Sound ICSE Class 8 Physics: Pitch, Frequency, Loudness and Musical Instruments

What Sound is about

Weightage: CISCE publishes no question paper, marking scheme or chapter weightage for Class VIII Physics: your school sets and marks the exam. Sound sits in the Theme 7: Sound theme of the CISCE Curriculum for Upper Primary Classes (VI-VIII).Source: CISCE Curriculum for Upper Primary Classes (VI-VIII), RDCD 2016

Key concepts

Frequency and pitch

Frequency is the number of complete vibrations made in one second:
$\text{frequency}=\frac{\text{number of vibrations}}{\text{time (s)}}$.

Unit: hertz (Hz); $1\text{Hz}=1$ vibration per second.

Pitch is the property of sound that lets us tell a shrill (high) sound from a deep (low) one.

• Higher frequency → higher pitch (a whistle, a bird's chirp, a child's voice).
• Lower frequency → lower pitch (a lion's roar, a man's deep voice, a large drum).

Worked example

A tuning fork makes 2560 vibrations in 5 seconds. Find its frequency.

Try it first, then show the solutionSolution

1. Frequency $=\frac{2560}{5}$.
2. $=512$ Hz.

Answer: 512 Hz

Amplitude and loudness

Amplitude is the greatest distance a vibrating object moves from its rest (mean) position.

• Larger amplitude → louder sound: strike a drum harder and its skin vibrates more widely.
• Loudness also falls as you move farther from the source.

Loudness is measured in decibels (dB). Rough everyday levels: rustling leaves or a whisper about 20–30 dB, normal conversation about 60 dB, heavy traffic about 80–90 dB. Long exposure to loud sounds (roughly above 85 dB) can damage hearing; very loud sounds are noise pollution.

Pitch and loudness are independent: you can sing the same note softly or loudly.

Worked example

The same drum is struck gently and then hard. What changes and what stays the same?

Try it first, then show the solutionSolution

1. Striking harder gives a larger amplitude, so the sound is louder.
2. The same drum vibrates at the same frequency, so the pitch stays the same.

Answer: Loudness increases; pitch is unchanged.

String instruments

In a sitar, guitar or violin, a stretched string vibrates. Its pitch (frequency) is higher when the string is:

• shorter (pressing a finger on the string shortens the vibrating part),
• tighter (turning the pegs increases the tension),
• thinner (thin strings give the high notes).

The hollow wooden body makes the sound louder by setting more air vibrating.

Worked example

A guitarist presses a string against the neck, halfway along it, and plucks it. How does the pitch change? What happens if the string is also tightened?

Try it first, then show the solutionSolution

1. Pressing shortens the vibrating length, so the frequency and pitch rise.
2. Tightening raises the frequency further, so the pitch rises more.

Answer: Pitch rises in both cases.

Wind instruments

In a flute, whistle or shehnai, a column of air vibrates.

• A shorter air column gives a higher pitch.
• Covering or uncovering the holes of a flute changes the length of the vibrating air column, and so the note.

A simple 'jal tarang' made of bottles: when you blow across the tops, a bottle with more water has a shorter air column and gives a higher pitch.

Worked example

Three identical bottles hold different amounts of water. When you blow across their tops, which gives the highest pitch?

Try it first, then show the solutionSolution

1. Blowing makes the air column above the water vibrate.
2. The bottle with the most water has the shortest air column, so it gives the highest pitch.

Answer: The bottle with the most water

Membrane instruments

In a tabla, drum or dholak, a stretched membrane (skin) vibrates when struck.

• A tighter membrane gives a higher pitch; tabla players tighten or loosen the skin by tapping the wooden blocks under the straps.
• A smaller drum generally gives a higher pitch than a large one.
• Striking harder gives a louder sound (bigger amplitude), not a higher pitch.

Worked example

A tabla player taps the blocks under the straps to tighten the skin. How does this change the sound?

Try it first, then show the solutionSolution

1. Tightening the membrane increases its frequency of vibration.
2. A higher frequency means a higher pitch.

Answer: The pitch becomes higher.

Monotone

A monotone is a sound that stays at one single pitch without rising or falling. A tuning fork gives a single, steady note of fixed frequency, and an alarm beep or a dial tone is a monotone.

Speech and music are pleasant because the pitch varies. Reading aloud in a monotone (all at one pitch) sounds dull and is hard to listen to.

Worked example

Which is a monotone: a song, a tuning fork's note, or a person telling an exciting story? Explain.

Try it first, then show the solutionSolution

1. A song and lively speech keep changing pitch.
2. A tuning fork gives one fixed frequency, so its note has one steady pitch.

Answer: The tuning fork's note

Physics in real life: Sound

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Frequency, Pitch, Amplitude and Loudness
• Musical Instruments and Monotone

Sound formula sheet

Frequency

$f=\frac{\text{vibrations}}{\text{time}}$

Hertz (Hz) = vibrations per second.

Pitch

$\text{higher }f\Rightarrow \text{higher pitch}$

Shrill sounds have high frequency.

Loudness

$\text{larger amplitude}\Rightarrow \text{louder}$

Measured in decibels (dB).

Sound: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Sound

The question patterns to expect, and the method that works for each.

Tricks to remember Sound

Short memory aids. Say each one aloud once, then use it on a question the same day.

• P-F, L-A

  Pitch goes with Frequency; Loudness goes with Amplitude.

  Use it when: Never mixing the two pairs.

• Short, tight, thin = high

  For strings, all three make the pitch higher.

  Use it when: String instrument questions.

• Short column, shrill sound

  Less air vibrating means a higher note in wind instruments.

  Use it when: Flutes and bottles.

