NCERT Solutions for Class 8 Science Chapter 13 - Sound

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Answer:

Sound needs material medium to travel. It can travel through solid (wood), liquid (water) and gas (air). But it can not travel in vacuum.
Hence, the correct answer is option (b).

Answer:

Loudness of a sound depends on its amplitude. Higher the amplitude, louder the sound. 
Hence, the correct answer is option (a).

Answer:

Sound can be  produced by vibrating objects. Sound requires material medium for propagation. it can not travel in vacuum. Speed of light is very high ($3\times {10}^8 \mathrm{m}/\mathrm{s}$) as compared to speed of sound (approximately 320 m/s). Light does not require material medium for propagation. 
Hence, the correct answer is option (d).
 

Answer:

Frequency (f) = 50 Hz
 $\mathrm{Frequency}\left(f\right)=\frac{1}{\mathrm{Time} \mathrm{period}\left(T\right)}$
$$\begin{gathered} \Rightarrow T=\frac{1}{f} \\ =\frac{1}{50} \\ =0.02 \mathrm{s} \end{gathered}$$
Hence, the correct answer is option (a).

Answer:

Loudness of the sound can be reduced by decreasing its amplitude.
Hence, the correct answer is option (c).

Answer:

Loudness of sound is measured in units of decibel (dB). Wavelength is measured in units of metre (m). Frequency is measured  in units of hertz (Hz). And the speed is measured  in units of metre/second (m/s).
Hence, the correct answer is option (a).
 

Answer:

The loudness of sound  is determined by the amplitude of vibration. Higher the amplitude, higher its loudness.
Hence, the correct answer is option (a).

Answer:

Frequency is defined as number of vibrations per second ( Hz). 
$$\begin{gathered} 1 \mathrm{Hz}=\frac{1\mathrm{vibration}}{1 \mathrm{s}} \\ =\frac{1\mathrm{vibration}}{{\displaystyle \frac{1}{60}} \mathrm{minute}} \\ =60 \mathrm{vibrations} \mathrm{per} \mathrm{minute} \end{gathered}$$
Hence, the correct answer is option (c).

Answer:

Pitch is determined by the frequency of the sound. High pitch means high frequency, and low pitch means low frequency.
Hence, the correct answer is option (a).

Answer:

Ultrasound has frequency of vibration above 20,000 Hz. It is above the maximum hearing range of normal person.
Hence, the correct answer is option (c).

Answer:

Speed of the sound, v = 330 m/s
Time, t = 5 s
Distance travelled,
$$\begin{gathered} s=vt \\ \Rightarrow s=330\times 5 \\ \Rightarrow s=1650 \mathrm{m} \end{gathered}$$
Paheli is 1650 m far from the place of lightning.

Answer:

Yes, in humans, the sound is produced by vibrating the vocal cords or the larynx. Muscles attached to the vocal cords can make the cords tight or loose. These function are governed by air pressure produced by lungs.

Answer:

Speed of light is very high as compared to sound. In other words, sound takes more time to cover the same distance as compared to light So, Boojho heard the sound of the cracker a little later after seeing the cracker burst.

Answer:

Yes, while hearing, sound vibrations make the eardrum vibrate. The eardrum sends vibrations to the inner ear. From there, the signal goes to the brain. That is how we hear sound.

Answer:

Guitar and Sitar are musical instruments which produce sound by vibrating strings.

Answer:

Number of oscillations, n = 10
Time, t = 20 s
Time period,
$$\begin{gathered} T=\frac{t}{n} \\ =\frac{20}{10} \\ =2 \mathrm{s} \\ \mathrm{Frequency}, \\ f=\frac{n}{t}=\frac{10}{20} \\ =\frac{1}{2} \mathrm{revolution}/\mathrm{s}=0.5 \mathrm{Hz} \end{gathered}$$

Answer:

Our ear is capable to hear the vibrations of the frequency range between 20 Hz to 20,000 Hz. If a vibrating body produces the sound of frequency range above 20,000 Hz or below 20 Hz, then the sound is not audible to us. So, we can not hear vibrations below 20 Hz or above 20,000 Hz.

Answer:

Yes, the frying pan will vibrate by striking the stick on it. As there is vacuum (i.e. no material medium) and sound will propagate in material medium only, so we can not hear the sound of vibrating pan.

Answer:

In space, there is absence of air i.e. there is vacuum and sound can not propagate in vacuum. So, the astronauts cannot hear sounds of each-other without using any special device.

Answer:

Loudspeakers during marriage functions and parties, horn of heavy vehicles, sound of metal cutting machines.

Answer:

Vibration produced will be of higher amplitude if the string is plucked with greater force. And we know that the higher the amplitude of vibrations, the louder the sound. So, the instruments in which the string is plucked with higher force causing higher amplitude, results the louder sound than the other. 

Answer:

Sound can be produced by vibrating objects. These vibrations transfers the vibrations to its neighboring material medium particles (air particles). These air particles starts vibrating to and fro and transfer the motion to the next neighboring air particles ...and so on. In this way the sound propagate and the air particle near our ear also starts vibrating and make the eardrum vibrate. 
The eardrum is like a stretched rubber sheet. Sound vibrations make the eardrum vibrate. The eardrum sends vibrations to the inner ear. From there, the signal goes to the brain. In this way we can hear the sound.
( Actually, in the whole process, the transfer of energy occurs)

Answer:

Sound needs medium to propagate. The loudness of the alarm clock sound will decrease as the air is removed slowly from the bottle. If the air in the bottle is removed completely, there is vacuum in the bottle. The sound cannot travel through vacuum, the loudness of the sound reduces to zero and hence the person cannot hear the sound of the alarm clock at all.

Answer:

In the day time, there are several other sounds of different frequency and loudness ( like...vehicle horn, speaker's sound, machinery sound etc.) apart from clock bell sound, create noise. These sounds affect the loudness and clarity of the clock bell sound.
The noise level is quite low at night. Therefore the sound of the clock appears much clearer at night than in the day.

Answer:

Following measures should be taken to limit the noise pollution:

• Strictly ban on use of loudspeakers during functions, parties and election rallies.
• Ban on un-necessary blowing of horn of vehicles in school or hospital premises.
• Development of soundproof design of machines.