Class 9 Science · Original EZT practice
Sound Waves (Class 9 Science Chapter 10): summary and practice
Why can't two astronauts hear each other during a spacewalk? Sound is a wave - a disturbance travelling through a medium as compressions and rarefactions. This chapter covers how sound is produced and propagated, why it needs a medium, and the wave quantities - wavelength, frequency, time period and amplitude - tied together by v = f x wavelength. All questions are original EZT tutor practice following the chapter's own section order.
6 original worked examples, 6 fresh self-check questions and 4 mixed questions, with explained answers. These are not copied textbook exercises, an official chapter index or a full answer key. No affiliation with NCERT is claimed.
Official sources for topic and edition context, checked 22 October 2026:
1. Production and propagation of sound
Sound is produced by vibrating objects and travels through a medium as compressions and rarefactions. It cannot travel through vacuum - there are no particles to pass the disturbance on.
2. Wavelength, frequency and time period
Wavelength is the distance between two consecutive compressions (or rarefactions), frequency is the number of oscillations per second measured in hertz, and the time period T = 1/f.
3. The wave equation v = f x wavelength
Wave speed equals frequency times wavelength. The speed of sound in air is about 344 m/s at 22 degC.
4. Amplitude, loudness and energy
Amplitude is the maximum disturbance of the medium; larger amplitude means more energy and a louder sound, while frequency decides the pitch.
Choose, solve, check, then try a fresh question.
Before calculating, read the method prompt and name the condition that makes it useful. Write your own attempt. Open the steps only when you need them, and use the check to catch a wrong sign, unit or interpretation. Finish with the fresh question before reading its answer.
Explain why sound needs a medium
Choose the method: No medium, no wave.
EZT practice question
Why do we not hear sound in space?
Show the worked solution
- Sound travels as compressions and rarefactions of particles.
- Space is nearly a vacuum - there are no particles to compress and rarefy.
- So the wave has nothing to carry it and cannot propagate.
Answer: Space is nearly a vacuum - sound needs a material medium whose particles can be compressed and rarefied, so it cannot propagate there.
Check: The same logic works anywhere: where there is a medium (air, water, steel), sound travels; where there is none, it does not.
Watch for this: Do not say sound is too weak in space - even the loudest sound cannot travel without a medium.
Now try it yourself
Two astronauts are floating outside their spacecraft. Can they hear each other without radio?
Check your answer
No - without air between them, sound cannot travel.
Sound needs a medium; the vacuum of space has none.
Describe the wave direction
Choose the method: Along the wave, not across it.
EZT practice question
In a sound wave in air, is the displacement of air particles parallel or perpendicular to the direction the wave travels?
Show the worked solution
- Watch one air particle as a sound passes.
- It oscillates back and forth around its rest position.
- That back-and-forth is along the same line the wave moves on.
Answer: Parallel - sound is a longitudinal wave.
Check: A quick contrast: ripples on water move up and down while the wave travels sideways (transverse); sound does the along-the-line version (longitudinal).
Watch for this: Do not picture air particles travelling all the way to your ear - they only oscillate; the disturbance is what travels.
Now try it yourself
What produces sound in a ringing bell?
Check your answer
Vibration of the bell's body, which sets the surrounding air into compressions and rarefactions.
Every sound starts with something vibrating.
Convert time period to frequency
Choose the method: f = 1/T.
EZT practice question
A source vibrates with a time period of 0.004 s. Find its frequency.
Show the worked solution
- Frequency and time period are reciprocals: f = 1/T.
- Here T = 0.004 s.
- f = 1/0.004 = 250 Hz.
Answer: 250 Hz.
Check: 250 oscillations each taking 0.004 s fill exactly one second - the numbers are consistent.
Watch for this: Do not multiply T by 1 - the reciprocal means divide 1 by T.
Now try it yourself
A tuning fork completes 500 oscillations per second. Find its time period.
Check your answer
T = 1/500 = 0.002 s.
Time period is the reciprocal of frequency.
Convert frequency to wavelength
Choose the method: wavelength = v/f.
