Physics Class 10 — Chapter 2: Sound
Complete Sindh Board notes — definitions, key points, formulas, examples, and numerical problems.
Introduction
Sound is a form of energy produced by vibrating bodies. It travels through a material medium (solid, liquid or gas) as mechanical waves. Sound cannot travel through vacuum.
Production of Sound
When an object vibrates it causes nearby particles of the medium to oscillate about their mean positions. These oscillations propagate as waves — causing sound to travel from the source to the listener.
Examples: Tuning fork, guitar string, vibrating vocal cords.
Types of Mechanical Waves
- Transverse waves: particle vibration is perpendicular to wave motion (e.g., waves on a rope).
- Longitudinal waves: particle vibration is parallel to wave motion. Sound travels as longitudinal waves having regions of compression and rarefaction.
Compression and Rarefaction
Compression (C): region of high pressure where particles are close together.
Rarefaction (R): region of low pressure where particles are farther apart.
Characteristics of Sound Waves
| Quantity | Meaning | Unit / Relation |
|---|---|---|
| Frequency (f) | Number of vibrations per second | Hertz (Hz) |
| Time period (T) | Time for one complete vibration | T = 1/f (s) |
| Wavelength (λ) | Distance between two successive compressions or rarefactions | metre (m) |
| Velocity (v) | Speed of sound in the medium | v = f × λ (m/s) |
| Amplitude (A) | Maximum displacement of particles from mean position | Related to loudness |
Loudness, Pitch and Quality (Timbre)
- Loudness depends on amplitude — larger amplitude = louder sound. Loudness is measured in decibels (dB).
- Pitch depends on frequency — higher frequency = higher pitch.
- Quality (Timbre) differentiates two sounds with same pitch and loudness but different sources (e.g., flute vs violin).
Range of Hearing
- Audible sound for humans: 20 Hz to 20,000 Hz (20 kHz).
- Infrasonic: < 20 Hz (used by elephants, some seismic events).
- Ultrasonic: > 20,000 Hz (used by bats, medical ultrasound, SONAR).
Speed of Sound in Different Media
Sound speed varies with medium; generally: solids > liquids > gases.
| Medium | Approx. Speed at 20°C |
|---|---|
| Air | ~343 m/s |
| Water | ~1480 m/s |
| Steel | ~5000 m/s (varies by alloy) |
Speed in air depends slightly on temperature: v ≈ 331 + 0.6 × T(°C).
Reflection of Sound — Echo
When sound reflects from a hard surface and returns to the listener, this is reflection of sound. A distinct reflected sound heard after the original is called an echo.
Condition for hearing an echo: The reflected sound must return at least 0.1 s after the original sound. For air, this implies a minimum one-way distance of about 17 m between source and reflector (at ~340 m/s).
Uses and Applications of Sound
- SONAR — detecting underwater objects, measuring depth using echoes of ultrasonic pulses.
- Ultrasound — medical imaging, cleaning small/micro parts, nondestructive testing.
- Musical instruments — production of musical notes through vibrating strings, air columns, or membranes.
Echo vs Reverberation
| Echo | Reverberation |
|---|---|
| Single reflected sound heard distinctly | Many reflections causing prolongation of sound |
| Heard when distance to reflector > ~17 m | Occurs in enclosed spaces with multiple nearby reflectors (distance < 17 m) |
Important Formulae
- v = f × λ
- T = 1 / f
- Distance to reflector (echo) = (v × t) / 2
Numerical Problems
-
Problem 1: A man hears an echo after 0.2 s. If speed of sound is 340 m/s, find distance to the reflecting wall.
Solution: distance = (340 × 0.2) / 2 = 34 m. -
Problem 2: A source emits sound of frequency 2 kHz in air. If speed of sound is 340 m/s, find the wavelength.
Solution: λ = v / f = 340 / 2000 = 0.17 m.
Summary — Key Points to Remember
- Sound is produced by vibrations and needs a medium to travel.
- Sound travels as longitudinal waves composed of compressions and rarefactions.
- Pitch depends on frequency; loudness depends on amplitude.
- Human audible range: 20 Hz to 20 kHz.
- Reflection of sound produces echoes — used in SONAR and depth measurement.
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