Study Material and Notes of Ch 13 Sound Class 8th Science
Topics in the chapter
• Introduction
• Vibration motion
• How sound is produced by Humans
• Propagation of Sound
• Frequency
• Loudness of Sound
→ Higher amplitude
→ Pitch
→ High frequency
→ Audible sound
• Noise
• Noise pollution
→ Measures to control noise pollution
• Human ear
Introduction
→ Sound helps us to communicate with one another and plays an important role in our life.
→ Vibrating body produces sound.
→ In this chapter, we will know about how sound is produced and how we hear them.
Vibration motion
→ to-and-fro or back-and-forth or up-and-down motion of a body.
→ Sound is a form of energy that is produced by producing vibration in an object.
→ Sound cannot move through vacuum.
→ Sound waves are longitudinal waves.
How sound is produced by Humans?
→ In humans, sound is produced by voice box or larynx.
→ When the lungs force air through the slit, the vocal cord vibrates which produce sound.
→ Sometimes, the amplitude of produced sound is so small that we can't hear them.
→ The muscles attached to the vocal cords can make the cords tight or loose to produce different types of sounds.
→ Different people have different vocal chords. Due to this reason, we all have a different voice quality.
Propagation of Sound
→ Sound requires material medium for propagation.
→ Sound can travel through solid, liquid or gas.
→ Sound cannot travel through vacuum.
→ No sound can be heard in outer spaces.
Frequency
→ Frequency is measure of number of oscillations per second.
→ It is measured in hertz(Hz).
Loudness of sound
→ It is measured in decibel (dB).
→ It depends on amplitude.
• Higher amplitude– louder sound
→ Pitch or shrillness depends on frequency.
• Higher frequency– higher pitch
• Audible sound– Human ear can hear sounds having frequency in the range of 20-20,000 Hz.
Noise
→ Unpleasant sound is called noise.
Noise pollution
→ Presence of unwanted and excessive sound in the environment
→ Noise pollution may cause many health related problems.
Measures to control noise pollution
→ Moving noise producing industries away from residential area.
→ Minimizing the usage of loud speakers.
→ Avoiding unnecessary usage of horns
→ Planting more and more trees.
Human ear
→ Human ear has three parts outer, middle and inner.
→ Shape of the outer part of the ear is like a funnel.
→ In human ear, the eardrum vibrates and passes vibration to the inner ear.
→ The eardrum is like a stretched rubber sheet.
→ Sound vibrations make the eardrum vibrate, from there the signal goes to the brain.
→ Noise level of 85 dB can damage the human ear.