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Class 9 Science Chapter 11 Question Answers - Sound

Q1: What does wave transfer - matter or energy?
Ans:
Energy.

Q2: What is intensity of sound?
Ans:
The amount of sound energy passing through unit area each second is called the intensity of sound.

Q3: Guess which sound has a higher pitch - guitar or car horn?
Ans:
Guitar has a higher pitch because it has higher frequency.

Q4: What do you mean by a wave?
Ans:
A wave is a vibratory disturbance in a medium which carries energy from one point to another without being a direct contact between the two points.

Q5: Where is density of air higher - at compression or at rarefaction?
Ans:
At compression.

Q6: Name the waves used by bats while flying in the dark.
Ans:
Bats use ultrasonic waves while flying in the dark.

Q7: A Sitarist tries to adjust the tension and pluck the string suitably, before playing the orchestra in a musical concert. By doing so, what is he adjusting?
Ans:
He is adjusting the frequency of the sitar string with the frequency of the other musical instrument.

Q8: Name two animals that communicate using infrasound?
Ans:
Rhinoceroses and whales communicate using infrasound.

Q9: If the tension in the wire is increased four times, how will the velocity of wave in a string vary?
Ans:
Velocity of the wave in string is directly proportional to the square root of the tension, thus if tension is increased 4 times, the velocity will be doubled.

Q10: What is SONAR?
Ans:
SONAR (Sound Navigation And Ranging) is a technique for determining water depth and locating underwater objects, such as reefs, submarines and schools of fish.

Q11: Define one hertz.
Ans:
One hertz is one vibration per second.

Q12: Define wavelength.
Ans:
It is the distance between two nearest points in a wave which are in the same phase of vibration.

Q13: What is the audible range of the average human ear?
Ans:
An average human ear can hear sound waves between frequencies 20 Hz to 20,000 Hz.

Q14: What is sound and how is it produced?
Ans:
Sound is mechanical energy which produces a sensation of hearing. When an object is set into vibrations, sound is produced.

Q15: Why is a sound wave called a longitudinal wave?
Ans:
A sound wave is called a longitudinal wave because the particles of the medium vibrate in the direction of the propagation of the wave.

Q16: How moths of certain families are able to escape capture?
Ans:
Moths of certain families can hear high frequency sounds (squeaks) of bats as they have sensitive hearing equipment. Thus, they get to know when a bat is nearby and hence, able to escape its capture.

Q17: What is a transverse wave?
Ans:
It is a wave in which the particles of the medium vibrate perpendicular to the direction of propagation of the wave.

Q18: What is a longitudinal wave?
Ans:
It is a wave in which the particles of the medium vibrate in the direction of propagation of the wave.

Q19: What is a trough?
Ans:
A trough is a depression in a wave, i.e., maximum displacement in the negative direction (below the mean position).

Q20: What do you understand by the term infrasonic vibrations?
Ans:
The sounds of frequency lower than 20 Hz are called the infrasonics or subsonics.

Q21: Which of the following sound waves we can hear: 10 Hz, 500 Hz, 1500 Hz, 12000 Hz, 25000 Hz?
Ans:
500 Hz, 1500 Hz, 12000 Hz.

Q22: What do you understand by the term ultrasonic vibrations?
Ans:
Sounds of frequency higher than 20,000 Hz are called the ultrasonics.

Q23: What do you understand by the term echo?
Ans:
The sound heard after reflection from a rigid obstacle is called an echo.

Q24: Name the two types of mechanical waves.
Ans:
The two types of mechanical waves are: (i) Transverse wave and (ii) Longitudinal wave.

Q25: What is a wave?
Ans:
A wave is a disturbance that travels in a medium due to repeated periodic motion of particles about their mean position – such that the disturbance is handed over from one particle to the other without the actual motion of the medium.

Q26: What is a transverse wave?
Ans:
It is a wave in which the particles of the medium vibrate perpendicular to the direction of propagation of the wave.

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