Samacheer Kalvi 9th Science Sound Exercise Solutions

Sound is a form of energy produced by vibrating objects that travels through a medium as longitudinal waves and is detected by the ear.

A longitudinal wave is one in which the particles of the medium vibrate parallel to the direction of wave propagation, creating compressions (high pressure) and rarefactions (low pressure).

Frequency is the number of complete oscillations per second. SI unit: hertz (Hz). 1 Hz = 1 oscillation per second.

An echo is the repetition of sound caused by reflection from a surface. For an echo to be heard distinctly, the reflected sound must reach the ear at least 0.1 seconds after the original sound.

Sound waves require a material medium (solid, liquid, or gas) for propagation. They cannot travel through vacuum. The energy is transferred through the mechanical vibration of particles, hence they are called mechanical waves.

Reverberation is the persistence of sound due to multiple reflections of sound waves in a hall or auditorium. It causes the sound to be heard for some time even after the source stops producing sound.

1. Medium: Speed of sound is highest in solids, less in liquids, and least in gases. (v_solid > v_liquid > v_gas)

2. Temperature: Speed increases with temperature. v ∝ √T. In air, v ≈ 331 + 0.6T (°C). 3. Humidity: Sound travels faster in moist air than dry air (water vapour is lighter than dry air). 4. Wind: If wind is in the direction of sound, effective speed increases; if opposite, it decreases.

• Steel ≈ 5960 m/s, Water ≈ 1500 m/s, Air ≈ 343 m/s

1. Medical: Ultrasound imaging (sonography) for examining internal organs and foetus. 2. Industrial: Detecting flaws in metal structures, cleaning delicate instruments. 3. SONAR: Sound Navigation And Ranging — used in ships for measuring ocean depth, detecting underwater objects. 4. Bats and dolphins: Use ultrasonic waves for navigation and hunting (echolocation). 5. Chemistry: Emulsification of immiscible liquids, accelerating chemical reactions.

Production: Sound is produced by vibrating objects. Examples: vibrating vocal cords, plucked string, struck tuning fork, beating drum membrane.

Propagation: When an object vibrates, it pushes and pulls the surrounding air particles, creating compressions (high pressure) and rarefactions (low pressure). These travel outward as a longitudinal wave through the medium, transferring energy without transferring matter.

SONAR (Sound Navigation And Ranging) is a technique that uses ultrasonic waves to detect objects underwater and measure ocean depth.

Like light, sound reflects off hard surfaces following the laws of reflection (angle of incidence = angle of reflection).

Examples: 1. Echo: Reflected sound heard after 0.1 s from a cliff, building, or well. 2. Megaphone: Horn-shaped tube — sound reflects from walls and emerges as a directed beam. 3. Stethoscope: Sound from the body reflects through the tube to the doctor's ears. 4. Sound boards: Curved surfaces in auditoriums reflect sound towards the audience.

Light travels at 3 × 10⁸ m/s, while sound travels at about 343 m/s in air. Since light is nearly a million times faster than sound, we see lightning almost instantly, but the sound of thunder takes much longer to reach us. The time delay between seeing lightning and hearing thunder can be used to estimate the distance of the storm: d ≈ v × t ≈ 343 × (time in seconds).

The human ear converts sound waves into electrical signals interpreted by the brain. It has three parts:

1. Outer Ear:

• Pinna (ear): Collects sound waves from the environment and directs them into the ear canal.
• Ear canal (meatus): Carries sound waves to the eardrum.

2. Middle Ear:

• Eardrum (tympanic membrane): Vibrates when sound waves strike it.
• Three ear ossicles: Hammer (malleus), Anvil (incus), Stirrup (stapes) — amplify vibrations approximately 20 times.
• Eustachian tube: Equalises air pressure on both sides of the eardrum.

3. Inner Ear:

• Cochlea: Snail-shaped, fluid-filled organ with hair cells that convert vibrations into electrical signals.
• Auditory nerve: Carries electrical signals to the brain, which interprets them as sound.

Reflection of sound: When sound waves hit a hard surface, they bounce back into the same medium, following the laws of reflection (angle of incidence = angle of reflection).

Applications: 1. Echo: Reflected sound heard after 0.1 s. Used to measure distance: d = vt/2. 2. SONAR: Ships send ultrasonic pulses; time for echo gives distance to objects or ocean floor. 3. Megaphone: Horn-shaped — sound waves reflect from walls and emerge as a directed beam. 4. Stethoscope: Sound from body reflects through tube to doctor's ears. 5. Sound boards: Curved surfaces in auditoriums reflect sound towards the audience.

Activity: An electric bell is placed inside an airtight glass jar connected to a vacuum pump.

1. When the bell is switched on with air inside the jar, we hear the sound clearly. 2. As air is gradually pumped out using the vacuum pump, the sound becomes fainter and fainter. 3. When nearly all air is removed (near vacuum), the sound is barely audible despite the bell still ringing (we can see the striker vibrating). 4. When air is let back in, the sound becomes loud again.

Conclusion: Sound needs a material medium for propagation. In vacuum, even though the bell vibrates, sound cannot reach our ears because there are no particles to carry the vibrations.

v = fλ = 500 × 0.68 = 340 m/s

λ = v/f = 340/1000 = 0.34 m

d = vt/2 = (340 × 3)/2 = 510 m

f = 1/T = 1/0.002 = 500 Hz λ = v/f = 340/500 = 0.68 m

d = vt/2 = (1500 × 4)/2 = 3000 m

T = 1/f = 1/256 = 0.0039 s ≈ 3.9 ms

T = 1/f = 1/2000 = 0.0005 s = 0.5 ms λ = v/f = 340/2000 = 0.17 m

d = vt/2 = (344 × 2.5)/2 = 430 m

f = v/λ = 340/1.7 = 200 Hz Yes, it is audible because 200 Hz falls within the audible range (20 Hz – 20,000 Hz).