SOUND (ध्वनि)

1. What is Sound?

• Sound is a form of Energy which produces a sensation of hearing in our ears.
• Law of Conservation of Energy: Energy can neither be created nor destroyed.
  So, when we clap, mechanical energy gets converted into sound energy.

2. Production of Sound

Sound is produced by Vibrating Objects.

Examples of Vibration:

• Vocal Cords: Vibrating cords in our throat produce voice.
• Guitar/Sitar: Plucking a stretched string makes it vibrate.
• Membrane: Striking a tabla or drum.

3. Propagation of Sound

• Sound needs a material Medium (Solid, Liquid, or Gas) to travel.
• Sound cannot travel through a vacuum. (Experiment: Electric bell in a vacuum jar).
• Sound waves are Mechanical Waves. (इन्हें medium चाहिए होता है).

4. How Sound Travels?

• When an object vibrates, it sets the particles of the medium around it in motion.
• Important: The particles do not travel from the source to the ear.
• Particle सिर्फ अपनी जगह पर oscillate करता है और disturbance को अगले particle को pass करता है।

Longitudinal Waves (अनुदैर्ध्य तरंग)

• Sound waves are longitudinal.
• Definition: Particles oscillate parallel to the direction of wave propagation.
• Example: A Slinky (spring) pushed and pulled.

• Compression (C): Region of high pressure (particles are crowded).
• Rarefaction (R): Region of low pressure (particles are spread apart).

5. Characteristics of Sound Wave

• 1. Wavelength ($\lambda$): Distance between two consecutive Compressions or Rarefactions. SI Unit: metre (m).
• 2. Frequency ($\nu$): Number of oscillations per unit time.
  Formula: $\nu = \frac{1}{T}$
  SI Unit: Hertz (Hz). Named after H.R. Hertz.
• 3. Time Period (T): Time taken for one complete oscillation. Unit: second (s).
• 4. Amplitude (A): Magnitude of max disturbance.
  Determines Loudness. (Higher Amp = Louder Sound).
• 5. Pitch: Depends on Frequency.
  High Freq = High Pitch (Shrill voice, e.g., Flute).
  Low Freq = Low Pitch (Deep voice, e.g., Drum).

6. Speed of Sound

• Speed depends on:
  1. Nature of Medium: Solid > Liquid > Gas.
     (Because particles are closer in solids).
  2. Temperature: Temp increases $\rightarrow$ Speed increases.

7. Reflection of Sound

• Sound bounces off obstacles just like light.
• Laws of Reflection:
  1. Angle of Incidence = Angle of Reflection ($\angle i = \angle r$).
  2. Incident sound, Reflected sound, and Normal lie in the same plane.

Echo (प्रतिध्वनि)

Reflection of sound that arrives at the listener with a delay.

• To hear a distinct echo, time gap must be at least 0.1 seconds. (Our brain holds sound for 0.1s).
• Speed in air = 344 m/s.
• Distance = Speed × Time = $344 \times 0.1 = 34.4$ m (Total path).
• So, min distance from obstacle = $\frac{34.4}{2} = 17.2$ meters.

Reverberation (गूँज)

• Persistence of sound due to repeated reflections.
• Often happens in big halls.
• Solution: Use sound-absorbent materials (fibreboard, heavy curtains) on walls/roofs.

8. Uses of Multiple Reflection

1. Megaphones / Horns: Designed to send sound in a particular direction without spreading.
2. Stethoscope: Used by doctors. Sound travels through the tube by multiple reflections.
3. Curved Ceilings: In concert halls, ceilings are curved so sound reflects and reaches every corner.

9. Range of Hearing

• Audible Range: 20 Hz to 20,000 Hz (20 kHz).
  (Humans सिर्फ इसी range में सुन सकते हैं).
• Infrasound (Infrasonic): Frequency < 20 Hz.
  • Produced by: Whales, Elephants, Rhinoceros (5 Hz).
  • Earthquakes produce infrasound before the main shockwave.
• Ultrasound (Ultrasonic): Frequency > 20 kHz.
  • Produced by: Bats, Dolphins, Porpoises.
  • Moths can hear bat squeaks to escape!

10. Applications of Ultrasound

Ultrasound can travel in well-defined paths even with obstacles.

• 1. Cleaning: Objects (like spiral tubes) are put in liquid. Ultrasonic waves stir the liquid, dusting off dirt from hard-to-reach places.
• 2. Detecting Cracks in Metal:
  Ultrasound is passed through metal blocks. If there is a crack, the wave reflects back and doesn’t reach the detector.
• 3. Medical Uses:
  • Echocardiography: Image of the heart using ultrasound.
  • Ultrasonography: Examining organs (liver, kidney) and foetus during pregnancy.
  • Lithotripsy: Breaking kidney stones into grains.
• 4. SONAR: (Sound Navigation And Ranging).
  Used to measure depth of ocean and locate underwater objects (submarines).
  Formula: $2d = v \times t$ (Speed $\times$ time).