Sound Class 9 Science Chapter 11 Notes: Sound is an important part of our daily life. We hear different types of sounds around us, such as the ringing of a bell, music from a speaker, the sound of vehicles, and even the voices of people. In Sound Class 9 Science Notes, students learn how sound is produced, how it travels from one place to another, and how our ears help us hear different sounds. This chapter builds a strong foundation for understanding the basic concepts of waves and vibrations.
These sound class 9 science Chapter 11 notes are prepared in simple language to help students understand every topic easily. The notes cover important concepts such as vibration, medium of sound propagation, sound waves, amplitude, frequency, time period, pitch, loudness, and the human ear. Students can also use these sound notes class 9 Science for quick revision before exams.
In these class 9 science chapter 11 sound notes, you will also find important formulas, diagrams, examples, and sound class 9 notes questions and answers that are frequently asked in exams. Whether you are looking for a quick revision guide or a sound class 9 science notes pdf download, these notes can help you prepare better. Designed according to the latest CBSE Class 9 syllabus, these Class 9 Notes make learning easy, clear, and exam-focused.
Nature of Sound
Sound is a form of energy that produces the sensation of hearing in our ears.
It is produced by longitudinal waves in an elastic medium waves in which the particles of the medium vibrate in the same direction as that of wave propagation.
Production of Sound
Sound is produced by vibrating bodies:
- A tuning fork struck on a rubber pad (laboratory standard).
- A plucked string (violin, guitar, sitar).
- A blown flute or shehnai.
- A struck table or drum.
Propagation of Sound
When a tuning fork vibrates, its prongs move outward and inward repeatedly:
- Outward motion → pushes air molecules together → compression (region of high pressure / high density).
- Inward motion → leaves space → rarefaction (region of low pressure / low density).
One compression + one rarefaction = one complete wave.
The number of waves produced per second = frequency of the source (engraved on every tuning fork).
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Sound Needs a Material Medium
Bell-jar experiment: An electric bell is suspended inside a bell jar connected to a vacuum pump.
- Initially (with air inside): the bell's sound is clearly heard.
- As air is pumped out: the sound becomes fainter and fainter.
- In near-vacuum: the bell hammer is still seen vibrating, but no sound is heard.
Conclusion: Sound cannot travel through vacuum it needs a material medium (solid, liquid, or gas).
Natural fact: The Moon has no atmosphere, so any explosion on the Moon would never be heard on Earth.
Characteristics of Sound Wave
(i) Pitch
- The brain's interpretation of the frequency of sound.
- High frequency → high pitch (shrill) humming of a bee, guitar.
- Low frequency → low pitch (hoarse) roar of a lion, car horn.
(ii) Loudness
- Depends on the amplitude of the sound wave.
- Larger amplitude → louder sound (carries more energy, audible at greater distance).
- Smaller amplitude → softer sound.
(iii) Timbre (Quality)
- The characteristic that allows us to distinguish between two sounds of the same pitch and loudness produced by different sources.
- This is why a flute and a violin playing the same note sound different and why we can recognise a friend's voice without seeing them.
(iv) Intensity
- Intensity = Sound energy transferred per unit area per unit time (perpendicular to wave propagation).
- SI unit: W/m² (watt per square metre).
- Intensity is an objective physical quantity (measurable).
Difference between Loudness and Intensity
| Loudness | Intensity |
|---|---|
| Subjective depends on the listener's ear sensitivity | Objective independent of the listener |
| Cannot be measured as a physical quantity | Can be measured as a physical quantity |
Range of Hearing Sound
| Type of Sound | Frequency Range |
|---|---|
| Infrasonic (below human hearing) | Less than 20 Hz |
| Audible (heard by humans) | 20 Hz to 20,000 Hz (20 kHz) |
| Ultrasonic (above human hearing) | More than 20 kHz |
- Children can hear up to ~30 kHz; in older people the upper limit may fall to 10–12 kHz.
