Sound Class 9 Notes Science Chapter 11

Introduction

Sound is a form of energy causing a hearing sensation. Other energy forms include mechanical and light energy.Energy conservation is the transformation, not creation or destruction, of energy. Sound production involves energy utilization.

Production of Sound

• Sound is generated by vibrations (e.g., clapping hands).
• Various methods produce sound: plucking, scratching, blowing, etc.
• Vibrations in vocal cords produce human voice.
• Sound in musical instruments comes from specific vibrating parts.

Propagation of Sound

• Waves: Disturbances moving through mediums.
• Sound waves: Mechanical waves characterized by particle motion.

Propagation of sound in Air:

• Vibrations create high-pressure compressions and low-pressure rarefactions.

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

• Formula: Speed (ν) = Frequency (ƒ) × Wavelength (λ).

Sound Waves as Longitudinal Waves

• In longitudinal waves, particle movement is parallel to wave propagation.
• Slinky example demonstrates longitudinal wave properties.

Characteristics of a Sound Wave

• Frequency: Number of oscillations per unit time (Hertz).
• Amplitude: Maximum disturbance magnitude in the medium.
• Speed: Distance a point on the wave travels per unit time.
• Loudness related to amplitude, pitch to frequency.
• Quality  (Timbre): Quality differentiating sounds with same pitch and loudness.
• Pitch: Higher frequency means higher pitch of the produced sound.
• Intensity: Energy passing through a unit area per second.

Speed of Sound in Different Media

Speed varies with medium and temperature. Faster in solids, slower in gases.Note: Speed of sound increases with increase in temperature.

Range of Hearing

• Human audible range: 20 Hz to 20 kHz.
• Infrasound: Below 20 Hz : Used by animals like whales and elephants.
• Ultrasound: Above 20 kHz : Used by bats, dolphins, and for industrial and medical applications.
  Age and species affect hearing range.

Reflection of Sound

Echo

• Reflected sound heard after 0.1 seconds.

• Minimum distance for an echo: 17.2 m (for air at 22°C).

Reverberation

• Continuous reflection causes prolonged sound.

• Reduced using sound-absorbing materials like curtains or fibreboard.

Applications of Multiple Reflections

1. Megaphones and Loudhailers: Direct sound using conical shapes.

2. Stethoscopes: Reflect sound to amplify body sounds.

3. Auditoriums: Curved ceilings distribute sound evenly.

4. Rolling Thunder: Repeated reflections of sound.

Applications of Ultrasound

1.Cleaning: Removes dust and grease from hard-to-reach areas.

2. Detecting Cracks: Identifies flaws in metal blocks.

3. Echocardiography: Creates images of the heart using reflected ultrasonic waves.

4. Ultrasonography:

• Images internal organs.
• Detects abnormalities (e.g., stones, tumours).
• Useful during pregnancy for checking fetal development.

5. Kidney Stone Treatment: Breaks stones into smaller fragments for natural expulsion.