CBSE Important Questions Class 9 Science Chapter 11 – Sound

Class 9 Science Chapter 11 Sound is part of the CBSE Class 9 Science syllabus. It explains how sound is produced by vibrating bodies and how it travels as a longitudinal wave through a medium. Students learn about compressions and rarefactions, the characteristics of a sound wave such as frequency, wavelength, amplitude, time period and speed, and about pitch, loudness and quality. The chapter also covers reflection of sound, echo, reverberation, the range of hearing, ultrasound and SONAR.

These CBSE important questions for Class 9 Science Chapter 11 cover frequency and its SI unit, the difference between frequency and pitch, reading the frequency from a wave diagram, why sound cannot travel through vacuum, longitudinal and transverse waves, wavelength and amplitude, echo with a numerical problem, and the uses of ultrasound. Every question has a clear step-by-step answer in simple English with figures, so students can practise on their own, parents can check the working, and teachers can use them for class tests.

Q1. The sound frequency determines its
(a) amplitude
(b) energy
(c) intensity
(d) pitch

Answer: (d) pitch
Pitch tells us how shrill or how flat a sound is, and it depends directly on the frequency. A higher frequency gives a higher (shriller) pitch and a lower frequency gives a lower (flatter) pitch. Loudness and intensity depend on the amplitude, not the frequency.

Q2. Define the frequency of a sound wave. What is its SI unit and how is it related to pitch? What is the frequency of the wave shown below?

Answer: The number of complete oscillations (vibrations) made by a sound wave in one second is called its frequency.
Its SI unit is the hertz (Hz). One hertz means one oscillation per second.
Frequency and pitch are directly related: the higher the frequency, the higher the pitch.
In the given wave, the number of complete oscillations in one second is 4.
So, frequency = 4 ÷ 1 = 4 Hz

Q3. Explain the difference between the frequency and the pitch of a sound wave.

Answer: Frequency is a physical quantity. It is the number of complete oscillations of a sound wave per unit time, and it is measured in hertz (Hz) with an instrument. It is a property of the wave itself.
Pitch is how our brain and ear interpret that frequency. It is a sensation, not something we can measure directly with an instrument. It tells us whether a sound seems shrill or flat.
The two are directly related: the faster the source vibrates, the higher the frequency, and the higher the pitch we hear.

Q4. The pitch of a sound is determined by its
(a) time period
(b) amplitude
(c) frequency
(d) vibrations

Answer: (c) frequency
Pitch depends on how many vibrations happen per second, which is exactly the frequency. Vibrations are the cause of sound, but it is the rate of vibration that decides the pitch. Amplitude decides loudness.

Q5. What is the relevance of frequency to sound? In the propagation of a sound wave, five compressions and five rarefactions cross a point in 5 s. Calculate the frequency of the sound wave.

Answer: The frequency of vibration of a body decides the pitch of the sound it produces. If the frequency increases, the sound becomes sharper and shriller; if it decreases, the sound becomes flat.
Calculation: One compression together with one rarefaction makes one complete oscillation (one wave).
Five compressions and five rarefactions make 5 complete oscillations in 5 s.
Frequency = number of oscillations ÷ time = 5 ÷ 5 = 1 Hz

Q6. The characteristic of sound which depends on frequency is
(a) velocity
(b) loudness
(c) pitch
(d) timbre

Answer: (c) pitch
Pitch is the only one of these that depends directly on frequency. Loudness depends on amplitude, the velocity of sound depends on the medium and its temperature, and timbre (quality) depends on the shape of the wave.

Q7. Define the term pitch. What is the pitch of a sound?

Answer: Pitch is the characteristic of sound that tells us how shrill or how flat a sound is. It depends on the frequency of vibration.
If the frequency of vibration is high, the sound produced is shrill and is said to be of higher pitch, like the voice of a child or the sound of a whistle.
If the frequency of vibration is low, the sound is dull and is said to be of lower pitch, like the roar of a lion or the voice of an adult man.

Q8. What is sound and how is it produced? Why can sound not travel through vacuum?

Answer: Sound is a form of energy that produces the sensation of hearing in our ears. It is produced when an object vibrates. For example, when we strike a tuning fork or pluck the string of a guitar, the vibrating object pushes the particles of the surrounding air back and forth, creating compressions and rarefactions that travel outwards as a wave.
Sound cannot travel through a vacuum because sound is a mechanical wave; it needs particles of a medium to pass the vibration from one place to the next. A vacuum has no particles at all, so there is nothing to carry the vibration. This is why no sound can be heard in outer space.

Q9. Distinguish between longitudinal and transverse waves. To which type does a sound wave belong?

A sound wave is a longitudinal wave, because the particles of the medium move back and forth in the same direction in which the sound travels.

Q10. Define wavelength, amplitude and time period of a sound wave. Write the relation between velocity, frequency and wavelength.

Answer: Wavelength (λ): the distance between two consecutive compressions or two consecutive rarefactions. Its SI unit is the metre (m).
Amplitude (A): the maximum displacement of the particles of the medium from their mean position. It decides the loudness of the sound.
Time period (T): the time taken for one complete oscillation. Its SI unit is the second (s). It is the reciprocal of frequency, that is T = 1/ν.
Relation: velocity = frequency × wavelength, that is v = νλ

Q11. What is an echo? Why is an echo not heard in a small room? A person claps near a cliff and hears the echo after 2 s. If the speed of sound is 346 m/s, find the distance of the cliff from the person.

Answer: An echo is the sound we hear again after it is reflected from a hard surface such as a wall or a cliff.
Our brain holds the sensation of a sound for about 0.1 s. For an echo to be heard separately, the reflected sound must take at least 0.1 s to come back, which needs a minimum distance of about 17.2 m between the source and the reflecting surface. In a small room the walls are much closer than this, so the reflected sound merges with the original sound and no separate echo is heard.
Numerical: In 2 s the sound travels to the cliff and back, so the total distance = 346 × 2 = 692 m.
Distance of the cliff = 692 ÷ 2 = 346 m

Q12. What is ultrasound? Write any three uses of ultrasound.

Answer: Ultrasound is sound of frequency higher than 20,000 Hz (20 kHz). Human beings cannot hear it, but animals such as bats, dolphins and porpoises can produce and hear it.
Uses:
1. In medicine, ultrasound scanners are used to see internal organs such as the liver, kidney and the growing baby in the womb, and to break small kidney stones into fine grains.
2. In industry, it is used to clean parts that are hard to reach, such as spiral tubes and odd-shaped machine parts, and to detect cracks or flaws inside metal blocks.
3. SONAR uses ultrasound to find the depth of the sea and to locate underwater objects such as submarines, icebergs and shoals of fish.

Related Study Material on Extramarks

• CBSE Important Questions Class 9 Science Chapter 1
• CBSE Important Questions Class 9 Science Chapter 2
• CBSE Important Questions Class 9 Science Chapter 3
• CBSE Important Questions Class 9 Science Chapter 4
• CBSE Important Questions Class 9 Science Chapter 5
• CBSE Important Questions Class 9 Science Chapter 6
• CBSE Important Questions Class 9 Science Chapter 7
• CBSE Important Questions Class 9 Science Chapter 8
• CBSE Important Questions Class 9 Science Chapter 9
• CBSE Important Questions Class 9 Science Chapter 10
• CBSE Important Questions Class 9 Science Chapter 12