NCTB Class 7 Science Chapter Eight Exercise 8 Solution by Science Expert. Bangladesh Board Class 7 Science Solution Chapter 8 ‘’About Sound’’ Exercise 8 Solution.
(1) Sound does not circulate without ——.
(2) The limit of audibility of man is from —– Hertz to 200000 Hertz.
(3) Unexpected & disturbing sound is called ——
(4) The velocity of sound in the air is the —-.
(5) The sound above 20000 hertz vibration is called ——.
Answer:- Hearing post-sound.
Short answer type question:-
(1) What are the difference between audible and inaudible and sound?
Answer:- The sound which creates 20 vibrations/ second or more than 200000 vibrations/second is called inaudible sound.
(2) What do you mean by act of hearing-pre sound and act of hearing-post sound?
Answer:- Below 20 Hz frequency sound is called hearing pre-sound. Above 20000 Hz frequency sound is called hearing post-sound.
(3) What are the differences between noise and pleasing sound?
Answer:- The difference between noise and sound —–
(a) Noise is unexpected and disturbing but pleasing sound is pleasing to ear and comfortable.
(b) Noise creates more than 200000 vibrations/second but pleasing sound creates 20 to 200000 vibrations/sounds.
(4) Can we hear the sound of all vibrations? What is the limit of our audibility?
Answer:- No we cannot hear the sound of all vibrations. Below 20 hertz vibration sound is inaudible and above 200000 hertz vibration sound is noise which is disturbing to ear. The limit of our audibility is 20 hertz to 200000 hertz.
Multiple choice questions:-
(1) Which medium circulates the highest velocity of sound?
Answer:- (b) Solid medium.
Read the passage given below attentively and answer the question no. 2 and 3.
The explosions on the surface of the moon and shooting a gun on the surface of the earth, in both cases the flashes of light are seen.
(2) To hear the sound of explosion on the surface of the moon from the earth –
Answer:- (d) (i) and (ii).
(4) Which instrument produces sound with the help of its inside vibration?
Answer:- (d) Flute.
The velocity of sound is 330m/sec (air). The velocity of sound in sea water is 1500m/sec. The man standing on the sea shore and the diver are 3300 meters away from the place of the bomb explosion.
(1) (a) What is sound?
Answer:- What we hear is called sound which is a kind of energy that produces the feeling of hearing.
(1) (b) Why do you hear the sound of railway movement from a distance if you put the ear on the rail?
Answer:- If we put the ear on the rail then we can hear the sound of railway movement from a distance due to the vibration of the rail.
(2) The sound of school bell was no heard for a long from Tapan’s house. Recently the shape of the bell has been changed keeping the weight as before. As a result now he can hear the sound of the school bell from his house.
(2) (a) what is pleasing audible sound?
Answer:- Some sounds that are pleasing to ear and comfortable are called pleasing audible sound.
(2) (b) What change will come to the flute if the length of the tube of the flute is lesser?
Answer:- If the length of the tube of the flute is lesser then the intensity of the sound of the flute become increased.
(c) Describe the techniques of reaching sound to Tapan’s ear from the school bell.
Answer:- The school bell is round metallic. If we hit the bell by a hammer then it produces sound Tung/Tang as a result of vibration of the round metallic bell. That’s how the sound of bell is reaching to tapan’s ear.
(d) What kind of change has been made to the bell for which Tapan can hear the sound from his house? Explain this with appropriate causes.
Answer:- If we increased the length and thickness of the bell then tapan can hear the sound his house because the intensity of the sound depends on the length and thickness of the bell.