Class 8th Science Chapter 13

Sound plays an important role in our life - It helps us to communicate with one another. We hear a variety of sounds in our surroundings.

—Vibrating objects produce sound.

—Vibration is the to and fro or back and forth movement of an object.

—Sound needs a medium to travel.

—Hence, it cannot travel in a vacuum.

—vacuum a space that is completely empty of all substances, including air or other gases.

—The matter or material through which sounds propagates is called a medium.

—There is no atmosphere on the surface of the Moon. Sound cannot travel without a medium. Hence, there is no sound on the Moon

—Introduction to waves

—As we know sound is produced by vibrating objects.

—They travel from one place to another in the form of waves. Hence, the name sound waves

.

—Mechanical waves are waves that travel through a material medium.

—It is of two types: depending on the direction of motion of the particle of the medium and the wave propagation:

—Transverse waves

—Particle motion is to perpendicular the direction of wave motion.

—This type of wave is a mechanical wave called a transverse wave. E.g.: Light, or even Mexican wave in a stadium.

—Longitudinal waves

—When the particles of the medium travel parallel to the direction of the wave motion by means of successive compression or rarefaction. Example: a slinky

Sounds by Humans

—How do humans produce sound?

—The sound produced in the voice box called larynx located at the upper end of the windpipe.

—2 vocal cords get stretched across in the voice box. Has a slit.

—When the lungs force air through the slit, the vocal cords vibrate, producing sound. Muscles attached to the vocal cords can make the cords tight or loose.

—HEARING

—Human ear

—Outer ear = pinna: collects sound from the surroundings.

—Sound passes through a tube called an auditory canal.

—Eardrum (tympanic membrane) → vibrates when the sound incident.

—Vibrations are sent to the inner ear, from there it goes to the brain as signals via the auditory nerve.

—Amplitude, Time Period and Frequency of a Vibration

—We have learnt that the to and fro motion of an object is known as vibration. This motion is also called oscillatory motion.

—The number of oscillations per second is called the frequency of oscillation. Frequency is expressed in hertz. Its symbol is Hz.

—A frequency of 1 Hz is one oscillation per second. If an object oscillates 20 times in one second, what would be its frequency?

—The frequency determines the shrillness or pitch of a sound. If the frequency of vibration is higher sound is shrill and has a higher pitch. If the frequency of vibration is lower sound has a lower pitch. For example, a drum vibrates with a low frequency a low-pitched sound. On the other hand, a whistle has a high frequency and therefore, produces a sound of higher pitch

——With sound waves, it is the extent to which air particles are displaced, and sound amplitude is experienced as the loudness of sound

—Loudness of sound is proportional to the square of the amplitude of the vibration producing the sound. if the amplitude becomes twice, the loudness increases by a factor of

4. The loudness is expressed in a unit called decibel (dB).

—Normal breathing 10 dB Soft whisper (at 5m) 30 dB Normal conversation 60 dB Busy traffic 70 dB Average factory 80 dB Above 80 dB the noise becomes physically painful.

Audible and Inaudible Sounds

—sounds of frequencies less than about 20 vibrations per second (20 Hz- infrasonic range) & sounds of frequencies higher than about 20,000 vibrations per second (20 kHz- Ultrasonic range) not audible to the human ear. For human ear, the range of audible frequencies is roughly from 20 to 20,000 Hz.

—Some animals can hear sounds of frequencies higher than 20,000 Hz. Dogs have this ability.

—Time taken for one complete oscillation to travel across a point is called time period. It is inverse of frequency

—Time Period = 1/ oscillation frequency

—Noise and Music

—Musical sound is one which is pleasing to the ear. Sound produced by a harmonium is a musical sound.

—Unpleasant sounds are called noise

—Noise Pollution

—– Presence of excessive unwanted noise in our surroundings is called as noise pollution.

–It Can cause hearing impairment, sleeplessness and also hypertension.

– Must be minimised by reducing noisy operations and honking in residential areas. Planting trees along roads also cut down the noise.