Sound Class 8 Notes - Chapter 13

According to the CBSE Syllabus 2023-24, this chapter has been renumbered as Chapter 10.

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.

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 waves

• Particle motion is to perpendicular the direction of wave motion.
• This type of wave is a mechanical wave called a transverse wave. For example, 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 is called the 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 them 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, and from there, it goes to the brain as signals via the auditory nerve.

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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, the 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)

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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.

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

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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 noise.

Noise pollution and measures to control it

– Presence of excessive unwanted noise in our surroundings is called 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 cuts down the noise.

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