Chapter 10: Sound

1. Introduction to Sound

• Sound is a form of energy that produces a sensation of hearing in our ears.
• Sound is produced by vibrating objects. Example: When a drum is beaten, its surface vibrates and produces sound.
• Sound cannot travel through vacuum (empty space). It needs a material medium (solid, liquid, or gas) to travel.
• Experiment: Bell-jar experiment – When an electric bell is rung inside a vacuum jar, no sound is heard when air is removed → proves sound needs a medium.

2. How Sound is Produced

• Vibration: To and fro or back and forth motion of an object.
• Every vibrating body produces sound. Example: Stretched rubber band, tabla, flute, vocal cords.
• Human voice: Sound is produced when air from lungs makes vocal cords vibrate.

3. Propagation of Sound (How Sound Travels)

• Sound travels as a longitudinal wave.
• Longitudinal wave: Particles of the medium vibrate parallel to the direction of wave propagation.
• Parts of a longitudinal wave:
  • Compression (C): Region of high pressure and high density.
  • Rarefaction (R): Region of low pressure and low density.
• Sound travels fastest in solids > liquids > gases because particles are closest in solids.
• Speed of sound in air ≈ 340–350 m/s (at room temperature).

4. Characteristics of Sound Waves

Sound can be described by three main characteristics:

5. Terms Related to Sound

• Loudness: Sensation of how strong or weak a sound is. Depends on amplitude.
  • Larger amplitude → louder sound.
• Pitch: How high or low the sound is. Depends on frequency.
  • Higher frequency → higher pitch (shrill sound, e.g., female voice, whistle).
  • Lower frequency → lower pitch (deep sound, e.g., male voice, drum).
• Quality (Timbre): Characteristic that helps us distinguish between sounds of same loudness and pitch produced by different sources (e.g., guitar vs flute).
• Noise: Unpleasant, irregular, and loud sound.
• Music: Pleasant, regular, and soothing sound.

6. Human Ear – How We Hear Sound

Structure of human ear (simplified):

1. Outer ear (Pinna) → collects sound waves.
2. Ear canal → sound travels to eardrum.
3. Eardrum → vibrates when sound waves hit it.
4. Middle ear → three tiny bones (hammer, anvil, stirrup) amplify vibrations.
5. Inner ear (Cochlea) → converts vibrations into electrical signals.
6. Auditory nerve → carries signals to brain → we perceive sound.

7. Audible, Infrasonic, and Ultrasonic Sounds

• Some animals can hear infrasound (elephants) and ultrasound (bats, dogs).

8. Reflection of Sound and Echo

• Sound gets reflected from hard surfaces following the same laws as light (angle of incidence = angle of reflection).
• Echo: Reflected sound heard distinctly after the original sound. Conditions for hearing a clear echo:

1. Reflecting surface should be large and hard.
2. Distance between source and reflector ≥ 17 m (in air at 340 m/s, time gap ≥ 0.1 second).

• Reverberation: Repeated reflection of sound causing persistence of sound (heard in empty halls). Reduced by carpets, curtains, etc.

9. Uses of Reflection of Sound

1. Megaphones, loudspeakers, horns – designed to send sound in a particular direction.
2. Stethoscope – sound of heartbeat travels through the tube by multiple reflections.
3. Soundboards – curved surfaces behind speaker reflect sound evenly in auditoriums.

10. SONAR (Sound Navigation and Ranging)

• Technique to measure depth of sea or locate underwater objects using ultrasonic waves. Working:
• Ultrasonic waves are sent from ship → reflected from seabed → echo received.
• Time interval (t) between sending and receiving is noted.
• Speed of sound in seawater (v) ≈ 1500 m/s.
• Distance/depth = (speed × time)/2 (divided by 2 because sound travels to object and back).

11. Applications of Ultrasound

• Cleaning delicate objects (jewellery, machine parts).
• Detecting cracks in metal blocks.
• Echocardiography (heart examination).
• Ultrasonography (imaging of foetus).
• Breaking kidney stones (lithotripsy).

12. Noise Pollution

• Causes:
  • Factories, vehicles, loudspeakers, bursting crackers, machines.
• Harmful effects:
  • Hearing impairment or deafness.
  • Lack of sleep, hypertension, anxiety.
  • Physiological and psychological stress.
• Control measures:
  • Use of silencers in vehicles.
  • Planting trees along roads.
  • Following noise level limits.
  • Avoiding loud music and horns.

Important Points to Remember

• Sound cannot travel in vacuum.
• Sound travels as longitudinal waves (compression and rarefaction).
• Speed of sound: solids > liquids > gases.
• Frequency determines pitch, amplitude determines loudness.
• Humans hear 20 Hz to 20 kHz only.
• Minimum distance for echo = 17 m (in air).
• SONAR uses ultrasound for underwater distance measurement.
• Excessive noise is harmful; we must reduce noise pollution.

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