Sound Waves & Acoustics

5 min

Master longitudinal waves, speed, Doppler effect and decibels for SSC & Defence exams.

Golden Rule

Sound is a mechanical longitudinal wave requiring a material medium. Its speed is maximum in solids, minimum in gases, and it cannot travel through a vacuum.

Exam High-Yield Points

Speed of sound in dry air is approx 332 m/s at 0°C.
Ultrasonic waves (>20 kHz) are used in SONAR and echocardiography.
Doppler effect explains apparent frequency change due to relative motion.

Memory Hooks

Solid > Liquid > Gas: Sound speeds up as particles pack tighter!
Infrasonic = Low (<20Hz), Ultrasonic = High (>20kHz).
Echo needs minimum 17-meter obstacle distance.

Sound Classification Table

Type	Frequency Range	Source / Application
Infrasonic	< 20 Hz	Earthquakes, whales
Audible	20 Hz - 20 kHz	Human vocalization
Ultrasonic	> 20 kHz	Bats, SONAR, scans

Acoustic Phenomena

Echo

(Reflected Sound Wave)

Reflected Sound Wave

Heard clearly in large empty halls.

Sonar

(Underwater Echo Ranging)

Underwater Echo Ranging

Used by ships to find depth.

Decibel

(Sound Intensity Unit)

Sound Intensity Unit

Normal talk is about 60 dB.

Doppler

(Frequency Shift Effect)

Frequency Shift Effect

Train whistle pitch changes.

Common Exam Traps

Sound waves are transverseSound waves are longitudinal

Light waves are transverse, but sound oscillates parallel to propagation.

Sound travels fastest in vacuumSound cannot travel in vacuum

Light needs no medium, but sound strictly requires physical particles.

Echo is heard at 5m distanceEcho requires at least 17m

Human ear persists sound for 0.1s; minimum distance formula yields 17m.

Quick Practice Quiz

Q1.Through which medium does sound travel with the maximum speed?

Q2.What is the audible frequency range for normal human beings?

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