PhysicsICSE Class 9 · Infrasonic, Sonic and Ultrasonic Sound

Infrasonic, Sonic and Ultrasonic Sound; Subsonic and Supersonic Speeds (ICSE Class 9)

Updated 17 October 2026 · Fact-checked

Infrasonic sound has frequency below 20 Hz, audible (sonic) sound lies between 20 Hz and 20 kHz and ultrasonic sound is above 20 kHz. These terms describe frequency; subsonic and supersonic describe the speed of a moving body relative to the speed of sound (Mach number below or above 1). Infrasound has very long wavelengths and travels far.

Understand Infrasonic, Sonic and Ultrasonic Ranges; Subsonic and Supersonic Speeds

Three frequency ranges.

Range	Frequency	In air ( $V = 340$ m s⁻¹)
Infrasonic	below 20 Hz	$λ > 17$ m
Sonic (audible)	20 Hz to 20 kHz	17 m down to 1.7 cm
Ultrasonic	above 20 kHz	$λ < 1.7$ cm

From $λ = \frac{V}{f}$ the wavelength falls as frequency rises. The limits 20 Hz and 20 kHz are standard values; they vary slightly between people and the upper limit falls with age.

Infrasound. Produced by earthquakes, volcanoes, ocean waves, storms, and by large animals such as elephants and whales. Its long wavelength means it is absorbed little and bends around obstacles, so it travels very long distances. Uses and effects:

elephants and whales communicate over long distances;
some animals sense infrasound from an earthquake or volcano and become restless;
detectors monitor storms, volcanic eruptions and large explosions;
strong infrasound, though not heard, can cause discomfort.

Ultrasound. Short wavelength, narrow beam, used in imaging, SONAR, cleaning and echolocation (see the ultrasound topic).

Ultrasonic is not supersonic; infrasonic is not subsonic.

*Ultrasonic* and *infrasonic* describe frequency of a sound.
*Supersonic* and *subsonic* describe the speed of a body compared with the speed of sound in the same medium.

Mach number. $Mach = \frac{speed of the body}{speed of sound}$ . Mach 1.8 means 1.8 times the speed of sound; below 1 is subsonic, above 1 is supersonic. A bullet or a jet can be supersonic; a sound wave itself is not "supersonic".

Wavelength in other media. The same frequency has a longer wavelength in a faster medium: at 20 Hz, $λ = \frac{1500}{20} = 75$ m in water and $\frac{5000}{20} = 250$ m in steel.

Infrasonic, Sonic and Ultrasonic Ranges; Subsonic and Supersonic Speeds: formulas and key facts

Ranges: $f < 20 Hz (infra); 20 Hz - 20 kHz (sonic); f > 20 kHz (ultra)$; Standard limits.
Wavelength: $λ = \frac{V}{f}$; Any medium.
Mach number: $M = \frac{v _{b o d y}}{V _{so u n d}}$; M < 1 subsonic; M > 1 supersonic.

How to solve Infrasonic, Sonic and Ultrasonic Ranges; Subsonic and Supersonic Speeds questions

Separate frequency questions from speed questions.

1Convert kHz to Hz before using the ranges.
2Classify by frequency: below 20 Hz, between, or above 20 kHz.
3Use $λ = V / f$ with the speed of the medium.
4For Mach, multiply Mach number by the local speed of sound.
5Use the words subsonic and supersonic only for speeds.

Infrasonic, Sonic and Ultrasonic Ranges; Subsonic and Supersonic Speeds in real life

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.

An elephant’s call

Suppose an elephant makes a call at 14 Hz and another elephant 6.8 km away stands in air where sound travels at 340 m s⁻¹.

Find the wavelength and the time for the call to reach the second elephant. Can a human hear it?

Try it first, then show the solutionSolution

$λ = \frac{340}{14} \approx 24$ m.
$t = \frac{6800}{340} = 20$ s.
14 Hz is below 20 Hz: infrasonic and not audible to humans.

Answer: About 24 m; 20 s; not audible.

