Strike a Tabla Harder and the Note Stays the Same, Only Louder
Learn how a sound wave carries energy without moving matter, how to read compressions and wavelength off a graph, how to use speed equals frequency times wavelength, and what sets pitch and loudness.
Why does hitting a drum harder change the loudness but not the note?
Because loudness and pitch are controlled by two different properties of the wave.
Strike a tabla gently and it gives a note. Strike the same spot harder and you hear the same note, more loudly. Nothing about the pitch has changed.
Hitting harder puts more energy into the membrane, so it swings further from its resting position — a larger amplitude, which your ear registers as greater loudness. But it swings back and forth the same number of times each second, so the frequency is unchanged, and frequency is what sets the pitch.
To change the note you must change the frequency — tighten the skin, or press it, or play a differently tuned drum.
So amplitude gives loudness and frequency gives pitch, and the two are independent. This page covers the second part of the CBSE Class 9 Science chapter on sound waves.
Strike a tabla gently and it gives a note. Strike the same spot harder and you hear the same note, more loudly. Nothing about the pitch has changed.
Hitting harder puts more energy into the membrane, so it swings further from its resting position — a larger amplitude, which your ear registers as greater loudness. But it swings back and forth the same number of times each second, so the frequency is unchanged, and frequency is what sets the pitch.
To change the note you must change the frequency — tighten the skin, or press it, or play a differently tuned drum.
So amplitude gives loudness and frequency gives pitch, and the two are independent. This page covers the second part of the CBSE Class 9 Science chapter on sound waves.
How does a sound wave carry energy without carrying matter?
Each particle does work on the next and then returns to where it started.
A vibrating source pushes the particles beside it, giving them energy. Those particles push their neighbours, passing the energy on, and spring back to their original positions. The energy moves steadily outward; the particles go nowhere.
The evidence that energy has arrived. A loud sound can rattle a window pane, and at a concert you can feel the bass in your chest. Something has done work on the glass and on you, so energy has travelled from the source. That energy came from the person who struck the drum or plucked the string.
The evidence that matter has not. Hold a candle flame in front of a loudspeaker playing loudly. The flame flickers back and forth — it does not get blown steadily sideways. The air is oscillating in place, not flowing across the room.
Everyday confirmation. A conversation across a room does not produce a draught, however animated. A temple bell can be heard a long way off without any air being transported that distance.
Where the energy finally goes. It spreads out over an ever larger area as the wave travels, which is why a sound gets fainter with distance, and it is gradually absorbed by the medium and by every surface it meets, ending as a very small amount of heat.
This is the definition of a wave, not a fact about sound in particular. A wave is a disturbance that transfers energy through a medium without transferring the medium itself. A water ripple carries energy across a pond while a floating leaf merely bobs; a rope wave carries energy along the rope while each bit of rope stays put. Sound is one member of a family, and the family is defined by this property — which is exactly the point the previous part of this chapter established with the slinky.
A vibrating source pushes the particles beside it, giving them energy. Those particles push their neighbours, passing the energy on, and spring back to their original positions. The energy moves steadily outward; the particles go nowhere.
The evidence that energy has arrived. A loud sound can rattle a window pane, and at a concert you can feel the bass in your chest. Something has done work on the glass and on you, so energy has travelled from the source. That energy came from the person who struck the drum or plucked the string.
The evidence that matter has not. Hold a candle flame in front of a loudspeaker playing loudly. The flame flickers back and forth — it does not get blown steadily sideways. The air is oscillating in place, not flowing across the room.
Everyday confirmation. A conversation across a room does not produce a draught, however animated. A temple bell can be heard a long way off without any air being transported that distance.
Where the energy finally goes. It spreads out over an ever larger area as the wave travels, which is why a sound gets fainter with distance, and it is gradually absorbed by the medium and by every surface it meets, ending as a very small amount of heat.
