Sound Book Back Questions 9th Science Lesson 8

9th Science Lesson 8

8] Sound

Book Back Questions with Answer and Do You Know Box Content

Do You Know?

Problem 1: A sound wave has a frequency of 2 kHz and wavelength of 15 cm. How much time will it take to travel 1.5 km?

Solution: Speed, v = nλ

Here, n = 2 kHz = 2000 Hz

λ = 15 cm = 0.15 m

v = 0.15 x 2000 = 300 ms^-1

Time (t) = Distance (d) /Velocity (v)

t = 1500/300 = 5s

The sound will take 5 s to travel a distance of 1.5 km.

Problem 2: What is the wavelength of a sound wave in air at 20^oC with a frequency of 22 MHz?

Solution: λ = v/n

Here, v = 344 ms^-1

n = 22 MHz = 22 x 10⁶ Hz

λ = 344/22 x 10⁶ = 15.64 x 10^-6 m = 15.64 µm.

Sonic boom: When the speed of any object exceeds the speed of sound in air (330 ms^-1) it is said to be travelling at supersonic speed. Bullets, jet, aircrafts etc., can travel at supersonic speeds. When an object travels at a speed higher than that of sound in air, it produces shock waves. These shock waves carry a large amount of energy. The air pressure variations associated with this type of shock waves produces a very sharp and loud sound called the ‘sonic boom’. The shock waves produced by an aircraft have energy to shatter glass and even damage buildings.

Sound travels about 5 times faster in water than in air. Since the speed of sound in sea water is very large (being about 1530 ms^-1 which is more than 5500 km/h^-1), two whales in the sea which are even hundreds of kilometres away can talk to each other very easily through the sea water.

Use of ear phones for long hours can cause infection in the inner parts of the ears, apart from damage to the ear drum. Your safety is in danger if you wear ear phones while crossing signals, walking on the roads and travelling. Using earphones while sleeping is all the more dangerous as current is passing in the wires. It may even lead to mental irritation. Hence, you are advised to deter from using earphones as far as possible.

Problem 3: A man fires a gun and hears its echo after 5s. The man then moves 310 m towards the hill and fires his gun again. If he hears the echo after 3 s, calculate the speed of sound.

Solution: Distance (d) = velocity (v) x time (t)

Distance travelled by sound when gun fires first time, 2d = v x 5 (1)

Distance travelled by sound when gun fires second time, 2d – 620 = v x 3 (2)

Rewriting equation (2) as, 2d = (v x 3) + 620 (3)

Equating (1) and (3), 5v = 3v + 620

2v = 620

Velocity of sound, v = 310 ms^-1

Animals, such as bats, dolphins, rats, whales and oil birds, use echolation, an ultrasound technique that uses echoes to identify and locate objects. Echolation allows bats to navigate through dark caves and find insectsfor food. Dolphins and whales emit a rapid series of underwater clicks in ultrasonic frequencies to locate their prey and navigate through water.

Problem 4: A ship sends out ultrasound that returns from the seabed and is detected after 3.42 s. If the speed of ultrasound through sea water is 1531 ms^-1, what is the distance of the seabed from the ship?

Solution: We know, distance = speed x time

2d = speed of ultrasound x time

2d = 1531 x 3.42

⸫d = 5236/2 = 2618 m

Thus, the distance of the seabed from the ship is 2618 m or 2.618 km.

Choose the best answers:

1. Which of the following vibrates when a musical note is produced by the cymbals in a orchestra?

(a) Stretched strings

(b) Stretched membranes

(c) Air columns

(d) Metal plates

2. Sound travels in air:

(a) If there is no moisture in the atmosphere

(b) If particles of medium travel from one place to another

(c) If both particles as well as disturbance move from one place to another

(d) If disturbance moves

3. A musical instrument is producing continuous note. This note cannot be heard by a person having a normal hearing range. This note must then be passing through

(a) Wax

(b) Vacuum

(c) Water

(d) Empty vessel

4. The maximum speed of vibrations which produces audible sound will be in

(a) Sea water

(b) Ground glass

(c) Dry air

(d) Human blood

5. The sound waves travel faster

(a) In liquids

(b) In gases

(c) In solids

(d) In vacuum

Fill in the blanks:

1. Sound is a ___________ wave and needs a material medium to travel.

2. Number of vibrations produced in one second is _________________.

3. The velocity of sound in solid is _______________ than the velocity of sound in air.

4. Vibration of object produces _______________.

5. Loudness is proportional to the square of the _________________.

6. _____________ is a medical instrument used for listening to sounds produced in the body.

7. The repeated reflection that results in persistence of sound is called ______________.

Match the following:

1. Tuning fork – the point where density of air is maximum

2. Sound – maximum displacement from the equilibrium position

3. Compressions – the sound whose frequency is greater than 20,000 Hz

4. Amplitude – longitudinal wave

5. Ultrasonics – production of sound

Answers:

Choose the best answers:

1. stretched strings 2. if particles of medium travel from one place to another 3. empty vessel 4. sea water 5. in solids

Fill in the blanks:

1. Longitudinal 2. Frequency of wave 3. Faster 4. Sound 5. Amplitude 6. ECG or Stethoscope 7. Reverberation

Match the following:

1.Tuning fork – Production of sound

2.Sound- The point where density of air is maximum

3.Compressions- Longitudinal wave

4.Amplitude- Maximum displacement from the equilibrium position

5.Ultasonics- The sound whose frequency is greater than 20,000 Hz