CG Board Class 8 Science Chapter 10 Solutions: Sound (ध्वनि) – 2026-27

दिया गया है:**

• दोलनों की संख्या = 20
• लिया गया समय = 10 सेकंड
• सूत्र:

1. आवृत्ति (f) = (दोलनों की संख्या)/(कुल समय (सेकंड में))
2. आवर्तकाल (T) = (1)/(आवृत्ति (f))

• गणना:
• आवृत्ति (f) = 20/10 = 2 हर्ट्ज (Hz)
• आवर्तकाल (T) = 1/2 = 0.5 सेकंड

वस्तु की आवृत्ति 2 हर्ट्ज (Hz) तथा इसका आवर्तकाल 0.5 सेकंड है।

Sound requires a material medium (solid, liquid, or gas) for its propagation. Since there is no atmosphere (vacuum) on the surface of the moon, sound vibrations cannot travel through the vacuum. Therefore, the voice of one astronaut cannot be heard by another astronaut standing nearby.

Scientist Robert Boyle was the first to demonstrate and suggest through his famous bell-jar experiment that a medium is necessary for the propagation of sound.

An echo is heard clearly under the following conditions:

1. The minimum distance between the source of sound and the reflecting surface must be at least 17 metres.
2. The reflected sound must reach our ears at least 0.1 second (1/10th of a second) after the original sound.
3. The reflecting surface must be large, hard, and a good reflector of sound.

(a) Clear echo will be heard from: Steel, Wood, and Asbestos** (because these are hard, smooth materials and act as good reflectors of sound).

• (b) Echo will not be heard from: Paper and Thermocole (because these are soft, porous materials that absorb sound rather than reflecting it).

High Pitch Sounds (High Frequency):**

1. Voice of a woman or a small child.
2. Whistle sound or buzzing of a mosquito.

• Low Pitch Sounds (Low Frequency):

1. Voice of an adult man.
2. Roar of a lion or beating of a drum.

When vocal chords are thin, tight, and tightly stretched, a sound of high frequency and high pitch (shrill voice) is produced.

Birds:** Produce sound with the help of a special vocal organ present in their breathing tube (windpipe).

• Fishes: Produce sound by forcefully pushing air out of their air bladders.
• Flies: Produce sound by rapidly vibrating their wings.

(1) The range of audible frequencies for human beings is:**

(d) 20 – 20,000 हर्ट्ज

(2) On increasing the frequency of a sound, what else increases:

(a) Its pitch

(b) Its intensity

(c) Its time period

(d) Its amplitude

(a) इसका तारत्व

(3) To hear the echo clearly, the minimum distance of the reflecting surface should be:

(a) 11 metre

(b) 34 metre

(c) 17 metre

(d) 50 metre

(c) 17 मीटर

(4) Sound propagation is not possible:

(a) In metals

(b) In water

(c) In vacuum

(d) In air

(c) निर्वात में

Amplitude:** The maximum displacement of a vibrating body from its mean (central) position is called its amplitude. Its S.I. unit is metre (m).

• Frequency: The total number of vibrations or oscillations completed by a vibrating object in one second is called its frequency. Its S.I. unit is Hertz (Hz).

Experiment Setup:** Take an airtight glass jar fitted with an electric bell inside it and connected to a vacuum pump at the bottom.

• Procedure:

1. Initially, when air is present inside the jar, press the bell switch. The sound of the ringing bell is clearly heard outside. This shows that sound propagates through air (gas).
2. Now, gradually pump out the air from inside the jar using the vacuum pump.

• Observation: As the air is removed, the sound of the bell becomes fainter and fainter, until it completely stops when a vacuum is created, even though the hammer is still seen striking the bell.
• Conclusion: This experiment proves that gas (air) is a medium through which sound propagates, and sound cannot travel in a vacuum.

In a big empty room without curtains, the hard bare walls act as good reflectors of sound. Sound reflects repeatedly from the walls, producing multiple echoes and reverberations that overlap with the original sound, making it indistinct and unclear.

• In contrast, when curtains (made of cloth/fabric) are hung on the walls, they act as good sound absorbers. The curtains absorb the reflected sound waves instead of bouncing them back, preventing echoes and allowing the original sound to be heard cleanly and clearly.