PhysicsICSE Class 9 · Chapter 11

Infrasonic, Sonic and Ultrasonic Sound ICSE Class 9: Ranges, Uses and Key Differences

Updated 15 October 2026 · Fact-checked

Humans hear sound from about 20 Hz to 20,000 Hz (sonic or audible range). Below 20 Hz is infrasonic; above 20 kHz is ultrasonic. ICSE Class 9 covers simple uses of each, such as ultrasound scans and SONAR, and the difference between ultrasonic (frequency) and supersonic (speed), and between infrasonic and subsonic.

What Infrasonic, Sonic and Ultrasonic Sound is about

Sound waves can have any frequency, but our ears respond only to a limited range. The CISCE 2028 Class IX syllabus asks for elementary ideas and simple applications of:

infrasonic sound (frequency below 20 Hz);
sonic or audible sound (about 20 Hz to 20 kHz);
ultrasonic sound (frequency above 20 kHz);

and for the differences between ultrasonic and supersonic, and between infrasonic and subsonic.

This is a short, mostly descriptive chapter. Expect MCQs on ranges, 2-mark differences, 3-mark lists of uses with reasons, and simple numericals linking frequency, wavelength and speed with $V = f λ$ , or a SONAR depth from an echo time.

Weightage: CISCE publishes no chapter weightage and no question-paper layout for ICSE Class 9 Physics. Class IX is examined by your school: one 80-mark paper of 2 hours plus 20 marks of internal assessment. Infrasonic, Sonic and Ultrasonic Sound is in the 7. Sound (ii) part of the 2028 syllabus.Source: CISCE ICSE 2028 Physics syllabus, Class IX

Key concepts

The three frequency ranges

Infrasonic: below about 20 Hz. Too low for human ears. Produced by earthquakes, volcanoes, ocean waves, and by large animals such as elephants and whales.
Sonic (audible): about 20 Hz to 20,000 Hz (20 kHz). The range of normal human hearing; the upper limit falls with age.
Ultrasonic: above 20 kHz. Too high for human ears. Heard by dogs (up to about 45 kHz), bats and dolphins (over 100 kHz).

The limits differ slightly from person to person; 20 Hz and 20 kHz are the standard values to use.

Worked example

Classify sounds of frequency 5 Hz, 500 Hz, 15 kHz and 40 kHz.

Try it first, then show the solutionSolution

5 Hz: below 20 Hz: infrasonic.
500 Hz and 15 kHz: between 20 Hz and 20 kHz: sonic (audible).
40 kHz: above 20 kHz: ultrasonic.

Answer: Infrasonic; sonic; sonic; ultrasonic

Wavelengths in each range

Since $V = f λ$ , at a given speed higher frequency means shorter wavelength.

In air ( $V \approx 340$ m s⁻¹):

20 Hz: $λ = 17$ m;
20 kHz: $λ = 1.7$ cm;
ultrasound of 100 kHz: $λ = 3.4$ mm.

The very short wavelengths of ultrasound let it travel in a narrow beam with little spreading and reflect from small objects, which is why it is useful for detecting and imaging things.

Worked example

A bat emits ultrasound of frequency 85 kHz. Find its wavelength in air (340 m s⁻¹).

Try it first, then show the solutionSolution

$λ = \frac{V}{f} = \frac{340}{85000}$ .
$λ = 0.004$ m $= 4$ mm.

Answer: 4 mm

Uses of ultrasound

Medical imaging (ultrasonography): ultrasound reflected from organs and from a baby in the womb forms images, without the harm of X-rays.
SONAR (Sound Navigation And Ranging): ships send ultrasound pulses into the sea and time the echoes to find the depth of the sea or to locate submarines and shoals of fish. Depth $= \frac{V \times t}{2}$ .
Cleaning: ultrasound in a liquid shakes dirt off small parts, jewellery and surgical instruments.
Detecting flaws (cracks) in metal parts.
Animals: bats and dolphins find their way and catch prey by echolocation; dog whistles use ultrasound that only dogs hear.

