RBSE Class 8 Science Notes Chapter 10 Sound

Rajasthan Board RBSE Class 8 Science Notes Chapter 10 Sound

Origin of Sound:
Sound is an important part of our daily life. As we take birth on earth, we start listening different types of sound like chirping of birds, bells of temples, music from sitar, harmonium etc. Hence, sound is an integral part of our life. If we kept a plate opposite and keep 2-3 balls of paper on it. If we play on it with a spoon, bits of paper starts shaking up and down. In the same manner if a rubber band is stretched by tying it on a wall with an iron nail. It started vibrating up and down, such movement is called vibration. Hence, it is clear that sound is produced by the vibration of objects.

Vocal Sound Produced in Human Activity:
Take two pieces of a balloon rubber of 43 cm size in rectangular shape. Stretch both together by hand, try to produce sound by blowing wind into its center speedly, due to blow pieces open and close, produce sound. In the same manner in man’s glottis two vocal cords are located. These vocal cords are the natural instrumental system. These vocal cords are stretched in such a way, when we speak that a thin gap is formed when air from lungs passes through this gap, vibration takes place in vocal cords and sound produced.

Transmission of Sound:
Sound travels through different medium

  • Sound transmission in air: Sound travels through air from its place of origin and reaches to our ears. In air it travels through vibration, when object vibrates, then atoms of air also vibrates, every vibrated atom transfer propagates this vibration to next atoms. In this way a wave of sound reaches to our ear where it strikes with the ear drum, and it starts vibrating and we listen sound.
  • Sound transmission in solid: Take 1m scale 2 cm long metal wire and 2 m long thread,keep one end of the scale on owner and ask your friend to create sound by light rubbing of other end of scale with nails. Repeat the same by stretching and rubbing metal wire and thread. The vibrations produced at the first end reaches to the other end. Hence, it is clear that sound also travels through solids. In labs when turning forks are vibrated and brought near to the ear, sound is heard.
  • Sound transmission in liquid: If two pieces of stones are strucked in a bucket filled with water. We can easily hear the sound, hence we can say that sound travels through liquid. From above discussion it is clear that sound travels through all three medium l.e. solid, liquid and gas. A medium is needed for sound travelling.
  • Amplitude, Frequency and Periodic time: The speed of sound is greatest in solid, then liquid and least in gas. At 0°C the speed of sound in air is 331 m/sec. As air is absent on moon hence astronauts can not talk with each other on moon. A vibrating object goes to its maximum displacement from its mean position O to A. and then come back to its mean position O and goes to its maximum displacement point B below and again showing its movement above again reaches to point O. In this way one oscillation (vibration) completes which can be represented as-
    \(A \rightleftharpoons O \rightleftharpoons B\)
  • Amplitude: The maximum displacement by a vibrating object to its mean position is called amplitude. In given figure oscillation is
    RBSE Class 8 Science Notes Chapter 10 Sound a
  • Frequency: The number of vibration done in a second is called frequency. It can be calculated by the following formula-Frequency = \(=\frac{\text { Total number of Oscillations }}{\text { Total time taken }}\)
    The unit of measurement of the frequency is vibration or oscillation per second which is called hertz (Hz).
  • Time period of vibration: The time period taken in one vibration is called time period of vibration. Time period of vibration and frequency are inversely proportional to each other. The unit of time period is second.
    Time period of vibration = \(=\frac{1}{\text { Frequency }}\)

Loudness and Pitch:
Every persons, animal, musical instruments have different sounds. We can experience these sounds by our ears on three bases they are
1. Intensity
2. Softness/Melody
3. Quality

  • Intensity of sound: Intensity of sound depends upon its displacement or Intensity. It is directly proportional to displacement. Its unit is decible (dB).
  • Softness/Melody: The shrillness or thickness of sound depends upon frequency. Due to high frequency in females their voice is soft and shrill and in males low frequency is the cause of loudness hence intensity of sound defines the different types of sounds.
  • Example: Sound of Sitar is soft and melodious than sound of Dholak. Cuckoo’s voice is melodious than crow.
  • Quality: It describes the characters of the the sound that allow the ear to distinguish sounds which have the same pitch and loudness.

Audible, Infrasonic, Ultrasonic:
On the basis of frequency, sound can be classified as follows:

  • Sonic
  • Infrasonic
  • Ultrasonic

Our ears can hear 20 hertz frequency sound and maximum of 20,000 hertz sound or we can hear sound between 20 hertz to 20,000 hertz, it is called Sonic sound. Sound less than 20 hertz frequency is called infrasonic and sound more than 20 hertz ultrasonics.

Ultrasonic sound is used to diagnose diseases in Ultra sonography in cammon language it is called sonography. These sounds are also used to measure the depth of the sea and location of submarine through an instrument ‘sonar’. Some animals like – dog, bat, mouse, whale can hear ultrasonic sounds. Policemen use whistles which produce ultrasonic sounds to trained dogs who search criminals. Bat can produce ultrasonic sounds and due to their reflection sounds are produced which can hear and detect the obstacle, therefore it can even fly in dark night.

Human Ear:
The human ear consists of a flap called the auricle or pinna and the auditory canal. The main functional parts are the middle and inner ears which are enclosed within the skull. The middle ear consists of three bones, known as auditory ossicles and the eustachian tube, which links the ear to the back of the nose. The inner ear consists of the spiral shaped cochlea and also the semicircular canals and the vestibule, which are the organs of balance. Sound waves entering the ear travel through the auditory canal to the tympanic membrane (eardrum) where they are converted to the vibrations that are transmitted via the ossicles to cochlea. Here the vibrations are converted by the millions of microscopic hair into electrical nerve signals interpreted by the brain. In this way the sound is heard.

Sound Pollution:
Melodious sounds are felt good and give pleasure to ears. Those sounds which are disliked by ears are called noise. For example, sounds created by automobiles, means of transport, very loud music, sound of construction sites, light sounds like breathing have intensity up to 10 decibel, slow-talks 20-30 decibel, normal radio music 50-60 decibel. Sounds upto 50 decibel are liked by ears. Such sounds are bearable. Sounds over 80 db are unbearable, whether it is in rhythm.

Noise pollution: It occurs due to high intensity sound like – sound of machines in industries, loudspeakers engines etc. Such noise affect the activities of life. It arouse the health problems like – frustration, non-sleepiness, high blood pressure, decrease in hearing capacity sometimes deafness.

Measures to Control Noise Pollution:

  • Use of silencer in all noise producing machines, appliances, automobiles etc.
  • Noise producing activities should take place away from residential areas.
  • Sound intensity of loudspeaker and television should be kept low.

Plantation should be done near road and residence so that sound could be absorbed. Indian music has seven notes. Different ragan and rajinis are based on these notes. Sa, Re, Ga, Ma, Pa, Dha, Ni.

RBSE Class 8 Science Notes