NCERT Solutions Class 8th Science Chapter – 10 Sound
Chapter – 10
Sound
Notes
Frequency – The number of oscillations or vibrations per second is called the frequency of oscillation.
Noise Pollution – The presence of excessive or unwanted sound in the atmosphere is called noise pollution. Plantation on the roadside and elsewhere is the best source to reduce the noise pollution. The loudness is expressed in a unit called decibel (dB). It also depends on the amplitude of the sound.
Amplitude – The maximum distance to which a vibrating body moves on either side of its mean position is called the amplitude of vibration.
Audible Frequencies – For human ears, the range of audible frequencies is roughly from 20 to 20,000 Hz.
Eardrum – A thin membrane which is stretched tightly to receive the waves bf sound at the end of ear canal.
Hertz – The frequency is expressed in hertz (Hz).
Larynx – In humans, the sound is produced by the larynx.
The loudness of Sound – Larger the amplitude of vibration, louder is the sound.
Noise – Unpleasant sounds are called noise.
Oscillation motion – The to and fro motion of an object is called oscillation motion.
Pitch of the Sound – Higher the frequency of vibration, the higher is the pitch, or shrill of the sound.
Shrillness – The frequency determines the shrillness or pitch of a sound. If the frequency of vibration is higher we can say that sound is shrill.
Time Period – The time taken by a pendulum to complete one oscillation is called the time period.
Vibration – The to and fro or back and forth motion of an object is termed as vibration.
Voice Box – Upper end of the windpipe, below the hard part on the throat is called the voice box.
Windpipe – It is the passage for the inlet and outlet of air in the lungs.
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 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
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.
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
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.
