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Question 3 of 5 · Waves & Sound
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SSC

Sound waves with frequencies less than 20 Hz are called:

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Answer: Option A — Infrasonic waves

Detailed Solution

Sound waves are classified according to their frequency, the number of vibrations per second, measured in hertz (Hz):

Infrasonic waves (infrasound): frequencies below 20 Hz. Humans cannot hear them.

Audible sound: frequencies from about 20 Hz to 20,000 Hz (20 kHz). This is the range that a healthy young human ear can hear. The upper limit falls with age.

Ultrasonic waves (ultrasound): frequencies above 20,000 Hz (20 kHz). Humans cannot hear them.

Sources and uses of infrasonic waves:

They are produced by earthquakes, volcanic eruptions, ocean waves, avalanches, thunder and large explosions. Earthquakes produce infrasound before the main shock waves, and some animals may sense it, which may explain why they seem to behave strangely before earthquakes.

Elephants and whales use infrasound to communicate over long distances, sometimes several kilometres. Rhinoceroses, giraffes and alligators also produce infrasound.

Infrasound monitoring stations are used to detect nuclear explosions under the Comprehensive Nuclear-Test-Ban Treaty.

Infrasound travels very long distances because it is absorbed very little by air.

Some animals can hear sounds that humans cannot:

  • Dogs can hear up to about 45,000 Hz; dog whistles use ultrasound.
  • Bats, dolphins and porpoises produce and hear ultrasound (up to 1,00,000 Hz or more).
  • Elephants can hear infrasound.
  • Uses of ultrasonic waves include ultrasound scanning in medicine, SONAR, cleaning delicate parts (ultrasonic cleaning), detecting cracks in metals and breaking kidney stones (lithotripsy).

"Supersonic" refers to speed, not frequency: an object moving faster than the speed of sound (about 343 m/s in air) is supersonic. A "sonic boom" is produced by supersonic aircraft. The loudness of sound is measured in decibels (dB), and its pitch depends on its frequency.

Why not the other options?

  • ✗Ultrasonic waves have frequencies above 20,000 Hz.
  • ✗Audible waves have frequencies between 20 Hz and 20,000 Hz.
  • ✗Supersonic refers to speeds greater than that of sound.

Quick tip

Remember: Infrasonic < 20 Hz (earthquakes, elephants, whales); audible 20 Hz-20 kHz; ultrasonic > 20 kHz (bats, dolphins, SONAR, sonography); supersonic = faster than sound (speed, not frequency).

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Waves & Sound — Preparation

This question tests your understanding of Waves & Sound, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.

This topic covers waves and their types (transverse and longitudinal), the properties of sound, the speed of sound in different media, echo, reverberation, the Doppler effect, pitch, loudness and the decibel scale, and infrasonic and ultrasonic sound and their uses. Questions often ask in which medium sound travels fastest (solids) and why it cannot travel through a vacuum, the human audible range (about 20 Hz to 20,000 Hz), the instrument used to detect underwater objects (SONAR), and the effect that explains the changing pitch of a passing train's horn (the Doppler effect). Practise the waves and sound questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.

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