Sound Notes Class 9 by Abhinav Sir

Sound - Comprehensive Notes

Created by Abhinav Anand Maths

1. Introduction to Sound

• A sound is a form of energy which produces a sensation of hearing in our ears.
• It is produced due to vibrations of different objects.
• It travels in the form of waves.

Production of Sound

• Sound is produced by the vibrations of objects.
• Vibrations create compressions and rarefactions in the surrounding air, leading to the formation of a sound wave.
• The energy required to make an object vibrate is provided by an outside source (like our hand, wind, etc.).

Examples:
- Sound of our voice is produced by the vibration of two vocal cords in our throat.
- Sound of a drum or tabla is produced by the vibration of its membrane when struck.

Sound can be produced by the following methods: By vibrating strings (sitar), vibrating air (flute), vibrating membranes (tabla, drum), vibrating plates (bicycle bell), by friction, or by scratching/scrubbing objects.

2. Propagation of Sound

• The matter or substance through which sound is transmitted is called a medium (solid, liquid, or gas). Air is the most common medium.
• A wave is a disturbance that moves through a medium when the particles set neighboring particles into motion. Sound waves are called mechanical waves.
• Compression (C): When a vibrating object moves forward, it pushes and compresses the air, creating a region of high pressure.
• Rarefaction (R): When the vibrating object moves backwards, it creates a region of low pressure.

Sound Needs a Medium: It cannot travel in a vacuum.
Experiment: An electric bell suspended in an airtight bell jar connected to a vacuum pump. As air is removed, the sound fades, proving a medium is necessary.

3. Types of Waves

• Longitudinal Waves: The particles of the medium oscillate in a direction parallel to the direction of propagation (e.g., Sound waves). They include compressions and rarefactions.
• Transverse Waves: The particles of the medium oscillate in a direction perpendicular to the direction of propagation in an up and down motion (e.g., Light waves).

4. Characteristics of Sound Waves

• Wavelength (λ): The distance between two consecutive compressions or rarefactions. SI unit: metre (m).
• Frequency (f or ν): The number of oscillations per unit time. SI unit: Hertz (Hz).
• Amplitude (A): The maximum displacement of the particle of the medium from their original undisturbed position. SI unit: metre (m).
• Time Period (T): The time taken for one complete oscillation through a medium. SI unit: second (s).

Relationship: f = 1 / T

Wave Velocity (v) = Frequency (f) × Wavelength (λ)

Pitch, Timbre, and Loudness

• Pitch: Depends upon the frequency of the sound, and the size/type of the object producing it.
• Timbre: The quality of sound that helps differentiate between different sounds even if they have the same pitch and amplitude.
• Loudness: Depends on the amplitude of the sound wave. Greater amplitude means louder sound.

5. Speed of Sound & Sonic Boom

• Sound travels at different speeds in different mediums. Speed is maximum in solids.
• Factors affecting speed: Density of the medium, Temperature (speed increases with temperature), and Humidity (speed increases with humidity).

Sonic Boom: When an object travels in the air with a speed greater than that of sound, it produces high energy shock waves. These loud sound waves are called Sonic booms (e.g., supersonic aircraft, bullets). The energy can break glass or damage buildings.

6. Reflection of Sound

Like light, sound also bounces back when it falls on a hard surface. The laws of reflection of light apply to sound:

1. The incident sound wave, the reflected sound wave, and the normal at the point of incidence all lie in the same plane.
2. Angle of reflection is always equal to the angle of incidence.

Echo and Reverberation

• Echo: The repetition of sound caused by the reflection of sound waves. To hear a distinct echo, the time interval must be at least 0.1s. The minimum distance from the obstacle must be 17.2 m.
• Reverberation: The persistence of sound in a big hall due to repeated/multiple reflections. Heavy curtains and carpets are used to absorb sound and reduce this.

Applications of Reflection of Sound

• Megaphones and Loudspeakers: Designed with funnel tubes to reflect sound waves repeatedly towards the audience.
• Stethoscope: Medical instrument where the sound of heartbeats reaches the doctor's ears by multiple reflections in the tube.
• Sound Boards: Curved boards placed behind speakers in big halls to reflect speech towards the audience. Concert hall ceilings are curved for the same reason.

7. Range of Hearing

• Human Range: 20 Hz to 20,000 Hz.
• Infrasonic Sounds: Frequencies lower than 20 Hz. Produced by simple pendulums, rhinoceroses, elephants, whales, and earthquakes.
• Ultrasonic Waves: Frequencies higher than 20 kHz. Heard and produced by dogs, dolphins, bats, and rats.

8. Structure of the Human Ear

The ear consists of three parts:

• Outer Ear: Includes the Pinna (gathers sound) and the Auditory Canal.
• Middle Ear: Includes the Eardrum (Tympanic Membrane) and three bones (hammer, anvil, stirrup) that amplify vibrations. The lower part connects to the Eustachian tube.
• Inner Ear: Includes the Cochlea, which converts vibrations into electrical signals sent to the brain via the auditory nerve.

