Maarg Manthan · Topic 8.7

Mechanical Wave Phenomena

Science & Technology › Physics – Mechanics, Waves and Thermodynamics · Topic 8.7

Mechanical waves are disturbances that carry energy through a material medium, such as air, water or the ground, and they cannot travel through a vacuum. In a transverse wave, particles move at right angles to the direction of the wave, as in ripples on water and in seismic S-waves. In a longitudinal wave, particles move back and forth along the direction of the wave, as in sound and in seismic P-waves. A wave is described by its wavelength, frequency, amplitude and speed, which are related by speed equals frequency times wavelength. Sound is a longitudinal mechanical wave that travels at about 343 metres a second in air at 20 degrees Celsius, faster in liquids and fastest in solids. Humans hear frequencies from about 20 hertz to 20 kilohertz.

5 min read · UPSC Prelims · Science & Technology · Physics – Mechanics, Waves and Thermodynamics

At a Glance

Needs a medium Yes
Sound in air About 343 m/s
Audible range 20 Hz to 20 kHz
Seismic P-wave Longitudinal
Seismic S-wave Transverse
Exam link UPSC Prelims, Science & Technology: Physics – Mechanics, Waves and Thermodynamics

Where Does It Fit?

This is the seventh topic of the macro on physics. It contrasts mechanical waves with the electromagnetic waves of earlier topics and links to earthquakes and sound technology. Previous topic: Heat and Thermodynamic Principles. Next topic: ionosphere Space…">Electromagnetic Wave Behaviour.

What Are the Types and Properties of Mechanical Waves?

Item	Detail
Transverse wave	Particles move perpendicular to the wave. Needs a medium that resists shear, so it travels in solids and on water surfaces but not through fluids. Water ripples, strings, seismic S-waves
Longitudinal wave	Particles move along the wave in compressions and rarefactions. Travels in solids, liquids and gases. Sound, seismic P-waves
Wavelength and frequency	Distance between crests and the number of waves per second in hertz
Amplitude	The maximum displacement, which relates to the loudness of sound or the energy of the wave
Wave speed	Equals frequency times wavelength and depends on the medium
Speed of sound	About 343 metres per second in air, about 1,500 in water and about 5,000 in steel

What Are the Properties and Phenomena of Sound?

Pitch and loudness: pitch depends on frequency, and loudness depends on amplitude, measured in decibels.
Echo: a reflected sound heard separately, needing a gap of at least about 0.1 second, so a reflecting surface at least 17 metres away. Reverberation is repeated reflection in halls.
Infrasound: below 20 hertz, used by elephants and whales, and produced by earthquakes and volcanoes.
Ultrasound: above 20 kilohertz, used by bats and dolphins for navigation. Applications include sonar, medical imaging of the foetus, cleaning of instruments and non-destructive testing, as in Imaging Methods: CT Scan, MRI, PET Scan.
Doppler effect: change in the observed frequency when the source and observer move relative to each other, as the pitch of a siren rises as it approaches. Used in radar guns and weather radar. Christian Doppler described it in 1842.
Resonance: a system vibrates with large amplitude when driven at its natural frequency, as in musical instruments and, destructively, in bridges and buildings.
Sonic boom: a shock wave made when an object travels faster than sound, which is Mach 1.

What Are Seismic Waves and What Else Should You Remember?

Wave	Features
P-wave (primary)	Longitudinal, fastest, arrives first and travels through solids and liquids
S-wave (secondary)	Transverse, slower, travels only through solids, so it does not pass through the liquid outer core
Surface waves	Slowest but cause the most damage on the surface, such as Love and Rayleigh waves
Measurement	Seismographs record the waves. Magnitude on the Richter or moment scale and intensity on the Mercalli scale
Indian seismic zones	The country is divided into zones II to V in the Bureau of Indian Standards code, with the Himalaya and the north-east in the highest zones
Early warning	P-waves arrive before the damaging S and surface waves, which gives a few seconds of warning in some systems

More Wave Facts

Sound waves: sound waves are produced by vibrating objects such as vocal cords, strings and membranes, and they carry the vibration to the ear through compressions and rarefactions.
Noise pollution: noise above safe levels harms hearing and sleep, and India has noise standards, such as 55 decibels by day and 45 at night for residential areas under the Noise Pollution Rules, 2000.
Tsunami waves: tsunami waves act as longitudinal waves in deep water and transverse in shallow water, as the Indian Ocean tsunami of 2004 showed.

