You Can Watch a Bell Ring and Hear Nothing at All
Learn how a vibrating body makes sound and why the bell-jar experiment proves a medium is needed, see what compressions and rarefactions really are, define the four wave quantities with units, and apply V equals f lambda.
Why does a ringing bell fall silent inside a sealed jar?
Put an electric bell inside a thick glass jar and switch it on. It rings loudly. Now start pumping the air out.
The sound gets fainter. Keep pumping and it becomes almost inaudible — and through the glass you can still see the hammer striking the gong, as vigorously as ever.
Nothing has gone wrong with the bell. What has been removed is the air, and without it the vibration has nothing to pass its energy to. Sound needs a material medium to travel through, and a vacuum offers none.
Let the air back in and the ringing returns at once.
Notice what the experiment also shows. You could see the hammer the whole time, so light crossed the vacuum perfectly well. Two kinds of wave, one needing a medium and one not — and that single contrast is the reason sound and light are treated as different families of wave.
So sound starts with something vibrating and travels as a disturbance passed from particle to particle. This page covers the first part of the ICSE Class 9 Physics chapter on the propagation of sound waves — how sound is produced, what a longitudinal wave is, the four quantities that describe a wave, and the relation connecting them.
The sound gets fainter. Keep pumping and it becomes almost inaudible — and through the glass you can still see the hammer striking the gong, as vigorously as ever.
Nothing has gone wrong with the bell. What has been removed is the air, and without it the vibration has nothing to pass its energy to. Sound needs a material medium to travel through, and a vacuum offers none.
Let the air back in and the ringing returns at once.
Notice what the experiment also shows. You could see the hammer the whole time, so light crossed the vacuum perfectly well. Two kinds of wave, one needing a medium and one not — and that single contrast is the reason sound and light are treated as different families of wave.
So sound starts with something vibrating and travels as a disturbance passed from particle to particle. This page covers the first part of the ICSE Class 9 Physics chapter on the propagation of sound waves — how sound is produced, what a longitudinal wave is, the four quantities that describe a wave, and the relation connecting them.
How does a vibrating body produce sound?
A vibrating surface pushes the air next to it back and forth, and that push is handed on from particle to particle as a travelling disturbance.
Every source of sound has something moving to and fro, even when the movement is too fast or too small to see:
- A stretched rubber band plucked and released — the vibration is plainly visible
- A tuning fork struck on a rubber pad — its prongs blur, and dipping a prong in water splashes it
- A drum or a tabla — the stretched membrane moves up and down
- A guitar or a sitar string — visibly shimmering when plucked
- Vocal cords in the larynx — put your fingers on your throat while humming and you feel them
- A bell — the metal rim vibrates after the hammer strikes
The test that settles it. Touch a ringing bell or a struck tuning fork and the sound stops almost at once. Your hand has absorbed the energy and stopped the vibration, and with the vibration gone there is nothing to produce sound. Damping the vibration silences the source, which shows that the vibration was the cause and not a side effect.
The bell-jar experiment, set out properly.
- An electric bell is suspended inside a bell jar standing on the plate of a vacuum pump
- Its wires pass out through an airtight seal, so it can be switched on from outside
- With air in the jar the bell is clearly heard
- As the pump removes the air, the sound becomes progressively fainter
- When the air is almost fully removed, the bell is almost inaudible, while the hammer is still seen to strike
- Letting air back in restores the sound
Conclusion: sound requires a material medium and cannot travel through a vacuum. Light, which was visible throughout, does not.
Why "almost" inaudible rather than silent. A real pump never removes every trace of air, and a faint sound also travels through the stand and the wires into the base plate and out. A perfect result would need a perfect vacuum and a bell touching nothing, which is why the observation is described as the sound becoming very faint rather than vanishing.
Sound travels through solids and liquids too, often better than through air. Put your ear to a long metal railing and tap the far end, and the tap arrives clearly. Whales call to each other across water. So "a medium" does not mean "air" — it means any material whose particles can be pushed, and the next part of this chapter compares how well the three states do it.
