# Chapter 13 Class 8 Sound Notes Pdf

## Short and Concise Class 8 Sound Notes

Firstly, let us see what we are going to study in this chapter. The overview of the chapter is very necessary for every student. This gives you an idea about the content we are going to study. These Short and Concise Class 8 Sound Notes will surely help you in your exams.

Here is the list of topics we are going to discuss in Chapter 13 Sound (Class 8 Sound Notes).

• What is Sound?
• Sound Production
• Propagation of Sound
• Time Period
• Frequency
• Amplitude
• Characteristics of Sound
• Pitch
• Loudness
• Quality of Sound
• How do Humans Speak?
• Human Ear
• Noise and Music
• Causes Noise Pollution
• Effects of Noise Pollution
• Prevention of Noise Pollution
• Echo
• Application of Echo

Class 8 Sound Notes | Short and Concise

### What is Sound?

A sound is a form of energy that helps us to communicate with each other.

Question: What type of energy is Sound?

Answer: Sound is a form of both Kinetic Energy and Potential Energy. (Kinetic>Potential).

In other words, Sound is a mechanical wave (longitudinal wave) that is produced by a vibrating source.

Knowledge Booster

A wave is a disturbance.

Longitudinal Wave: The type of wave in which particles vibrate in the direction of the motion of the wave.

### Production of Sound

Sound is produced when the source vibrates or the object producing the sound vibrates.

Vibration: Vibration is the to and fro or back and forth motion. The best example is the motion of the pendulum about its mean central position.

### Propagation of Sound

Now, let us talk about the propagation of sound waves.

What is the propagation of sound?

The propagation of sound takes place in a medium.

The propagation of sound takes place in all the mediums- solid, liquid, and gas.

The propagation of sound waves is faster in Solids than in Liquid and Gas.

SOLID > LIQUID > GAS

### Amplitude

Amplitude Meaning

The definition of Amplitude says it is the maximum displacement of a particle from its mean position.

### Time Period

It is the time taken to repeat an event or It is the time taken to complete one vibration or oscillation.

### Frequency

It is the number of events occurring in a second or It is the number of oscillations or vibrations taking place in 1 second.

In other words, it is the number of waves (in this case sound waves) passing a fixed point in a unit of time.

Frequency can also be defined as the number of cycles in unit time or one second.

Unit of Frequency: Hertz (Hz)

Note: per second (1/second) = Hertz

### Characteristics of Sound

The characteristics of sound help us to distinguish different sounds coming from different sources of Sound.

The characteristics that help us to distinguish the sounds are – Loudness, Pitch, and Quality.

#### Loudness

Two sounds can be differentiated with the help of loudness and depends upon the sound energy. The greater the sound energy, the larger is the loudness.

• The Loudness of the Sound depends on the Amplitude of the Sound wave.
• Loudness is measured in decibels (dB).
• The drop of a pin is considered 0 dB.
• When we increase the volume, we increase the amplitude.
• When we decrease the volume we decrease the Amplitude.

Loudness depends upon the following factors-

1. Amplitude
2. Area of vibrating body (Larger Area means larger loudness)
3. Distance from the sound source (With distance loudness decreases)
4. Density of the medium (Solid>Liquid>Gas)

#### Pitch

Two sounds having the same loudness (Amplitude) can be differentiated with the help of Pitch.

Pitch helps in distinguishing a Shrill Sound (thin Sound) from a Grave Sound (Thick or Heavy Sound).

• The pitch of a sound depends on the frequency of the sound.
• Shrill Sound produces high frequency whereas Grave Sound produces low frequency.
• High-pitched sound has a high frequency.
• Low-pitched sound has a low frequency.
• Examples of High-Frequency Sounds- Sounds of Women, Birds, Cat, Babies
• Examples of Low Frequency Sounds- Sounds of Men, Dogs, Frog

#### Quality of Sound (Timbre)

Quality of Sound” which is also called Timbre, helps in distinguishing two sounds of the same Pitch and Loudness.

• Different sources have different Quality of Sound because of the Waveform.
• Quality of Sound is related to Music.

### How do Humans Speak?

• Humans speak with the help of a voice box also called the Larynx.
• The voice box has two Vocal Cords and a Slit in between them.
• When the exhaled air passes through this slit, the two Vocal Cords vibrate (just like air passing through two paper folds) and thus produce sound.
• The type of sound, i.e., high-pitched or low-pitched, depends upon the size of the vocal cords.
• The size of Vocal cords in Women is 15 mm.
• The size of Vocal cords in Men is 20 mm.

### Human Ear

The Human Ear can be divided into two three parts- Outer Ear, Middle Ear, and Inner Ear.

Outer Ear: The outer ear consists of the Pinna, Ear canal, and Eardrum (a stretched membrane).

• The eardrum separates the outer ear from the middle ear.

Middle Ear: It has three interlocked delicate bones- Malleus (Hammer), Incus (Anvil), and Stapes (Stirrup).

• The vibration of the eardrum makes these bones vibrate.

Inner Ear: It consists of a Cochlea and Auditory Nerves.

• Cochlea: It is a snailed-shaped organ filled with a fluid that moves in response to the vibrations.
• Auditory Nerves: It consists of a bundle of nerve fibers that carries hearing information from the inner ear (Cochlea) to the brain.

### Noise and Music

Music

• Music is the sound that is pleasant to our ears.
• Any sound that has a loudness below 30 dB is considered Music.
• The waveform of Music is- Regular, Smooth, Continuous, and periodic.

Noise

• Noise is the sound that is not pleasant to our ears.
• Any sound that has a loudness above 80 dB is considered Noise.
• The waveform of Music is- Irregular, harsh, discontinuous, and non-periodic.

### Noise Pollution

It is the presence of lots of noises around us or in the environment that make us uncomfortable or cause discomfort to our ears.

### Causes Noise Pollution

There are many sources that lead to noise pollution such as

• Transportation- Traffic in the cities
• Industrial Works-
• Construction- Use of Heavy machines like drilling machines,
• Launching of Satellites
• Social Events- Political Rallies, Religious Events, etc
• Natural Activities- Thunderstorms, Earthquake, etc

### Effects of Noise Pollution

Noise pollution can cause-

• deafness or hearing problems
• High Blood Pressure
• decreases the work efficiency
• Sleeplessness or Insomnia
• Death

### Prevention of Noise Pollution

We can control sound pollution by methods like-

• Planting Trees
• Making people aware of the effects of Sound Pollution
• minimum use or ban of Horns at places like hospitals, schools, etc.
• Minimize the use of Horns, loudspeakers

### Audible and Inaudible Sounds

What do you understand by Audible and Inaudible Sounds?

Sounds that humans can hear are called Audible Sounds. The range of sound waves that humans can hear is 20 Hz to 20000 Hz.

The sounds below 20 Hz and the sound having frequencies above 20 kHz that cannot be heard by humans are called Inaudible Sounds.

Ultrasound

• sound waves having a frequency range above 20 kilohertz
• Used by animals like cats, bats

Infrasound

• sounds having a frequency range below 20 hertz
• Used by animals like giraffes, rhinoceros, and whales

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