1. Introduction — What is Sound?

Definition: Sound is a form of energy produced by vibrating objects that travels through a medium (solid, liquid, or gas) as longitudinal waves and is heard when it reaches the ear.

Key points: Sound requires a medium (no sound in vacuum). It is created by vibrations which produce compressions and rarefactions in the medium.

Example: Plucking a guitar string produces vibrations; these reach the ear as sound.

2. Production and Propagation of Sound

1. Production: Vibration of a source (string, membrane, column of air).
2. Propagation: Vibrations create compressions and rarefactions; these travel as longitudinal waves.
3. Nature of waves: Sound waves are longitudinal — particles vibrate parallel to wave direction.

3. Characteristics of Sound

Sound is described by:

• Frequency (f): Number of vibrations per second (Hz). Determines pitch.
• Loudness: Perceived intensity; depends on amplitude. Measured in decibels (dB).
• Pitch: How high or low a sound is perceived; depends on frequency.
• Quality (Timbre): Distinguishes different sources producing the same pitch and loudness.

4. Audible Range and Types of Sound

Human hearing range: 20 Hz to 20,000 Hz (20 kHz).

5. Speed of Sound

Speed depends on the medium and its temperature. Generally: solids > liquids > gases.

Relation: v = f × λ (v = speed, f = frequency, λ = wavelength)

When temperature increases, the speed of sound in air increases (molecules move faster).

6. Reflection of Sound — Echo and Reverberation

Reflection: When sound strikes a hard surface and bounces back.

Echo: A reflected sound that is heard separately after the original sound. Requirement: time interval ≥ 0.1 s (approx.).

Minimum distance for echo: For air (v ≈ 343 m/s), minimum distance to reflecting surface ≈ 17 m, because sound must travel to the surface and back in ≥ 0.1 s.

Reverberation: Multiple reflections in an enclosed space causing prolonged sound. Good acoustics control reverberation for clarity.

7. Noise and Noise Pollution

Noise: Unwanted or unpleasant sound (irregular and often loud).

Noise pollution: Excessive environmental noise causing harm to health and wildlife.

Effects

• Hearing impairment
• Stress, sleep disturbance
• Reduced productivity and concentration

Control measures

• Use of silencers and mufflers
• Planting trees and creating green belts
• Soundproofing (walls, windows)
• Regulations on industrial and traffic noise

8. Acoustics — Designing for Sound

Acoustics is the science of sound and its behavior in buildings. Important for designing halls, auditoriums and recording rooms.

• Absorption and diffusion materials reduce echoes and produce clear sound.
• Shape of the hall and placement of audience affects sound distribution.

9. Ultrasonic Waves (Ultrasound)

Definition: Sound waves with frequency above 20 kHz (20,000 Hz). Humans cannot hear ultrasound.

Major applications

• Medical imaging: Ultrasound scans for fetus and internal organs.
• Industrial testing: Detecting cracks and flaws in metal (non-destructive testing).
• Cleaning: Ultrasonic cleaners remove dirt from delicate objects.
• SONAR: Uses ultrasound for navigation and distance measurement underwater.

10. Important Formulas & Worked Example

Worked example

Problem: You shout and receive echo after 0.6 s. Find distance to reflecting wall. (Use v = 343 m/s)

Solution: Distance = v × t / 2 = 343 × 0.6 / 2 = 102.9 m

11. Important Questions and Detailed Answers

Q1: Define sound and explain how it travels.

Answer: Sound is produced by vibrating bodies. These vibrations cause compressions and rarefactions in the medium (air, water, or solids). The pattern of compressions and rarefactions moves through the medium as longitudinal waves and reaches the ear, producing the sensation of sound. Sound cannot travel through vacuum because there are no particles to carry the vibrations.

Q2: Explain frequency, loudness, pitch and quality with examples.

Answer: Frequency is vibrations per second (Hz) and determines pitch; loudness depends on amplitude and is measured in decibels; pitch is how high or low a sound seems (higher frequency = higher pitch); quality or timbre lets us distinguish different instruments playing the same note (piano vs violin).

Q3: What is echo? Explain conditions required for an echo and one application.

Answer: An echo is a reflected sound heard after the original sound. Conditions include a hard reflecting surface placed at sufficient distance so that time delay ≥ 0.1 s. Application: SONAR uses reflected sound to measure distance underwater.

Q4: Describe noise pollution and how it can be controlled.

Answer: Noise pollution is harmful or annoying levels of noise. It can be reduced by planting trees, using soundproofing materials, enforcing noise regulations, and designing quieter machines and vehicles.

Q5: Define ultrasound and discuss two medical uses.

Answer: Ultrasound has frequency > 20 kHz. Medical uses: (1) Diagnostic imaging (ultrasound scans), (2) Therapeutic uses (breaking kidney stones with focused ultrasound).