Waves and sound · Investigation
Sound needs a medium
A buzzer in a jar goes quiet while you watch it still ringing. Nothing has been done to the buzzer at all — something has been taken away from the space around it.
Start here
The buzzer is still going. The sound is not.
A small buzzer hangs on a thread inside a thick glass jar, ringing away. A pump starts pulling the air out of the jar. Through the glass you can still see the little hammer beating against the bell, exactly as fast as before.
As the last of the air leaves the jar, what happens to what you hear?
The hammer never stops. What stops is the delivery: sound is a squeeze handed from one particle to the next, and the pump is taking the particles away. Let the air back in and the ringing returns at once, exactly as loud as before, which proves the buzzer was working the whole time.
Sound is a disturbance passed from one particle of a material to the next. A vibrating surface pushes the particles touching it, those push the next ones along, and the squeeze travels. Take the particles away and there is nothing to do the passing on: sound cannot travel through a vacuum. The buzzer is still vibrating, and the vibration has nowhere to go.
Give it particles and it travels — through gases, through liquids and through solids. How fast depends on the material, and the pattern is the same one every time: the closer the particles sit and the more strongly they are held to each other, the more quickly each one passes the shove on. Sound is slowest in gases, faster in liquids, and fastest in solids. In air it manages about 340 metres every second; in steel, about 5000.
At the bench · a striker and a microphone, with a measured gap between them
Same bang. Change what is in the way.
Change a control to begin
A hammer strikes a plate at one end and a microphone records the arrival at the other. Set what fills the gap, and set how long the gap is.
Commit first. The same striker and microphone are set 100 m apart, once through air and once through steel. Which arrives first, and why?
What fills the gap
200 m
The gap
—
measured striker to microphone
Speed of sound in it
—
—
Time to arrive
—
—
At the microphone
—
The figure
All five side by side, to one scale
The order runs with the particles, not against them. A gas has its particles far apart and free, so each shove takes a while to reach the next one. A solid has them close and strongly linked, so the shove is handed on almost at once. A vacuum has none, and the bar has nothing to draw.
Writing it down · the shape of this relationship
Distance = speed × time
The triangle
Cover the one you want
d = v × t
v = d ÷ t
t = d ÷ v
Two things side by side means multiply. One thing over another means divide.
d · distance · m
v · speed of sound in that material · m/s
t · time · s
The speed belongs to the MATERIAL, not to the sound: change what fills the gap and v changes, whatever the note.
Worked example · one step at a time
A hammer strikes one end of a steel rail 1000 m long. The blow is heard through the rail 0.20 s later. What is the speed of sound in steel?
Step 0 of 5
Convert
1000 m stays 1000 m · 0.20 s stays 0.20 s
The length is already in metres and the time already in seconds, so there is nothing to convert.
Formula
speed = distance ÷ time
Cover v on the triangle: d sits over t, so you divide.
Insert
speed = 1000 m ÷ 0.20 s
The distance is the length of the rail, because the sound went through the steel.
Fine-tune
1000 ÷ 0.20 = 5000
Metres divided by seconds leaves metres per second.
Answer
speed = 5000 m/s
About fifteen times the speed of the same blow through the air.
Worked example · one step at a time
A blow travels 2.4 km along a steel pipe in 0.48 s. What is the speed of sound in the pipe?
Step 0 of 5
Convert
2.4 km × 1000 = 2400 m
Metres per second needs the distance in metres, and a kilometre is a thousand of them.
Formula
speed = distance ÷ time
Cover v on the triangle: d sits over t, so you divide.
Insert
speed = 2400 m ÷ 0.48 s
The converted distance goes in. The 2.4 never does.
Fine-tune
2400 ÷ 0.48 = 5000
Metres divided by seconds leaves metres per second.
Answer
speed = 5000 m/s
Insert 2.4 instead of 2400 and the answer comes out 5 m/s — slower than walking.
Your turn · the same five steps
Your gap: 200 m of air.
Write all five lines before you check. The gap and the material are the ones your own bench is showing.
The five lines, marked
Convert
the distance is already in metres · the time is already in seconds
The bench measures in metres and seconds, which is what m/s needs, so there is nothing to convert.
Formula
t = d ÷ v
Cover t on the triangle: d sits over v, so you divide.
Insert
t = 200 m ÷ 340 m/s
The gap comes from your slider; the speed is the one for air.
Fine-tune
200 ÷ 340 = 0.5882
Metres divided by metres per second leaves seconds.