Why Does It Matter for the Exam?

Type questions: which waves are longitudinal or transverse, mechanical or electromagnetic.
Sound questions: audible range, echo condition, speed in different media and ultrasound uses.
Disaster link: seismic waves and the interior of the Earth.
Technology link: sonar for ocean mapping, Doppler radar for cyclones, and noise pollution rules.
Common errors: believing that sound travels in a vacuum or is faster in air than in water.

As an analytical point, mechanical waves show how energy moves without matter moving along with it. They reveal the interior of the Earth, the sea floor and the human body.

Why Is This Topic Important?

Facts Wave types and speeds asked
Concept Doppler and resonance
Link Earthquakes and sonar

What Came Next?

The next topic covers electromagnetic wave behaviour. Next topic: Electromagnetic Wave Behaviour.

Key People and Terms

Amplitude Maximum displacement
Compression Region of high pressure
Infrasound Below 20 Hz
Ultrasound Above 20 kHz
Doppler effect Frequency shift from relative motion
Resonance Large response at natural frequency

Exam Corner

Points to Remember

Sound is longitudinal.
Audible: 20 Hz to 20 kHz.
P-waves arrive first; S-waves not in liquids.
Echo needs about 17 m.
Doppler effect: 1842.

Do Not Confuse With

Transverse and longitudinal Perpendicular versus parallel motion
Infrasound and ultrasound Below versus above audible range
P-wave and S-wave Longitudinal versus transverse

Memory Hook

Sound is longitudinal, light transverse; P comes first, S stays solid; Doppler shifts the siren, ultrasound for the sonar.

Mains Angle

Questions link wave physics with earthquakes and technology. These points add depth.

Types Transverse and longitudinal
Sound Properties and uses
Seismic Earth interior

A Question You May Face

An original practice question, not a past paper question.

“Distinguish between transverse and longitudinal waves with examples, and explain how seismic waves give information about the interior of the Earth.”

How to Answer

Types Two kinds.
Sound Properties.
Doppler Effect.
Seismic P and S.

GS Relevance

Prelims: types of waves, sound properties, speed in media, Doppler effect, ultrasound uses, seismic waves. GS Paper 3: science and technology, disaster management.

Frequently Asked Questions

What is the difference between transverse and longitudinal waves?

In a transverse wave the particles of the medium vibrate at right angles to the direction in which the wave travels, as in ripples on water and light waves. In a longitudinal wave the particles vibrate back and forth along the direction of travel, forming compressions and rarefactions, as in sound waves.

What is the range of human hearing?

Humans can hear sound with frequencies from about 20 hertz to 20,000 hertz. Sounds below this range are called infrasound and those above it are called ultrasound, and animals such as elephants use infrasound while bats and dolphins use ultrasound to communicate and navigate.

What is the Doppler effect?

The Doppler effect is the change in the observed frequency of a wave when the source and the observer are moving relative to each other. The pitch of a siren seems higher as it approaches and lower as it moves away, and the effect is used in radar, medical blood flow tests and the study of expanding galaxies.

How do P-waves and S-waves differ?

P-waves are primary seismic waves that are longitudinal, travel fastest and pass through solids and liquids. S-waves are secondary waves that are transverse, slower and move only through solids, so their failure to pass through the Earth's outer core showed that the core is liquid.

Why does an echo need a minimum distance?

The human ear can separate two sounds only if they arrive at least about 0.1 second apart. As sound travels about 343 metres per second, the sound must travel to a reflecting surface and back in that time, so the surface must be at least about 17 metres away for an echo to be heard.

PYQ Practice — Statement Analysis

1 Sound waves are longitudinal waves.

True

They need a medium.

2 Sound travels faster in air than in steel.

False

It is faster in solids.

3 S-waves cannot pass through liquids.