📘Class 9 Science – Chapter: Sound Notes

✨ Introduction to Sound

• Sound is a form of energy that produces a sensation of hearing.

• It is produced due to the vibration of objects.

Example: A ringing bell, plucked guitar string.

🔹 Production of Sound

• Sound is produced by vibrating bodies.

Examples:

• Vocal cords vibrate to produce sound.

• Tuning fork produces sound by vibration.

🔹 Propagation of Sound

• Sound travels as a longitudinal wave through a medium (solid, liquid, gas).

• It requires a medium to travel; cannot travel in vacuum.

Example: We hear thunder after seeing lightning because light travels faster than sound.

🔹 Characteristics of Sound Waves

• Wavelength (λ): Distance between two consecutive compressions or rarefactions.

• Frequency (f): Number of oscillations per second.

• Time Period (T): Time taken for one oscillation.

• Amplitude (A): Maximum displacement of particles.

Equations:

• Speed (v) = Frequency (f) × Wavelength (λ)

• Frequency (f) = 1 / Time Period (T)

Example: A wave with frequency 100 Hz and wavelength 2 m has speed = 100 × 2 = 200 m/s

🔹 Types of Waves

• Mechanical Waves: Require a medium (e.g., sound).

• Electromagnetic Waves: Do not require a medium (e.g., light).

Mechanical Waves can be:

• Longitudinal (sound)

• Transverse (water waves)

🔹 Reflection of Sound

• Sound reflects off surfaces, following the laws of reflection.

Laws of Reflection:

• Angle of incidence = Angle of reflection

• Incident wave, reflected wave, and normal lie in the same plane

Example: Echoes, soundboards in auditoriums.

🔹 Echo and Reverberation

• Echo: Reflected sound heard after some time (minimum 0.1 s).

  • Minimum distance = 17.2 m (when speed = 343 m/s)

Formula:
Distance = Speed × Time / 2

Example: Shouting in a valley and hearing your voice again.

• Reverberation: Persistence of sound due to multiple reflections.

🔹 Range of Hearing

Examples:

• Elephants use infrasonic sounds.

• Bats use ultrasonic sounds for echolocation.

🔹 Applications of Ultrasound

• Cleaning narrow parts (like watches, teeth).

• Medical imaging (ultrasound scan).

• SONAR (Sound Navigation and Ranging).

SONAR Working Principle:

• Ultrasonic waves sent and reflected back from object.

Formula:
Distance = (Speed × Time) / 2

Example: Depth of sea measured using SONAR.

🔹 Structure of the Human Ear

• Collects sound and sends electrical signals to the brain.

Parts:

• Outer ear (Pinna)

• Ear canal

• Ear drum

• Middle ear (3 bones)

• Inner ear (cochlea, auditory nerve)

Process:

1. Pinna collects sound

2. Vibrations hit the ear drum

3. Bones amplify vibrations

4. Cochlea converts to electrical signals

5. Brain interprets them