• Harder hit, louder, not higher

  Hitting harder changes amplitude only.

  Use it when: Drum and tabla questions.

Sound: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 8-mark question split into 2 + 3 + 3 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Sound

The slips examiners see most often, and what to write instead.

• ✗Mistake: Linking loudness with frequency

  Loudness depends on amplitude, pitch on frequency.

  Wrong working

  A louder sound has a higher frequency

  Right working

  A louder sound has a larger amplitude

  ✓Do this: Remember P-F, L-A.

  Usually costs: 1 mark.

• ✗Mistake: Writing the unit of loudness as hertz

  Hertz measures frequency.

  ✓Do this: Loudness is measured in decibels (dB).

  Usually costs: 1 mark.

• ✗Mistake: Saying a longer string gives a higher note

  A longer string vibrates more slowly.

  ✓Do this: Shorter, tighter, thinner gives a higher pitch.

  Usually costs: 1 mark.

• ✗Mistake: Confusing blowing and tapping bottles

  When you blow across bottles, the air column vibrates (more water → higher pitch).

  ✓Do this: State clearly that the air column is the vibrating part when blowing.

  Usually costs: 1 mark.

• ✗Mistake: Defining monotone as 'a loud sound'

  A monotone is a sound of one unchanging pitch.

  ✓Do this: Monotone = single pitch.

  Usually costs: 1 mark.

Practice questions on Sound

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   The loudness of a sound depends on its:
   (a) frequency
   (b) amplitude
   (c) pitch
   (d) speed

   Hint

   Size of vibration.

   Answer

   (b)

2. Q2 · 1 mark · Very short answerExam level

   What is the unit of loudness?

   Hint

   Not hertz.

   Answer

   Decibel (dB).

3. Q3 · 2 marks · Short answerExam level

   Define amplitude. How does it affect a sound?

   Hint

   Distance from rest position.

   Answer

   Amplitude is the maximum distance a vibrating body moves from its rest position. A larger amplitude gives a louder sound.

4. Q4 · 2 marks · Short answerHarder exam level

   Why does a thin string on a sitar give a higher note than a thick one?

   Hint

   Frequency.

   Answer

   A thin string vibrates faster (higher frequency) than a thick one under similar conditions, so its pitch is higher.

5. Q5 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): Striking a drum harder makes its sound louder.
   Reason (R): Striking harder increases the amplitude of vibration of the drum skin.
   (a) (A) is true and (R) is false.
   (b) (A) is false and (R) is true.
   (c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).

   Hint

   Loudness ↔ amplitude.

   Answer

   (c)

6. Q6 · 2 marks · Short answerHarder exam level

   A source vibrates 500 times each second. How many vibrations does it make in one minute?

   Hint

   1 min = 60 s.

   Answer

   $500\times 60=30000$.

7. Q7 · 2 marks · Short answerHarder exam level

   How does a tabla player raise the pitch of the tabla? Which part vibrates?

   Hint

   Membrane.

   Answer

   The stretched skin (membrane) vibrates. Tightening it, by tapping the blocks under the straps, raises the frequency and pitch.

8. Q8 · 2 marks · Short answerHarder exam level

   A body makes 3000 vibrations in 10 s. Find its frequency.

   Hint

   Divide.

   Answer

   300 Hz.

9. Q9 · 3 marks · Short answerHarder exam level

   Explain why a flute gives different notes when different holes are covered.

   Hint

   Air column.

   Answer

   Covering or uncovering holes changes the length of the vibrating air column. A shorter column vibrates faster (higher frequency), giving a higher note; a longer column gives a lower note.

10. Q10 · 3 marks · Short answerHarder exam level

    A note of frequency 440 Hz is played for 5 s. How many vibrations is that? What would change if it were played louder?

    Hint

    Frequency × time.

    Answer

    $440\times 5=2200$ vibrations. Playing louder increases the amplitude, not the frequency.

11. Q11 · 3 marks · Short answerHOTS / competency

    Two sitar strings have the same length and thickness. One is tighter. Which gives the higher pitch? If the looser one is pressed to make it shorter, could it match the tighter one? Explain.

    Hint

    Both tension and length change frequency.

    Answer

    The tighter string gives the higher pitch. Yes: shortening the looser string raises its frequency, so at a suitable length it can match the tighter one.

12. Q12 · 3 marks · Short answerHOTS / competency

    A teacher reads a story in a monotone and students lose interest. Using the idea of pitch, explain why, and suggest how the teacher could make it livelier.

    Hint

    Single pitch.

    Answer

    A monotone stays at one pitch, so it gives the listener no changes to follow and sounds dull. Varying the pitch (and loudness) to match the mood of the story makes it livelier.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What produces sound?Show answer

   Vibrating objects.

2. 2.Define pitch.Show answer

   The property of sound that distinguishes a shrill (high) sound from a deep (low) one.

3. 3.On what does pitch depend?Show answer

   Frequency.

4. 4.On what does loudness depend?Show answer

   Amplitude.

5. 5.Unit of loudness?Show answer

   Decibel (dB).

6. 6.What is a monotone?Show answer

   A sound of a single, unchanging pitch.

7. 7.Name one string, one wind and one membrane instrument.Show answer

   Sitar; flute; tabla.

8. 8.What is 1 Hz?Show answer

   One vibration per second.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you define frequency and give its unit?
• ☐Do you link pitch–frequency and loudness–amplitude correctly?
• ☐Can you raise the pitch of a string in three ways?
• ☐Can you explain the bottle experiment?
• ☐Do you know the unit of loudness?
• ☐Can you define monotone?

Sound: frequently asked questions