Sound
Sound is not a thing that travels — it is a squeeze passing through matter. That one shift explains why space is silent, why pitch is not loudness, and how a ship measures the sea floor.
The big picture
Something vibrates. The air next to it gets squeezed. That squeeze pushes the air next to it, which squeezes the air next to that — and a pattern of squeezes travels outward until one of them reaches your ear.
Hold on to one idea and most of this chapter follows from it: the air does not travel to you. The squeeze does. Each bit of air only jiggles back and forth about where it already was. What moves across the room is a pattern, not a substance — the same way a stadium wave crosses a crowd while every person stays in their seat.
Production
- A vibrating object
- No vibration, no sound
Propagation
- Needs a medium
- Longitudinal wave
- Compressions and rarefactions
- Silent in vacuum
Characteristics
- Wavelength λ
- Frequency f (Hz)
- Time period T = 1/f
- Amplitude
- Pitch
- Loudness
- Quality
- v = f λ
Reflection
- Echo
- Reverberation
- Megaphone, stethoscope, soundboard
Range of hearing
- 20 Hz – 20 000 Hz
- Infrasound below
- Ultrasound above
Ultrasound at work
- Cleaning
- Finding cracks
- Scans of the body
- SONAR
The human ear
- Pinna
- Auditory canal
- Eardrum
- Hammer, anvil, stirrup
- Cochlea
- Auditory nerve
Sound is made by something vibrating
Every sound you have ever heard began with something moving back and forth. Not a special kind of movement — just fast, small, repeated movement.
You can check this without any apparatus. Press your fingers gently to your throat and hum: you will feel the buzz. Strike a tuning fork and touch its prong to the surface of water, and the water sprays — the prong is moving, even though it looks still. Pluck a stretched rubber band and you can watch it blur.
Stop the vibration and the sound stops instantly. Grip a struck tuning fork and the note dies the moment your hand touches it, because you have taken the movement away. There is no such thing as a silent vibration making sound, and no such thing as sound without something vibrating.
How the vibration gets to you
The vibrating object pushes the layer of air touching it. That layer gets crowded — a compression, a region where the particles are pressed closer together and the pressure is higher than normal.
Then the object swings back the other way, leaving room behind it. The crowded layer spreads out into that space, creating a rarefaction — a region where the particles are further apart than normal and the pressure is lower.
Because the object keeps vibrating, compressions and rarefactions leave it one after another, and each one shoves the region ahead of it. Step through it and watch which thing actually moves.
Watch two things at once: the grey bracket, which marks a compression, and the red particle. Step forward.
- Wave has moved
- 0.00 λ
- Each particle
- in place
Notice which way the particles move: back and forth along the same line the wave is travelling. A wave like that is called a longitudinal wave, and sound in air is always one. (In a wave on a rope, the rope moves up and down while the wave goes sideways — that is a transverse wave, and sound in air is not one.)
Sound needs something to travel through
Because sound is particles shoving other particles, it needs particles. Take them away and there is nothing to hand the squeeze on.
This is not a guess — it is a demonstration you can watch. Put an electric bell inside a glass jar and start pumping the air out. As the air thins, the ringing gets fainter and fainter, until you can see the hammer striking the gong and hear nothing at all.
A material that sound can travel through is called a medium — a solid, a liquid or a gas. Sound is therefore a mechanical wave: it cannot exist without matter to travel in.
The four numbers that describe a sound
Any sound wave can be described by a handful of quantities, and two of them decide what you actually hear.
Wavelength (λ) is the distance between two consecutive compressions — one complete repeat of the pattern. Metres.
Frequency (f) is how many complete waves pass a point each second, which is also how many times per second the source vibrates. Its unit is the hertz (Hz): 1 Hz is one wave per second.
Time period (T) is how long one complete wave takes. Since frequency counts waves per second and the period is seconds per wave, each is the other upside down:
T = 1 / f
Amplitude is how far each particle moves from its rest position — how hard the squeeze is.
Frequency decides pitch
Pitch is how high or low a sound seems. A faster vibration means more waves per second means a higher pitch. That is the whole relationship.
At 200 Hz the trace is more tightly packed — a higher pitch. The height never changes, because pitch is not loudness.
- Frequency
- 200 Hz
- Time period
- 5.00 ms
- Wavelength in air
- 1.72 m
- Pitch
- low
Amplitude decides loudness
Loudness is how strong the sound seems. A bigger amplitude means the particles are pushed further, the compressions are harder, and the sound carries more energy — so it sounds louder.
Quality tells one instrument from another
Play the same note at the same volume on a flute and on a violin and you can still tell them apart instantly. What differs is the quality (or timbre) of the sound — the particular shape of its wave. A sound of a single frequency is called a tone; a sound made of several frequencies together is a note, and its shape is what your ear recognises as “violin”.
The speed of sound
The three quantities are tied together by one equation that is worth recognising as ordinary common sense: how far the wave gets in a second equals the length of one wave times the number of waves per second.
v = f × λ
Speed depends on the medium, not on how loud or high the sound is. Sound travels fastest where the particles are closest together and most tightly linked, because each one passes the shove on sooner.
| Part | What it does | Why it's there |
|---|---|---|
| Solids (steel ≈ 5960 m/s) | Fastest | Particles are packed tightly and strongly bound, so a push is handed on almost at once |
| Liquids (water ≈ 1500 m/s) | Slower | Particles are close but free to slide, so the push is passed on less directly |
| Gases (air ≈ 344 m/s) | Slowest | Particles are far apart and must travel before they collide with the next one |
| Vacuum | No sound at all | Nothing to pass the push to — there is no medium |
Temperature matters too. Warmer air means faster-moving particles, which collide sooner and pass the compression on more quickly.
