1. Waves-
ü The continuous motion of particles or energy through a medium (sound wave, water wave) or through vacuum (electro-magnetic wave) is called as a wave.
ü The sound is produced by vibration of objects.
ü They travel from one place to another in the form of waves.
ü These waves are called as sound waves, they can only move through a medium.
2. Mechanical Nature Of Waves-
ü Mechanical waves are waves that travel through a medium.
ü For sound wave, particles present in air act as medium, for water wave water act as a medium.
ü Depending on the direction of motion of the particle of the medium and the wave propagation, mechanical waves are of two types viz- transverse waves and longitudinal waves.
ü 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.
ü Longitudinal Waves- When the particles of the medium travel parallel to the direction of the wave motion by means of successive compression or rarefaction.
Table No. 01- Transverse vs longitudinal waves
3. Sound Properties-
ü Sound needs a medium to propagate.
ü The matter or material through which sounds propagates is called a medium.
ü Sound cannot travel in a vacuum. The moon does not have an atmosphere, hence, you can hear on the moon.
4. Production Of Sounds In Humans-
ü The sound produced in the voice box called larynx located at the upper end of the windpipe.
ü Two vocal cords get stretched across in the voice box.
ü Has a slit, through which air is forced out by the lungs.
ü Muscles attached to vocal cords make it tight or loose.
5. Hearing Ability-
ü Human Ear-
n Outer ear = pinna: collects sound from the surroundings.
n Sound passes through a tube called an auditory canal.
n Eardrum (tympanic membrane) → vibrates when the sound incident.
ü Vibrations are sent to the inner ear, from there it goes to the brain as signals via the auditory nerve.
Figure 01- Human ear
6. Wave Properties - Amplitude, Time Period And Frequency -
ü 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.
ü The number of oscillations per second is called frequency. Expressed in Hertz (Hz).
ü Time taken for one complete oscillation to travel across a point. T = 1/f. (Seconds).
Figure 02- Wave properties
7. Loudness And Pitch Of Sound-
ü Volume or loudness of a sound depends on the amplitude. The force with which an object is made to vibrate gives the loudness.
ü The number of oscillations per unit time. Directly proportional to frequency.
Figure 03- Loudness and pitch
8. Sound Range-
ü Audible range ( that human can hear) 20Hz to 20kHz known as the Sonic range.
ü Sound below 20 Hz is called infrasonic range.
ü Whereas sound above 20 kHz is called Ultrasonic range.
9. Noise-
Unwanted and unpleasant sounds are called as noise.
Sounds with the same pitch and loudness can be distinguished based on the quality. Music is pleasant to the ears while noise is not.
Figure 04- Voice and noise
NCERT PRACTICE QUESTIONS
1. Choose the correct answer. Sound can travel through-
(a) gases only (b) solids only (c) liquids only (d) solids, liquids, and gases
Ans.- (d) solids, liquids, and gases.
2. Voice of which of the following is likely to have a minimum frequency?
(a) Baby girl (b) Baby boy (c) A man (d) A woman
Ans.- (c) A man
3. In the following statements, tick ‘T’ against those which are true and ‘F’ against those which are false.
a) Sound cannot travel in a vacuum.
b) The number of oscillations per second of a vibrating object is called its time period.
c) If the amplitude of vibration is large, the sound is feeble.
d) For human ears, the audible range is 20 Hz to 20,000 Hz.
e) The lower the frequency of vibration, the higher is the pitch.
f) Unwanted or unpleasant sound is termed as music.
g) Noise pollution may cause partial hearing impairment.
Ans. - True, False, False, True, False, False, True
4. Fill in the blanks with suitable words.
a) Time taken by an object to complete one oscillation is called _______
b) Loudness is determined by the ________ of vibration.
c) The unit of frequency is ________
d) Unwanted sound is called _______
e) The shrillness of a sound is determined by the ______ of vibration.
Ans.- Time period, Amplitude, Hertz (Hz), Noise, Frequency
5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Ans.- No. of oscillation = 40 and Total time is taken = 4 seconds
6. The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?
Ans.- Number of vibrations per second = 500
7. Identify the part which vibrates to produce sound in the following instruments- Dholak, Sitar and Flute.
Ans.- Stretched membrane; String of sitar and Air column.
8. What is the difference between noise and music? Can music become noise sometimes?
Ans.- The type of sound which are unpleasant to listen is known as noise whereas music is a pleasant sound, which produces a sensation.
Yes, music can become noise when it’s too loud.
9. List the sources of noise pollution in your surroundings.
Ans.- Following are the major sources of noise pollution-
ü Sound of vehicles
ü Sound of kitchen appliances
ü Sound of bursting crackers
ü Sound of loudspeakers, TV, transistors
10. Explain in what way noise pollution is harmful to humans.
Ans.- Noise pollution causes- Lack of sleep, anxiety, hypertension all of these are harmful to health.
11. Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.
Ans.- I would suggest my parents buy a house three lanes away from the roadside because house on the roadside would be much noisy in both days and night due to running vehicles. Whereas, a house three lanes away would be comparatively quieter as the intensity of noise decreases with the distance between the source and the listener.
12. Sketch larynx and explain its function in your own words.
Ans.- Larynx is also known as voice box. It is at the upper end of the windpipe. Two vocal cords are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for passage of air. When lung force air through the slit, the vocal cords vibrate, producing sound. Muscles attached to the vocal cords can make the cords tight or loose. When the vocal cords are tight and thin, the type or quality of voice is different from that when they are loose and thick.
13. Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why?
Ans.- The speed of light is more than that of the speed of sound. Thus, due to more speed of light it reaches us before sound. So, lightning is seen earlier and thunder is heard later.
Junior Science Topic - Sound. Chapter notes, video lectures, power point presentation and NCERT exercise solution. (Class 8, CBSE syllabus, chapter 13)
By - EduPorm India - The Educational Platform
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