Science Question Answer (3 Marks)

1. Infrasounds include sounds of frequencies below 20Hz.
2. Audible sounds include sounds of frequencies between 20Hz and 20,000Hz.
3. Ultrasounds include sounds of frequencies above 20,000Hz.

We hear more clearly in a room with curtains than in a room without curtains because curtains are bad reflectors of sound. They absorb most of the sound falling on them, and hence do not produce echoes. On the other hand, in rooms without curtains, there is a greater reflection of sound due to which some echoes are produced.
These echoes cause a hindrance to hearing.

In most of the cases, a sound producing object vibrates so fast that we cannot see its vibrations with our eyes. The time interval between two successive vibration is lower than the persistence of vision. Hence we see the object in static state and not in vibration mode.

Sound can be produced by the following methods:

1. By vibrating strings (as in a sitar).
2. By vibrating air (as in a flute).
3. By vibrating membranes (as in a drum).
4. By vibrating plates (as in cymbals).

When we put our ear to a railway line, we can hear the sound of an approaching train even when the train is far off but its sound cannot be heard through the air. This is due to the fact that sound travels much more fast through the railway line made of steel than through air.

Ultrasound waves are made to pass through one face of the metal block and ultrasound detectors are placed on the opposite face of the block to detect the transmitted ultrasound waves.

1. If the ultrasound waves pass uninterrupted through all parts of the metal block, then the block is flawless.
2. However, if the ultrasound waves are not able to pass through a part of the metal block and get reflected back, then there is a flaw in the metal block.

If the air is gradually pumped out of the glass vessel, no sound of the electric bell can be heard because vacuum is created in the vessel and there are no air molecules to carry sound vibrations.

Bats emit high frequency ultrasonic squeaks while flying and listen to the echoes produced by the reflection of their squeaks from their prey. From the time taken by the echo to be heard, bats can judge the distance of the prey in their path and catch it.

Sound cannot be heard on the surface of moon because there is no air on the moon to carry the sound waves.
Astronauts talk to one another on the surface of moon through wireless sets using radio waves. This is because radio waves can travel even through vacuum though sound waves cannot travel through vacuum.

1. Echolocation: Echolocation is the method used by some animals to locate the objects by hearing the echoes of their ultrasonic squeaks.
2. Echocardiography: Echocardiography is the use of ultrasound waves to investigate the action of the heart.
3. Ultra sonography: Ultra sonography is the technique of obtaining pictures of internal organs of the body by using echoes of ultrasound pulses.

Crest: The 'elevation' or 'hump' in a transverse wave is called crest. It is that part of the transverse wave which is above the line of zero disturbance of the medium.
Trough: The 'depression' or 'hollow' in a transverse wave is called trough. It is that part of the transverse wave which is below the line of zero disturbance of medium.
A transverse wave consists of crests and troughs.

Ultrasounds: Ultrasounds are the sounds having very high frequency which cannot be heard by human beings.

Ordinary sound	Ultrasound
These include sounds of frequencies in the range of 20Hz to 20,000hz.	These include sounds of frequencies above 20,000Hz.
These sounds are audiible to human ears.	These sounds are inaudible to human ears.

Bats are able to fly at night without colliding with other objects because they emit high frequency ultrasonic squeaks while flying and listen to the echoes produced by the reflection of their squeaks from the objects or obstacles in their path. From the time taken by the echo to be heard, bats can judge the distance of the object in their path and avoid it by changing the direction.

The maximum displacement of the particles of the medium from their original undisturbed positions, when a wave passes through the medium, is called amplitude (A) of the wave.

Sound is produced by the following objects:

1. Vibrating stretched strings of sitar.
2. Vibrating stretched membranes of tabla.
3. Vibrating prongs of a tuning fork.
4. Vibrating wings of mosquito.
5. Vibrating air columns in flute.

Applications of ultrasound are:

1. Ultrasound is used in industry for detecting flaws in metal blocks without damaging them.
2. In hospitals, ultrasounds are used to investigate the internal organs of the human body such as liver, kidneys, uterus, etc.
3. Ultrasounds are also used to monitor the growth of fetus inside the mother’s uterus.