Chapter 13 - Sound

Sounds play an important role in our life. It helps us to communicate with one another.

SOUND IS PRODUCED BY VIBRATIG BODY

• To and fro or back and forth motion of an object is termed as vibration.
• Vibrating objects produces sound.
• Example- when a tightly stretched band is plucked, it vibrates and produces sound.

SOUND PRODUCED BY HUMANS

    

• 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.
• When air passes through those stretched membranes they produce different types of sound with different combinations of stretching.

SOUND NEEDS MEDIUM TO PROPOGATE

• The travelling 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. It travels through solid, liquid and gas.

WE HEAR SOUND THROUGH OUR EAR

• The shape of outer part of the ear is like a funnel.
• The tympanum is like a stretched membrane; which vibrates when sound waves strike it.
• Then the sound waves reach the ear drum or tympanum.
• Ear drum is like a stretched rubber sheet.
• From the ear drum, sound waves reach inner ear; through middle ear. From inner ear, sound waves are sent to the brain.

AMPLITUDE, TIME PERIOD AND FREQUENCY OF A VIBRATION

• To and fro motion of object is also known as oscillatory motion.
• 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.

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

Frequency is expressed in hertz.

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

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

SHRILLNESS OR PITCH

• Frequency determines the shrillness and pitch of sound.
• Sound with greater frequency is shriller and has higher pitch. Sound with lower frequency is less shrill and of lower pitch.

LOUDNESS

• Loudness of sound depends on the amplitude of the sound wave.
• When amplitude is large, sound produced is loud.
• Loudness of sound is measured in decibel (dB).

AUDIBLE AND INAUDIBLE SOUND

• Sounds frequency between 20 Hz to 20,000 Hz are called audible sound.
• Sound with frequency below 20 hertz and above 20,000 hertz is called sound of inaudible range.

NOISE AND MUSIC

• Sound that is unpleasant to us is called
• Example sound of horn, sound near the site of construction work, etc.
• Sound that is pleasant to our ear is called musical sound, such as sound of musical instrument, song of a good singer, etc.

NOISE POLLUTION

• 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 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 by controlling the noise coming from source. Noise pollution can be limited by taking some measures like:
• TV or radio should be played at low volume.
• Minimum use of vehicle horns.
• By installing high quality silencer in vehicles.
• Plantation of trees along the road sides because trees absorb sound.