Sound and waves: echo, ultrasound, Doppler effect…

Core facts

Sound needs a medium (solid, liquid or gas). It cannot travel in a vacuum.
Sound is a longitudinal wave (compressions and rarefactions). Light and all EM waves are transverse.
Speed of sound is greatest in solids, less in liquids, least in gases. Approximate values: air about 340 m/s (at room temperature), water about 1,500 m/s, steel about 5,000 m/s. It rises with temperature.
Audible range for humans: 20 Hz to 20,000 Hz.
- Infrasound: below 20 Hz (elephants, whales, earthquakes).
- Ultrasound: above 20 kHz (bats, dolphins, porpoises).
Pitch depends on frequency; loudness on amplitude (measured in decibel, dB); quality (timbre) distinguishes two sounds of the same pitch and loudness.
Echo: reflected sound heard separately; needs a reflector at least about 17 m away (the ear retains a sound for 0.1 s).
Uses of ultrasound: SONAR, ultrasonography (scanning), cleaning delicate parts, welding, detecting flaws in metal.
Doppler effect: apparent change in frequency when source and observer are in relative motion.
EM spectrum, in order of increasing frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.
All EM waves travel at 3 x 10⁸ m/s in vacuum.

Explained

Noise and its limits. Noise Pollution (Regulation and Control) Rules, 2000 set ambient standards:

Area	Day (6 am-10 pm)	Night
Industrial	75 dB	70 dB
Commercial	65 dB	55 dB
Residential	55 dB	45 dB
Silence zone	50 dB	40 dB

Normal conversation is about 60 dB; prolonged exposure above about 85 dB can damage hearing.

Reflection of sound in practice. Echoes are used in SONAR; reverberation (persistence from repeated reflections) is controlled in halls by curtains, carpets and curved ceilings. Stethoscopes, megaphones and horns use reflection.

SONAR (Sound Navigation and Ranging) sends ultrasound and times the echo to find depth of sea, shoals and submarines. RADAR uses radio waves and microwaves, not sound.

Doppler effect in use: police speed guns, Doppler weather radar, medical blood-flow scans, the redshift of receding galaxies.

Electromagnetic spectrum and uses

Region	Use / fact
Radio	broadcast, mobile and TV communication
Microwave	radar, microwave oven, satellite links
Infrared	remote controls, night vision, heat
Visible (about 400-700 nm)	vision
Ultraviolet	sterilisation, vitamin D synthesis; absorbed by ozone
X-rays	medical imaging, airport security
Gamma rays	cancer therapy, sterilisation

Sonic boom: an object moving faster than sound (Mach number above 1) produces a shock wave heard as a boom.

Common traps

✗ "Sound travels fastest in air." ✓ Fastest in solids, slowest in gases.
✗ "Ultrasound is below 20 Hz." ✓ Infrasound is below 20 Hz; ultrasound is above 20 kHz.
✗ "Sound is a transverse wave." ✓ It is longitudinal.
✗ "RADAR uses ultrasound." ✓ RADAR uses radio/microwaves; SONAR uses ultrasound.
✗ "Gamma rays have the longest wavelength." ✓ They have the shortest; radio waves are the longest.
✗ "Loudness is measured in hertz." ✓ Loudness in decibel; hertz is frequency.

Quick revision

Sound: mechanical, longitudinal, needs a medium.
Audible 20 Hz to 20 kHz; infrasound below, ultrasound above.
Speed: solid > liquid > gas.
Echo needs about 17 m distance.
Loudness in dB; pitch is frequency.
SONAR = sound; RADAR = radio waves.
EM order (low to high frequency): radio, microwave, IR, visible, UV, X-ray, gamma.
All EM waves 3 x 10⁸ m/s in vacuum.

Go deeper: Deeper, Mains writing (beyond what APPSC Paper I usually asks)

Deeper Advanced

Wave terms: wavelength, frequency and speed are linked by v = f x wavelength. Frequency is unchanged when a wave enters a new medium; speed and wavelength change.
Interference, beats and resonance. Two close frequencies give beats; matching a natural frequency gives resonance (a swing, a glass shattered by sound, soldiers breaking step on a bridge).
Medical ultrasound uses frequencies of a few MHz; it avoids ionising radiation, so is used in pregnancy scans. Lithotripsy breaks kidney stones with shock waves.
Maxwell predicted EM waves (1860s); Hertz produced radio waves (1887). The ozone layer absorbs harmful short-wave UV (UV-C and most UV-B).
Microwave oven works by making water molecules in food rotate; it heats foods rich in water. Mobile networks use radio and microwave bands; their exposure limits are set by regulators.
Redshift and Hubble: spectral lines of distant galaxies shift to longer wavelengths, evidence of an expanding universe.
Tsunami and earthquake waves (P-waves longitudinal, S-waves transverse) are mechanical, not EM; S-waves cannot travel through liquid, which showed Earth's outer core is liquid.

Mains writing Mains writing

Frame: waves underpin communication, health and defence technology, and noise is a recognised pollutant.
Analysis:
- Ultrasound and X-rays have made diagnostics affordable and non-invasive, while misuse of scanning for sex determination led to the PCPNDT Act, 1994.
- Radio and microwave spectrum is a scarce public resource, allocated by auction and regulation.
- Urban noise from traffic and loudspeakers affects health; rules exist but enforcement is weak.
Way forward: strict enforcement of noise rules, silent zones near hospitals and schools, and responsible spectrum policy.
Example: Doppler weather radars help forecast cyclones along the east coast and give early warning.

Medha AI writes its own notes and checks them against the sources listed. Medha AI is an independent preparation service. It is not affiliated with, endorsed by or connected to APPSC, TSPSC, MPSC, UPSC or any other commission or government body.

2.1.4 Sound and waves: echo, ultrasound, Doppler effect and the electromagnetic spectrum

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