Worked example

A ship's SONAR sends an ultrasonic pulse and receives the echo from the sea bed 2 s later. If ultrasound travels at 1500 m s⁻¹ in sea water, find the depth.

Try it first, then show the solutionSolution

The pulse travels down and back: distance $= 1500 \times 2 = 3000$ m.
Depth $= \frac{3000}{2} = 1500$ m.

Answer: 1500 m

Uses and effects of infrasound

Earthquakes and volcanoes produce infrasound before and during the main event; some animals (such as dogs, elephants) appear to sense it and behave restlessly.
Elephants and whales communicate over long distances using infrasound, which travels far with little loss.
Monitoring: infrasound detectors help track storms, volcanic eruptions and large explosions.
Infrasound is not heard, but strong infrasound can cause discomfort.

The long wavelengths of infrasound (several metres or more) let it travel very long distances.

Worked example

An elephant's call has a frequency of 14 Hz. Can a person hear it? Find its wavelength in air (340 m s⁻¹).

Try it first, then show the solutionSolution

14 Hz is below 20 Hz: infrasonic, so a person cannot hear it.
$λ = \frac{340}{14} \approx 24.3$ m.

Answer: No; about 24 m

Ultrasonic vs supersonic; infrasonic vs subsonic

These pairs sound alike but describe different things:

Ultrasonic refers to frequency: sound above 20 kHz.
Supersonic refers to speed: a body (such as a jet aircraft or a bullet) moving faster than the speed of sound in that medium.

Infrasonic refers to frequency: sound below 20 Hz.
Subsonic refers to speed: a body moving slower than the speed of sound.

The speed of a moving body compared with the speed of sound is called its Mach number: Mach 2 means twice the speed of sound.

Worked example

A jet flies at 850 m s⁻¹ where the speed of sound is 340 m s⁻¹. Is it subsonic or supersonic? What is its Mach number?

Try it first, then show the solutionSolution

850 > 340, so it is supersonic.
Mach number $= \frac{850}{340} = 2.5$ .

Answer: Supersonic; Mach 2.5

Physics in real life: Infrasonic, Sonic and Ultrasonic Sound

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Ultrasound scanning

Suppose a doctor uses a 3 MHz ultrasound probe on a patient, and ultrasound travels at about 1500 m s⁻¹ in body tissue.

Find the wavelength in tissue, and give one reason ultrasound is preferred to X-rays for checking a baby in the womb.

Try it first, then show the solutionSolution

$λ = \frac{1500}{3 \times 1 0 ^{6}} = 5 \times 1 0^{- 4}$ m $= 0.5$ mm.
Ultrasound is not known to harm the baby, while X-rays can damage cells.

Answer: 0.5 mm; ultrasound is safe for the baby.

Memory hook: Tiny wavelength, fine detail, safe scan.

SONAR

Imagine a fishing boat's SONAR receives an echo from a shoal of fish 0.4 s after sending a pulse (speed in sea water 1500 m s⁻¹).

How deep is the shoal?

Try it first, then show the solutionSolution

Total path $= 1500 \times 0.4 = 600$ m.
Depth $= 300$ m.

Answer: 300 m.

Memory hook: Halve the round trip.

Dog whistle

Suppose a trainer blows a whistle of frequency 25 kHz.

Why does the dog respond while the people nearby hear nothing?

Try it first, then show the solutionSolution

25 kHz is above 20 kHz, so it is ultrasonic for humans.
Dogs can hear up to about 45 kHz, so the dog hears it.

Answer: It is ultrasonic: beyond human but within dog hearing.

Memory hook: Different animals, different hearing ranges.