- Human ear is most sensitive to frequencies around 2000–3000 Hz.
Hearing Range of Animals
| Animal | Audible Range |
|---|---|
| Dog | Up to ~50 kHz |
| Bat | Up to ~100 kHz |
| Dolphin | Even higher |
| Elephant, whale | Less than 20 Hz (infrasonic) |
| Some fish | As low as 1–25 Hz |
Sonic Boom
When a body (e.g., a jet fighter, bullet) travels at supersonic speed (faster than sound), it produces shock waves a high-pressure layer followed by a low-pressure layer that travels along the surface of a cone. When this shock wave reaches a person, the sudden pressure change is heard as a sharp, loud sonic boom.
Shock waves from supersonic aircraft can have enough energy to shatter glass and damage weak buildings.
Reflection of Sound
When sound waves strike a surface, they bounce back into the same medium this is reflection.
Laws of Reflection of Sound
- Angle of incidence = Angle of reflection.
- Incident wave, reflected wave, and the normal all lie in the same plane.
Sound waves require larger reflecting surfaces than light, because their wavelength is much longer.
Applications of Reflection
| Device | Use |
|---|---|
| Megaphone / Speaking tube | Horn-shaped tube prevents sound from spreading; sound energy is directed forward by successive reflections. |
| Stethoscope | Sounds from a patient's heart and lungs reach the doctor's ears through multiple reflections inside the tubes. |
| Sound board | A curved (concave) reflector placed behind a speaker in halls; sound waves reflect parallel to the principal axis, spreading sound evenly. |
Speed of Sound in Different Media
Sound travels by transfer of energy from one particle to the next.
vgas < vliquid < vsolid
- Solids: Particles are tightly packed → energy transfers fastest → highest speed.
- Liquids: Particles farther apart → slower.
- Gases: Particles farthest apart → slowest.
Effect of Temperature
- Speed of sound increases as temperature increases (particles collide more frequently).
- In air, speed increases by 0.61 m/s for every 1°C rise in temperature.
- At 0°C, speed of sound in air ≈ 330 m/s; at 25°C ≈ 345 m/s.
Speed of sound does not depend on pressure (at constant temperature).
Echo Sound
An echo is the sound heard after reflection from a rigid obstacle.
Types of Echo
| Type | Description |
|---|---|
| Instantaneous Echo | Reflection of short-duration sounds like a clap or pistol shot. |
| Syllabic Echo | Reflection of spoken syllables; obstacle at least 22 m away. |
| Successive Echo | Multiple reflections heard between two parallel rows of buildings or hills. |
Persistence of Hearing
The sensation of sound persists in the ear for 1/15 second (typical value used for calculations).
Minimum Distance to Hear an Echo
d = v × t/2
For v = 340 m/s and t = 1/15 s:
d = 340 × (1/15)/2 = 22.67/2 ≈ 11m
Minimum distance to hear a distinct echo = 11 metres (in air at room temperature).
Conditions for Formation of an Echo
- Minimum distance between source and reflector should be 11 m.
- Wavelength of sound should be less than the height of the reflector.
- Sound must be intense enough to be heard after reflection.
Reverberation
Reverberation is the persistence of sound due to repeated reflections inside a closed space (like a big hall), even after the source has stopped.
- A small amount of reverberation adds volume to music or speech.
- Too much reverberation causes confusion and overlap of sounds.
- To reduce reverberation in auditoriums, walls and ceilings are covered with sound-absorbing materials like rough plaster, thick curtains, and porous tiles.
Audible, Infrasonic and Ultrasonic Waves
| Type | Frequency | Source / Example |
|---|---|---|
| Audible | 20 Hz – 20 kHz | Sitar, guitar, organ pipes, flute, shehnai |
| Infrasonic | Below 20 Hz | Earthquakes; usually from large sources |
| Ultrasonic | Above 20 kHz | Quartz crystal oscillators; usually small sources |
Ultrasound and its Applications
Ultrasound = sound of frequency greater than 20 kHz.
