Sound Travels Across a Room Without a Single Air Molecule Crossing It
Learn how vibrating objects produce sound, how the bell jar experiment proves a medium is needed, how compressions and rarefactions carry the wave, and how longitudinal differs from transverse.
Does the air itself travel from a speaker to your ear?
No. Not one air molecule makes that journey.
What travels is the disturbance. A speaker cone pushes the air next to it, crowding those molecules together. They push their neighbours and spring back to where they were. The neighbours do the same to the next layer along, and so on across the room.
Each molecule only jiggles back and forth about its own position. The pattern of crowding and thinning moves outward, and your eardrum is pushed and pulled by it when it arrives.
That is why standing in front of a loudspeaker does not produce a wind, however loud the sound. A wave carries energy through a medium without carrying the medium with it — and that one sentence is the definition every part of this chapter depends on. It covers the first part of the CBSE Class 9 Science chapter on sound waves.
What travels is the disturbance. A speaker cone pushes the air next to it, crowding those molecules together. They push their neighbours and spring back to where they were. The neighbours do the same to the next layer along, and so on across the room.
Each molecule only jiggles back and forth about its own position. The pattern of crowding and thinning moves outward, and your eardrum is pushed and pulled by it when it arrives.
That is why standing in front of a loudspeaker does not produce a wind, however loud the sound. A wave carries energy through a medium without carrying the medium with it — and that one sentence is the definition every part of this chapter depends on. It covers the first part of the CBSE Class 9 Science chapter on sound waves.
How does a vibrating object produce sound?
By pushing the medium around it back and forth, so that the disturbance travels outward.
Sound is always produced by a vibrating object. If something is making a sound, something in it is vibrating — and if the vibration is stopped, the sound stops at once.
A tuning fork. Strike it and the prongs vibrate too fast to see clearly. Two simple tests reveal the motion:
- Touch a vibrating prong gently to the surface of water in a beaker, and the water splashes
- Touch it to your fingertip, and you feel the vibration
Hold the prongs firmly and both the vibration and the sound stop together.
A stretched string. Pluck the string of a sitar, a veena or a guitar and you can see it blur as it vibrates. Damp it with a finger and the note dies instantly.
A stretched membrane. The skin of a tabla, a dholak or a drum vibrates when struck. Sprinkle a few grains of rice on the surface and they jump as it sounds.
An air column. A flute or a shehnai has no moving part you can see — the air inside vibrates, set going by the player's breath.
Your own voice. The vocal cords in the larynx vibrate as air passes over them. Place your fingers lightly on the front of your throat and hum: the vibration is easy to feel, and it stops the moment you stop.
The vibration must be fast enough to hear. A pendulum swinging back and forth is vibrating, and it produces no audible sound at all, because its frequency is far below the range human ears respond to. Vibration is necessary for sound but not sufficient for a sound you can hear — which is the distinction the second part of this chapter makes precise with the audible range.
Sound is always produced by a vibrating object. If something is making a sound, something in it is vibrating — and if the vibration is stopped, the sound stops at once.
A tuning fork. Strike it and the prongs vibrate too fast to see clearly. Two simple tests reveal the motion:
- Touch a vibrating prong gently to the surface of water in a beaker, and the water splashes
- Touch it to your fingertip, and you feel the vibration
Hold the prongs firmly and both the vibration and the sound stop together.
A stretched string. Pluck the string of a sitar, a veena or a guitar and you can see it blur as it vibrates. Damp it with a finger and the note dies instantly.
A stretched membrane. The skin of a tabla, a dholak or a drum vibrates when struck. Sprinkle a few grains of rice on the surface and they jump as it sounds.
An air column. A flute or a shehnai has no moving part you can see — the air inside vibrates, set going by the player's breath.
Your own voice. The vocal cords in the larynx vibrate as air passes over them. Place your fingers lightly on the front of your throat and hum: the vibration is easy to feel, and it stops the moment you stop.
The vibration must be fast enough to hear. A pendulum swinging back and forth is vibrating, and it produces no audible sound at all, because its frequency is far below the range human ears respond to. Vibration is necessary for sound but not sufficient for a sound you can hear — which is the distinction the second part of this chapter makes precise with the audible range.
How can you show that sound needs a material medium?
With the bell jar experiment, which removes the medium and watches the sound disappear.
The design. An electric bell is suspended inside a thick glass bell jar standing on a sealed base. The jar is connected to a vacuum pump that can remove the air. The bell's wires pass out through the base so it can be switched on from outside.