Topics in this chapter

Infrasonic, Sonic and Ultrasonic Sound formula sheet

Audible range: $20 Hz \leq f \leq 20 kHz$; Sonic sound.
Wave equation: $V = f λ$; Higher f, shorter λ.
SONAR depth: $d = \frac{V \times t}{2}$; t = time for the echo to return.
Mach number: $Mach = \frac{speed of body}{speed of sound}$; > 1 supersonic; < 1 subsonic.

Infrasonic, Sonic and Ultrasonic Sound: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

SONAR on a ship

Show how SONAR finds the depth of the sea; 2 to 3 marks.

Draw

A ship on the sea surface with a transmitter and a receiver underneath.
An arrow from the transmitter down to the sea bed (ultrasonic pulse).
An arrow from the sea bed back up to the receiver (echo).
Mark the depth d between the ship and the sea bed.

Label

Ship
Transmitter
Receiver
Ultrasonic pulse
Reflected pulse (echo)
Sea bed
depth d

Watch out: Write d = Vt/2 beside the figure and explain why the time is halved.

How the school exam asks Infrasonic, Sonic and Ultrasonic Sound

The question patterns to expect, and the method that works for each.

Question type	How it is asked	Method
Ranges	State the audible range; classify given frequencies as infrasonic, sonic or ultrasonic.	20 Hz and 20 kHz are the boundaries.
Uses	Give uses of ultrasound (medicine, SONAR, cleaning, flaws, animals) or of infrasound.	One use per line with a short reason (short wavelength, narrow beam, travels far).
Differences	Distinguish ultrasonic and supersonic, or infrasonic and subsonic.	Frequency terms vs speed terms.
Simple numericals	Wavelength of an ultrasonic or infrasonic wave; SONAR depth; Mach number.	V = fλ; d = Vt/2; Mach = v ÷ speed of sound.

Tricks to remember Infrasonic, Sonic and Ultrasonic Sound

Short memory aids. Say each one aloud once, then use it on a question the same day.

Infra = below, ultra = beyond
Infra means below (like infrared); ultra means beyond (like ultraviolet).
Use it when: Naming the ranges.
-sonic for pitch, super/sub for speed
Infrasonic and ultrasonic: frequency. Subsonic and supersonic: speed.
Use it when: The two differences.
20 to 20 thousand
Human hearing: 20 Hz to 20,000 Hz.
Use it when: Range questions.
There and back
SONAR time covers the trip down and back, so halve it.
Use it when: Depth calculations.

Infrasonic, Sonic and Ultrasonic Sound: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Example 1: Audible range · 1 mark · MCQExam level

The audible range of frequencies for a normal human ear is:
(a) 2 Hz to 2000 Hz
(b) 20 Hz to 20,000 Hz
(c) 200 Hz to 200,000 Hz
(d) 20 kHz to 200 kHz

Try it first, then show the solutionSolution

Human hearing extends from about 20 Hz to 20 kHz.

Answer: (b)

Example 2: Ultrasonic vs supersonic · 2 marks · Short answerExam level

Distinguish between ultrasonic and supersonic, with one example of each.

Try it first, then show the solutionSolution

Ultrasonic: sound of frequency above 20 kHz (e.g. sound used in ultrasonography).
Supersonic: speed of a body greater than the speed of sound (e.g. a fighter jet flying at Mach 2).

Answer: Frequency above 20 kHz vs speed above the speed of sound.

Example 3: Uses of ultrasound · 3 marks · Short answerExam level

Give three uses of ultrasound and state the property that makes each possible.

Try it first, then show the solutionSolution

Ultrasonography: short wavelength gives a narrow beam that reflects from organs to form images.
SONAR: ultrasound travels far in water as a narrow beam and its echo can be timed.
Cleaning small parts: high-frequency vibrations in a liquid shake off dirt.

Answer: Any three uses with reasons.

Example 4: Wavelength at the limits · 3 marks · Short answerHarder exam level

Find the wavelengths in air of the lowest and highest audible frequencies (speed 340 m s⁻¹). Which is longer, and by what factor?

