Sound
Subject: Physics | Topic: Sound
For TLT, ECT, PST, JEST & All Competitive Exams
Sound is a form of energy that produces the sensation of hearing. It is produced by vibrating objects and needs a material medium (solid, liquid or gas) to travel. Sound is a regular topic in ECT, BPS-15, PST and JEST papers, with questions on wave type, speed, echo, pitch, loudness and ultrasound.
1. Production and Transmission of Sound
- Sound is produced when an object vibrates: a guitar string, a tuning fork, vocal cords, a drum skin.
- The vibrating object makes the particles of the surrounding medium vibrate, and the disturbance moves forward as a wave.
- The particles themselves do not travel with the wave; only energy is transferred.
- Sound cannot travel in a vacuum (proved by the electric bell in a bell jar experiment: the sound fades as air is pumped out).
2. Sound is a Longitudinal Wave
In a longitudinal wave, the particles of the medium vibrate parallel to the direction of travel of the wave. Sound waves consist of alternate compressions and rarefactions.
Compressions and Rarefactions
- Compression: region where particles are crowded together (high pressure).
- Rarefaction: region where particles are spread apart (low pressure).
3. Characteristics of a Wave
| Term | Meaning | Unit |
|---|---|---|
| Wavelength (λ) | Distance between two successive compressions (or rarefactions) | metre (m) |
| Frequency (f) | Number of vibrations (waves) per second | hertz (Hz) |
| Time period (T) | Time for one complete vibration | second (s) |
| Amplitude (A) | Maximum displacement of a particle from its mean position | metre (m) |
| Speed (v) | Distance travelled by the wave per second | m/s |
4. Speed of Sound
| Medium | Approximate speed (m/s) |
|---|---|
| Air at 0 °C | 331 |
| Air at 20 °C | 343 |
| Water | 1480 |
| Iron / Steel | about 5000 |
Order of speed: Solids > Liquids > Gases. Sound moves fastest in solids because their particles are closely packed.
Factors affecting the speed of sound
- Nature of medium: density and elasticity of the medium.
- Temperature: speed increases with temperature (about 0.6 m/s for each 1 °C rise in air).
- Humidity: sound travels slightly faster in moist air than in dry air.
- Not affected by: pressure (at constant temperature), loudness, pitch (frequency) or amplitude.
Light travels much faster than sound (3 × 10⁸ m/s), so we see lightning before we hear thunder.
5. Characteristics of Sound
| Characteristic | Depends on | Meaning |
|---|---|---|
| Loudness | Amplitude (and intensity) | How loud or soft a sound is. Measured in decibels (dB). |
| Pitch | Frequency | How high (shrill) or low (flat) a sound is. High frequency = high pitch. |
| Quality (Timbre) | Waveform / overtones | Lets us tell two sounds of the same pitch and loudness apart (e.g. guitar vs. piano). |
Sound Level (approximate)
| Source | Level (dB) |
|---|---|
| Whisper | 30 |
| Normal conversation | 60 |
| Busy traffic | 80 |
| Threshold of pain | about 120 |
6. Range of Hearing
- Audible sound: 20 Hz to 20,000 Hz (20 kHz) for humans.
- Infrasonic sound: frequency below 20 Hz. Produced by earthquakes; elephants and whales use it.
- Ultrasonic sound: frequency above 20,000 Hz. Bats, dolphins and dogs can hear or produce it.
Uses of Ultrasound
- SONAR (Sound Navigation and Ranging): finding the depth of the sea and locating submarines.
- Medical: ultrasonography (scanning of organs, unborn babies) and breaking kidney stones.
- Industry: cleaning delicate parts and detecting cracks in metals.
7. Reflection of Sound and Echo
Sound reflects from hard surfaces (walls, cliffs, hills) according to the laws of reflection: the angle of incidence equals the angle of reflection, and the incident ray, reflected ray and normal lie in the same plane.
An echo is the repetition of sound heard after reflection from a distant obstacle. The human ear retains a sound for about 0.1 second (persistence of hearing), so the reflected sound must arrive after at least 0.1 s to be heard separately.
Distance of obstacle: d = (v × t) ÷ 2 (t = total time to go and return)
- Reverberation: repeated reflections that make a sound persist in a large hall; reduced using curtains, carpets and sound-absorbing panels.
- Applications of reflection: megaphones, stethoscope, soundboards and curved ceilings of auditoriums.
8. Doppler Effect and Resonance
- Doppler effect: the apparent change in frequency (pitch) when there is relative motion between the source and the listener. A siren sounds higher in pitch as it comes closer and lower as it moves away.
- Resonance: when the frequency of an external force equals the natural frequency of an object, the object vibrates with a very large amplitude. Example: a glass breaking by a singer's note, or marching soldiers on a bridge being told to break step.
9. Solved Examples
Example 1: A sound has frequency 256 Hz and speed 340 m/s. Find its wavelength.
λ = v ÷ f = 340 ÷ 256 = 1.33 m
Example 2: An echo is heard 2 seconds after a shout. Speed of sound = 340 m/s. How far is the cliff?
d = (v × t) ÷ 2 = (340 × 2) ÷ 2 = 340 m
Example 3: Thunder is heard 5 s after lightning is seen. How far is the lightning? (v = 343 m/s)
d = v × t = 343 × 5 = 1715 m
Example 4: A wave has frequency 4 Hz. Find its time period.
T = 1 ÷ f = 1 ÷ 4 = 0.25 s
10. Noise Pollution
Noise is unpleasant, irregular sound. Excessive noise causes hearing loss, stress, headache and loss of sleep. It can be reduced by planting trees, controlling horns, using silencers and sound-proofing buildings.
11. Quick Exam Tricks
- Sound = longitudinal, mechanical wave. Light = transverse, non-mechanical.
- Speed: solids > liquids > gases. Temperature raises speed; frequency and amplitude do not.
- Pitch ↔ frequency. Loudness ↔ amplitude. Quality ↔ waveform.
- In echo questions, always divide by 2 because the sound travels to the obstacle and back.
- Audible range: 20 Hz – 20,000 Hz.
Practice MCQs
1. Sound cannot travel through:
a) Water b) Steel c) Vacuum d) Air
2. Sound waves are:
a) Transverse b) Longitudinal c) Electromagnetic d) None of these
3. The pitch of a sound depends on its:
a) Amplitude b) Frequency c) Speed d) Medium
4. The audible range of a human ear is:
a) 2 – 2000 Hz b) 20 – 20,000 Hz c) 200 – 2000 Hz d) Above 20,000 Hz
5. The minimum distance of a reflector for hearing an echo is about:
a) 5 m b) 10 m c) 17 m d) 34 m
Answers: 1-c, 2-b, 3-b, 4-b, 5-c
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