The procedure and what is observed.
- With air in the jar, switch on the bell: it is heard clearly
- Start the pump. As the air is gradually removed, the sound becomes fainter and fainter
- When the jar is nearly evacuated, the sound is almost inaudible — yet through the glass the hammer can still be seen striking the gong
- Let the air back in and the sound returns to full loudness
The prediction for a perfect vacuum: no sound at all. The experiment cannot quite reach a perfect vacuum, so a trace of sound remains, but the trend is unmistakable — less air means less sound, and no air would mean none.
The conclusion. Sound needs a material medium to travel through. Solids, liquids and gases will all do; vacuum will not.
Why the bell is still visibly working. This is the detail that makes the experiment convincing. The hammer strikes as vigorously as before, so the source has not changed. What has been removed is the medium between the source and your ear, and that alone has silenced it.
Everyday evidence for a solid and a liquid medium. Put your ear to a railway line and you hear an approaching train long before you hear it through the air, because sound travels faster and more strongly through steel. Clap two stones together underwater and a swimmer nearby hears it clearly, because water carries sound well.
The speed depends on the medium. Sound travels fastest in solids, slower in liquids and slowest in gases — roughly m/s in iron, m/s in water and m/s in air at room temperature. Particles that are closer together and more strongly linked pass the disturbance on more quickly.
Light can cross a vacuum and sound cannot. That contrast is the cleanest way to remember the result. We see the Sun and hear nothing from it, and an explosion in space would be silent however violent. Sound is a mechanical wave and light is not, and this experiment is what establishes the difference.
The design. An electric bell is suspended inside a thick glass bell jar standing on a sealed base. The jar is connected to a vacuum pump that can remove the air. The bell's wires pass out through the base so it can be switched on from outside.
The procedure and what is observed.
- With air in the jar, switch on the bell: it is heard clearly
- Start the pump. As the air is gradually removed, the sound becomes fainter and fainter
- When the jar is nearly evacuated, the sound is almost inaudible — yet through the glass the hammer can still be seen striking the gong
- Let the air back in and the sound returns to full loudness
The prediction for a perfect vacuum: no sound at all. The experiment cannot quite reach a perfect vacuum, so a trace of sound remains, but the trend is unmistakable — less air means less sound, and no air would mean none.
The conclusion. Sound needs a material medium to travel through. Solids, liquids and gases will all do; vacuum will not.
Why the bell is still visibly working. This is the detail that makes the experiment convincing. The hammer strikes as vigorously as before, so the source has not changed. What has been removed is the medium between the source and your ear, and that alone has silenced it.
Everyday evidence for a solid and a liquid medium. Put your ear to a railway line and you hear an approaching train long before you hear it through the air, because sound travels faster and more strongly through steel. Clap two stones together underwater and a swimmer nearby hears it clearly, because water carries sound well.
The speed depends on the medium. Sound travels fastest in solids, slower in liquids and slowest in gases — roughly m/s in iron, m/s in water and m/s in air at room temperature. Particles that are closer together and more strongly linked pass the disturbance on more quickly.
Light can cross a vacuum and sound cannot. That contrast is the cleanest way to remember the result. We see the Sun and hear nothing from it, and an explosion in space would be silent however violent. Sound is a mechanical wave and light is not, and this experiment is what establishes the difference.
What are compressions and rarefactions?
Regions of crowded and thinned-out particles, travelling outward one after the other.
Follow one cycle of a vibrating object:
- As it moves forward, it pushes the particles next to it together. That region has higher density and higher pressure, and it is called a compression
- As it moves back, it leaves the particles behind it more widely spaced. That region has lower density and lower pressure, and it is called a rarefaction
Repeat the vibration and a train of alternating compressions and rarefactions moves away from the source. When they reach your eardrum they push it in and let it out in turn, and you hear a sound.
A sound wave is therefore a longitudinal wave. The particles of the medium vibrate along the same line that the wave travels, parallel to its direction of motion.
It is also called a mechanical wave, because it needs a medium, and a pressure wave, because what varies from place to place is the pressure.
Everyday demonstration — a slinky spring. Lay a long slinky on a table, hold one end, and push it sharply along its own length. A region of squashed coils travels down the spring, followed by a region of stretched coils. Each coil only moves back and forth a little; the pattern travels the whole length.
No matter is transported. Each particle returns to its original position after each cycle, and the disturbance moves on. That is why the opening section could say that no air molecule crosses the room — the compressions cross it, and the molecules stay where they are.
