Class IX Solutions of Sound

I. Very Short Answer Type Questions: 

1. If 20 waves are produced per second, what is the frequency in Hertz?Ans. Frequency = 20 Hz

2. The frequency of a source of sound is 10 Hz. How many times does it vibrate in a minute?

Ans. Vibrations per second = 10 Vibrations per minute = 10 × 60 = 600 times

3. What type of wave is a sound?

Ans. Longitudinal wave

4. What is the audible range of the human ear?

Ans. 20 Hz to 20 kHz (20,000 Hz)

5. Define mechanical wave.

Ans. A mechanical wave is a wave that requires a medium (solid, liquid, or gas) to travel.

6. Define wave motion.

Ans. Wave motion is the transfer of energy and momentum from one point to another without the actual transport of matter.

7. What is supersonic speed?

Ans. Supersonic speed is the speed greater than the speed of sound in a given medium (more than 343 m/s in air at room temperature).

8. Write a difference between longitudinal and transverse 

waves.

Ans. Longitudinal Wave: Particles of the medium vibrate parallel to the direction of wave propagation (e.g., sound waves).

Transverse Wave: Particles of the medium vibrate perpendicular to the direction of wave propagation (e.g., light waves, water waves).

9. What are the three quantities that play an important role in describing the nature of a wave?

Ans. Wavelength (λ), Frequency (f), and Wave Speed (v)

10. Define frequency.
Ans. Frequency is the number of waves produced per second. It is measured in Hertz (Hz).

11. What is the time period?
Ans. Time period (T) is the time taken to complete one wave cycle. It is the inverse of frequency: T = 1/f

12. What is the unit of frequency?
Ans. Hertz (Hz)

13. Write a relation between wavelength, frequency, and wave speed.
Ans. Wave speed (v) = Frequency (f) × Wavelength (λ)

14. What are the factors that make the sound seem different to us?
Ans. Pitch, loudness, and quality (or timbre)

15. Define pitch.
Ans. Pitch is the perception of how high or low a sound is, determined by its frequency.

16. What is the relation between amplitude and loudness?
Ans. Loudness is directly proportional to the square of the amplitude:

$$\text{Loudness} \propto A^2$$

17. What do you mean by the quality of musical sound?
Ans. Quality (Timbre) of sound is the characteristic that helps us distinguish between different sources of sound, even if they have the same pitch and loudness.

18. What is the frequency range of hearing of the human ear?
Ans. 20 Hz to 20 kHz

19. Give one example of each of longitudinal and transverse waves.
Ans. Longitudinal wave: Sound wave
Transverse wave: Light wave

20. What is the frequency of a wave with a time period of 0.025 s?
Ans. Frequency = 

$\frac{1}{T}=\frac{1}{0.025}=40$ Hz

21. Why do we hear the sound of an approaching car before the car reaches us?

Ans. Because sound travels faster than the car itself in the air.

22. A baby recognizes her mother by her voice. Name the characteristic of sound involved.
Ans. Quality (Timbre)

23. What is the unit of wavelength of sound?
Ans. Meter (m)

24. Infrasound is produced by:
Ans. (c) Rhinoceros

25. Which of the following is used in echocardiography?
Ans.(a) Ultrasound waves

26. Speed of sound is maximum in:
Ans. (a) Solids

27. Light is a:
Ans. (b) Transverse wave

28. In compression, pressure density is:
Ans. (a) High

29. Frequency of an ultrasonic sound wave is:
Ans. (b) Greater than 20,000 Hz

30. Stethoscope works on the principle of:
Ans. (a) Multiple reflections of sound

31. Audible range of the human ear is:

Ans. (d) Both (a) and (c) → 20 Hz – 20 kHz or 20 Hz – 20,000 Hz

32. The order of bones in the human ear from outside to inside:
Ans. (b) Hammer, Anvil, and Stirrup