Gr.8 Sub Science Ch 10 Sound W.B. Work

 

1.      Choose the correct option.

a)      Which of the following frequency can be heard by humans?

iii. 3000 Hz

b)     Which of the following is used to hear sounds generated by the internal body organs of humans as a heartbeat?

ii. Stethoscope

c)      The voice box is also called as:

iii. larynx

d)     Pitch of sound is determined by:

i.frequency

2.      Name the following

a)      A sound with a low pitch.

bass

b)     The maximum distance moved by a vibrating body from its mean position 

 amplitude

c)      A sound with a high pitch or frequency.

shrill sound

d)     The type of musical instruments that produce sounds by vibration of a tightly stretched membrane.

Membranophones

3.      Fill in the blanks

a)      The back-and-forth movement of a body in the air, which produces sound is called oscillatory motion.

 

b)     Continuous exposure to sound levels of about 85dB can cause serious damage.

 

c)      The word timbre also means quality.

 

d)     The bouncing back of sound waves from a surface is called reflection of sound.

 

4.      Define the following

a)      Amplitude

The maximum distance moved by a vibrating body from its mean position is called its amplitude.

b)     Noise

Sounds which are unpleasant to the ear are called noise.

c)      Inaudible sounds

Sound frequencies lower than 20Hz and higher than 20,000Hz cannot be heard by human ears are called inaudible sounds.

d)     Time period

Time period is the time taken by a vibrating body for one complete oscillation.

 

5.      Answer the following questions

a)      Find the frequency of a wave where one oscillation is completed in 2.5s.

Given: Time = 0.5s

Formula: f = 1 / T

               f = 1 / 2.5

               f = 0.4

Hence the frequency of the wave is 0.4 Hz.

 

b)     Explain chordophone. Give two examples of chordophones.

·         A chordophone is a musical instrument in which sound is produced by the vibration of a string or strings which are stretched between fixed points.

·         Guitar, violin, cello and harp are a few examples of string-based instruments.

 

c)      State two adverse effect of noise pollution.

Noise pollution can cause temporary and permanent damage to health like the inability to concentrate, increase in blood pressure or heart rate, hearing loss, sleep disturbances, dementia, psychological dysfunctions, etc.

 

d)     State the application of ultrasound.

Ultrasounds are very useful in various applications like:

•            In the field of medical science, ultrasounds are used as a diagnostic tool to investigate the human body from the inside.

•            They are also used to measure the depth of the sea and to detect underwater objects.

 

 

6.      Observe the diagram of the ear shown below and answer the following question.

a)      Label the above diagram.

Pinna, Ear canal, Ear drum, Interlocked bones, Organ of balance, Cochlea, Auditory nerve

b)     Describe the middle ear and list the parts under the middle ear?

A thin membrane known as the eardrum is present in the cavity of the middle

ear. When the incoming sound waves from the outer ear fall on the eardrum membrane, it vibrates and transfers these vibrations to the three tiny bones in the middle ears. These three bones are known as malleus, incus, and stapes. These bones amplify the sound vibrations and transfer them to the inner ear.

 

7.      Real life learning

 

                                                                                              

The town hall building is situated near Raj’s house. The building has a big bell on the top. During the night time, Raj is able to hear the sound of the bell more clearly as compared to the daytime.

During day time, there are many vibrations present in the air. These vibrations reduce the intensity of the loudness and pitch of the bell. Therefore, Raj hears the sound of the bell much clearer at night than in the day.

 

8.      Challenge time

Three astronauts A, B and C are in space.

Astronaut A was outside the spacecraft while astronaut B and C was inside the spacecraft. Astronaut B can easily hear astronaut C but he is not able to hear astronaut A. Why?

There is no air in space (outside the spacecraft). Astronaut A was outside the spacecraft and hence there isa vacuum and sound cannot propagate through a vacuum. In spacecraft oxygen (air) was present and thus astronaut B could hear astronaut C as sound can propagate in gases.

 

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