Memory hook: Low f, long λ, long range.

Tricks to remember Infrasonic, Sonic and Ultrasonic Ranges; Subsonic and Supersonic Speeds

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

Infra below, ultra above
Infra: under 20 Hz. Ultra: over 20 kHz.
Use it when: Classifying.
Sonic is speed, ultrasonic is frequency
Super/sub tell speed; ultra/infra tell frequency.
Use it when: Term pairs.
Mach times 340
Speed = Mach number × speed of sound.
Use it when: Mach questions.

Infrasonic, Sonic and Ultrasonic Ranges; Subsonic and Supersonic Speeds: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Upper limit of hearing · 1 mark · MCQBoard level

The upper limit of the frequency range audible to a normal human ear is about:
(a) 2 kHz
(b) 20 kHz
(c) 45 kHz
(d) 100 kHz

Try it first, then show the solutionSolution

The audible range is about 20 Hz to 20 kHz.

Answer: (b) 20 kHz

Example 2: Classifying · 2 marks · Short answerBoard level

Classify as infrasonic, audible or ultrasonic: 8 Hz, 800 Hz, 40 kHz, 18 kHz.

Try it first, then show the solutionSolution

8 Hz: infrasonic. (½)
800 Hz: audible. (½)
40 kHz: ultrasonic. (½)
18 kHz: audible (below 20 kHz). (½)

Answer: Infrasonic; audible; ultrasonic; audible

Example 3: Wavelengths in water and steel · 3 marks · Short answerHarder board level

Find the wavelengths of sounds of 20 Hz and 20 kHz in (a) water (1500 m s⁻¹) and (b) steel (5000 m s⁻¹).

Try it first, then show the solutionSolution

(a) $\frac{1500}{20} = 75$ m and $\frac{1500}{20 000} = 0.075$ m. (1½)
(b) $\frac{5000}{20} = 250$ m and $\frac{5000}{20 000} = 0.25$ m. (1½)

Answer: Water: 75 m, 7.5 cm. Steel: 250 m, 25 cm

Example 4: Mach number · 3 marks · Short answerHarder board level

An aircraft flies at Mach 1.8 where the speed of sound is 330 m s⁻¹. Find its speed in m s⁻¹ and km h⁻¹, the time to cover 10 km, and the speed of a second aircraft flying at Mach 0.6 there. Classify the two flights.

Try it first, then show the solutionSolution

Speed $= 1.8 \times 330 = 594$ m s⁻¹ $= 594 \times 3.6 \approx 2138$ km h⁻¹. (1)
$t = \frac{10 000}{594} \approx 16.8$ s. (1)
Second: $0.6 \times 330 = 198$ m s⁻¹. First flight supersonic, second subsonic. (1)

Answer: 594 m s⁻¹ (2138 km h⁻¹); 16.8 s; 198 m s⁻¹

Example 5: Elephant communication · 4 marks · Case-basedHOTS / competency

An elephant calls at 14 Hz in air (340 m s⁻¹). Another elephant is 6.8 km away.
(a) Find the wavelength.
(b) Find the time for the call to arrive.
(c) Give a reason why infrasound travels far.
(d) Can a human standing nearby hear the call? Why?

Try it first, then show the solutionSolution

(a) $λ = \frac{340}{14} \approx 24.3$ m. (1)
(b) $t = \frac{6800}{340} = 20$ s. (1)
(c) Long wavelength: little absorption and it bends around obstacles. (1)
(d) No, 14 Hz is below the lower limit of audibility, 20 Hz. (1)

Answer: (a) 24.3 m (b) 20 s (c) long wavelength, low absorption (d) no, infrasonic

Where students lose marks in Infrasonic, Sonic and Ultrasonic Ranges; Subsonic and Supersonic Speeds

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

Mistake: Supersonic used for sound frequency
Supersonic is a speed term.
Wrong working
Sound above 20 kHz is supersonic.
Right working
Sound above 20 kHz is ultrasonic.
Do this: Pair infra/ultra with frequency.
Usually costs: 1 mark.
Mistake: Upper audible limit given as 2000 Hz
It is 20 000 Hz.
Wrong working
Audible range 20 Hz to 2000 Hz.
Right working
20 Hz to 20 kHz.
Do this: Write both units.
Usually costs: 1 mark.
Mistake: Mach number multiplied by the wrong speed
Use the local speed of sound.
Wrong working
Mach 1.8 = 1.8 × 3 × 10⁸
Right working
1.8 × 330 m s⁻¹
Do this: Read the speed of sound in the question.
Usually costs: The answer.

