Notes on Sound | Grade 8 > Science > Sound | KULLABS.COM

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A sound is a vibration that travels through the air or another medium and can be heard when they reach in a person's or animal's ear.The sound is produced by rapid to and fro movements, called vibrations. The sound travels through the air at a speed of about 332 meters per second.

Sound needs a medium to travel. The medium can be solid, liquid or gases. It cannot travel in a vacuum. Sound travels faster in solids and liquids than in gases. Sound travels at a speed of 1500 m/s in water and at 5000 m/s in steel. Sound travels roughly at a velocity of 330 m/s in the air at 00 Celsius. The velocity of sound is maximum in solid and least in gas. The sound is reflected from the hard surfaces. Soft surfaces are better on absorbing sound.

#### Sound wave A sound wave is a periodic disturbance which travels through a medium transferring energy from one point to another without causing any permanent displacement of the medium. A sound wave is produced by vibrating bodies. The vibration causes the surrounding air molecules to vibrate also producing a disturbance of the air. This disturbance travels out from the source of vibration in the form of longitudinal waves, which is the sound you hear.

#### Frequency (f)

The number of complete waves sets up in a medium in one second is called frequency of the wave. Its SI unit is hertz (Hz).

#### Time period (T)

The time taken by the wave to complete one cycle or a complete wave is called time period. Its unit is second.

#### Wavelength (λ) The distance between two consecutive troughs or crests in a transverse wave or the distance between two consecutive compressions or rarefactions in a longitudinal wave is called wavelength. It is denoted by λ. If λ is the wavelength, f is the frequency and v is the propagation speed. Its SI unit is meter (m).

then,

#### Velocity of Sound Wave (v)

The velocity with which sound wave propagates in a medium is called the velocity of a sound wave. Its SI unit is m/s. If λ is the wavelength, f is the frequency and v is the velocity of a sound wave.

then,

#### v =λf

Transmission of sound: The sound is produced by a vibrating body. It is propagated from the source to the listener through a medium in the form of sound waves. In the absence of a medium, no sound waves can travel.

• Sound is produced by rapid to and fro movements, called vibrations. Sound needs a medium to travel. The medium can be solid, liquid or gas.
• Soundwave is a disturbance which travels through a medium transferring energy from one point to another without causing any permanent displacement of the medium.
• The number of complete waves, set up in a medium in one second is called frequency of the wave.
• The time taken by the wave to complete one cycle or a complete wave is called time period.
• The distance between two consecutive troughs or crests in a transverse wave or the distance between two consecutive compressions or rarefactions in a longitudinal wave is called wavelength.
• The velocity of sound is maximum in solid and least in gas. Sound is reflected from the hard surfaces. Soft surfaces are better absorbers of sound.
.

### Very Short Questions

When ringing bell is touched, it stops because sound is produced by molecular vibration and when we touch it, the vibration stops, ultimately sound stops.

Let your friend put his ear on the desk and you tap on the desk. Ask to your friend whether he heard the sound or not? Surely, you get the answer that he heard. Hence, we can say that sound travels through solid with high speed. Let's put an electric bell in a bell jar and switch it on. At that time, we can listen the sound then take out all air from the jar using a vacuum pump. Again switch it on, but this time, we cannot listen the sound because sound needs a medium to travel. It cannot travel through a vacuum.

A bell and a hammer is taken inside water. Firstly, bell is hammered when it is not touching the water. We hear a very weak sound but when the bell touches the water, we hear a louder sound. This shows that sound travels in liquid.

Sound is produced due to molecular vibration of a body. Solids have large number of molecules than liquids and gases. So, vibration will also be large. Hence, sound moves faster in solids than in liquids and gases.

Sound moves faster in water than air because molecules of the water is largely packed. Speed of sound in air is 332m/s and in water is 1500m/s.

In space, there is no medium and sound needs medium to travel. So, astronauts use electronic medium to talk in space.

As we know that moon has no atmosphere, it means there is no medium and without medium sound cannot travel. So, we cannot hear each other on the surface of the medium.

When we hit long pole then sound travels through two medium. One is solid (rod) and another is air. Speed of sound in solid is greater than air. Hence, a person at another end hears two sounds one from pole and another from air.

When a sound strikes on hard surface, it returns to the same medium. This phenomenon is known as reflection of sound.

The speed of sound in air is 332 m/s.

When a bell is hammered inside water, we hear very weak sound than that in solid.

Sound needs material medium to travel but in vacuum nothing is present. It means there is no vibration in vacuum. Hence, sound cannot travel in vacuum.

Solution

\begin{align*} \text {Frequency} = 100\: Hz \\ \text {wavelength} \: (\lambda ) = 6.6\: m \\ \text {velocity of sound}\:(v)\: =\: ? \\ V &= f \times \lambda \\ &= 100 \times 6.6 \\ &= 660 \: m/s \\ \therefore \text {velocity of sound is}\: 660\: m/s \\ \end{align*}

Solution

\begin{align*} \text {velocity of sound} \: (v) = 360\: m/s \\ \text {frequency} \: = 100\: Hz\\ \text {wavelength} \: (\lambda ) =\: ? \\ \text {according to formula} \\ V &= f \times \lambda \\ \text {or,} \: \lambda &= \frac vf \\ \lambda &= \frac {360}{100} \\ &= 3.6 m \\ \therefore \text {The wavelength of the sound is} \: 3.6 \: m. \\ \end{align*}

Solution

\begin{align*} \text {Distance travelled,}\:(s) = 600 m/s \\ \text {speed,}\: (v) = 330 m/s \\ \text {speed}\:(v) &= \frac {s}{t} = \frac {600\: m}{330\:m/s} \\ &= 2\: \text {seconds} \\ \end{align*}

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• ### Sound can travel through ______.

gases only

solids only

solid, liquid and gas

liquids only

• ### Music becomes noise when ______.

it is low

it is loud

it is loud and unpleasant to hear

none of these

• ### A loud sound has _____.

higher pitch

higher frequency

higher amplitude

• ### When a sound falls on the surface ______.

it is partly reflected and partly absorbed

it is highly reflected and partially observed

it is partly reflected and highly observed

it is high reflected and highly observed

• ### The velocity of sound is ______ in solids and least in gas.

minimum
average
neither minimum nor maximum
maximum
• ### Which of the following need a medium to propagate?

light
magnetic field
sound
electric field

432 m/s

232 m/s

332 m/s

132 m/s

reflection

diffusion

echo

refraction

absorption

refraction

reflection

rotation

vibration

tension

rarefaction

• ### Which of the following statements best explains why lightning is seen before thunder is heard?

Electromagnetic waves travel faster than mechanical waves in air.
Electromagnetic waves form faster than mechanical waves during a thunderstorm.
Electromagnetic waves experience less interference than mechanical waves.
Electromagnetic waves have a higher frequency than mechanical waves.

## ASK ANY QUESTION ON Sound

Forum Time Replies Report
##### samyam

sound is herd louder in newly furnished room, why?

##### Namrata

What is meant by pitch?