This is the definition of a wave, not a fact about sound in particular. A wave is a disturbance that transfers energy through a medium without transferring the medium itself. A water ripple carries energy across a pond while a floating leaf merely bobs; a rope wave carries energy along the rope while each bit of rope stays put. Sound is one member of a family, and the family is defined by this property — which is exactly the point the previous part of this chapter established with the slinky.
How do you read a sound wave off its graph?
The graph plots density or pressure against distance, and every quantity in the chapter can be read from it.
- A crest of the curve is a compression — maximum density and pressure
- A trough is a rarefaction — minimum density and pressure
- The horizontal line through the middle is the undisturbed density of the medium
Wavelength, written with the symbol lambda, is the distance between two consecutive compressions — or equally between two consecutive rarefactions. It is measured in metres.
Amplitude is the maximum change in density or pressure from the undisturbed value — the height of a crest above the middle line.
Frequency is the number of complete oscillations per second, measured in hertz (Hz). One hertz is one oscillation per second.
Time period is the time for one complete oscillation, in seconds, and it is the reciprocal of the frequency:
Worked example. A wave has a time period of s. Then
and conversely a frequency of Hz means a period of s.
Where each property comes from. The frequency is set entirely by the source — how fast the object vibrates. The amplitude is set by how much energy the source was given. Neither is a property of the air.
The graph is not a picture of the air moving up and down. The curve rising above the line means the density there is higher than normal, not that anything has moved upwards. Sound is longitudinal, as the previous part of this chapter showed, and the particles move along the direction of travel. The drawing is a plot, and reading it as a photograph is the commonest error in the chapter.
A useful check on any diagram question. The wavelength must be measured crest to crest or trough to trough, never crest to trough — that distance is only half a wavelength. Marking it wrongly halves every answer that follows from it.
- A crest of the curve is a compression — maximum density and pressure
- A trough is a rarefaction — minimum density and pressure
- The horizontal line through the middle is the undisturbed density of the medium
Wavelength, written with the symbol lambda, is the distance between two consecutive compressions — or equally between two consecutive rarefactions. It is measured in metres.
Amplitude is the maximum change in density or pressure from the undisturbed value — the height of a crest above the middle line.
Frequency is the number of complete oscillations per second, measured in hertz (Hz). One hertz is one oscillation per second.
Time period is the time for one complete oscillation, in seconds, and it is the reciprocal of the frequency:
Worked example. A wave has a time period of s. Then
and conversely a frequency of Hz means a period of s.
Where each property comes from. The frequency is set entirely by the source — how fast the object vibrates. The amplitude is set by how much energy the source was given. Neither is a property of the air.
The graph is not a picture of the air moving up and down. The curve rising above the line means the density there is higher than normal, not that anything has moved upwards. Sound is longitudinal, as the previous part of this chapter showed, and the particles move along the direction of travel. The drawing is a plot, and reading it as a photograph is the commonest error in the chapter.
A useful check on any diagram question. The wavelength must be measured crest to crest or trough to trough, never crest to trough — that distance is only half a wavelength. Marking it wrongly halves every answer that follows from it.
Formula
How do you use the relation between speed, frequency and wavelength?
where is the speed in m/s, the frequency in Hz and lambda the wavelength in metres. Combined with , that one relation answers every numerical in this section.
Why it is true. In one second the source completes oscillations, and each sends out one full wave of length lambda. So the front of the wave has moved times lambda in that second — which is its speed.
Worked example 1 — finding the speed. A tuning fork of frequency Hz produces sound of wavelength m.
Worked example 2 — finding the wavelength. Taking the speed of sound in air as m/s, what wavelengths correspond to the two ends of the audible range?
So audible sound spans wavelengths from m down to under cm — a range of a thousand to one.
Worked example 3 — via the time period. A wave has a period of s and travels at m/s.
Worked example 4 — counting vibrations. A source completes vibrations in s.
At Hz this is below the audible range, so it would not be heard however close you stood.
Worked example 5 — the same note in two media. A source of frequency Hz is sounded in air, where the speed is m/s, and in water, where it is m/s.