Try it first, then show the solutionSolution

20 Hz: $λ = 17$ m.
20 kHz: $λ = 0.017$ m (1.7 cm).
The 20 Hz wavelength is 1000 times longer.

Answer: 17 m and 1.7 cm; factor 1000

Example 5: SONAR depth · 3 marks · Short answerHarder exam level

A SONAR pulse returns 3 s after being sent from a ship. If the speed of ultrasound in sea water is 1520 m s⁻¹, find the depth of the sea there. Why is ultrasound, not audible sound, used?

Try it first, then show the solutionSolution

Depth $= \frac{1520 \times 3}{2} = 2280$ m.
Ultrasound has a very short wavelength, so it travels as a narrow beam with little spreading and is not confused with the ship's own audible noise.

Answer: 2280 m

Example 6: Same wave, two media · 3 marks · Short answerHarder exam level

An ultrasonic wave of frequency 50 kHz goes from air (340 m s⁻¹) into water (1500 m s⁻¹). Find its wavelength in each and say whether it is still ultrasonic in water.

Try it first, then show the solutionSolution

Air: $λ = \frac{340}{50000} = 6.8$ mm.
Water: $λ = \frac{1500}{50000} = 30$ mm $= 3$ cm.
Frequency is unchanged (50 kHz), so it is still ultrasonic.

Answer: 6.8 mm; 3 cm; yes

Example 7: Bat echolocation · 4 marks · Long answerHOTS / competency

A bat flying in a cave emits an ultrasonic squeak and hears its echo from a wall 0.02 s later. If sound travels at 340 m s⁻¹, how far is the wall? Why can't the bat use a 2 kHz sound to detect a small moth 2 cm across?

Try it first, then show the solutionSolution

Distance $= \frac{340 \times 0.02}{2} = 3.4$ m.
At 2 kHz, $λ = \frac{340}{2000} = 17$ cm, much larger than the moth.
Waves much longer than an object pass round it with little reflection; ultrasound (λ of a few mm) reflects well from small objects.

Answer: 3.4 m; audible sound's wavelength is too long to reflect well from a moth

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 9-mark question split into 3 + 3 + 3 marks.

Sounds we cannot hear

A science exhibit lists four sources: (A) an earthquake rumble of 8 Hz, (B) a flute note of 1.2 kHz, (C) a bat call of 60 kHz, and (D) a fighter jet flying at 680 m s⁻¹. Take the speed of sound in air as 340 m s⁻¹.

(i) · 3 marks
Classify A, B and C by frequency, and say which a person can hear.
Answer
Answer: A infrasonic; B sonic (audible); C ultrasonic
A: below 20 Hz, infrasonic. B: within 20 Hz–20 kHz, sonic, heard. C: above 20 kHz, ultrasonic. Only B is heard.
(ii) · 3 marks
Find the wavelengths of A and C in air.
Answer
Answer: 42.5 m; about 5.7 mm
A: $\frac{340}{8} = 42.5$ m. C: $\frac{340}{60000} \approx 5.7 \times 1 0^{- 3}$ m.
(iii) · 3 marks
Is the jet subsonic or supersonic? Explain why describing it as 'ultrasonic' would be wrong.
Answer
Answer: Supersonic (Mach 2)
$\frac{680}{340} = 2 > 1$ : supersonic. 'Ultrasonic' describes sound of frequency above 20 kHz, not the speed of a moving body.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

What are infrasonic and ultrasonic sounds? (2 marks)

Step	Marks
Infrasonic: sound of frequency below 20 Hz, not heard by humans	1
Ultrasonic: sound of frequency above 20 kHz, not heard by humans	1

State three uses of ultrasound. (3 marks)

Step	Marks
Imaging internal organs or a baby in the womb (ultrasonography)	1
SONAR: measuring sea depth, locating submarines or fish	1
Cleaning delicate parts, or detecting cracks in metals (any one)	1

A ship's SONAR receives an echo 1.6 s after sending a pulse. The speed of ultrasound in sea water is 1500 m s⁻¹. Find the depth of the sea, and the wavelength if the frequency is 30 kHz. (4 marks)

Step	Marks
Total distance $= 1500 \times 1.6 = 2400$ m	1
Depth $= \frac{2400}{2} = 1200$ m	1
$λ = \frac{V}{f} = \frac{1500}{30000}$	1
$λ = 0.05$ m $= 5$ cm	1

Where students lose marks in Infrasonic, Sonic and Ultrasonic Sound

The slips examiners see most often, and what to write instead.