Every source of sound has something moving to and fro, even when the movement is too fast or too small to see:
- A stretched rubber band plucked and released — the vibration is plainly visible
- A tuning fork struck on a rubber pad — its prongs blur, and dipping a prong in water splashes it
- A drum or a tabla — the stretched membrane moves up and down
- A guitar or a sitar string — visibly shimmering when plucked
- Vocal cords in the larynx — put your fingers on your throat while humming and you feel them
- A bell — the metal rim vibrates after the hammer strikes
The test that settles it. Touch a ringing bell or a struck tuning fork and the sound stops almost at once. Your hand has absorbed the energy and stopped the vibration, and with the vibration gone there is nothing to produce sound. Damping the vibration silences the source, which shows that the vibration was the cause and not a side effect.
The bell-jar experiment, set out properly.
- An electric bell is suspended inside a bell jar standing on the plate of a vacuum pump
- Its wires pass out through an airtight seal, so it can be switched on from outside
- With air in the jar the bell is clearly heard
- As the pump removes the air, the sound becomes progressively fainter
- When the air is almost fully removed, the bell is almost inaudible, while the hammer is still seen to strike
- Letting air back in restores the sound
Conclusion: sound requires a material medium and cannot travel through a vacuum. Light, which was visible throughout, does not.
Why "almost" inaudible rather than silent. A real pump never removes every trace of air, and a faint sound also travels through the stand and the wires into the base plate and out. A perfect result would need a perfect vacuum and a bell touching nothing, which is why the observation is described as the sound becoming very faint rather than vanishing.
Sound travels through solids and liquids too, often better than through air. Put your ear to a long metal railing and tap the far end, and the tap arrives clearly. Whales call to each other across water. So "a medium" does not mean "air" — it means any material whose particles can be pushed, and the next part of this chapter compares how well the three states do it.
What are compressions and rarefactions?
Regions where the particles of the medium are crowded together and regions where they are spread apart — and they travel outward one after another as the wave.
Sound is a longitudinal wave: the particles of the medium vibrate along the same direction that the wave travels. Compare that with a wave on a rope, where the rope moves across the direction of travel — that is a transverse wave.
A compression is a region where the particles are crowded:
- Density is higher than normal
- Pressure is higher than normal
A rarefaction is a region where the particles are spread out:
- Density is lower than normal
- Pressure is lower than normal
How a tuning fork makes them. As a prong swings outward it pushes the air in front of it together, making a compression. As it swings back it leaves room behind, and the air spreads into it, making a rarefaction. One complete vibration of the prong therefore sends out one compression and one rarefaction, and they travel away as a pair.
The wavelength on this picture. The distance between two consecutive compressions — or equally between two consecutive rarefactions — is one wavelength.
A compression and the nearest rarefaction are half a wavelength apart, so a question giving the distance between a compression and the next rarefaction as m is telling you that
Pressure and displacement are out of step, and this is the subtle point. At the centre of a compression or a rarefaction:
- the particle displacement is zero — the particles there are momentarily at their mean positions
- the pressure change is at its maximum
Midway between a compression and a rarefaction the reverse holds: the displacement is maximum and the pressure change is zero. So the pressure wave and the displacement wave describe the same sound a quarter of a wavelength apart, and a diagram of one is not a diagram of the other.
The particles do not travel with the sound. Each one vibrates about its own fixed mean position and hands the disturbance to its neighbour. A shout does not blow air across a room — the air molecules near your mouth stay near your mouth, and only the pattern of crowding travels. That is why sound can cross a hall in a fraction of a second while the air itself barely moves at all.
Sound is a longitudinal wave: the particles of the medium vibrate along the same direction that the wave travels. Compare that with a wave on a rope, where the rope moves across the direction of travel — that is a transverse wave.
A compression is a region where the particles are crowded:
- Density is higher than normal
- Pressure is higher than normal
A rarefaction is a region where the particles are spread out:
- Density is lower than normal
- Pressure is lower than normal
How a tuning fork makes them. As a prong swings outward it pushes the air in front of it together, making a compression. As it swings back it leaves room behind, and the air spreads into it, making a rarefaction. One complete vibration of the prong therefore sends out one compression and one rarefaction, and they travel away as a pair.
The wavelength on this picture. The distance between two consecutive compressions — or equally between two consecutive rarefactions — is one wavelength.