The frequency did not change and the wavelength did. Frequency is fixed by the source, so it is carried unchanged into a new medium. The speed is fixed by the medium. So when sound passes from air into water, the wavelength must adjust to keep true — and it stretches in exactly the ratio of the speeds, here , which is the same as .
Worked example 6. A wave of frequency Hz travels at m/s along a stretched wire.
Convert units before substituting. A wavelength given in centimetres must become metres, and a frequency in kilohertz must become hertz — kHz is Hz. Substituting mixed units is the standard arithmetic error here, and it is worth writing each quantity with its unit on a separate line before using the formula.
What decides the pitch and the loudness of a sound?
Pitch comes from frequency and loudness from amplitude. They are independent, and that is the whole point of this section.
Pitch is how high or low a note sounds.
- Higher frequency means higher pitch, heard as shriller
- Lower frequency means lower pitch, heard as deeper
- A mosquito's whine is high-pitched; a lion's roar is low-pitched
- A child's voice is generally higher-pitched than an adult man's, because the vocal cords are shorter and vibrate faster
Loudness is how strong the sound seems.
- Larger amplitude means louder
- A larger amplitude means the source was given more energy, so the wave carries more energy to your ear
Quality, also called timbre, is what distinguishes two sounds of the same pitch and loudness. A sitar and a flute playing the same note at the same volume still sound different, and it is why you can recognise a familiar voice on a telephone.
The audible range. Human ears respond to frequencies from about ** Hz to Hz**. Outside it:
- Below Hz is infrasonic
- Above Hz is ultrasonic
Both are real sounds; they are simply outside the range human ears detect, and the third part of this chapter uses both.
Everyday evidence that the two are independent. A tabla struck harder gives the same note louder — frequency unchanged, amplitude increased. Two strings of a sitar plucked with equal force give different notes at similar loudness — amplitude similar, frequency different. You can vary either one while holding the other fixed, which is exactly what independent means.
Pitch and loudness are the commonest pair of terms to be swapped. Fix them by their cause: frequency sets pitch, amplitude sets loudness. A question about tightening a string or changing the length of an air column is a pitch question; a question about striking or blowing harder is a loudness question.
Loudness is not the same as intensity. Intensity is a physical quantity — the sound energy passing through unit area each second — and it can be measured with an instrument. Loudness is how strong that sound seems to a listener, and it depends on the ear as well as on the wave. Two people can receive the same intensity and report different loudness, which is why the syllabus keeps the two words apart.
Pitch is how high or low a note sounds.
- Higher frequency means higher pitch, heard as shriller
- Lower frequency means lower pitch, heard as deeper
- A mosquito's whine is high-pitched; a lion's roar is low-pitched
- A child's voice is generally higher-pitched than an adult man's, because the vocal cords are shorter and vibrate faster
Loudness is how strong the sound seems.
- Larger amplitude means louder
- A larger amplitude means the source was given more energy, so the wave carries more energy to your ear
Quality, also called timbre, is what distinguishes two sounds of the same pitch and loudness. A sitar and a flute playing the same note at the same volume still sound different, and it is why you can recognise a familiar voice on a telephone.
The audible range. Human ears respond to frequencies from about ** Hz to Hz**. Outside it:
- Below Hz is infrasonic
- Above Hz is ultrasonic
Both are real sounds; they are simply outside the range human ears detect, and the third part of this chapter uses both.
Everyday evidence that the two are independent. A tabla struck harder gives the same note louder — frequency unchanged, amplitude increased. Two strings of a sitar plucked with equal force give different notes at similar loudness — amplitude similar, frequency different. You can vary either one while holding the other fixed, which is exactly what independent means.
Pitch and loudness are the commonest pair of terms to be swapped. Fix them by their cause: frequency sets pitch, amplitude sets loudness. A question about tightening a string or changing the length of an air column is a pitch question; a question about striking or blowing harder is a loudness question.
