Class 9 Science Chapter 11 Question Answers - Sound

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

Q2: What is the intensity of sound?
Ans: Sound intensity is the sound energy passing per unit area per second, perpendicular to the direction of wave propagation.

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

Q4: Name the waves used by bats while flying in the dark.
Ans: Bats use ultrasonic waves while flying in the dark.

Q5: Which of the following sound waves can we hear: 10 Hz, 500 Hz, 1500 Hz, 12000 Hz, 25000 Hz?
Ans:  We can hear sound waves of frequency: 500 Hz, 1500 Hz, 12000 Hz. These are within the audible range of 20 Hz to 20,000 Hz for a typical human ear.

Q6: What do you understand by the term ultrasonic vibrations?
Ans: Sounds having frequencies higher than 20,000 Hz are called ultrasonic vibrations.

Q7: What do you understand by the term echo?
Ans: The sound heard after reflection from a rigid obstacle is called an echo.

Q8: Name the two types of mechanical waves.
Ans: The two types of mechanical waves are: (i) Transverse wave and (ii) Longitudinal wave.

Q9: What is a wave?
Ans: A wave is a disturbance that travels through a medium, transferring energy from one point to another without the net movement of particles of the medium.

Q10: Define one hertz.
Ans: One hertz is one vibration per second. It is the SI unit of Frequency.

Q11: Define wavelength.
Ans: It is the distance between two nearest points in a wave which are in the same phase of vibration.

Q12: 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.

Q13: What is sound, and how is it produced?
Ans: Sound is a form of mechanical energy that produces a sensation of hearing. It is produced when an object vibrates.

Q14: Why is a sound wave called a longitudinal wave?
Ans: Sound is a form of energy that produces a sensation of hearing. It is generated when an object vibrates and these vibrations travel through a medium.

Q15: How are moths of certain families are able to escape capture?
Ans: Moths of certain families can hear the high-frequency sounds made by bats using sensitive hearing organs. This helps them detect the presence of bats and escape capture.

Q16: 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.

Q17: 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.

Q18: What is a trough?
Ans: A trough is a depression (lowest point) in a wave, i.e., maximum displacement in the negative direction (below the mean position).

Q19: What do you understand by the term infrasonic vibrations?
Ans: Sounds having frequencies lower than 20 Hz are called infrasonic or subsonic vibrations.

Q20: Why do we see lightning before we hear thunder during a storm?
Ans: Because light travels much faster than sound in air.