2 — wavelength in air, λ = v / fλ = 340 / 500 = 0.68 mStep 3 — wavelength in steelλ = 5000 / 500 = 10 mair: 0.68 m | steel: 10 mThe note sounds identical — same f, same pitch. Only v and λ changed, and they changed together.
💡 Top tips
Pitch = frequency, volume = amplitude. Write it in the margin before you start the question.
Echo questions: the sound travels there and back, so use 2 × distance.
Convert kHz to Hz before substituting into v = fλ.
Changing medium changes v and λ, never f.
Remember the ordering solid > liquid > gas and be ready to justify it with particle spacing.
⚠ Common mistakes
Saying a louder sound is a higher-pitched sound — louder means bigger amplitude only
Thinking a louder sound travels faster — amplitude has no effect on v
Claiming frequency changes when sound enters water or steel — the source fixes f
Forgetting the factor of 2 in echo and sonar calculations
Saying ultrasound and infrasound are not sound — they are, we just can’t hear them
Assuming sound is faster in gases because “particles move freely” — it’s the slowest there
Quick recap: Sound is a longitudinal mechanical wave made by an oscillating source changing the density of a medium, producing compressions and rarefactions. Frequency gives pitch, amplitude gives volume. Humans hear 20 Hz – 20 kHz; below is infrasound, above is ultrasound. Sound moves at about 340 m s−1 in air, faster when hotter, and fastest in solids — but never at all through a vacuum.
Sound needed a medium. Now meet the family that needs nothing at all: electromagnetic waves. They are transverse, they cross empty space, and every single one of them travels at the same enormous speed — the speed of light. That’s the next page.
Want sound waves to feel this clear in the exam?
Book a free meeting and we’ll work through pitch, volume, echoes and past-paper wave questions together.