Loudness is not the same as intensity. Intensity is a physical quantity — the sound energy passing through unit area each second — and it can be measured with an instrument. Loudness is how strong that sound seems to a listener, and it depends on the ear as well as on the wave. Two people can receive the same intensity and report different loudness, which is why the syllabus keeps the two words apart.
Exam tip
Exam tip: measure the wavelength crest to crest
** and answer every numerical here. Write each quantity with its unit on a separate line before substituting.
Convert first**: centimetres to metres, kilohertz to hertz. kHz is Hz.
On a graph, a crest is a compression and a trough is a rarefaction. Measure the wavelength crest to crest or trough to trough — crest to trough is only half a wavelength.
Amplitude is the maximum change from the undisturbed line, and it is set by the energy given to the source.
The graph plots density against distance — it is not a picture of air moving sideways.
Frequency is fixed by the source; speed is fixed by the medium. So when sound enters a new medium the wavelength changes and the frequency does not. Hz gives m in air and m in water.
Frequency sets pitch; amplitude sets loudness. Tightening a string changes the pitch; striking harder changes the loudness.
Name quality as the third characteristic, which distinguishes two instruments playing the same note.
The **audible range is Hz to Hz — below is infrasonic, above is ultrasonic.
And distinguish loudness from intensity** — one is perceived, the other measured.
Convert first**: centimetres to metres, kilohertz to hertz. kHz is Hz.
On a graph, a crest is a compression and a trough is a rarefaction. Measure the wavelength crest to crest or trough to trough — crest to trough is only half a wavelength.
Amplitude is the maximum change from the undisturbed line, and it is set by the energy given to the source.
The graph plots density against distance — it is not a picture of air moving sideways.
Frequency is fixed by the source; speed is fixed by the medium. So when sound enters a new medium the wavelength changes and the frequency does not. Hz gives m in air and m in water.
Frequency sets pitch; amplitude sets loudness. Tightening a string changes the pitch; striking harder changes the loudness.
Name quality as the third characteristic, which distinguishes two instruments playing the same note.
The **audible range is Hz to Hz — below is infrasonic, above is ultrasonic.
And distinguish loudness from intensity** — one is perceived, the other measured.
Did you know
Why a shout carries further than a whisper but not further than a horn
A shout carries further than a whisper because it has a larger amplitude, and therefore more energy to spend crossing the distance. That much is straightforward. What is less obvious is why a vehicle horn cuts through a noisy street far better than a shout of the same loudness.
Part of the answer is frequency. Human hearing is not equally sensitive across its whole range — it responds most readily to frequencies in the middle of the range, around those of speech, and less readily to very low and very high notes. A horn is deliberately pitched where the ear is most sensitive, so it seems louder than its energy alone would suggest.
That is why loudness and intensity had to be kept apart in the last section. The horn and the shout can deliver the same energy per second to your ear and still not seem equally loud, because loudness involves the ear's response as well as the wave.
The same reasoning explains a design choice in emergency sirens. A single steady note can be masked by traffic noise at a similar frequency, so sirens sweep up and down through a range instead. Whatever the background noise happens to be, some part of the sweep stands clear of it.
And it explains why an alarm clock is shrill rather than deep. A low note needs a large object to produce it — long organ pipes and big drums make low sounds — while a small buzzer can only manage a high frequency, and high frequencies happen to be very hard to ignore.
So carrying power is not amplitude alone. It is amplitude, frequency and the ear's own response taken together — which is why a whisper at the right frequency close by is perfectly clear, and a shout across a noisy hall may not be.
Part of the answer is frequency. Human hearing is not equally sensitive across its whole range — it responds most readily to frequencies in the middle of the range, around those of speech, and less readily to very low and very high notes. A horn is deliberately pitched where the ear is most sensitive, so it seems louder than its energy alone would suggest.
That is why loudness and intensity had to be kept apart in the last section. The horn and the shout can deliver the same energy per second to your ear and still not seem equally loud, because loudness involves the ear's response as well as the wave.
