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Class 9 Science: Sound Waves, Characteristics and Applications, 8 practice questions with answers
These are 8 practice questions on Chapter 10, Sound Waves: Characteristics and Applications, of the new NCERT Class 9 Science textbook, Exploration (First Edition, April 2026). Each has a worked answer and the textbook page it comes from. They cover frequency and time period, the relation v = λν in air and in water, the delay between lightning and thunder, the distance to a wall from an echo, the audible range, and finding a depth with sonar. Try each one on paper before you open the answer.
These questions are our own, drafted with AI help. Every number was re-computed by a test script and none of the numbers is taken from the chapter, but no practising teacher has reviewed them yet. If you find a mistake, please tell us.
Where a question needs the speed of sound, it uses the textbook's own values: 340 m s⁻¹ in air and 1500 m s⁻¹ in water (Table 10.1, page 197).
What the chapter says you need
Facts from the NCERT textbook (Exploration: Textbook of Science for Grade 9, Chapter 10, printed pages 184 to 207):
- The wavelength λ is the distance between two consecutive crests (or troughs), in metres. The frequency ν is the number of oscillations per unit time, in hertz (Hz). The time period T is the time for one complete oscillation, and ν = 1/T (Eq. 10.1, page 194).
- Speed = wavelength × frequency, v = λν (Eq. 10.2, page 196). Sound travels fastest in solids, slower in liquids and slowest in gases (page 196); Table 10.1 gives about 5000 m s⁻¹ in steel, 1500 m s⁻¹ in water and 340 m s⁻¹ in air (page 197).
- Humans hear from 20 Hz to 20,000 Hz (20 kHz). Waves below 20 Hz are infrasonic and waves above 20 kHz are ultrasonic (page 199; also page 202).
- An echo is sound reflected back from a distant surface. To hear it separately, it must arrive at least 0.1 s after the original sound, which needs a reflecting surface at least 17 m away (page 201). Since the sound goes to the surface and back, the distance to the surface is speed × time ÷ 2 (Example 10.5, page 201).
- Sonar sends ultrasonic waves into water and uses the reflected waves to find the distance of objects (page 203).
Frequency, wavelength and speed
Question 1. A sound wave has a time period of 0.0016 s. What is its frequency?
Answer: 625 Hz. ν = 1/T = 1 ÷ 0.0016 = 625 Hz. (Eq. 10.1, page 194.)
Question 2. A sound wave in air has a wavelength of 0.4 m. What is its frequency? Use 340 m s⁻¹ for the speed of sound in air.
Answer: 850 Hz. From v = λν, ν = v/λ = 340 ÷ 0.4 = 850 Hz. (Eq. 10.2, page 196; speed in air, Table 10.1, page 197.)
Question 3. A tuning fork of frequency 425 Hz is sounded in air. What is the wavelength of the sound? Use 340 m s⁻¹.
Answer: 0.8 m. λ = v/ν = 340 ÷ 425 = 0.8 m. (Eq. 10.2, page 196.)
Question 4. A sound of frequency 2500 Hz travels through water. What is its wavelength there? Use 1500 m s⁻¹ for the speed of sound in water.
Answer: 0.6 m. λ = v/ν = 1500 ÷ 2500 = 0.6 m. The same frequency has a longer wavelength in water than in air, because sound is faster in water. (Eq. 10.2, page 196; speed in water, Table 10.1, page 197.)
Hearing and echoes
Question 5. During a storm you see a lightning flash and hear the thunder 3.5 s later. Roughly how far away was the lightning? Use 340 m s⁻¹, and treat the light as arriving instantly.
Answer: about 1190 m. Distance = speed × time = 340 × 3.5 = 1190 m, a little over 1 km. (Speed of sound in air, page 197; the same reasoning as Example 10.3, page 197.)
Question 6. You shout towards a cliff and hear the echo 1.3 s later. How far away is the cliff? Use 340 m s⁻¹.
Answer: 221 m. In 1.3 s the sound goes to the cliff and back: 340 × 1.3 = 442 m. The cliff is half of that away: 442 ÷ 2 = 221 m. This is more than 17 m, so the echo can be heard separately. (Echo, page 201.)
Question 7. One sound has a frequency of 26 kHz and another has 16 Hz. Can a human hear either of them? What is each one called?
Answer: neither; 26 kHz is ultrasonic and 16 Hz is infrasonic. Humans hear from 20 Hz to 20 kHz. 26 kHz is above 20 kHz, so it is an ultrasonic wave; 16 Hz is below 20 Hz, so it is an infrasonic wave. (Audible range, page 199; page 202.)
Sonar
Question 8. A ship's sonar sends an ultrasonic pulse straight down, and the echo from the sea floor returns after 2.6 s. How deep is the sea there? Use 1500 m s⁻¹ for the speed of sound in water.
Answer: 1950 m. The pulse goes down and back up: 1500 × 2.6 = 3900 m. The depth is half of that: 3900 ÷ 2 = 1950 m. (Sonar, page 203; speed in water, Table 10.1, page 197.)
Practise more, and check your marks
- Free practice, marked instantly in your browser: Class 9 Sound Waves questions, a separate short set on the same chapter.
- Printable Class 9 worksheets with answer keys, in English, Hindi and Marathi: Class 9 worksheets.
- More Class 9 physics in the same style: Work, Energy, and Simple Machines, How Forces Affect Motion and Describing Motion Around Us.
Where CMDLearn fits
CMDLearn is building an AI tutor for Classes 1 to 10 that explains a step and checks the child's working, but leaves the writing to the child. It is still in development, and no child uses it yet. If you would like to hear when there is something to try, you can join the waitlist.
Sources
All opened and read on 21 October 2026. Page numbers are the printed page numbers of the textbook; printed page = PDF page + 183.
- NCERT, Exploration: Textbook of Science for Grade 9 (First Edition, April 2026), Chapter 10 "Sound Waves: Characteristics and Applications", printed pages 184-207 (wavelength, frequency, time period and Eq. 10.1, page 194; the speed of sound and Eq. 10.2, page 196; Table 10.1 and Example 10.3, page 197; the audible range, infrasonic and ultrasonic waves, page 199; the echo, the 0.1 s and 17 m limit and Example 10.5, page 201; ultrasonic and infrasonic waves, page 202; sonar, page 203): ncert.nic.in/textbook/pdf/iesc110.pdf
- NCERT, Exploration: Textbook of Science for Grade 9, front matter and contents (chapter titles and starting pages): ncert.nic.in/textbook/pdf/iesc1ps.pdf
The definitions, equations and constants are from the textbook pages above. The questions, worked answers and checks are our own, and every number was re-computed by script. This guide is general information; follow your teacher and the textbook.