Sound — Science Class 9 Notes (CBSE & HBSE)
Free NCERT Science notes for Sound (Class 9) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 3 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — Sound (CBSE & HBSE)
CBSE emphasises the wave nature of sound, characteristics (pitch, loudness, timbre), v=λf derivation-style numericals, echo and SONAR; HBSE focuses on production/propagation, reflection of sound, and direct numericals on speed, frequency and echo distance.
Production and Propagation of Sound
Production of Sound
Sound is produced by vibrating objects. When a body vibrates, it makes the surrounding particles of the medium vibrate, and this disturbance travels as sound. Example: a vibrating tuning fork, a stretched rubber band, or vocal cords.
Propagation of Sound — A Mechanical Wave
Sound travels as a wave through a material medium (solid, liquid or gas). The particles of the medium do not travel; they only vibrate about their mean position and pass the energy forward. Such a wave is called a mechanical wave.
Longitudinal Wave: Compressions and Rarefactions
Sound is a longitudinal wave — the particles of the medium vibrate parallel to the direction of wave propagation.
| Region | Description | Density & Pressure |
|---|---|---|
| Compression (C) | Particles crowded together | High density, high pressure |
| Rarefaction (R) | Particles spread apart | Low density, low pressure |
As the tuning fork's prong moves forward it pushes the air, creating a compression; as it moves back it creates a rarefaction. A series of compressions and rarefactions travels outward, carrying the sound.
Sound Needs a Medium
Sound cannot travel through vacuum. In the classic bell-jar experiment, an electric bell ringing inside a glass jar becomes fainter and finally inaudible as the air is pumped out, proving a material medium is essential.
Speed of Sound
Sound travels fastest in solids, slower in liquids, and slowest in gases (because particles are closest in solids). Speed also increases with temperature of the medium.
Tip: Light can travel through vacuum but sound cannot — this is why we see lightning before we hear thunder.
Characteristics of a Sound Wave
Describing a Sound Wave
A sound wave can be represented by a graph of density/pressure versus distance, giving a curve with crests (compressions) and troughs (rarefactions).
Key Terms
| Term | Symbol | Definition | Unit |
|---|---|---|---|
| Wavelength | λ | Distance between two consecutive compressions or rarefactions | metre (m) |
| Frequency | f (or ν) | Number of complete oscillations per second | hertz (Hz) |
| Time period | T | Time for one complete oscillation | second (s) |
| Amplitude | A | Maximum displacement of particles from mean position | metre (m) |
Relation: Frequency and time period are reciprocals → f = 1/T.
Wave Equation (Speed–Wavelength–Frequency)
v = λ f
Speed of a sound wave = wavelength × frequency. This is the most important numerical relation in this chapter.
Characteristics of Sound
- Pitch — decided by frequency. Higher frequency → higher pitch (shrill, like a whistle); lower frequency → lower pitch (e.g., a drum).
- Loudness — decided by amplitude. Larger amplitude → louder sound. Loudness is measured in decibels (dB).
- Timbre (quality) — the characteristic that lets us distinguish two sounds of the same pitch and loudness (e.g., a flute vs a violin playing the same note).
CBSE/HBSE Trap: Pitch depends on frequency (not amplitude), while loudness depends on amplitude (not frequency). Do not mix these up. A 'tone' is a single-frequency sound; a 'note' is a mix of several frequencies.
Reflection of Sound, Echo, SONAR and Range of Hearing
Reflection of Sound
Sound bounces back (reflects) from a hard surface, obeying the laws of reflection: the angle of incidence equals the angle of reflection, and the incident sound, reflected sound and the normal lie in the same plane.
Echo
An echo is the repetition of sound caused by the reflection of sound waves from an obstacle.
To hear a distinct echo, the time gap between the original and reflected sound must be at least 0.1 second. Taking the speed of sound as 344 m/s at 22 °C, the minimum distance of the reflecting surface = (344 × 0.1)/2 = 17.2 m.
Reverberation
The persistence of sound due to repeated/multiple reflections (e.g., in a big hall) is called reverberation. Excessive reverberation is reduced by covering walls and ceilings with sound-absorbing materials like compressed fibreboard, curtains and perforated panels.
Uses of Multiple Reflection
- Megaphones, loudhailers, horns, trumpets (send sound in one direction).
- Stethoscope (sound of heartbeat reaches doctor's ears by multiple reflection).
- Curved ceilings of concert halls (spread sound evenly).
SONAR
SONAR = Sound Navigation And Ranging. It uses ultrasound to find the distance, direction and speed of underwater objects (submarines, hills, icebergs, shipwrecks). It sends ultrasonic pulses and detects the echo.
Distance, d = (v × t) / 2, where t is the total time for the pulse to go and return, and v is the speed of sound in water (about 1500 m/s).
Range of Hearing
| Type | Frequency Range |
|---|---|
| Infrasound | below 20 Hz (e.g., produced by rhinoceros, whales, earthquakes) |
| Audible sound | 20 Hz to 20,000 Hz (range for normal human ear) |
| Ultrasound | above 20,000 Hz (used by bats, dolphins; in medical imaging) |
CBSE/HBSE Trap: Remember to divide by 2 in echo/SONAR distance problems, because the sound travels to the obstacle and back.
Frequently asked questions
Are these Sound notes free?
Yes — the Sound notes for Science (Class 9) on Siksha Sarovar are completely free to read, with no account required.
Do these notes follow CBSE and HBSE?
Yes. The Sound notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.
What does the Sound chapter cover?
Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Sound.