Sound Class 8 Notes - Chapter 13

Sound plays an important role in our life. It is through sound we know that a period in school is over or if someone is approaching you by just listening to the footsteps. 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.

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Introduction to Sound

Introduction to waves

  • The sound is produced by vibrating objects.
  • They travel from one place to another in the form of waves. Hence, the name sound waves.

Introduction to waves

Wave and particle motion of 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
*longitudinal
Transverse and longitudinal waves

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.
  • It is also a mechanical wave.
  • Example: a slinky

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Sound Properties

Introduction to sound waves

– Sound needs a medium to propagate. The matter or material through which sounds propagates is called a medium.
– Sound cannot travel in a vacuum. The moon does not have an atmosphere, hence, you can hear on the moon.

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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, through which air is forced out by the lungs.
  • Muscles attached to vocal cords make it 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.

Human ear

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Amplitude, Time Period and Frequency

Amplitude, frequency and time period of vibrations

  • The magnitude of disturbance in the medium on either side of the mean value is called as Amplitude(A). Larger the amplitude, louder the sound.
  • The number of oscillations per second is called frequency. Expressed in Hertz (Hz).
  • Time taken for one complete oscillation to travel across a point. T = 1/f. (Seconds)

Amplitude, frequency, and time period

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Loudness and Pitch

  • Volume or loudness of a sound depends on the amplitude. The force with which an object is made to vibrate gives the loudness.
  • The number of oscillations per unit time. Directly proportional to frequency.

Loudness and pitch

Audible and inaudible sounds

  • Audible range = 20Hz to 20kHz known as the Sonic range.
  • Below 20 Hz (inaudible) → infrasonic range
  • Above 20 kHz (inaudible) → Ultrasonic range

To know more about “Characteristics of Sound Waves”, visit here.

Noise Pollution

Noise and music

– Sounds with the same pitch and loudness can be distinguished based on the quality. Music is pleasant to the ears while noise is not.
– Unpleasant sounds are called as noise.
Noise and music

Noise pollution and measures to control it

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

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