Reflection of sound
Sound bounces off a hard surface much as light bounces off a mirror, and it obeys the same two rules: the reflected wave leaves at the same angle it arrived, and the incoming wave, the reflected wave and the normal to the surface all lie in one plane. Unlike light, sound reflects well off any large hard surface — it does not need a polished one.
Echo
An echo is a reflected sound heard as a separate sound from the original.
Whether you hear one is a question about your ear, not about the wall. The sensation of a sound persists in your brain for about 0.1 seconds. If the reflection arrives sooner than that, it merges with the original and you hear one sound. Later, and you hear two.
So there is a minimum distance for an echo, and it is a calculation, not a fact to memorise. Move the wall and watch the threshold arrive.
The reflection is back in 0.058 s, inside the 0.1 s your brain holds the original for. The two merge and you hear one sound.
- Distance
- 10 m
- There and back
- 20 m
- Time taken
- 0.058 s
- Result
- one sound
Reverberation
In a large hall, sound reflects repeatedly off the walls, ceiling and floor. These reflections keep arriving after the original has stopped, and the sound seems to persist — this is reverberation. Too much of it and speech turns to mush, because each word is still echoing while the next is spoken.
Halls are treated for it: curtains, carpets, cushioned seats, and rough or panelled surfaces that absorb sound rather than bouncing it back cleanly.
Uses of multiple reflection
- A megaphone is a cone that stops sound spreading in all directions, reflecting it repeatedly forward so more of it reaches the audience.
- A stethoscope carries the sound of a heartbeat along its tubes by repeated reflection off the inside walls, instead of letting it escape.
- A soundboard — a curved surface behind a speaker on a stage — reflects sound out towards the audience instead of letting it disappear upwards.
- Concert hall ceilings are curved for the same reason, so reflected sound reaches the back rows evenly.
What we can and cannot hear
Human ears respond only to a band of frequencies, roughly 20 Hz to 20 000 Hz. This is the audible range, and it narrows with age — most adults lose the top of it.
Infrasound is sound below 20 Hz. Rhinoceroses call to each other at about 5 Hz, whales use it across oceans, and earthquakes produce it before the main tremor — which is one reason some animals appear to know before we do.
Ultrasound is sound above 20 000 Hz. Bats and dolphins use it to navigate and hunt: they emit bursts and read the reflections to build a picture of what is around them, in complete darkness.
Putting ultrasound to work
Because ultrasound has a short wavelength and can be sent in a narrow beam, it can be aimed and can pick out small details.
- Cleaning parts with awkward shapes — spiral tubes, odd corners. The object sits in a cleaning liquid and ultrasound is passed through it; the vibration shakes the dirt loose from places no brush reaches.
- Detecting cracks in metal blocks. Ultrasound passes through metal but reflects off a crack, so a gap in the transmitted beam reveals a flaw that is invisible from the outside.
- Seeing inside the body. An ultrasound scanner sends pulses into the body and builds an image from the reflections off different organs — an ultrasonograph. The same technique applied to the heart is called echocardiography. Ultrasound is also used to break small kidney stones into grains that leave with the urine.
SONAR
SONAR — sound navigation and ranging — measures the depth of the sea, and finds submarines, hills and wrecks, using nothing but a pulse and a stopwatch.
A transmitter sends an ultrasound pulse downwards. It reflects off the sea bed and a detector picks it up. The only measurement is the time between sending and receiving. Work out the distance from it:
1What is measured
t = time between sending and receiving
A stopwatch, essentially. The speed of sound in sea water is known — about 1531 m/s — so the time is the only unknown quantity.
2Distance covered
total distance = v × t
Speed times time gives the distance the pulse travelled. But that is the whole journey.
3Halve it
d = (v × t) / 2
The pulse went down AND came back, so it covered the depth twice. The depth is half the distance travelled — forgetting to divide by two is the single commonest error in these questions.
How the ear hears it
Everything so far has been about the sound. This last section is about the receiver — and it is really a story about conversion. A pressure wave in air has to end up as an electrical signal in a nerve, and the ear does that in stages, each solving a different problem.
The outer ear. Its funnel shape collects sound from a wide area and directs it into a much narrower opening — so more of the sound that reaches your head actually gets in.
- Section
- outer ear
- Signal
- mechanical
Go deeper
Beyond the syllabus. Nothing here is examinable in Class 9 — everything you are assessed on is above this line. These are threads worth pulling if the chapter has clicked:
- Why does a sound get quieter as you walk away, when the speed and frequency have not changed? The energy spreads over a larger and larger sphere. Work out how the area of a sphere grows with radius and you have the answer.
- What happens to the pitch when the source is moving? An ambulance siren drops as it passes you, though the siren itself never changes. The effect has a name and a neat explanation in terms of the compressions being bunched up ahead of the source.
- Why does your voice sound wrong on a recording? When you speak you hear yourself partly through the bones of your skull, which carry the low frequencies better than air does. Everyone else has only ever heard the other version.