50 Numerical Practice Questions

Topic: Time Period & Frequency (T = 1/f, f = 1/T)

1. Find the time period of a tuning fork whose frequency is 50 Hz.
2. A sound wave has a frequency of 200 Hz. What is its time period?
3. Calculate the time period of a wave if its frequency is 1000 Hz.
4. If a pendulum completes 20 oscillations in 10 seconds, what is its frequency?
5. Calculate the frequency of a wave whose time period is 0.05 seconds.
6. A wave takes 0.02 seconds to complete one cycle. What is its frequency?
7. A bat produces ultrasonic sound of frequency 50 kHz. Find its time period.
8. Find the frequency of a vibrating body that has a time period of 0.004 s.
9. If a guitar string vibrates 250 times in one second, what is its time period?
10. A source produces 500 compressions and 500 rarefactions in 2 seconds. Find its frequency.

Topic: Wave Velocity (v = f × λ)

11. A sound wave has a frequency of 250 Hz and a wavelength of 1.4 m. Calculate its speed.
12. Find the wavelength of a sound wave whose frequency is 150 Hz and speed is 330 m/s.
13. A wave travels at a speed of 340 m/s. If its wavelength is 2 m, what is its frequency?
14. The speed of sound in steel is 5000 m/s. If the frequency is 1000 Hz, find the wavelength.
15. A tuning fork has a frequency of 512 Hz. Find the wavelength of the sound produced in air if speed is 344 m/s.
16. Calculate the speed of a wave with a time period of 0.01 s and a wavelength of 5 m.
17. If the frequency of a sound is 20 Hz, find its wavelength in air (v = 340 m/s).
18. A sound wave of wavelength 0.5 m travels with a speed of 350 m/s. Find its frequency.
19. Find the time period of a wave traveling at 300 m/s with a wavelength of 3 m.
20. A source emits a sound of wavelength 1.5 m. If the time period is 0.005 s, calculate the wave speed.
21. Find the wavelength of a radio wave traveling at 3 × 10⁸ m/s with a frequency of 10⁶ Hz.
22. An underwater sonar emits a frequency of 50 kHz. If sound speed in water is 1500 m/s, find the wavelength.
23. A sound wave has a frequency of 2 kHz and wavelength 15 cm. How fast is it traveling?
24. If a wave's speed doubles while its frequency remains constant, what happens to its wavelength?
25. A wave travels 100 meters in 2 seconds. If its wavelength is 2 meters, find its frequency.

Topic: Echoes and Reflection (d = vt / 2)

26. A boy shouts near a cliff and hears an echo after 4 seconds. If the speed of sound is 340 m/s, how far is the cliff?
27. A ship sends a sonar signal which returns in 2 seconds. If sound speed in water is 1500 m/s, find the depth of the ocean.
28. An echo is heard after 0.8 seconds. How far is the reflecting surface? (Speed of sound = 344 m/s).
29. A person fires a gun 170 m away from a tall building. After what time will they hear the echo? (v = 340 m/s).
30. A submarine sends a signal to a target 3000 m away. How long will it take for the echo to return? (v = 1500 m/s).
31. To hear a distinct echo, what is the minimum distance required if the speed of sound is 350 m/s?
32. A girl claps her hands near a mountain and hears the echo 3 seconds later. Calculate the distance (v = 346 m/s).
33. A radar sends a signal to an airplane at a distance of 30 km. If the signal travels at 3 × 10⁸ m/s, calculate the total time taken for the echo to return.
34. Sound from an explosion reflects off a cliff 680 m away. Find the time taken to hear the echo (v = 340 m/s).
35. A man stands between two parallel cliffs and fires a gun. He hears the first echo after 1.5 s and the second after 2.5 s. Find the distance between the cliffs (v = 340 m/s).
36. An ultrasonic pulse is sent to the seabed and returns in 0.4 seconds. Find the depth (v in water = 1530 m/s).
37. If an echo is heard after 0.1 seconds, and the reflecting object is 17 m away, what is the speed of sound?
38. A bat emits a click and hears the echo from an insect in 0.02 seconds. How far is the insect? (v = 344 m/s).
39. A child hears an echo from a wall 85 meters away. How much time elapsed between shouting and hearing the echo? (v = 340 m/s).
40. Calculate the depth of a well if a stone dropped takes 4 seconds to splash, and the splash is heard 0.2 seconds later.

Topic: Mixed & Advanced Application Numericals

41. A sound wave has a wavelength of 0.8 m. If 500 waves pass a point in 2 seconds, calculate the speed of the wave.
42. The frequency of a source is 100 Hz. How many times does it vibrate in 1 minute?
43. Sound travels in solid medium A at 4000 m/s and liquid medium B at 1000 m/s. If frequency is 200 Hz, find the difference in wavelengths in the two mediums.
44. A source produces sound of 500 Hz. How long does it take for the sound to travel 1.5 km in air? (v = 340 m/s).
45. If a wave has a speed of 330 m/s and a time period of 0.02 s, find the distance between a consecutive compression and rarefaction.
46. An observer sees a lightning flash and hears the thunder 5 seconds later. Find the distance of the storm (v = 340 m/s).
47. The minimum audible frequency for humans is 20 Hz. Find its wavelength in air (v = 344 m/s).
48. The maximum audible frequency for humans is 20,000 Hz. Find its wavelength in water (v = 1500 m/s).
49. If the distance between two consecutive compressions is 10 cm and the wave travels at 300 m/s, calculate its frequency.
50. A tuning fork makes 256 vibrations per second. Find the distance traveled by the sound in the time the fork completes 50 vibrations (v = 340 m/s).