What is an echo?
(a) State two conditions necessary for hearing an echo.
(b) What are the medical applications of echo?
c) How can you calculate the speed of sound using echo?

Explanation

Echo:
An echo is the sound reproduced due to the rigid surfaces such as walls, ceilings, surfaces of mountains, etc.
(a) (i) The persistence of hearing for human ears is 0.1 second. This means that we can hear two sound waves clearly, if the time interval between the two sounds is atleast 0.1 s. Thus, the minimum time gap between the original sound and an echo must be 0.1s
(ii) The above criterion can be satisfied only when the distance between the source of sound and the reflecting surface would satisfy the following equation:
Velocity `("Distance travelled by sound")/("Time taken")`
`v=(2d)/(t)`
`d=(vt)/(2)`
Since, t=0.1 second, then `d=(vxx0.1)/(2)=(v)/(20)`
(b) (i) The principle of echo is used in obstetric ultrasonography, which is used to create real-time visual images of the developing embryo or fetus in the mother.s uterus.
(ii) This is a safe testing tool, as it does not use any harmful radiations.
(c) Apparatus required:
A source of sound pulses, a measuring tape, a sound receiver and a stop watch.
Calculation of speed of sound:
The sound pulse emitted by the source travels a total distance of 2d while travelling from the source to the wall and then back to the receiver.
The time taken for this has been observed to be .t.. Hence, the speed of sound wave is given by
Speed of sound `=("Distance travelled")/("Time taken")=(2d)/(t)`