The same reasoning explains a design choice in emergency sirens. A single steady note can be masked by traffic noise at a similar frequency, so sirens sweep up and down through a range instead. Whatever the background noise happens to be, some part of the sweep stands clear of it.
And it explains why an alarm clock is shrill rather than deep. A low note needs a large object to produce it — long organ pipes and big drums make low sounds — while a small buzzer can only manage a high frequency, and high frequencies happen to be very hard to ignore.
So carrying power is not amplitude alone. It is amplitude, frequency and the ear's own response taken together — which is why a whisper at the right frequency close by is perfectly clear, and a shout across a noisy hall may not be.
Exam relevance
How is the wave relation tested in JEE Main and NEET?
Because is used in every wave chapter for the next three years, and this page is where it is first applied to numbers.
This is the foundation for the Class 11 Physics chapter Waves, examined in JEE Main and in NEET Physics. That chapter writes a wave as a function of position and time, defines the angular frequency and the wave number, and shows that follows from them. The relation itself is unchanged, and so are all the definitions on this page.
The most reused idea here is the last one. That frequency is fixed by the source while speed is fixed by the medium, so the wavelength changes when a wave crosses into a new medium, is examined again and again. In Class 11 it explains what happens to a sound wave entering water; in Class 12 Wave Optics the identical argument explains why light's wavelength shortens inside glass while its frequency stays the same, which is the basis of refraction. A student who settles this here meets refraction as familiar rather than new.
Pitch and loudness become frequency and intensity treated quantitatively, with intensity measured in watts per square metre and loudness on a logarithmic scale. The Class 9 distinction between the perceived and the measured quantity is exactly what that scale exists for.
The audible range leads into the ultrasonic applications of the next part of this chapter and into Class 11 discussions of the Doppler effect, where an approaching source raises the received frequency.
What the questions look like. Numericals dominate, and the standard shapes are find the wavelength given the speed and frequency, find the frequency from a time period, and find the new wavelength when the medium changes. Assertion-reason items favour two statements from this page: that frequency does not change when a wave enters a new medium, and that loudness depends on amplitude while pitch depends on frequency. Graph-based questions ask you to read a wavelength or an amplitude from a drawn wave.
How board and competitive emphasis differ. A board paper asks you to define wavelength, amplitude, frequency and time period, state the audible range, and compute one substitution into . A competitive paper sets the medium-change problem, or asks which quantity stays constant when a wave refracts — so the reasoning about what the source controls and what the medium controls matters more than the formula.
The single trap that costs the most marks. Assuming the frequency changes when sound enters a new medium. It does not; the wavelength does. Students who think otherwise get every medium-change numerical wrong, and the same error reappears in optics.
A second trap worth naming. Measuring the wavelength from a crest to the next trough. That is half a wavelength, so every answer built on it is out by a factor of two. Mark two consecutive crests on the diagram before measuring anything, and the mistake becomes impossible.
This is the foundation for the Class 11 Physics chapter Waves, examined in JEE Main and in NEET Physics. That chapter writes a wave as a function of position and time, defines the angular frequency and the wave number, and shows that follows from them. The relation itself is unchanged, and so are all the definitions on this page.
The most reused idea here is the last one. That frequency is fixed by the source while speed is fixed by the medium, so the wavelength changes when a wave crosses into a new medium, is examined again and again. In Class 11 it explains what happens to a sound wave entering water; in Class 12 Wave Optics the identical argument explains why light's wavelength shortens inside glass while its frequency stays the same, which is the basis of refraction. A student who settles this here meets refraction as familiar rather than new.
Pitch and loudness become frequency and intensity treated quantitatively, with intensity measured in watts per square metre and loudness on a logarithmic scale. The Class 9 distinction between the perceived and the measured quantity is exactly what that scale exists for.
The audible range leads into the ultrasonic applications of the next part of this chapter and into Class 11 discussions of the Doppler effect, where an approaching source raises the received frequency.