A compression and the nearest rarefaction are half a wavelength apart, so a question giving the distance between a compression and the next rarefaction as m is telling you that
Pressure and displacement are out of step, and this is the subtle point. At the centre of a compression or a rarefaction:
- the particle displacement is zero — the particles there are momentarily at their mean positions
- the pressure change is at its maximum
Midway between a compression and a rarefaction the reverse holds: the displacement is maximum and the pressure change is zero. So the pressure wave and the displacement wave describe the same sound a quarter of a wavelength apart, and a diagram of one is not a diagram of the other.
The particles do not travel with the sound. Each one vibrates about its own fixed mean position and hands the disturbance to its neighbour. A shout does not blow air across a room — the air molecules near your mouth stay near your mouth, and only the pattern of crowding travels. That is why sound can cross a hall in a fraction of a second while the air itself barely moves at all.
Formula
What do wavelength, amplitude, frequency and time period mean?
Four quantities describe any wave completely, and each has its own SI unit.
- **Wavelength () — the distance between two consecutive points in the same phase, such as two successive compressions. SI unit the metre (m)
- Amplitude () — the maximum displacement of a particle of the medium from its mean position. SI unit the metre (m)
- Frequency () — the number of complete vibrations per second. SI unit the hertz** (Hz), which is
- **Time period () — the time taken for one complete vibration. SI unit the second (s)
Frequency and time period are reciprocals:**
Worked examples. Hz gives s. s gives Hz. Hz gives s.
The wave relation. In one time period the wave advances exactly one wavelength, so
Worked example 1 — finding the speed. A wave of frequency Hz has a wavelength of m:
Worked example 2 — finding the wavelength. Sound of frequency Hz travels at :
Worked example 3 — finding the frequency. A wave of wavelength m travels at :
Worked example 4 — a tuning fork. A fork of Hz sounds in air at :
Worked example 5 — the two ends of the audible range. In air at :
- at Hz: m
- at Hz: cm
So the sounds a human ear can hear span wavelengths from about ** cm to m — a range of a thousand to one.
Worked example 6 — from the period.** A wave has s, so Hz, and in air
Amplitude is the odd one out. Wavelength, frequency and speed are tied together by , and amplitude appears nowhere in it. Changing the amplitude changes how loud the sound is and leaves its speed, its frequency and its wavelength untouched — which is why a shout and a whisper of the same note reach you together.
When sound passes from one medium to another, the frequency does NOT change. The frequency is set by the source, and the source is unaltered. The speed changes, so by the wavelength must change with it.
Worked example 7. A Hz fork sounds into water, where sound travels at about :
against m in air. Same frequency, different wavelength — and treating the wavelength as the fixed property is the standard error in these questions.
- **Wavelength () — the distance between two consecutive points in the same phase, such as two successive compressions. SI unit the metre (m)
- Amplitude () — the maximum displacement of a particle of the medium from its mean position. SI unit the metre (m)
- Frequency () — the number of complete vibrations per second. SI unit the hertz** (Hz), which is
- **Time period () — the time taken for one complete vibration. SI unit the second (s)
Frequency and time period are reciprocals:**
Worked examples. Hz gives s. s gives Hz. Hz gives s.
The wave relation. In one time period the wave advances exactly one wavelength, so
Worked example 1 — finding the speed. A wave of frequency Hz has a wavelength of m:
Worked example 2 — finding the wavelength. Sound of frequency Hz travels at :
Worked example 3 — finding the frequency. A wave of wavelength m travels at :
Worked example 4 — a tuning fork. A fork of Hz sounds in air at :
Worked example 5 — the two ends of the audible range. In air at :
- at Hz: m
- at Hz: cm
So the sounds a human ear can hear span wavelengths from about ** cm to m — a range of a thousand to one.
Worked example 6 — from the period.** A wave has s, so Hz, and in air
Amplitude is the odd one out. Wavelength, frequency and speed are tied together by , and amplitude appears nowhere in it. Changing the amplitude changes how loud the sound is and leaves its speed, its frequency and its wavelength untouched — which is why a shout and a whisper of the same note reach you together.
When sound passes from one medium to another, the frequency does NOT change. The frequency is set by the source, and the source is unaltered. The speed changes, so by the wavelength must change with it.
Worked example 7. A Hz fork sounds into water, where sound travels at about :
against m in air. Same frequency, different wavelength — and treating the wavelength as the fixed property is the standard error in these questions.
