Class 8 SCIENCE NCERT NOTES SOUND

Sound

Sound is very important for our life. It is the sound that helps us to communicate with each other.

Production of sound –

Sound is produced because of vibration of an object. In other words, a vibrating object produces sound.

Example –

When a table is beaten, it starts vibrating and produces sound.

When a drum is beaten, it starts vibrating and produces sound.

When a stretched rubber band is struck, it starts vibrating and produces sound.

Musical instruments produce different types of sound because of vibration. For example; a flute produce sound because of vibration in its air column, a guitar produces sound because of vibration of its string, a drum produces sound because of vibration of its diaphragm.

Sound Produced by Human:

A human being produces sound because of vibration of his voice box. The voice box is also known as larynx. Larynx is situated at the upper end of windpipe. There are two stretched membranes attached in larynx; with a narrow slit between them. When air passes through those stretched membranes they produce different types of sound with different combinations of stretching.

Sound needs a medium to propagate:

The traveling of sound is called propagation of sound. Sound cannot propagate in the absence of a medium. The place where there is no air or air is removed, is called vacuum. Sound does not propagate (travels) through vacuum.

Sound travels through solid, liquid and gas.

We usually hear sound which comes to us through air.

Aquatic animals communicate as sound travels through water also.

We hear the sound through ear

The external ear works like a funnel. Due to its unique shape, sound waves are concentrated once they enter through the external ears. Then the sound waves reach the ear drum or tympanum. The tympanum is like a stretched membrane; which vibrates when sound waves strike it. From the ear drum, sound waves reach inner ear; through middle ear. From inner ear, sound waves are sent to the brain via auditory nerve. The brain interprets these signals and we hear the sound.

Amplitude, Time Period and Frequency of a Sound

Sound travels in the form of wave. When a pebble is dropped in pond water, it produces ripples in water. The ripple is called wave. Sound travels producing similar waves.

Amplitude and frequency are two important characters of sound. Sounds produced by different object are differentiated by amplitude and frequency of sound.

Amplitude – The distance from normal to peak is called amplitude. Since, sound travels in the form of wave, so sound has amplitude.

Frequency: The number of vibrations or oscillations per second is called frequency.

Frequency is expressed in hertz.

If an object oscillates or vibrates 40 times in 1 second, then its frequency will be equal to 40 hertz.

Time period - Time required to produce one complete oscillation is called time period.

Loudness and Pitch

Loudness of sound depends on the amplitude of the sound wave. Greater amplitude produces louder sound and smaller amplitude produces feeble sound.

Loudness of sound is measured in decibel (dB). Loudness of some types of sound is given here in decibel.

Shrillness or pitch

Frequency of a sound determines the shrillness or pitch of the sound. Shrillness or pitch increases with increase in frequency of sound. Sound with greater frequency is shriller and has higher pitch. Sound with lower frequency is less shrill and of lower pitch.

Examples:

Children and women produce sound of high frequency and their sound is shriller and of higher pitch. On the other hand, an adult male produces sound of lower frequency and his sound is less shrill and has lower pitch.

A drum produces sound of lower frequency which is less shrill and has lower pitch, while a whistle produces sound of higher frequency which is shriller and is of higher pitch.

A lion produces a sound of lower frequency which is less shrill and has lower pitch, while a bird produces sound of high frequency which is shriller and has higher pitch. However, sound of lion is louder than the sound of a bird.

Audible and Inaudible Sound

Sounds with frequency between 20 Hz to 20,000 Hz are called audible sound. The hearing range of human beings is between 20 hertz to 20,000 hertz.

Sound with frequency below 20 hertz and above 20,000 hertz is called sound of inaudible range. Humans cannot hear the sound of inaudible range.

Many animals, such as dogs, cats, etc. can hear the sound with frequency above 20,000 hertz.

Noise and Music

Sound that appears unpleasant to us is called noise, such as sound of horn, sound near the site of construction work, sound of aeroplane, etc.

Sound that appears pleasant to our ear is called musical sound, such as sound of musical instrument, song of a good singer, etc.

Noise Pollution

Loud and excessive sound is unbearable to our ears, and is called noise. Unwanted and excessive sound in our environment creates noise pollution. Sounds of crackers, factories, vehicles, desert coolers, air conditioners, aeroplane, transistors or television with high volume, loudspeakers, etc. create sound pollution.

Problems due to Noise Pollution:

Noise pollution can create many types of health related problems, such as lack of sleep (insomnia), hypertension, loss of hearing, anxiety, etc. Sound above 80 dB is very painful to hear.

A person who is exposed to loud sound continuously may get permanent or temporary loss of hearing (impairment of hearing).

Measure to Limit Noise Pollution

Noise pollution can be limited or controlled by controlling the noise coming from source. Noise pollution can be limited by taking following measures:

TV or radio should be played at low volume.

By installing high quality silencer in vehicles.

Minimum use of vehicle horns.

Plantation of trees along the road sides and around buildings. Trees absorb sound.

Awareness campaign to make people aware; about the harmful effects of noise pollution and measures to control noise pollution.

Question: Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain?

Answer: We know that the speed of light much more than the speed of sound. Due to this, light reaches to us faster than sound. Hence, during lightning we see the streak of light earlier than hearing the sound of thunder.