The compressions are where the pressure is highest, not where the particles are moving fastest. Those are different things, and a diagram question asking you to mark the compressions wants the regions of greatest crowding, which is what the next part of this chapter draws as the crests of a graph.
Follow one cycle of a vibrating object:
- As it moves forward, it pushes the particles next to it together. That region has higher density and higher pressure, and it is called a compression
- As it moves back, it leaves the particles behind it more widely spaced. That region has lower density and lower pressure, and it is called a rarefaction
Repeat the vibration and a train of alternating compressions and rarefactions moves away from the source. When they reach your eardrum they push it in and let it out in turn, and you hear a sound.
A sound wave is therefore a longitudinal wave. The particles of the medium vibrate along the same line that the wave travels, parallel to its direction of motion.
It is also called a mechanical wave, because it needs a medium, and a pressure wave, because what varies from place to place is the pressure.
Everyday demonstration — a slinky spring. Lay a long slinky on a table, hold one end, and push it sharply along its own length. A region of squashed coils travels down the spring, followed by a region of stretched coils. Each coil only moves back and forth a little; the pattern travels the whole length.
No matter is transported. Each particle returns to its original position after each cycle, and the disturbance moves on. That is why the opening section could say that no air molecule crosses the room — the compressions cross it, and the molecules stay where they are.
The compressions are where the pressure is highest, not where the particles are moving fastest. Those are different things, and a diagram question asking you to mark the compressions wants the regions of greatest crowding, which is what the next part of this chapter draws as the crests of a graph.
How does a longitudinal wave differ from a transverse wave?
By the direction in which the particles vibrate relative to the direction the wave travels.
Longitudinal wave:
- Particles vibrate parallel to the direction of wave travel
- Made of compressions and rarefactions
- Example: sound in any medium
- Travels through solids, liquids and gases
Transverse wave:
- Particles vibrate perpendicular to the direction of wave travel
- Made of crests and troughs
- Examples: waves on a stretched rope, ripples on a water surface, and light
- Travels through solids and along surfaces; light also crosses a vacuum
One spring, both waves. The slinky demonstrates each in turn:
- Push and pull it along its length and you get compressions and rarefactions travelling along it — longitudinal
- Flick one end sideways and a sideways hump travels along while each coil moves across the spring — transverse
The spring has not changed. Only the direction in which you disturb it has.
Everyday evidence. Drop a stone into a pond and the ripples spread outward while a floating leaf bobs up and down — the leaf moves at right angles to the direction the ripple travels, which is what makes a water ripple transverse.
Sound in air can only be longitudinal. A gas cannot support a sideways disturbance, because there is nothing to stop one layer of gas simply sliding past the next — there is no sideways grip. So a sound wave in air is always a pressure wave, and a question asking whether sound can be transverse in air is asking about that.
In a solid, both are possible. A solid can resist a sideways shear, so a disturbance in a steel rail can travel as a longitudinal wave, a transverse wave, or both at once. That is why an earthquake produces two kinds of wave travelling at different speeds through rock, and why they arrive at a recording station at different times.
The two names describe the particle motion, not the shape of a drawing. A sound wave is drawn as a curve with crests and troughs in the next part of this chapter, and it is still longitudinal — the curve is a graph of density, not a picture of particles moving up and down. Mistaking that graph for the wave's shape is the commonest confusion in the whole chapter.
Longitudinal wave:
- Particles vibrate parallel to the direction of wave travel
- Made of compressions and rarefactions
- Example: sound in any medium
- Travels through solids, liquids and gases
Transverse wave:
- Particles vibrate perpendicular to the direction of wave travel
- Made of crests and troughs
- Examples: waves on a stretched rope, ripples on a water surface, and light
- Travels through solids and along surfaces; light also crosses a vacuum
One spring, both waves. The slinky demonstrates each in turn:
- Push and pull it along its length and you get compressions and rarefactions travelling along it — longitudinal
- Flick one end sideways and a sideways hump travels along while each coil moves across the spring — transverse
The spring has not changed. Only the direction in which you disturb it has.
Everyday evidence. Drop a stone into a pond and the ripples spread outward while a floating leaf bobs up and down — the leaf moves at right angles to the direction the ripple travels, which is what makes a water ripple transverse.
Sound in air can only be longitudinal. A gas cannot support a sideways disturbance, because there is nothing to stop one layer of gas simply sliding past the next — there is no sideways grip. So a sound wave in air is always a pressure wave, and a question asking whether sound can be transverse in air is asking about that.