Answer
t = 0.588 s
Seconds, because that is what is left when the metres cancel.
The five lines above give 0.588 s for 200 m of air.
A sound travels 1.5 km through sea water in 1.0 s. What is its speed?
This one needs the Convert line to do some work.
The five lines, marked
Convert
1.5 km × 1000 = 1500 m
Metres per second needs the distance in metres, so multiply the kilometres by 1000.
Formula
speed = distance ÷ time
Cover v on the triangle: d sits over t, so you divide.
Insert
speed = 1500 m ÷ 1.0 s
The converted distance goes in. The 1.5 never does.
Fine-tune
1500 ÷ 1.0 = 1500
Metres divided by seconds leaves metres per second.
Answer
speed = 1500 m/s
Insert 1.5 instead of 1500 and the answer comes out 1.5 m/s.
The five lines give 1500 m/s — about four times the speed of sound in air.
Key fact
Sound needs a material to travel through and cannot cross a vacuum, because there are no particles to pass the disturbance on. It goes fastest where the particles are closest together and most strongly linked: about 340 m/s in air, about 1500 m/s in water and about 5000 m/s in steel.
Think again
“Sound crosses a vacuum, just very slowly and faintly.”
There is no slow, faint version. Sound is particles shoving their neighbours, so with no particles there is no mechanism at all, and the loudness does not tail off towards a whisper — it goes to nothing. That is why an astronaut outside a spacecraft hears their own breathing and their radio and nothing else, no matter how violent the thing happening a few metres away, and why the explosions in space films are a sound-effects decision rather than physics. Light does cross a vacuum, which is why you can see the thing you cannot hear.
“Sound goes fastest through air, because air is the easiest thing to get through.”
Easy to walk through is not the same as easy to pass a shove along. Getting through the air is easy precisely because its particles are far apart and barely hold on to each other — and that is exactly what makes it slow at handing a disturbance on. In steel every particle is packed tight against its neighbours and firmly linked to them, so the shove is passed on almost immediately and sound manages about 5000 m/s, roughly fifteen times its speed in air. Put your ear to a long metal fence and have someone tap the far end: you hear it through the metal first and through the air a moment later.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Calculate
A shout crosses 680 m of still air. Sound travels at about 340 m/s in air. How long does it take?
Rung 2 · The one that catches people
A spacecraft explodes a hundred metres from an astronaut on a spacewalk. Which statement is right?
Rung 3 · Explain
A ringing buzzer hangs inside a glass jar. A pump slowly removes the air. Describe what happens and explain why, using the word particles.
Rung 4 · Take it somewhere new
Standing beside a long steel railway rail, you hear a hammer blow from far down the track twice: once through the rail and once through the air. The rail is 1700 m long. Work out both arrival times and the gap between them, then say which arrives first and why.
Key note
Sound is a disturbance passed from particle to particle, so it needs a material to travel through and cannot cross a vacuum at all. It travels through gases, liquids and solids, and it goes faster where the particles are closer together and more strongly linked: about 340 m/s in air, about 1500 m/s in water and about 5000 m/s in steel. Distance = speed of sound × time.
Going further
Sound going faster in solids is what makes a stethoscope, a train-track trick and a whole branch of engineering work. Ultrasonic testers send a pulse into a steel casting and time what comes back: a crack inside sends a reflection home early, and the timing gives its depth to within a millimetre. Nobody has to cut the casting open.
The speed in air is not quite a constant. It rises with temperature, by roughly 0.6 m/s for every degree Celsius, because warmer particles are already moving faster and hand the shove on more quickly. On a hot day sound outruns its cold-morning self by several metres a second, and on a cold clear night the layer of warm air above can bend sound back down to the ground and carry a distant conversation much further than it has any right to go.
Before this lesson
Connects to
At GCSE this becomes
- Measuring the speed of sound in air and in solids, ultrasound imaging and non-destructive testing, and the wave equation applied to sound.
Where to next
Ask Mr Badmus AI
Got a gap of your own to time?
The bench is a teaching model. Every speed in it is an approximate value for one ordinary sample at about 20 degrees Celsius: air varies by roughly 0.6 m/s per degree, sea water differs from fresh, oak differs across the grain from along it, and steel differs with its alloy. The particle drawings show relative spacing only and are not to scale in size or number, and real particles are in constant random motion rather than in rows. The gap is treated as filled with one material end to end, with no losses, so the blow arrives at full strength however far away it is; over long distances in real air a sound both spreads out and is absorbed.
Lesson content © MrBadmusAI.