## Important topics covered in class 8 sound notes:

• What is sound?
• Production of sound
• Propagation of sound
• Human ear
• Amplitude
• Time Period
• Frequency of a Vibration
• Loudness and Pitch
• Audible and Inaudible Sounds
• Noise and Music
• Noise Pollution
• Harms of Noise Pollution
• Measures to control noise pollution

## What is sound?

• Sound is the form of energy which causes sensation of hearing to our ears.
• Sound helps the living organism to communicate with each other.
• Vibrating body produces sound.
• After a bell being hit by the gong, the bell starts vibrating thus produces sound.
• Sound doesn’t travel through a vacuum.
• These waves are longitudinal waves.

## Vibration

• The to and fro or back and forth motion of a body is said to be vibration.
• Different musical instruments produce different types of sound due to vibration of its parts.

## Production of sound by humans

• In humans sound is produced by larynx also called voice box.
• It is situated in the upper part of the windpipe.
• Our vocal cord vibrates when lung force air through the slit.
• Muscles which are attached to the vocal cord enable us to make the vocal cords tight or loose as per our need.
• The voices of men, women and children are different from each other because vocal cords in men are longer (about 20 mm) but they are shorter in women and children.

## Propagation of sound:

• Sound needs a material medium to travel.
• Through vacuum, sound cannot travel. We can’t hear sound in space.
• Sound can also travel through solid and liquid.
• Sound propagation happens in all directions in a medium.

## Human Ear

Human ear consists of three parts i.e. outer ear, inner ear, and middle ear.

### External ear

• Outer part of the ear looks like funnel.
• It is also called Pinna.
• It collects the sound from surrounding and transfers to middle ear.

### Middle ear

• The middle ear is composed of a stretched membrane which is called eardrum.
• Eardrum appears as the stretched rubber sheet.
• The vibration of eardrum helps in passing the vibration to inner ear.
• Middle ear also consists of three small bones- hammer, anvil, and stirrup.

### Inner ear

• The internal ear is consists of cochlea and semicircular canals.
• From outside, cochlea appears like a snail.
• Vibrations derived from middle ear reach the cochlea.
• Signals from cochlea reach the brain. The brain interprets message and makes us listen to it.
• Semicircular canals have no role in sense of hearing, rather they maintain the balance of the body.

## Amplitude, Time Period and Frequency of a sound

• The to and fro motion of any object produces sound and this motion is said to be as vibration.
• Sound propagates from one place to other in the form of waves.
• The phenomenon or disturbance in which energy is transferred from one place to other is said to be wave.
• So, sound is also considered as wave.

### Amplitude

• Maximum displacement of a wave on either side from mean position is said to be amplitude.
• We can conclude that it shows how far the vibrating object moves from the mean position.
• It is denoted by ‘A’.
• The SI unit of Amplitude is metre.

### Time period

• The total time taken by the particle of medium to complete one oscillation is said to be time period.
• We denote it by ‘T’.
• The SI unit of time is second.
• Time period = Time / Number of oscillation or vibration

### Frequency

• The number of complete oscillation done by an object is said to be frequency of the sound.
• Frequency = $$\frac{Number\: of \:Oscillation}{Time\:taken}$$
• The unit used for frequency is hertz (Hz).
• Oscillation of x Hz is called as the x oscillation in a second.
• For same amplitude, the frequency may vary.

## Loudness

• The loudness of sound is softness which depends on the amplitude of vibration.
• Loudness is directly proportional to the square of the amplitude of vibration.
• The louder sound has a higher amplitude, while lower sound has lower amplitude.
• We express loudness in terms of decibel.
• Loudness of some common sounds is in the following table.

Loudness ∝ (Amplitude)2

## Pitch or Shrillness

• Pitch is the sensation (Brain interpretation) of the frequency of emitted sound.
• Pitch depends on the frequency of vibration of sound.
• Sound with greater frequency is shriller and has higher pitch whereas the lower frequency sound has a lower pitch.

## Audible and inaudible sound

• Sounds with frequency range 20 Hz to 20,000 HZ is said to be audible sound.
• Also, Sound with frequency below 20 Hz or above 20,000 Hz can’t be listened by humans and hence comes in inaudible range.
• Humans can’t hear the sound of inaudible range.
• Some animals like dogs, flies, cats can even listen the sound having frequency above 20,000 Hz.

## Noise and Music

• The sound which appears unpleasant to listen is said to be noise.
• Sounds produced by vehicles, train etc. are some examples of noise.
• The sound which appears pleasant to listen is called music.
• Sound which comes out from musical instruments appears pleasant to listen and hence is music.

## Noise pollution:

• The presence of excessive, loud, unwanted or unpleasant sound to our ears in the environment is said to be noise pollution.
• Sound produced from crackers, loudspeakers, explosions etc. cause noise pollution.

### Effects of noise pollution

• Due to noise pollution, many type of disease/health related problems take place.
• Lack of sleep (insomnia), hypertension (high blood pressure), loss of hearing etc. are some common effects seen in humans as the cause of noise pollution.
• A person who is exposed to loud sound continuously may get temporary or permanent hearing loss.
• Class 8 sound notes will simplify your exam.