Science textbook solutions for Class 7 science Chapter 18 Sound : Production of Sound

Important points to remember :
  1. Sound  is generated by the rhythmic vibration of an object. 
  2. We can hear the sound as long as the object vibrates. But, when we touch the vibrating object with  our  hand, the  vibrations  stop  and  we no  longer  hear  the  sound. 
  3. Sometimes we can see the vibrations,  but sometimes,  the vibrations  are so minute  that  we cannot see them with our eyes. 
  4. A  swing that moves back and forth like this, is an  oscillator. When the swing moves from one  end to the  other  and returns to its  starting  point,  it  is said to  have completed one  oscillation. 
  5. The maximum distance  between the original position A  and the position B on stretching the rubber, is called  the  amplitude  of vibration. 
  6. Frequency is  the number of  oscillations occurring in one second. The unit of frequency is Hertz (Hz). 1 Hz means there is one oscillation  in one second; 100 Hz means hundred oscillations  in one second. 
  7. The  decibel  (dB) is the  unit  for measuring sound level. 
  8. It was  named  decibel  in  honour of the  work of the scientist Alexander  Graham  Bell. The magnitude of  sound  level, ‘decibels’ can be deduced from the intensity of the  sound using a mathematical  formula.

Question 1: Fill in the blanks .

a) Sound is generated by the rhythmic vibration of an object.

b) The frequency of sound is measured in Hertz (Hz).

c) If amplitude of sound is decreased, its loudness also decreases.

d) A medium is necessary for propagation of sound.

Question 2: Match the pairs
Group 'A'                      Group 'B'
(a) Flute                         1) Frequency less than 20 Hz

(b) Frequency                2) Frequency more than 20000 Hz

(c) Sound level              3) Vibrations in the air

(d) Ultrasonic sound     4) Measured in Hz

(e) Infrasonic sound     5) Decibel

Answer :
a) Flute  - 3) Vibrations in the air

b) Frequency - 4) Measured in Hz

c) Sound level - 5) Decibel

d) Ultrasonic sound - 2) Frequency more than 20000 Hz

e) Infrasonic sound - 1) Frequency less than 20 Hz

Question 3: Give scientific reasons.
a) In earlier times, people used to listen for the arrival of a distant train by putting their ear to the rail.
Answer : The train run on rail and its wheel bang on rail and the friction  occurs.

These produces vibration and the sound is produced.

The sound travel over a large distance through the rail.

If the year is put to the Rain sound can be clearly heard at the arrival of a train can be guessed.

b) The sounds generated by a tabla and a sitar are different.
Answer : Sitar has metal strings - stretched over pegs - that are plucked while the tabla has a piece of dried skin - stretched over a round wooden hollow - that is struck. Thus the timbre or the tonal quality of the sound produced is different. That is to say they 'sound' different.

c) If you were both on the moon , your friend will not be able to hear you call.
Answer : This is because sound needs a material medium.In case of moon , there is no material medium on the surface of moon . So , sound doesn't propagate there and you don't hear your friend's call.

d) We can hear the movement of a mosquito's wings but we cannot hear the movement of our hands.
Answer : The movement of Mosquito's wing is very fast and having a frequency of 10 khz which lies in the audible range so, we can hear the movement of mosquito wing. The movement of human hand is to slow and having a frequency less than the 1 hz which is not lies in the audible range so we cannot hear the movement of our hands.

Question 4. Write  answers to the following questions. 

a)  How is sound produced ? 
Answer : Sound is produced when something vibrates. around it to vibrate. Vibrations in air are called traveling longitudinal waves, which we can hear. Sound waves consist of areas of high and low pressure called compressions and rarefactions, respectively.

b)  What does the intensity of sound depend upon? 
Answer : The intensity of a sound wave depends on the pressure of the wave,density of the medium and speed of sound in the medium. Higher density and higher sound speed both give a lower intensity. and may be it is because that sound wave is more characterize by wavelength than frequency.

c)  Explain how  the frequency of oscillation  is related  to the length of a pendulum and the amplitude of its oscillation. 
Answer : The period of swing of a simple gravity pendulum depends on its length, the local strength of gravity, and to a small extent on the maximum angle that the pendulum swings away from vertical, θ0, called the amplitude. It is independent of the mass of the bob.

d)  Explain the two ways  by which the pitch of the sound generated by a stretched  string can be changed.
Answer : When the length of a string is changed, it will vibrate with a different frequency. Shorter strings have higher frequency and therefore higher pitch. When a musician presses her finger on a string, she shortens its length. Tightening the string gives it a higher frequency while loosening it lowers the frequency.

# Use your brain power
1.  Will  a sound be generated  no matter  how the ruler is kept on the table ? 
Answer : Yes ,Sound will be generated.
If the ruler is kept on the table,Then ruler definitely influence the frequency of the vibrations.

2. Is there any correlation  between the length of the free part of the ruler and the  sound generated ? 
Answer : The frequency of the ruler Generated by the vibrations is inversely proportional to the length of the ruler. Where f is the frequency and l is the length of the ruler. This means that if the length of the ruler increase ,than the frequency of the vibration will decrease.

3. If the ruler is plucked while it is held with 25 cm of it off the table, does it make any sound ? If there  is no sound, look for the reason why it is so.
Answer : After placing the ruler with 25cm of it on the table and 5cm in air. When we vibrate, we may notice that number of vibrations produced by 25cm blade is less. so, frequency is less and hence pitch of sound is low and hence low pitch sound is produced by blade.

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