Infrasonic vs. Ultrasonic

The Hook: Humans are partially deaf. We live in a narrow slice of the sound spectrum.
While we chat, Elephants are whispering secrets below our hearing (Infrasonic), and Bats are screaming above our hearing (Ultrasonic).

1. The Frequency Ruler

Sound is measured in Hertz (Hz). Here is where humans stand compared to the rest of nature.

Infrasonic

Audible

Ultrasonic

20 Hz 20,000 Hz

🐘 INFRASONIC < 20 Hz

Produced by Elephants, Whales, and Earthquakes. These waves travel long distances.

🙎‍♂️ AUDIBLE 20 Hz - 20 kHz

The Human Range. Anything above this is silent to us.

🦇 ULTRASONIC > 20 kHz

Produced by Bats, Dogs, and Dolphins. High energy, short distance.

2. How Bats "See" with Sound

Bats are nocturnal (active at night). They don't use eyes to hunt mosquitoes; they use Echolocation.

[Image of bat echolocation diagram]

1. Scream: The bat emits a high-pitched Ultrasonic squeak.
2. Bounce: The sound hits a moth and bounces back (Reflection).
3. Calculate: The bat's brain calculates the time delay to know exactly where the prey is.

Note: This is exactly how SONAR (Sound Navigation And Ranging) works on ships to detect submarines.

[Image of SONAR working principle]

3. Applications of Ultrasound (Exam Yield)

• Medical: Ultrasonography (seeing the fetus in the womb).
• Cleaning: To clean spiral tubes and electronic components (High-frequency vibrations shake off dust).
• Drilling: To drill holes in glass or hard metals.
• SONAR: To measure the depth of the ocean.

4. Speed of Sound (The Medium Matters)

Does Sound Travel in Space?

NO. Sound needs a medium (solid, liquid, gas) to travel. Space is a vacuum, so it is silent.

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Speed Hierarchy:

• Solids (Aluminium/Steel) ~5000 - 6000 m/s
• Liquids (Water) ~1500 m/s
• Gases (Air at 20°C) ~343 m/s

Tip: Sound travels fastest in Solids because molecules are packed tight!

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Revision Flashcards

SONAR

Full Form?

SOund Navigation And Ranging

Uses Ultrasonic waves to measure ocean depth.

20 Hz

What is below this frequency?

Infrasonic

Sounds produced by Earthquakes and Elephants.

Vacuum

Speed of sound in vacuum?

Zero

Sound is a mechanical wave; it cannot travel without a medium.

Galton Whistle

Used for whom?

Dogs

It produces Ultrasonic sound which dogs can hear, but humans cannot.

Exam-Style MCQs

Q1. Which of the following has the maximum speed of sound?

A) Air

B) Water

C) Steel

Correct! Sound travels fastest in solids (Steel > Water > Air).

Q2. The frequency of Ultrasonic waves is:

A) Less than 20 Hz

B) More than 20,000 Hz

Correct! (20 kHz).

C) Between 20 Hz and 2000 Hz

Q3. Bats detect obstacles in their path by receiving:

A) Infrasonic waves

B) Ultrasonic waves

Correct! They use echolocation via reflected ultrasonic waves.

C) Radio waves

Part of the Physics Revision Series.