Exam tips

Give ranges with units: 20 Hz and 20 kHz.
In "distinguish" answers write both columns.
Mention the long wavelength when explaining the range of infrasound.
Use km h⁻¹ conversions with × 3.6 explicitly.

Practice questions on Infrasonic, Sonic and Ultrasonic Ranges; Subsonic and Supersonic Speeds

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 · MCQBoard level
Subsonic describes:
(a) a sound of low frequency
(b) a body moving slower than sound
(c) a body moving faster than sound
(d) a sound of high frequency
Hint
Speed word.
Answer
(b)
Q2 · 1 mark · Very short answerBoard level
Which has the longer wavelength in air, infrasound or ultrasound?
Hint
λ = V/f.
Answer
Infrasound.
Q3 · 2 marks · Short answerHarder board level
A sound wave in air has a wavelength of 25 m. Find its frequency and classify it (speed 340 m s⁻¹).
Hint
f = V/λ.
Answer
$f = \frac{340}{25} = 13.6$ Hz: infrasonic.
Q4 · 3 marks · Short answerHarder board level
A dog whistle emits sound of frequency 30 kHz. Find its wavelength in air and say who can hear it.
Hint
Compare with 20 kHz and the dog’s upper limit of about 45 kHz.
Answer
$λ = \frac{340}{30 000} \approx 1.13$ cm. It is ultrasonic: not audible to humans (limit 20 kHz) but within a dog’s range (up to about 45 kHz).
Q5 · 2 marks · Assertion-reasonHarder board level
Assertion (A): The term "ultrasonic aircraft" is incorrect.
Reason (R): Ultrasonic refers to the frequency of a sound, not to the speed of a body.
(a) Both true, R explains A.
(b) Both true, R does not explain A.
(c) A true, R false.
(d) A false, R true.
Hint
Frequency vs speed.
Answer
(a)
Q6 · 3 marks · Short answerHOTS / competency
Source A emits 15 Hz and source B emits 25 kHz in air (340 m s⁻¹). Find both wavelengths and the ratio $λ_{A} : λ_{B}$ . Which of the two will bend more readily around a 10 m wide building, and why?
Hint
Diffraction is greater when wavelength is comparable to or larger than the obstacle.
Answer
$λ_{A} = \frac{340}{15} \approx 22.7$ m; $λ_{B} = \frac{340}{25 000} = 1.36$ cm; ratio $\approx 1667 : 1$ . A bends around the building (its wavelength exceeds the building’s width); B, with a tiny wavelength, travels almost in straight lines and casts a sound shadow.

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.Infrasonic range?Show answer
Below 20 Hz.
2.Audible range?Show answer
20 Hz to 20 kHz.
3.Supersonic means?Show answer
Faster than the speed of sound.
4.Subsonic means?Show answer
Slower than the speed of sound.
5.Mach number?Show answer
Speed of the body ÷ speed of sound.

Infrasonic, Sonic and Ultrasonic Ranges; Subsonic and Supersonic Speeds: frequently asked questions

What is the difference between ultrasonic and supersonic?

Ultrasonic describes sound above 20 kHz (frequency); supersonic describes a body moving faster than sound (speed).

What is the difference between infrasonic and subsonic?

Infrasonic is sound below 20 Hz; subsonic is a body moving slower than the speed of sound.

Why can elephants communicate over long distances?

They use infrasound, whose long wavelength is absorbed little and travels far.