Exam tip
Exam tip: keep frequency fixed and let the wavelength change
**Use ** and rearrange rather than remembering three formulas: and .
Frequency is set by the SOURCE and does not change between media. The speed changes, so the wavelength changes.
Give units every time: in metres, in hertz, in seconds, in .
**** — check it as a free verification: Hz means s.
**Take in air unless the question states otherwise, and say which value you used.
A compression to the next rarefaction is HALF a wavelength**, so double that distance to get .
Compression means high pressure AND high density; rarefaction means both are low. Give both.
At the centre of a compression the displacement is zero and the pressure change is maximum — the two waves are a quarter wavelength apart.
Amplitude decides loudness only — it does not appear in .
For the bell jar, give the observation before the conclusion: the sound fades, the hammer is still seen, so sound needs a medium and light does not.
And say the bell becomes almost inaudible — a real pump leaves a little air, and some sound travels through the stand.
Frequency is set by the SOURCE and does not change between media. The speed changes, so the wavelength changes.
Give units every time: in metres, in hertz, in seconds, in .
**** — check it as a free verification: Hz means s.
**Take in air unless the question states otherwise, and say which value you used.
A compression to the next rarefaction is HALF a wavelength**, so double that distance to get .
Compression means high pressure AND high density; rarefaction means both are low. Give both.
At the centre of a compression the displacement is zero and the pressure change is maximum — the two waves are a quarter wavelength apart.
Amplitude decides loudness only — it does not appear in .
For the bell jar, give the observation before the conclusion: the sound fades, the hammer is still seen, so sound needs a medium and light does not.
And say the bell becomes almost inaudible — a real pump leaves a little air, and some sound travels through the stand.
Did you know
Why the particles of air barely move when you shout
Shout across a room and the sound arrives in a fraction of a second. It is tempting to picture a puff of air travelling from your mouth to the listener's ear.
Nothing of the kind happens. The air molecules near your mouth stay near your mouth. Each one vibrates back and forth about its own position by a tiny distance and passes the disturbance to its neighbour, which passes it on again.
A useful comparison is a line of people standing shoulder to shoulder. Give one end a shove and everybody sways and steadies in turn, so the sway travels right down the line at speed. Nobody has walked anywhere.
The amplitude of an ordinary conversation moves air molecules by an almost unimaginably small distance — very much less than the thickness of a sheet of paper — while the pattern of crowding races along at about .
This is why sound and wind are quite separate things. A gale is air genuinely travelling from one place to another; a shout is a pattern travelling through air that stays put. You can hear someone perfectly well upwind of you, which would be impossible if hearing depended on air arriving.
And it explains why a candle flame in front of a loudspeaker flickers rather than being blown out. The flame is nudged one way and then the other, many times a second, and ends up roughly where it started.
The energy travels and the matter does not — and that is the defining feature of a wave. It is equally true of a ripple on a pond, where the water does not flow outward, and of a wave on a rope, where the rope stays in your hand.
Nothing of the kind happens. The air molecules near your mouth stay near your mouth. Each one vibrates back and forth about its own position by a tiny distance and passes the disturbance to its neighbour, which passes it on again.
A useful comparison is a line of people standing shoulder to shoulder. Give one end a shove and everybody sways and steadies in turn, so the sway travels right down the line at speed. Nobody has walked anywhere.
The amplitude of an ordinary conversation moves air molecules by an almost unimaginably small distance — very much less than the thickness of a sheet of paper — while the pattern of crowding races along at about .
This is why sound and wind are quite separate things. A gale is air genuinely travelling from one place to another; a shout is a pattern travelling through air that stays put. You can hear someone perfectly well upwind of you, which would be impossible if hearing depended on air arriving.
And it explains why a candle flame in front of a loudspeaker flickers rather than being blown out. The flame is nudged one way and then the other, many times a second, and ends up roughly where it started.
The energy travels and the matter does not — and that is the defining feature of a wave. It is equally true of a ripple on a pond, where the water does not flow outward, and of a wave on a rope, where the rope stays in your hand.
Exam relevance
How does the propagation of sound feed into JEE Main and NEET?
Because is used in every wave chapter that follows, and the longitudinal-against-transverse distinction organises the whole of wave physics.