In a solid, both are possible. A solid can resist a sideways shear, so a disturbance in a steel rail can travel as a longitudinal wave, a transverse wave, or both at once. That is why an earthquake produces two kinds of wave travelling at different speeds through rock, and why they arrive at a recording station at different times.
The two names describe the particle motion, not the shape of a drawing. A sound wave is drawn as a curve with crests and troughs in the next part of this chapter, and it is still longitudinal — the curve is a graph of density, not a picture of particles moving up and down. Mistaking that graph for the wave's shape is the commonest confusion in the whole chapter.
Exam tip
Exam tip: name the medium and the direction of vibration
Sound is always produced by a vibrating object, and stopping the vibration stops the sound. Name the vibrating part — prongs, string, membrane, air column, vocal cords.
For the bell jar experiment, state all three observations: the sound is loud with air, fainter as air is removed, almost inaudible in near vacuum — while the hammer is still seen striking. That last detail is what proves the source is unchanged.
Sound needs a material medium; it cannot travel through vacuum. Light can — quote that contrast.
Speed order: fastest in solids, then liquids, slowest in gases — roughly m/s in iron, m/s in water, m/s in air.
Compression means high density and high pressure; rarefaction means low density and low pressure.
Longitudinal means particles vibrate parallel to the wave's travel, with compressions and rarefactions. Transverse means perpendicular, with crests and troughs.
Give one example of each: sound and a water ripple or a wave on a rope.
Sound in air is always longitudinal, because a gas cannot resist a sideways disturbance.
No matter is transported — only energy and the disturbance move.
And remember the graph of a sound wave is a plot of density against distance, not a picture of particles moving sideways.
For the bell jar experiment, state all three observations: the sound is loud with air, fainter as air is removed, almost inaudible in near vacuum — while the hammer is still seen striking. That last detail is what proves the source is unchanged.
Sound needs a material medium; it cannot travel through vacuum. Light can — quote that contrast.
Speed order: fastest in solids, then liquids, slowest in gases — roughly m/s in iron, m/s in water, m/s in air.
Compression means high density and high pressure; rarefaction means low density and low pressure.
Longitudinal means particles vibrate parallel to the wave's travel, with compressions and rarefactions. Transverse means perpendicular, with crests and troughs.
Give one example of each: sound and a water ripple or a wave on a rope.
Sound in air is always longitudinal, because a gas cannot resist a sideways disturbance.
No matter is transported — only energy and the disturbance move.
And remember the graph of a sound wave is a plot of density against distance, not a picture of particles moving sideways.
Did you know
Why you hear a train through the rail before you hear it through the air
Put an ear to a railway line and an approaching train can be heard well before any sound arrives through the air. The same event, two media, two arrival times.
Sound is passed from particle to particle, so how quickly it travels depends on how closely those particles are packed and how strongly they are linked. In a solid the particles are close together and tightly bound, so a push is handed on almost immediately. In a gas they are far apart and weakly linked, so each has to travel some distance before it affects the next.
The numbers make the gap plain. Sound moves through iron at roughly m/s, through water at roughly m/s and through air at roughly m/s. A steel rail therefore carries the sound more than ten times faster than the air beside it.
The same ordering explains several familiar things. Swimmers underwater hear a splash sharply because water carries sound well. A whisper travels surprisingly far along a long metal pipe or a taut string, which is why a simple string telephone made from two cups works at all. And sound passes through a closed wooden door more easily than one might expect, because the wood itself carries it.
There is a limit to the comparison worth noting. A denser medium does not automatically mean faster sound — what matters is the combination of how stiff the medium is and how heavy its particles are. Sound travels faster in warm air than in cold air, although warm air is less dense, because its particles move faster and pass the disturbance on more quickly.
So the rule to carry is the ordering by state, not by density alone: solids fastest, liquids next, gases slowest — and vacuum, with no particles at all, not at all.
Sound is passed from particle to particle, so how quickly it travels depends on how closely those particles are packed and how strongly they are linked. In a solid the particles are close together and tightly bound, so a push is handed on almost immediately. In a gas they are far apart and weakly linked, so each has to travel some distance before it affects the next.
The numbers make the gap plain. Sound moves through iron at roughly m/s, through water at roughly m/s and through air at roughly m/s. A steel rail therefore carries the sound more than ten times faster than the air beside it.
