CBSE Class 8 Science Chapter 13 Sound NCERT Solutions

Solution: Sound is a form of energy that requires a medium to travel. It can propagate through solids, liquids, and gases because these mediums consist of particles that can vibrate and transmit the sound energy. Therefore, sound can travel through solids, liquids, and gases.

The correct option is (d) solids, liquids and gases.

Solution: The frequency of a sound is related to its pitch. A lower frequency corresponds to a lower pitch (a deeper voice), while a higher frequency corresponds to a higher pitch (a shriller voice). Adult men typically have larger and thicker vocal cords compared to women and children. This results in slower vibrations of the vocal cords, producing a sound with a lower frequency and thus a deeper voice. Baby girls generally have the highest pitched voices among the options due to their smaller vocal cords.

Therefore, the voice of a man is likely to have the minimum frequency.

The correct option is (c) A man.

Solution: Let's evaluate each statement:

1. Sound cannot travel in vacuum. (T) Sound waves are mechanical waves, meaning they require a medium (like solids, liquids, or gases) to propagate. They cannot travel through a vacuum where there are no particles to vibrate.
2. The number of oscillations per second of a vibrating object is called its time period. (F) The number of oscillations per second is called frequency. The time period is the time taken to complete one oscillation.
3. If the amplitude of vibration is large, sound is feeble. (F) The loudness of a sound is determined by the amplitude of vibration. A large amplitude results in a loud sound, not a feeble one. A small amplitude produces a feeble sound.
4. For human ears, the audible range is 20 Hz to 20,000 Hz. (T) This is the generally accepted range of frequencies that the average human ear can detect.
5. The lower the frequency of vibration, the higher is the pitch. (F) Pitch is directly related to frequency. A lower frequency results in a lower pitch (deeper sound), while a higher frequency results in a higher pitch (shriller sound).
6. Unwanted or unpleasant sound is termed as music. (F) Unwanted or unpleasant sound is termed as noise. Music is generally considered pleasant to hear.
7. Noise pollution may cause partial hearing impairment. (T) Prolonged exposure to loud noise (noise pollution) can damage the sensitive structures in the ear, leading to temporary or permanent hearing loss.

Solution:

(a) Time taken by an object to complete one oscillation is called time period.

(b) Loudness is determined by the amplitude of vibration.

(c) The unit of frequency is hertz (Hz).

(d) Unwanted sound is called noise.

(e) Shrillness of a sound is determined by the frequency of vibration.

Solution:

We are given that a pendulum completes 40 oscillations in 4 seconds.

To find the time period, we use the formula:

$$Time\backslash \; period\; =\; \backslash frac\{Time\backslash \; taken\}\{Number\backslash \; of\backslash \; oscillations\}$$

Substituting the given values:

$$Time\backslash \; period\; =\; \backslash frac\{4\backslash \; seconds\}\{40\backslash \; oscillations\}\; =\; 0.1\backslash \; seconds$$

The time period of the pendulum is 0.1 seconds.

To find the frequency, we use the formula:

$$Frequency\; =\; \backslash frac\{Number\backslash \; of\backslash \; oscillations\}\{Time\backslash \; taken\}$$

Substituting the given values:

$$Frequency\; =\; \backslash frac\{40\backslash \; oscillations\}\{4\backslash \; seconds\}\; =\; 10\backslash \; Hz$$

The frequency of the pendulum is 10 Hz.