This is the foundation for Class 11 Physics Waves, examined in JEE Main and NEET. The relation is carried forward unchanged and joined by the wave equation
where is the amplitude of this page, and . The four quantities defined here are the four constants in that equation, and a question giving and and asking for the speed is asking for , which is again.
The pressure-against-displacement quarter-wavelength shift becomes a formal result. Class 11 shows that the pressure wave in a gas leads the displacement wave by , so where displacement is maximum the pressure change is zero — exactly the statement made on this page. Assertion-reason questions on it appear in both papers, and the diagram-based version asks which curve is which.
Standing waves and resonance are the main extension. An organ pipe and a vibrating string set up compressions and rarefactions that do not travel, and the positions where the pressure change is maximum are displacement nodes — so the distinction drawn here decides where a node sits. Numericals on the frequency of a pipe are recurring JEE Main material.
The frequency-stays-fixed rule is used constantly. When sound crosses from air into water, or light from air into glass, the frequency is set by the source and the wavelength adjusts to the new speed. That is the same reasoning as the refractive index in Class 12 optics, and the worked water example on this page is its acoustic twin.
The Doppler effect in Class 11 is the one case where the observed frequency does change, and it changes because the source or the observer moves — not because the medium changed. Keeping those two situations apart is what the rule on this page makes possible.
For NEET Biology, the ear is examined as a sense organ: the eardrum responds to the pressure variations described here, and the cochlea separates frequencies. Questions on the audible range and on how the ear detects pitch and loudness sit in that chapter, and pitch is frequency while loudness is amplitude.
What the questions look like. For board work, expect explain how sound is produced, describe the bell-jar experiment with its conclusion, define compression and rarefaction in terms of pressure and density, define the four quantities with units, and **numericals on in all three rearrangements. For JEE Main and NEET, expect the wave equation, standing waves, the Doppler effect and pressure-displacement questions.
How board and competitive emphasis differ. A board paper rewards the stated observation before the conclusion** in the bell-jar answer and the units on every definition. A competitive paper assumes all of it and works from and rather than from and .
The single trap that costs the most marks. Treating the wavelength as unchanged when sound enters a new medium. A Hz fork gives m in air and about m in water, because the frequency is what stays fixed and the speed changed. The defence is to write down which quantity the source controls before substituting — frequency from the source, speed from the medium, and wavelength as whatever then requires.
This is the foundation for Class 11 Physics Waves, examined in JEE Main and NEET. The relation is carried forward unchanged and joined by the wave equation
where is the amplitude of this page, and . The four quantities defined here are the four constants in that equation, and a question giving and and asking for the speed is asking for , which is again.
The pressure-against-displacement quarter-wavelength shift becomes a formal result. Class 11 shows that the pressure wave in a gas leads the displacement wave by , so where displacement is maximum the pressure change is zero — exactly the statement made on this page. Assertion-reason questions on it appear in both papers, and the diagram-based version asks which curve is which.
Standing waves and resonance are the main extension. An organ pipe and a vibrating string set up compressions and rarefactions that do not travel, and the positions where the pressure change is maximum are displacement nodes — so the distinction drawn here decides where a node sits. Numericals on the frequency of a pipe are recurring JEE Main material.
The frequency-stays-fixed rule is used constantly. When sound crosses from air into water, or light from air into glass, the frequency is set by the source and the wavelength adjusts to the new speed. That is the same reasoning as the refractive index in Class 12 optics, and the worked water example on this page is its acoustic twin.
The Doppler effect in Class 11 is the one case where the observed frequency does change, and it changes because the source or the observer moves — not because the medium changed. Keeping those two situations apart is what the rule on this page makes possible.
For NEET Biology, the ear is examined as a sense organ: the eardrum responds to the pressure variations described here, and the cochlea separates frequencies. Questions on the audible range and on how the ear detects pitch and loudness sit in that chapter, and pitch is frequency while loudness is amplitude.
What the questions look like. For board work, expect explain how sound is produced, describe the bell-jar experiment with its conclusion, define compression and rarefaction in terms of pressure and density, define the four quantities with units, and **numericals on in all three rearrangements. For JEE Main and NEET, expect the wave equation, standing waves, the Doppler effect and pressure-displacement questions.
