CBSE Class 7 Science Advanced Physics Sound Very Short Answer Questions Set 02

Very Short Answer Questions

Question 1. What is reflection of sound?
Answer:
When sound strikes a surface it bounces back into the same medium. This is reflection of sound, and the angle of incidence equals the angle of reflection.
In simple words: Sound hits a surface and comes back, like a ball off a wall.

Teacher's Note:
1. Hard, smooth surfaces reflect sound best.
2. An echo is a result of reflection.

Question 2. What is an echo?
Answer:
The repetition of a sound heard a short time after the original, caused by reflection from a hard surface such as a wall or a cliff.
In simple words: An echo is your own sound coming back to you.

Teacher's Note:
1. The surface must be at least about 17 m away in air.
2. The gap between the sounds must be at least 0.1 s.

Question 3. What is the frequency of infrasonic sound?
Answer:
Infrasonic sound has a frequency less than 20 Hz.
In simple words: It is too low for human ears.

Teacher's Note:
1. Ultrasonic sound is above 20 000 Hz.
2. Humans hear between these two limits.

Question 4. What is loudness?
Answer:
Loudness is the property of sound that lets us tell a feeble sound from a strong one. It increases with the amplitude of vibration and is measured in decibels (dB).
In simple words: Loudness says how strong a sound seems.

Teacher's Note:
1. Loudness depends on amplitude, distance and surface area of the source.
2. It is not the same as pitch.

Question 5. What is a pitch?
Answer:
Pitch is the shrillness of a sound. It depends on the frequency: the higher the frequency, the higher the pitch.
In simple words: Pitch tells whether a note is high and thin or low and deep.

Teacher's Note:
1. A flute gives a higher pitch than a drum.
2. Women and children have higher-pitched voices.

Question 6. What is quality of sound?
Answer:
Quality (timbre) is the characteristic that distinguishes two sounds of the same pitch and loudness, such as a violin and a sitar playing the same note.
In simple words: Quality is the tone colour of a sound.

Teacher's Note:
1. It lets us recognise voices and instruments.
2. It is independent of pitch and loudness.

Question 7. What is noise pollution?
Answer:
The undesirable, loud and harsh sounds produced in the environment by different sources, such as vehicles, machines and loudspeakers, are called noise pollution.
In simple words: Unwanted loud sound in our surroundings.

Teacher's Note:
1. It can cause loss of hearing and stress.
2. Silencers and lower volume reduce it.

Question 8. Why the oscillations of a simple pendulum are not perceived by the human ear?
Answer:
Its frequency is below 20 Hz. It makes far fewer than 20 oscillations per second, so the sound is infrasonic and the ear cannot detect it.
In simple words: The pendulum swings too slowly to be heard.

Teacher's Note:
1. Humans hear only 20 Hz to 20 000 Hz.
2. A pendulum swings about once a second.

Question 9. What is the persistence of hearing?
Answer:
The sensation of a sound stays in the brain for about 0.1 s after the sound has stopped. This is the persistence of hearing.
In simple words: We keep hearing a sound in the mind for a tenth of a second.

Teacher's Note:
1. Two sounds closer than 0.1 s apart seem to be one.
2. This is why an echo needs a minimum distance.

Question 10. What is the minimum distance between the reflector and the listener to hear the echo, when the speed of sound in air is 330 m s⁻¹?
Answer:
The echo must come back after 0.1 s.
2d = 330 × 0.1 = 33 m, so d = 33 ÷ 2 = 16.5 m.
In simple words: The wall must be at least 16.5 metres away.

Teacher's Note:
1. The sound travels to the wall and back, so use 2d.
2. In practice about 17 m is quoted.

Question 11. What is the principle on which megaphone works?
Answer:
A megaphone works on the multiple reflection of sound from the walls of its horn-shaped tube, which sends the sound out in one direction.
In simple words: The horn keeps bouncing the sound forward.

Teacher's Note:
1. The sound does not spread in all directions.
2. Loudspeakers and hearing aids use a similar idea.

Question 12. What happens to the pitch of the sound when the length of the vibrating column is increased?
Answer:
The pitch decreases because a longer air column vibrates with a smaller frequency.
In simple words: Longer column, lower note.

Teacher's Note:
1. The burette experiment shows this.
2. Shorter column raises the pitch.

Question 13. A body completes 50 oscillations in 20 s. What is the frequency?
Answer:
f = number of oscillations ÷ time = 50 ÷ 20 = 2.5 Hz.
In simple words: Divide the number of oscillations by the time in seconds.

Teacher's Note:
1. Frequency is the number of oscillations in one second.
2. Check the unit: Hz.

Question 14. The time period of a vibrating body is 0.1 s. Find its frequency.
Answer:
f = 1 ÷ T = 1 ÷ 0.1 = 10 Hz.
In simple words: Take the reciprocal of the time period.

Teacher's Note:
1. A small time period means a high frequency.
2. 10 Hz is just below the audible range of 20 Hz.

Question 15. State whether the following statement is true or false.
The to and fro motion of an object is called vibration.
Answer:
True
In simple words: Vibration is a back-and-forth motion.

Teacher's Note:
1. Oscillation is another word for it.
2. The to and fro path is about a mean position.

Question 16. State whether the following statement is true or false.
The maximum displacement of a bob from its mean position is called its amplitude.
Answer:
True
In simple words: The farthest the bob goes from its rest position is the amplitude.

Teacher's Note:
1. Measure it on one side of the mean position.
2. Its unit is m or cm.

Question 17. State whether the following statement is true or false.
The unit of frequency is second.
Answer:
False. The unit of frequency is hertz (cycles per second); the second is the unit of time period.
In simple words: Frequency is counted per second, so its unit is hertz.

Teacher's Note:
1. Do not mix frequency with time period.
2. T is in seconds and f is in hertz.

Question 18. State whether the following statement is true or false.
The time taken to perform 30 vibrations is 30 times the time taken for a single vibration.
Answer:
True. If one vibration takes T, then 30 vibrations take 30 T.
In simple words: Thirty equal vibrations take thirty times as long.

Teacher's Note:
1. The time for each vibration is the same.
2. Multiply T by the number of vibrations.

Question 19. State whether the following statement is true or false.
If a body vibrates 20 times in a second, then its frequency is 20 Hz.
Answer:
True
In simple words: Twenty vibrations per second is 20 hertz.

Teacher's Note:
1. Frequency is by definition vibrations per second.
2. 20 Hz is the lowest frequency we can hear.