EZT practice question
Human hearing roughly spans 20 Hz to 20 kHz. Find the corresponding wavelengths in air (v = 344 m/s).
Show the worked solution
- Use wavelength = v/f for each end of the range.
- For 20 Hz: wavelength = 344/20 = 17.2 m.
- For 20 kHz: wavelength = 344/20000 = 0.0172 m = 1.72 cm.
Answer: 17.2 m for 20 Hz and 1.72 cm for 20 kHz.
Check: Higher frequency gives shorter wavelength - the 1000-times-higher frequency gives a 1000-times-shorter wave.
Watch for this: Do not flip the ends - the low frequency has the long wavelength.
Now try it yourself
A sound of frequency 86 Hz travels in air at 344 m/s. Find its wavelength.
Check your answer
wavelength = 344/86 = 4 m.
wavelength = v/f.
Compute wave speed
Choose the method: v = f x wavelength.
EZT practice question
A sound wave has frequency 170 Hz and wavelength 2 m. Find its speed.
Show the worked solution
- The wave equation links the three: v = f x wavelength.
- Substitute f = 170 Hz and wavelength = 2 m.
- v = 170 x 2 = 340 m/s.
Answer: 340 m/s.
Check: 340 m/s is close to the usual speed of sound in air (344 m/s at 22 degC) - sensible for a sound wave.
Watch for this: Do not divide here - speed is frequency times wavelength, not frequency over wavelength.
Now try it yourself
A wave has frequency 50 Hz and speed 344 m/s. Find its wavelength.
Check your answer
wavelength = 344/50 = 6.88 m.
Rearrange v = f x wavelength to wavelength = v/f.
Relate amplitude to loudness
Choose the method: Bigger amplitude, more energy, louder sound.
EZT practice question
Two sounds have the same frequency, but one has twice the amplitude of the other. How do they differ?
Show the worked solution
- Amplitude measures the maximum disturbance of the medium.
- Twice the amplitude means the wave carries more energy.
- Frequency is unchanged, so the pitch stays the same.
Answer: The larger-amplitude sound carries more energy and is louder; both have the same pitch since frequency is unchanged.
Check: Loudness tracks amplitude, pitch tracks frequency - changing only amplitude changes only loudness.
Watch for this: Do not say the louder sound has a higher pitch - pitch comes from frequency, which did not change.
Now try it yourself
A supersonic aircraft flies faster than the speed of sound. What is the loud sound it creates called?
Check your answer
A sonic boom.
The waves pile up into a single powerful shock - heard as a boom.
Try a mixed set
Choose a method or explanation before opening each answer.
1. Can sound travel through vacuum?
Show answer and reason
No - it needs a material medium.
Compressions and rarefactions need particles.
2. Frequency from time period?
Show answer and reason
f = 1/T.
They are reciprocals.
3. Wave equation?
Show answer and reason
v = f x wavelength.
Speed equals frequency times wavelength.
4. Loudness vs pitch?
Show answer and reason
Loudness follows amplitude; pitch follows frequency.
More amplitude means more energy.
Try it yourself
Work these on paper before checking any notes. Answers are deliberately not shown here.
- What produces sound in a ringing bell?
- A sound of frequency 86 Hz travels in air at 344 m/s. Find its wavelength.
- Which property of a sound wave decides its pitch?
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Questions about this practice page
- Can sound travel through vacuum?
- No - sound needs a material medium; it travels as compressions and rarefactions of particles.
- What is the formula connecting speed, frequency and wavelength?
- v = f x wavelength (wave speed = frequency x wavelength).
- What is the speed of sound in air?
- About 344 m/s at 22 degC, the value used in the textbook's examples.
- What is the range of human hearing?
- Roughly 20 Hz to 20 kHz - corresponding to wavelengths from 17.2 m down to 1.72 cm in air.
- Are these the NCERT exercise questions for Class 9 Science Chapter 10?
- No. EZT pages are original practice following the chapter's section order - use them to prepare for the textbook exercises, not as a substitute for your book.