Mistake: Confusing ultrasonic with supersonic
Ultrasonic is about frequency; supersonic is about speed.
Wrong working
'A supersonic wave of 40 kHz'
Right working
'An ultrasonic wave of 40 kHz'
Do this: Ask: is the question about pitch (frequency) or speed?
Usually costs: 1 to 2 marks.
Mistake: Not halving the SONAR time
The echo time covers the journey down and back.
Do this: Depth = speed × time ÷ 2.
Usually costs: The answer.
Mistake: Wrong range limits
Writing 20 Hz to 2000 Hz, or 2 Hz to 20 kHz.
Do this: Remember 20 to 20,000.
Usually costs: 1 mark.
Mistake: Saying ultrasound travels faster than audible sound
In the same medium all frequencies travel at the same speed; only the wavelength differs.
Do this: V depends on the medium, not on frequency.
Usually costs: 1 mark.
Mistake: Forgetting kHz to Hz conversion
Using 50 instead of 50,000 gives a wavelength 1000 times too large.
Do this: 1 kHz = 1000 Hz; 1 MHz = 10⁶ Hz.
Usually costs: The answer.

Practice questions on Infrasonic, Sonic and Ultrasonic Sound

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQExam level
Sound of frequency 15 Hz is:
(a) audible
(b) ultrasonic
(c) infrasonic
(d) supersonic
Hint
Below 20 Hz.
Answer
(c)
Q2 · 1 mark · MCQExam level
Which animal uses ultrasound for echolocation?
(a) elephant
(b) bat
(c) cow
(d) whale
Hint
Night flyer.
Answer
(b)
Q3 · 2 marks · Short answerExam level
Distinguish between infrasonic and subsonic.
Hint
Frequency vs speed.
Answer
Infrasonic: sound with frequency below 20 Hz. Subsonic: speed of a body less than the speed of sound.
Q4 · 1 mark · Assertion-reasonHarder exam level
Assertion (A): Ultrasound is used to find the depth of the sea.
Reason (R): Ultrasound travels faster in water than audible sound.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
Hint
Speed depends on the medium.
Answer
(a): ultrasound is used because it forms a narrow beam, not because it is faster.
Q5 · 2 marks · Short answerHarder exam level
A ship sends ultrasound to the sea bed and receives the echo after 4 s. If the depth is 3000 m, find the speed of ultrasound in sea water.
Hint
2d = Vt.
Answer
$V = \frac{2 \times 3000}{4} = 1500$ m s⁻¹.
Q6 · 2 marks · Short answerHarder exam level
Find the frequency of a sound of wavelength 2 cm in air (340 m s⁻¹) and classify it.
Hint
f = V/λ.
Answer
$f = \frac{340}{0.02} = 17000$ Hz: audible (sonic).
Q7 · 3 marks · Short answerExam level
Give two uses of ultrasound in industry and one in medicine.
Hint
Cleaning, flaws, scanning.
Answer
Industry: cleaning small or delicate parts; detecting cracks and flaws in metal blocks. Medicine: ultrasonography to image organs or a foetus.
Q8 · 3 marks · Short answerHarder exam level
An aircraft flies at Mach 1.5 where the speed of sound is 320 m s⁻¹. Find its speed in m s⁻¹ and in km h⁻¹. Is it subsonic or supersonic?
Hint
Speed = Mach × speed of sound.
Answer
$1.5 \times 320 = 480$ m s⁻¹ $= 1728$ km h⁻¹; supersonic.
Q9 · 3 marks · Short answerHarder exam level
A dolphin emits a click of frequency 120 kHz under water (1500 m s⁻¹) and receives an echo from a fish 0.04 s later. Find the wavelength and the distance of the fish.
Hint
Halve the echo time.
Answer
$λ = \frac{1500}{120000} = 0.0125$ m $= 1.25$ cm. Distance $= \frac{1500 \times 0.04}{2} = 30$ m.
Q10 · 2 marks · Short answerHarder exam level