Production: Electric oscillator using high-frequency vibrations of a quartz crystal.
Applications
| Application | How it Works |
|---|---|
| Welding metals (e.g., tungsten) | Friction from ultrasonic vibration melts the contact points; metals fuse on stopping vibration. |
| Medical imaging (ultrasonography) | Reflected ultrasound from internal organs is converted into electrical signals and displayed as images. Safer than X-rays. |
| Breaking kidney stones | High-intensity ultrasound shatters stones into fine grains that pass out with urine. |
| Drilling and cutting | Ultrasonic "horn" hammers fragile materials like glass thousands of times per second to make precise holes. |
| Ultrasonic cleaning | Watches, electronic parts, and odd-shaped objects are placed in a solution; ultrasonic vibrations detach dirt and grease. |
| Detection of metal defects | Ultrasound passes through metal; cracks reflect waves, reducing intensity at the detector → defect identified. |
| Echolocation by bats | Bats emit ultrasonic squeaks; reflected echoes help them locate prey and avoid obstacles in the dark. |
SONAR
SONAR stands for Sound Navigation And Ranging.
Principle
SONAR uses ultrasonic waves to find the depth of the sea or to locate underwater objects such as shoals of fish, submarines, sunken ships, or icebergs.
Working
A SONAR device has two parts:
- Transmitter emits a pulse of ultrasound (~50,000 Hz) into the water.
- Receiver detects the reflected echo from the underwater object.
Formula:
d = v × t/2
where d = depth, v = speed of sound in water, t = total time between sending the pulse and receiving the echo.
Why Ultrasonic Waves are Used in SONAR
- Their high frequency allows them to penetrate deep into seawater without being absorbed.
- They are not perceived by the human ear, so they cannot be confused with the noise of ship engines.
Solved Example (from the source notes)
Q. Ultrasonic waves take 4 seconds to travel from a ship to the sea bottom and back. Find the depth of the sea. (Speed of sound in water = 1500 m/s)
Solution:
- Total time = 4 s → one-way time = 4/2 = 2 s.
- Speed = Distance / Time → 1500 = d / 2
- Depth, d = 1500 × 2 = 3000 m
The Human Ear
Structure
The human ear has three compartments:
| Part | Components | Function |
|---|---|---|
| Outer Ear | Pinna, ear canal, ear-drum (tympanum) | Collects sound waves and directs them onto the ear-drum |
| Middle Ear | Three tiny bones hammer, anvil, stirrup; Eustachian tube (to throat) | Amplifies and transmits vibrations; equalises air pressure |
| Inner Ear | Oval window, cochlea (coiled, liquid-filled tube), auditory nerve | Converts vibrations into electrical impulses sent to brain |
Working
- Pinna collects sound waves → directs them through the ear canal.
- Sound waves hit the ear-drum, which vibrates back and forth (compressions push it in, rarefactions pull it out).
- Vibrations pass to the hammer → anvil → stirrup (each bone amplifies vibrations).
- Stirrup hits the oval window, transferring vibrations to the liquid inside the cochlea.
- Vibrating liquid stimulates nerve cells → generates electrical impulses.
- Auditory nerve carries these impulses to the brain, which interprets them as sound.
Sound Class 9 Science Formulas
| Quantity / Concept | Value / Formula |
|---|---|
| Audible range | 20 Hz – 20,000 Hz |
| Infrasonic | < 20 Hz |
| Ultrasonic | > 20,000 Hz |
| Speed of sound in air at 0°C | ≈ 330 m/s |
| Speed of sound in air at 25°C | ≈ 345 m/s |
| Temperature rise effect | +0.61 m/s per 1°C |
| Speed order in media | v_gas < v_liquid < v_solid |
| Persistence of hearing | 1/15 second |
| Minimum distance for echo | 11 m (in air) |
| Echo / SONAR distance formula | d = (v × t)/2 |
| 1 W | 1 J/s |