The same ordering explains several familiar things. Swimmers underwater hear a splash sharply because water carries sound well. A whisper travels surprisingly far along a long metal pipe or a taut string, which is why a simple string telephone made from two cups works at all. And sound passes through a closed wooden door more easily than one might expect, because the wood itself carries it.
There is a limit to the comparison worth noting. A denser medium does not automatically mean faster sound — what matters is the combination of how stiff the medium is and how heavy its particles are. Sound travels faster in warm air than in cold air, although warm air is less dense, because its particles move faster and pass the disturbance on more quickly.
So the rule to carry is the ordering by state, not by density alone: solids fastest, liquids next, gases slowest — and vacuum, with no particles at all, not at all.
Exam relevance
How does wave motion in Class 9 feed into JEE Main and NEET?
This page is the foundation for the Class 11 Physics chapter Waves, examined in JEE Main and in NEET Physics, and every definition established here is used there without restatement.
The longitudinal and transverse distinction becomes formal in that chapter, where a wave is written as a mathematical function of position and time. The Class 9 statement that sound in a gas must be longitudinal is explained there by the fact that a fluid has no shear modulus, and it becomes the reason the speed of sound in a gas depends on pressure and density in the way it does.
The speed in different media becomes a set of formulas — the speed depending on the elastic property and the density of the medium — and the ordering solids, liquids, gases follows from them. The remark above that warm air carries sound faster despite being less dense is a standard JEE Main assertion-reason item, and it is unanswerable if speed is thought to depend on density alone.
Compressions and rarefactions become the pressure variation of a sound wave, and the relationship between the displacement wave and the pressure wave — which are a quarter cycle apart — is a recurring JEE Main question. The Class 9 warning that the drawn curve is a graph of density rather than a picture of particles is exactly what makes that later idea approachable.
The bell jar result connects to Class 11 and Class 12 in a broader way: sound is a mechanical wave and light is an electromagnetic one, and the contrast drawn here is the first statement of a distinction that runs through Electromagnetic Waves and Wave Optics.
What the questions look like. For NEET, assertion-reason items are the commonest form on this material, and favourites are that sound cannot travel through vacuum, and that sound in air is longitudinal. Match-the-column questions pair a wave with its type or a medium with a speed. For JEE Main, this page is assumed rather than tested — it appears as the first step of a numerical on speed, wavelength or the pressure amplitude.
How board and competitive emphasis differ. A board paper asks you to describe the bell jar experiment with a diagram, define compression and rarefaction, and give two differences between longitudinal and transverse waves. A competitive paper never asks for the description; it asks which of four statements about wave types is correct, or sets a numerical in which the medium's speed has to be chosen correctly. Board papers reward the labelled diagram; competitive papers reward knowing which quantity the medium controls.
The single trap that costs the most marks. Treating the drawn curve of a sound wave as the path the particles follow. It is a graph of density or pressure against distance, and the particles move along the wave, not across it. Students who picture air moving up and down go on to misread every sound-wave diagram, and the error survives into Class 11.
A second trap worth naming. Concluding that a denser medium always carries sound faster. Warm air is less dense than cold air and carries sound faster. What governs the speed is the combination of stiffness and density, and a question comparing two gases at different temperatures is testing precisely that.
The longitudinal and transverse distinction becomes formal in that chapter, where a wave is written as a mathematical function of position and time. The Class 9 statement that sound in a gas must be longitudinal is explained there by the fact that a fluid has no shear modulus, and it becomes the reason the speed of sound in a gas depends on pressure and density in the way it does.
The speed in different media becomes a set of formulas — the speed depending on the elastic property and the density of the medium — and the ordering solids, liquids, gases follows from them. The remark above that warm air carries sound faster despite being less dense is a standard JEE Main assertion-reason item, and it is unanswerable if speed is thought to depend on density alone.
Compressions and rarefactions become the pressure variation of a sound wave, and the relationship between the displacement wave and the pressure wave — which are a quarter cycle apart — is a recurring JEE Main question. The Class 9 warning that the drawn curve is a graph of density rather than a picture of particles is exactly what makes that later idea approachable.
The bell jar result connects to Class 11 and Class 12 in a broader way: sound is a mechanical wave and light is an electromagnetic one, and the contrast drawn here is the first statement of a distinction that runs through Electromagnetic Waves and Wave Optics.