How board and competitive emphasis differ. A board paper rewards the stated observation before the conclusion** in the bell-jar answer and the units on every definition. A competitive paper assumes all of it and works from and rather than from and .
The single trap that costs the most marks. Treating the wavelength as unchanged when sound enters a new medium. A Hz fork gives m in air and about m in water, because the frequency is what stays fixed and the speed changed. The defence is to write down which quantity the source controls before substituting — frequency from the source, speed from the medium, and wavelength as whatever then requires.
Key takeaways
Sound production, longitudinal waves and V = f lambda: quick revision
- Sound is produced by a vibrating body — a rubber band, a tuning fork, a drum membrane, a string, vocal cords, a bell.
- Touching a ringing source stops the sound, which shows the vibration is the cause.
- Bell-jar experiment: a bell in a jar on a vacuum pump grows fainter as air is removed and is almost inaudible in a near vacuum, while the hammer is still seen to strike. Letting air in restores it.
- Conclusion: sound needs a material medium; light does not.
- It is almost inaudible because a real pump leaves some air and some sound passes through the stand and wires.
- Sound travels through solids and liquids too, often better than through air.
- Sound is a LONGITUDINAL wave — particles vibrate along the direction of travel, unlike a transverse wave on a rope.
- Compression: particles crowded, high density, high pressure. Rarefaction: particles spread, low density, low pressure.
- One wavelength is the distance between two consecutive compressions or two consecutive rarefactions.
- A compression to the next rarefaction is half a wavelength — m apart means m.
- At the centre of a compression the displacement is zero and the pressure change is maximum; midway between, the reverse. The two waves are a quarter wavelength apart.
- The particles do not travel with the wave — only the pattern of crowding does, which is why a shout does not blow air across a room.
- **Wavelength () in m; amplitude () in m; frequency () in Hz; time period () in s.
- **: Hz gives s; s gives Hz.
- ****, since the wave advances one wavelength in one period.
- Hz with m gives ; Hz at gives m; m gives Hz.
- A Hz fork in air gives m.
- The audible range corresponds to wavelengths from ** cm** ( kHz) to ** m** ( Hz).
- **Amplitude does not appear in — it sets the loudness only, so a shout and a whisper of the same note arrive together.
- Frequency is fixed by the source and does not change between media; the speed changes and so the wavelength** changes. In water the Hz fork gives m.
Strike a spoon against a table and then again while holding the handle tightly, and notice how much sooner the second sound dies — you have just damped a vibration.
- Touching a ringing source stops the sound, which shows the vibration is the cause.
- Bell-jar experiment: a bell in a jar on a vacuum pump grows fainter as air is removed and is almost inaudible in a near vacuum, while the hammer is still seen to strike. Letting air in restores it.
- Conclusion: sound needs a material medium; light does not.
- It is almost inaudible because a real pump leaves some air and some sound passes through the stand and wires.
- Sound travels through solids and liquids too, often better than through air.
- Sound is a LONGITUDINAL wave — particles vibrate along the direction of travel, unlike a transverse wave on a rope.
- Compression: particles crowded, high density, high pressure. Rarefaction: particles spread, low density, low pressure.
- One wavelength is the distance between two consecutive compressions or two consecutive rarefactions.
- A compression to the next rarefaction is half a wavelength — m apart means m.
- At the centre of a compression the displacement is zero and the pressure change is maximum; midway between, the reverse. The two waves are a quarter wavelength apart.
- The particles do not travel with the wave — only the pattern of crowding does, which is why a shout does not blow air across a room.
- **Wavelength () in m; amplitude () in m; frequency () in Hz; time period () in s.
- **: Hz gives s; s gives Hz.
- ****, since the wave advances one wavelength in one period.
- Hz with m gives ; Hz at gives m; m gives Hz.
- A Hz fork in air gives m.
- The audible range corresponds to wavelengths from ** cm** ( kHz) to ** m** ( Hz).
- **Amplitude does not appear in — it sets the loudness only, so a shout and a whisper of the same note arrive together.
- Frequency is fixed by the source and does not change between media; the speed changes and so the wavelength** changes. In water the Hz fork gives m.
Strike a spoon against a table and then again while holding the handle tightly, and notice how much sooner the second sound dies — you have just damped a vibration.