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CBSE · Class 9 · Science

Sound Waves: Characteristics and Applications

Standard difficulty · 20 questions · 40 marks

Name: Date: Score: ____ / 40

Show your working for Sections B and C. Answers are on the last page — try every question first!

Section A — Choose the correct answer8 × 1 = 8 marks
1.
A sound has frequency 220 Hz and wavelength 1.5 m. Its speed is:
  1. 146.67 m/s
  2. 165 m/s
  3. 660 m/s
  4. 330 m/s
2.
The time period of a sound wave is 0.4 ms. Its frequency is:
  1. 2,500 Hz
  2. 25,000 Hz
  3. 1,250 Hz
  4. 5,000 Hz
3.
Sound cannot travel through:
  1. steel
  2. water
  3. a vacuum
  4. air
4.
A person claps near a cliff and hears the echo 2.5 s later. If sound travels at 330 m/s, the cliff is:
  1. 206.25 m away
  2. 132 m away
  3. 825 m away
  4. 412.5 m away
5.
A sound of frequency 50,000 Hz is:
  1. ultrasound
  2. infrasound
  3. audible sound
  4. a light wave
6.
In which of these does sound travel slowest (at 25 °C)?
  1. Distilled water
  2. Sulphur dioxide
  3. Helium
  4. Sea water
7.
Sound travels at 5,960 m/s in steel. How long does it take to travel 2,980 m in steel?
  1. 5 s
  2. 1 s
  3. 0.25 s
  4. 0.5 s
8.
Assertion (A): An echo heard 2.5 s after a shout means the wall is 430 m away (speed of sound 344 m/s).
Reason (R): Sound needs a material medium to travel.
  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is not the correct explanation of A.
  3. A is true, but R is false.
  4. A is false, but R is true.
Section B — Short answer8 × 2 = 16 marks
9.
A sound of frequency 100 Hz travels through air at 344 m/s. Find its wavelength and its time period.
10.
A boy stands 196 m from a tall building and claps. After how long will he hear the echo? (Speed of sound = 344 m/s)
11.
A ship's SONAR sends an ultrasonic pulse to the sea bed and receives the echo 1.5 s later. If sound travels at 1,531 m/s in sea water, how deep is the sea?
12.
Why do we see the lightning before we hear the thunder?
13.
A tuning fork vibrates at 100 Hz. Find its time period, and the number of vibrations it makes in 30 s.
14.
Thunder is heard 2 s after a flash of lightning is seen. How far away is the storm? (Speed of sound = 346 m/s; light arrives almost instantly.) Give the answer in km.
15.
To hear a distinct echo, the reflected sound must arrive at least 0.1 s after the original. What is the minimum distance of the reflecting surface if sound travels at 346 m/s?
16.
A hammer strikes one end of a 500 m long brass rail. Sound reaches the other end through the brass (4,700 m/s) and through the air (346 m/s). Find both times and the gap between them, in milliseconds.
Section C — Word problems4 × 4 = 16 marks
17.
Describe an experiment to show that sound needs a medium to travel.
18.
A person stands between two cliffs and fires a cracker. The first echo is heard after 2 s and the second after 3.5 s. If sound travels at 346 m/s, find (a) the distance to each cliff and (b) the distance between the cliffs.
19.
A SONAR sends ultrasound of frequency 50,000 Hz into sea water (speed 1,531 m/s). The echo from a shoal of fish returns after 3 s. (a) How far away are the fish? (b) What is the wavelength of the ultrasound in water, in cm? (c) Why is ultrasound used and not audible sound?
20.
A sound of frequency 5,000 Hz travels through methanol, where its speed is 1,103 m/s. Find (a) its wavelength, (b) its time period, (c) the distance it travels in 0.5 s, and (d) say whether a human can hear it.
AmbifulWorksheets
Practice that never repeats
CBSE · Class 9 · Science

Answer key

Sound Waves: Characteristics and Applications · AW-9S-SOU-2026-V1 · For parents — fold back or tear off before handing the sheet to your child.

QAnswerWorking
1(d) 330 m/sv = fλ
2(a) 2,500 Hzf = 1 ÷ T
3(c) a vacuumit needs a medium
4(d) 412.5 m away2d = vt, d = 330 × 2.5 ÷ 2
5(a) ultrasoundhumans hear 20 Hz to 20,000 Hz
6(b) Sulphur dioxidehelium 965 m/s, sea water 1,531 m/s, sulphur dioxide 213 m/s, distilled water 1,498 m/s
7(d) 0.5 st = d ÷ v
8(b) Both A and R are true, but R is not the correct explanation of A.A is true: d = 344 × 2.5 ÷ 2 = 430 m. R is true but does not explain A.
9λ = 3.44 m; T = 10 msλ = v ÷ f; T = 1 ÷ f
101.14 st = 2d ÷ v = 392 ÷ 344
111,148.25 md = vt ÷ 2 = 1531 × 1.5 ÷ 2
12Light travels at about 3 × 108 m/s, far faster than sound (about 340 m/s in air), so the flash reaches us almost at once while the sound takes longer.—
1310 ms; 3,000 vibrationsT = 1 ÷ 100; 100 × 30
14692 m = 0.69 kmd = 346 × 2
1517.3 msound travels 346 × 0.1 = 34.6 m there and back; ÷ 2
16brass: 106.38 ms; air: 1,445.09 ms; gap 1,338.70 mst = d ÷ v for each
17Hang an electric bell inside an airtight glass jar connected to a vacuum pump. Ring the bell: you hear it. Pump the air out slowly: the sound becomes fainter and almost stops, though the bell can still be seen ringing. So sound cannot travel through a vacuum.—
18(a) 346 m and 605.5 m. (b) 951.5 md = vt ÷ 2 for each echo; add
19(a) 2,296.5 m. (b) 3.06 cm. (c) Ultrasound has a very high frequency and short wavelength, so it travels in a narrow beam without spreading much and is reflected well by small objects; it also cannot be heard, so it does not disturb people.d = vt ÷ 2; λ = v ÷ f
20(a) 0.22 m. (b) 0.2 ms. (c) 551.5 m. (d) Yes, it is in the audible range (20 Hz to 20,000 Hz).λ = v ÷ f; T = 1 ÷ f; d = v × 0.5