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UPPCS Physics Gap Sheet: Sound and Acoustics.
UPPCS has asked 21 of these. 9 have not been asked yet. Lock the first list, then read the second: it holds what UPPCS has not used yet.
First, the concept
Sound is a mechanical disturbance and needs a material medium. In air, compressions and rarefactions carry it as a longitudinal wave. Frequency sets pitch; amplitude strongly affects loudness; the waveform gives timbre.
Reflection produces echoes and reverberation. Resonance gives a large response when a driving frequency matches a natural frequency. Relative motion changes heard pitch through the Doppler effect; it does not turn sound into light.
✅ Already asked (21)
Wave properties
- Sound in vacuum · Sound cannot travel through vacuum because there is no material medium.
- Longitudinal sound in air · Air particles vibrate parallel to the direction of wave travel.
- Pitch and frequency · Higher frequency gives a higher pitch.
- Loudness and amplitude · Greater amplitude generally makes sound louder; perceived loudness also depends on the ear.
- Sound-speed order · For air, water and steel under usual conditions, steel is fastest and air slowest.
- Sound in a sitar string · The string supports transverse standing waves; sound leaving it travels through air.
Hearing and ultrasound
- Human hearing range · A young healthy ear typically hears about 20 Hz to 20,000 Hz.
- Infrasound · Its frequency is below 20 Hz.
- Ultrasound · Its frequency is above 20,000 Hz.
- Bat echolocation · Bats use emitted ultrasound and returning echoes to locate prey and obstacles.
- SONAR · Uses sound pulses and echoes to locate underwater objects or measure depth.
- Ultrasonic imaging · Medical ultrasound uses reflected sound and does not use ionising radiation.
- Ultrasonic cleaning · Rapid vibrations and cavitation help remove dirt from small parts.
Reflection and relative motion
- Distinct echo · A return delay of about 0.1 second separates an echo; the reflector is roughly 17 metres away in ordinary air.
- Reverberation · Many closely spaced reflections prolong sound after the source stops.
- Concert-hall treatment · Sound-absorbing materials control excessive reverberation.
- Doppler effect · An approaching source is heard at higher frequency; a receding one at lower frequency.
- Resonance · A matching driving frequency can produce large vibrations.
- Beats · Two close frequencies produce periodic rises and falls in loudness.
- Sonic boom · Supersonic motion produces shock waves heard as a boom.
Further acoustic facts
- Human speech phoneme · A phoneme is a basic contrastive sound unit in language, not an SI unit.
❌ Not asked yet (9)
Items marked 'asked in other exams' have come in UPSC or other state exams, not yet in UPPCS.
🔒 9 items not asked yet
Wave properties: 2 · Further acoustic facts: 7
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⚠️ Ultrasound means high frequency, not high loudness. SONAR uses sound; RADAR uses radio waves.
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