Reflection of sound and echo 9th class(lec 4)

 video lecture for Reflection on sound and echo

Question 1: The velocity of ultrasonic sound in water is `1400 ms^{−1}`. The depth of the ocean as detected by `SONAR`, if the time taken to receive the reflected wave is `15 s`, is_____.

(a) `21 km`
(b) `10.5 km`
(c) `105 m`
(d) `1500 m`

Question 2: The minimum distance to hear an echo is (Taking the velocity of sound in air to be `330 ms^{–1}`)  

(a) `1/20 m`
(b) `16.5 m `
(c) `20 m`
(d) Cannot be determined

Question 3: In `SONAR`, we use

(a) ultrasonic waves
(b) infrasonic waves
(c) radio waves
(d) audible sound waves

Question 4: Sound produced by a thunderstorm is heard `10 s` after the lightning is seen. Calculate the approximate distance of the thundercloud. (Given the speed of sound` = 340 ms^{-1}`.)

Question 5: For hearing the loudest ticking sound heard by the ear, find the angle `x` in the figure.

Question 6: A sonar device mounted on a submarine sends out signals and receives their echo after `5 s`. If the distance of the object from the submarine is `3625m`, calculate the speed of sound.

a) `7250 ms^{-1}`

b) `1450 ms^{-1}`

c) `1750 ms^{-1}`

d) `1050 ms^{-1}`

Question 7: How do bats sense their path?

Question 8: Jet planes that move with speeds greater than the speed of sound are called ________.

Question 9: The audible frequency range of sound for human beings is _______. 

Question 10: Sound travels in air if
(a) particles of medium travel from one place to another
(b) there is no moisture in the atmosphere
(c) disturbance moves
(d) both particles as well as disturbance travel from one place to another.

Question 11: When we change feeble sound to loud sound we increase its
(a) frequency
(b) amplitude
(c) velocity
(d) wavelength

Question 12:  If any explosion takes place at the bottom of a lake, what type of shock waves in water will take place?

Question 13: What is the range of frequencies related to the following?
(a) inaudible sound
(b) Ultrasound.

Answer Key 

Q1: (b)  

Q2: (b)

Q3: (a) Ultrasonic waves are used in `SONAR`.

Q4: Solution:

Let the distance of the thunder cloud from the observer be `x m`.

Now, speed of light` = 3\times10^{8} ms^{-1}`

Speed of sound `= 340 ms^{-1}`

Time taken by the light to reach observer,

`t_{l}=\frac{x}{3\times10^{8}s`

Time taken by the sound to reach observer,

`t_{s}=\frac{x}{340}s`

Given `t_{s}-t_{l}=10s`

`\frac{x}{340}-\frac{x}{3\times10^{8}}=10  or x(\frac{1}{340}-\frac{1}{3\times10^{8}})=10`

`\frac{x}{340}=10`                           `( ∴  3\times10^{8}＞＞＞＞＞340, \frac{1}{340}＞＞＞＞\frac{1}{3\times10^{8}})`

Q5: Solution:

For hearing the loudest ticking sound, angle made by incident sound with normal = angle made by reflected sound wave with normal `= x`

Now, angle made by incident sound with normal

`= 90° – 50° = 40°`

`∴ x = 40°`

Q6:  (b)

Q7:  Bats have the ability to produce and hear ultrasonic sounds. It is because of these vibrations of ultrasonic sound that bats perceive their path of movement.

Q8: Answer: supersonic jets

Q9: Answer:  20 Hz to 20 kHz

Q10: Solution:
(c) Sound travels in air if disturbance moves, not the particles of medium.

Q11: Solution:

(b) As the loudness and softness of sound or intensity of sound is directly proportional to the square of amplitude of sound wave, therefore, feeble sound can be changed to loud sound by increasing its amplitude.

Q12: Solution:

Shock waves in water are longitudinal waves, when any explosion takes place at the bottom of a lake.

Q13: Answer- 

(a) Infrasound = frequency less than 20 Hz 

(b) Ultrasound = frequency more than 20K Hz