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Sound 

Class 9 

1. Sound and its Production

• Sound is a form of energy produced by vibrating objects.
• Example: Tuning fork, a stretched string, and vocal cords.
• A medium (solid, liquid, or gas) is required for sound to travel. Sound cannot travel in a vacuum.
• Sound waves are mechanical waves as they require a medium for propagation.

2. Propagation of Sound

• Sound propagates in the form of waves.
• These waves travel as compressions and rarefactions in a medium.
• Compression (C): Region of high pressure and high particle density.
• Rarefaction (R): Region of low pressure and low particle density.

Wave Motion in Sound

• Longitudinal waves: The particles of the medium vibrate parallel to the direction of wave propagation. Example: Sound waves.
• Transverse waves: The particles of the medium vibrate perpendicular to the direction of wave propagation. Example: Water waves, light waves. (Sound waves are not transverse!)

3. Characteristics of a Sound Wave

(a) Amplitude (A)

• The maximum displacement of particles from their mean position.
• Determines the loudness of the sound.
• More amplitude = Louder sound and less amplitude = Fainter sound.

(b) Frequency (f)

• Number of vibrations per second. Measured in Hertz (Hz).
• Determines the pitch of the sound.
• More frequency = High-pitched sound (e.g., whistle).
• Less frequency = Low-pitched sound (e.g., drum beat).

(c) Time Period (T)

• The time taken to complete one vibration (or one wave cycle).
• Formula: $$T = \frac{1}{f}$$
• SI Unit: Seconds (s).

(d) Wavelength (λ – Lambda)

• Distance between two consecutive compressions or rarefactions.
• Measured in meters (m).

(e) Wave Velocity (v)

• The speed at which sound waves travel in a medium.
• Formula: $$v = f \times \lambda$$
  
• Unit: m/s.

(f) Loudness

• Related to amplitude of the wave.
• Measured in decibels (dB).
• Threshold of hearing = 0 dB, normal conversation = 60 dB, jet engine = 130 dB.

4. Speed of Sound in Different Mediums

• Sound travels fastest in solids, slower in liquids, and slowest in gases.
• Speed of sound in different mediums at 25°C:
  • Air: 340 m/s
  • Water: 1450 m/s
  • Iron: 5000 m/s
  • Steel: 5130 m/s

5. Reflection of Sound

• Sound follows the laws of reflection, similar to light:
  1. Angle of incidence = Angle of reflection.
  2. The incident wave, reflected wave, and normal all lie in the same plane.
• Reflection of sound is used in SONAR and echolocation.

6. Echo and Reverberation

Echo

• When sound waves reflect from a surface and return to the listener after 0.1 seconds or more, an echo is heard.
• Minimum distance for an echo: 17.2 m (in air at 25°C).
• Examples of echoes:
  • Sound in an empty hall.
  • Echo in mountains or tunnels.

Reverberation

• Prolonged persistence of sound due to multiple reflections.
• Causes sound to become unclear in large halls.
• Reduced by:
  • Using soft materials like carpets, curtains, foam panels, etc.
  • Designing halls with sound-absorbing materials.

7. Applications of Sound

(a) SONAR (Sound Navigation and Ranging)

• Uses ultrasound to detect objects underwater.
• Principle: Ultrasonic waves reflect from underwater objects and return as echoes.
• Formula for calculating depth: $$\text{Distance} = \frac{\text{Velocity of Sound} \times \text{Time}}{2}$$
• Used in submarines, oceanography, and marine life study.

(b) Medical Uses of Ultrasound

• Ultrasound scanning: Used for imaging internal organs and fetus development.
• Lithotripsy: Used to break kidney stones using ultrasound waves.

(c) Bats and Echolocation

• Bats emit ultrasonic waves to detect objects and prey in the dark.
• They hear echoes to determine the distance and direction of objects.

8. Structure of the Human Ear

Parts of the Ear:

1. Outer Ear (Pinna): Collects sound and directs it to the eardrum.
2. Middle Ear: Contains three bones (malleus, incus, stapes) that amplify vibrations.
3. Inner Ear (Cochlea): Converts vibrations into electrical signals sent to the brain via the auditory nerve.

(A labeled diagram of the human ear should be included.)

9. Hearing Range and Ultrasound

• Humans: Can hear frequencies between 20 Hz – 20,000 Hz (Audible range).
• Infrasound (< 20 Hz): Elephants and whales communicate using infrasound.
• Ultrasound (> 20,000 Hz): Dogs, bats, and dolphins can hear these high-frequency sounds.

10. Why Do We See Lightning Before We Hear Thunder?

• Speed of light = $3 \times 10^8$ m/s (very fast).
• Speed of sound = 340 m/s (much slower).
• Thus, light reaches us before sound.

11. Pitch, Loudness, and Quality of Sound

• Pitch: Depends on frequency (higher frequency = higher pitch).
• Loudness: Depends on amplitude (higher amplitude = louder sound).
• Quality/Timbre: Determines how we differentiate between different sounds of the same pitch and loudness.

12. Noise vs. Music

Noise	Music
Unpleasant sound	pleasant sound
Irregular wave pattern	Regular wave pattern
Example: Horns, drills	Example: Musical instruments

13. Mechanical Nature of Sound Waves

• Sound waves are mechanical waves because they require a medium to travel.
• Unlike light, sound cannot travel in a vacuum.

Experiment to Prove Sound Needs a Medium (Bell Jar Experiment)

• A bell is placed inside a glass jar connected to a vacuum pump.
• When air is removed, the sound of the bell fades and eventually stops.
• This proves that sound cannot travel in a vacuum.

14. Laws of Sound Reflection

1. Angle of incidence = Angle of reflection.
2. Incident wave, reflected wave, and normal lie in the same plane.