Why can elephants communicate over several kilometres using infrasound?
Hint
Long wavelength.
Answer
Infrasound has long wavelengths and loses little energy as it travels, so it can be detected far away by other elephants.
Q11 · 3 marks · Short answerHOTS / competency
A doctor wants to see details about 1 mm in size inside the body, where sound travels at 1540 m s⁻¹. Suggest a suitable minimum frequency, given that the wavelength should be no larger than the detail.
Hint
f = V/λ with λ = 1 mm.
Answer
$f = \frac{1540}{0.001} = 1.54 \times 1 0^{6}$ Hz, about 1.5 MHz or higher (ultrasonic).
Q12 · 3 marks · Short answerHOTS / competency
Explain why the terms 'ultrasonic jet' and 'supersonic sound wave' are both used wrongly.
Hint
Match each word to frequency or speed.
Answer
'Ultrasonic' describes frequency above 20 kHz, so it cannot describe a jet's speed: a fast jet is supersonic. 'Supersonic' describes a speed greater than that of sound, but a sound wave always travels at the speed of sound in its medium, so a sound wave cannot be supersonic; a high-frequency sound is ultrasonic.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Lower limit of human hearing?Show answer
About 20 Hz.
2.Upper limit of human hearing?Show answer
About 20 kHz.
3.What is ultrasound?Show answer
Sound of frequency above 20 kHz.
4.Name an animal that uses infrasound.Show answer
Elephant (or whale).
5.What does SONAR stand for?Show answer
Sound Navigation And Ranging.
6.What does 'supersonic' mean?Show answer
Moving faster than the speed of sound.
7.Formula for depth by SONAR?Show answer
d = Vt/2.
8.Does ultrasound travel faster than audible sound in air?Show answer
No; the same speed, shorter wavelength.

Best order to study it

Learn the three frequency ranges.
Learn uses of ultrasound with reasons.
Learn effects and uses of infrasound.
Learn the two pairs of differences (ultrasonic/supersonic, infrasonic/subsonic).
Practise V = fλ, SONAR and Mach-number numericals.

Quick revision checklist

The night before the exam: can you do each of these without looking?

Can you state the audible range?
Can you classify any frequency?
Can you list three uses of ultrasound with reasons?
Can you give two examples of infrasound?
Can you distinguish ultrasonic from supersonic?
Do you halve the time in SONAR problems?
Do you convert kHz and MHz to Hz?

Infrasonic, Sonic and Ultrasonic Sound: frequently asked questions

Is echo (reflection of sound) part of this chapter?

Only as needed for simple applications such as SONAR. Reflection of sound and echoes are studied in more detail in Class X.

Why can't humans hear ultrasound?

Our ears respond only to frequencies from about 20 Hz to 20 kHz; ultrasound is above that range.

Is ultrasound dangerous?

At the levels used in medical scans it is considered safe, which is why it is used for checking babies before birth.

Does a supersonic plane make ultrasonic sound?

Not necessarily. Supersonic describes the plane's speed; the sounds it makes can be of any frequency.

Why do animals sometimes sense earthquakes early?

Earthquakes can produce infrasound and small vibrations that some animals detect before humans notice anything.

Based on: CISCE ICSE 2028 Science (52) Physics syllabus (Classes IX and X); CISCE Regulations for ICSE Examination Year 2028