What the questions look like. Numericals dominate, and the standard shapes are find the wavelength given the speed and frequency, find the frequency from a time period, and find the new wavelength when the medium changes. Assertion-reason items favour two statements from this page: that frequency does not change when a wave enters a new medium, and that loudness depends on amplitude while pitch depends on frequency. Graph-based questions ask you to read a wavelength or an amplitude from a drawn wave.
How board and competitive emphasis differ. A board paper asks you to define wavelength, amplitude, frequency and time period, state the audible range, and compute one substitution into . A competitive paper sets the medium-change problem, or asks which quantity stays constant when a wave refracts — so the reasoning about what the source controls and what the medium controls matters more than the formula.
The single trap that costs the most marks. Assuming the frequency changes when sound enters a new medium. It does not; the wavelength does. Students who think otherwise get every medium-change numerical wrong, and the same error reappears in optics.
A second trap worth naming. Measuring the wavelength from a crest to the next trough. That is half a wavelength, so every answer built on it is out by a factor of two. Mark two consecutive crests on the diagram before measuring anything, and the mistake becomes impossible.
Key takeaways
Wave properties, the wave relation, pitch and loudness: quick revision
- A wave transfers energy without transferring matter. A loud sound rattles a window (energy arrives) and a candle flame in front of a speaker only flickers (no air flows).
- On a graph of density against distance: a crest is a compression, a trough is a rarefaction, and the middle line is the undisturbed density.
- Wavelength is the distance between two consecutive compressions, in metres. Amplitude is the maximum change from the undisturbed value.
- Frequency is oscillations per second in hertz; time period is in seconds. A period of s means Hz.
- The graph is a plot, not a picture — sound is longitudinal and the particles move along the wave.
- Measure the wavelength crest to crest — crest to trough is only half.
- ****, because the source sends out complete waves each second.
- Hz with m gives m/s.
- At m/s, the audible range spans m down to cm.
- A period of s at m/s gives Hz and m. And vibrations in s gives Hz, which is inaudible.
- Frequency is set by the source; speed by the medium. So Hz gives m in air and m in water — the wavelength changes, not the frequency.
- Hz at m/s gives m. Convert units before substituting — kHz is Hz.
- Pitch depends on frequency: higher frequency means shriller. Loudness depends on amplitude: bigger swing means louder.
- The two are independent — a tabla struck harder gives the same note louder.
- Quality (timbre) distinguishes two instruments playing the same note at the same loudness.
- **Audible range: Hz to Hz. Below is infrasonic, above ultrasonic.
- Loudness is perceived; intensity is measured.** The same intensity can seem differently loud.
Take one frequency and work out its wavelength in air and in water, then explain in one sentence which quantity refused to change and why.
- On a graph of density against distance: a crest is a compression, a trough is a rarefaction, and the middle line is the undisturbed density.
- Wavelength is the distance between two consecutive compressions, in metres. Amplitude is the maximum change from the undisturbed value.
- Frequency is oscillations per second in hertz; time period is in seconds. A period of s means Hz.
- The graph is a plot, not a picture — sound is longitudinal and the particles move along the wave.
- Measure the wavelength crest to crest — crest to trough is only half.
- ****, because the source sends out complete waves each second.
- Hz with m gives m/s.
- At m/s, the audible range spans m down to cm.
- A period of s at m/s gives Hz and m. And vibrations in s gives Hz, which is inaudible.
- Frequency is set by the source; speed by the medium. So Hz gives m in air and m in water — the wavelength changes, not the frequency.
- Hz at m/s gives m. Convert units before substituting — kHz is Hz.
- Pitch depends on frequency: higher frequency means shriller. Loudness depends on amplitude: bigger swing means louder.
- The two are independent — a tabla struck harder gives the same note louder.
- Quality (timbre) distinguishes two instruments playing the same note at the same loudness.
- **Audible range: Hz to Hz. Below is infrasonic, above ultrasonic.
- Loudness is perceived; intensity is measured.** The same intensity can seem differently loud.
Take one frequency and work out its wavelength in air and in water, then explain in one sentence which quantity refused to change and why.