What the questions look like. For NEET, assertion-reason items are the commonest form on this material, and favourites are that sound cannot travel through vacuum, and that sound in air is longitudinal. Match-the-column questions pair a wave with its type or a medium with a speed. For JEE Main, this page is assumed rather than tested — it appears as the first step of a numerical on speed, wavelength or the pressure amplitude.
How board and competitive emphasis differ. A board paper asks you to describe the bell jar experiment with a diagram, define compression and rarefaction, and give two differences between longitudinal and transverse waves. A competitive paper never asks for the description; it asks which of four statements about wave types is correct, or sets a numerical in which the medium's speed has to be chosen correctly. Board papers reward the labelled diagram; competitive papers reward knowing which quantity the medium controls.
The single trap that costs the most marks. Treating the drawn curve of a sound wave as the path the particles follow. It is a graph of density or pressure against distance, and the particles move along the wave, not across it. Students who picture air moving up and down go on to misread every sound-wave diagram, and the error survives into Class 11.
A second trap worth naming. Concluding that a denser medium always carries sound faster. Warm air is less dense than cold air and carries sound faster. What governs the speed is the combination of stiffness and density, and a question comparing two gases at different temperatures is testing precisely that.
Key takeaways
Sound production, medium and wave types: quick revision
- Sound is always produced by a vibrating object — tuning fork prongs, a plucked string, a drum membrane, an air column in a flute, or the vocal cords.
- Stop the vibration and the sound stops. A vibrating fork splashes water and can be felt on a fingertip.
- Vibration alone is not enough to be heard — a swinging pendulum vibrates too slowly.
- Bell jar experiment: the bell is loud with air, fainter as air is pumped out, and almost inaudible in near vacuum — while the hammer is still seen striking, proving the source is unchanged.
- Sound needs a material medium; light does not. An explosion in space would be silent.
- Speed order: solids fastest, then liquids, then gases — roughly m/s in iron, m/s in water, m/s in air at room temperature.
- A compression is a region of high density and pressure; a rarefaction is a region of low density and pressure.
- Alternating compressions and rarefactions travel outward, and each particle only oscillates about its position — no matter is transported.
- Sound is therefore a longitudinal, mechanical, pressure wave.
- Longitudinal: particles vibrate parallel to the travel direction, with compressions and rarefactions. Example: sound.
- Transverse: particles vibrate perpendicular, with crests and troughs. Examples: a rope wave, a water ripple, light.
- A slinky shows both — push along its length for longitudinal, flick it sideways for transverse.
- Sound in a gas can only be longitudinal, because a gas cannot resist a sideways disturbance. A solid can carry both, which is why an earthquake sends two kinds of wave.
- The drawn curve of a sound wave is a graph of density against distance, not a picture of particles moving sideways.
- Warm air carries sound faster than cold air, although it is less dense — speed is not decided by density alone.
Strike a tuning fork and find three different ways to show that it is vibrating before the sound dies away — proving the vibration yourself is what makes the rest of this chapter obvious.
- Stop the vibration and the sound stops. A vibrating fork splashes water and can be felt on a fingertip.
- Vibration alone is not enough to be heard — a swinging pendulum vibrates too slowly.
- Bell jar experiment: the bell is loud with air, fainter as air is pumped out, and almost inaudible in near vacuum — while the hammer is still seen striking, proving the source is unchanged.
- Sound needs a material medium; light does not. An explosion in space would be silent.
- Speed order: solids fastest, then liquids, then gases — roughly m/s in iron, m/s in water, m/s in air at room temperature.
- A compression is a region of high density and pressure; a rarefaction is a region of low density and pressure.
- Alternating compressions and rarefactions travel outward, and each particle only oscillates about its position — no matter is transported.
- Sound is therefore a longitudinal, mechanical, pressure wave.
- Longitudinal: particles vibrate parallel to the travel direction, with compressions and rarefactions. Example: sound.
- Transverse: particles vibrate perpendicular, with crests and troughs. Examples: a rope wave, a water ripple, light.
- A slinky shows both — push along its length for longitudinal, flick it sideways for transverse.
- Sound in a gas can only be longitudinal, because a gas cannot resist a sideways disturbance. A solid can carry both, which is why an earthquake sends two kinds of wave.
- The drawn curve of a sound wave is a graph of density against distance, not a picture of particles moving sideways.
- Warm air carries sound faster than cold air, although it is less dense — speed is not decided by density alone.
Strike a tuning fork and find three different ways to show that it is vibrating before the sound dies away — proving the vibration yourself is what makes the rest of this chapter obvious.