These DAV Class 8 Science Notes and DAV Class 8 Science Chapter 12 Notes – Sound act as excellent revision resources, particularly in preparation for board exams.
Sound Class 8 DAV Notes
→ Sound: Sound is a form of energy which produces the sensation of hearing in our ears. Vibrations are the basic cause of sound.
→ Sounds produced by humans: Sound is produced by the voice box or larynx, which is situated in the throat. There are two vocal chords stretched across the voice box. There is a narrow gap between them. When air passes through the slit, it creates vibrations in the vocal chords. The vibrations in the vocal chords produce sound. Muscles are attached to the vocal chords so that the vocal chords can become tighter or loosened up and can become thinner or thicker. These changes help in changing the pitch of the sound.
Vocal chords are smaller in women and children and are longer in adult men.
→ Sounds Produced by Animals:
- Many other animals have vocal chords like human beings, e.g. dog, cat, horse, lion, cow, etc.
- Birds have a ring of cartilage, called syrinx, in their windpipe. These rings are used by birds to produce different types of sound.
- Insects; like mosquitoes and cockroaches produce sound by vibrating their wings.
- Frogs puff up their vocal sacs; like a balloon; to produce croaking sound.
→ Propagation of Sound:
- Sound needs a medium for propagation. Sound cannot travel through vacuum. Sound can propagate through all the three states of matter, i.e. solid, liquid and gas.
- Speed of sound through air is about 330 m/s.
- Speed of sound through water is about 1500 m/s.
- Speed of sound is highest in solid; followed by liquid and gas.
- Speed of light is more than that of sound.
→ Human Ear
The human ear is a sense organ which gives the perception of sound. The human ear is composed of three parts, i.e. outer ear, middle ear and inner ear.
→ Outer Ear: This is also called pinna. It collects sound vibrations from air. The collected sound vibrations pass through the auditory canal. There is a thin membrane at the end of the auditory canal. This membrane is called tympanum or ear drum.
→ Middle Ear: Middle ear is composed of three bony structures. They are; hammer, anvil and stirrup. Through these bony structures, the middle ear transfers sound waves to the inner ear.
→ Inner Ear: The inner ear is made up of cochlea and semi-circular canals. The sound waves reach the cochlea from where they are sent to the brain via auditory nerve. The brain interprets these signals and we are able to hear sound. The semi-circular canal plays a role in maintaining the balance of the body.
→ Persistence of Hearing: The sound stays on the ear drum for nearly 1/10th of a second. Echo: Sound waves also get reflected; like light. When a reflecting surface is 17 m or more from the source of sound, we get to hear the reflected sound. This sound is a repetition of the original sound and is known as echo.
→ Oscillation: When a body moves on either side of a fixed point in repetition, its motion is called oscillation or oscillatory motion. Oscillation is also known as vibration.
→ Amplitude: The maximum displacement of a vibrating oody on either side of the mean position is called its amplitude. Amplitude is measured in metre.
→ Time Period: The time taken to complete one oscillation is called time period. One cycle of to and fro motion along a mean position is counted as one oscillation. The SI unit of time period is second.
→ Frequency: The number of oscillations in one second is called frequency. The SI unit of frequency is hertz.
Frequency = (No. of vibrations completed/time taken) or, hertz = (1/second)
→ Loudness of Sound: The amplitude of sound determines its loudness. Larger the amplitude, louder is the sound and vice-versa is also true.
→ Pitch of Sound: The shrillness or flatness of sound is called the pitch of the sound. The frequency of a sound determines its pitch. A higher pitch sound has a higher frequency, while a lower pitch sound has a lower frequency. Voices of women and children are of high pitch while the voice of adult male is of low pitch.
→ Audible and Inaudible Sound: Human beings can hear the sound between 20 Hz to 20,000 Hz. The sound within this range of frequencies is called audible sound. Human beings cannot hear sound with frequency below 20 Hz or more than 20,000 Hz. The sound below 20 Hz and more than 20,000 Hz is called inaudible sound. The sound below 20 Hz is called infrasonic and that beyond 20,000 Hz is called ultrasonic.
→ Noise and Music: Any unpleasant sound is called noise. Noise is produced due to sounds produced by irregular and non-periodic vibrations. A sound which is pleasant to hear and has soothing effect is called music.
→ Noise Pollution: The disturbances produced in our environment; due to the presence of excessive or undesirable loud sounds; is called noise pollution.
→ Sources of Noise Pollution:
- Various appliances and gadgets at home; like mixer grinder, vacuum cleaner, lawn mowers, television sets and music system (at high volume).
- Street noise, traffic noise, public transport, loud speakers, shopping malls.
- Smaii factories, large factories, construction activities.
- Dripping taps, ticking of clock, coolers, etc.
→ Effects of Noise Pollution:
- Continuous noise can create panic. This can lead to irritation of mind.
- Noise pollution affects human heart. It can result in lack of sleep, hypertension, nervousness, anxiety and extreme emotional behaviour.
- Noise pollution can lead to partial or complete hearing impairment.
→ Measures to Limit Noise Pollution:
- Silencing devices in aircraft engines, motor vehicles, industrial machines and home appliances.
Plantation of trees along roadside. - Staying away from noisy places, as far as possible.
→ Hearing Impairment: Total hearing impairment is a rare defect and is often from birth itself. Partial hearing impairment can result from disease, injury or age. Sign languages can help such persons to communicate effectively. Hearing aids also help a person with partial hearing impairment.
→ Oscillation : The to and fr0 motion along a mean point is called oscillation.
→ Sound : Sound is a form of energy which gives us the sensation of hearing.
→ Amplitude : Maximum displacement of a vibrating body along the mean point is called amplitude.
→ Time period : Time taken to complete one oscillation.
→ Frequency : Number of oscillations per second.
→ Echo : Reflection of sound which results in repetitive sound is called echo.