Unit Test(Solutions): Sound | Science Class 9 PDF Download

Time: 1 hour 
M.M. 30 
Attempt all questions. 
Question numbers 1 to 5 carry 1 mark each. 
Question numbers 6 to 8 carry 2 marks each. 
Question numbers 9 to 11 carry 3 marks each. 
Question numbers 12 & 13 carry 5 marks each.

Q1. What is the SI unit of frequency? (1 Mark)

Ans: Hertz (Hz)

Q2. Define the term "wavelength." (1 Mark)

Ans: Distance between two consecutive compressions or rarefactions in a sound wave, denoted by λ, measured in metres (m).

Q3. Which type of wave is sound? (1 Mark)
(i) Transverse
(ii) Longitudinal
(iii) Electromagnetic
(iv) Stationary

Ans: (ii) Longitudinal

Q4. Fill in the blank: Sound travels fastest in _________. (1 Mark)

Ans: Solids

Q5. Which of the following is an application of ultrasound? (1 Mark)
(i) Measuring temperature
(ii) Detecting flaws in metal blocks
(iii) Generating visible light
(iv) Producing low-frequency sound

Ans: (ii) Detecting flaws in metal blocks

Q6. Why are sound waves called mechanical waves? (2 Marks)

Ans: Sound waves are called mechanical waves because they need a material medium like solid, liquid, or gas to travel. They propagate by vibrating the particles of the medium.

Q7. How is an echo produced? (2 Marks)

Ans: An echo is produced when sound is reflected from a hard surface and returns to the listener's ears. To hear an echo, the reflected sound must reach after 0.1 seconds, which requires the reflecting surface to be at least 17.2 m away in air at room temperature.

Q8. What is the difference between a tone and a note? (2 Marks)

Ans: A tone is a sound of single frequency, while a note is a sound that is produced due to a mixture of different frequencies and is pleasant to hear.

Q9. Explain how sound is produced and propagated through a medium. (3 Marks)

Ans: Sound is produced by vibrating objects, like vocal cords or a tuning fork, which set nearby medium particles into vibration. These particles create compressions (high-pressure regions) and rarefactions (low-pressure regions), forming a longitudinal wave. The wave propagates as particles oscillate, transferring energy through the medium without moving the particles themselves from the source to the listener.

Q10. Describe how a stethoscope uses multiple reflections of sound to function. (3 Marks)

Ans: A stethoscope works on the principle of multiple reflections of sound. The sound of the heartbeat enters the chest piece, travels through the tube, and gets reflected many times inside the tube. These multiple reflections help in guiding the sound clearly to the doctor’s ears without loss.

Q11. Calculate the time taken by a sound wave with a frequency of 2 kHz and a wavelength of 35 cm to travel 1.5 km in air. (3 Marks)

Ans: Frequency, ν = 2 kHz = 2000 Hz
Wavelength, λ = 35 cm = 0.35 m
Speed, v = λν = 0.35 × 2000 = 700 m/s
Distance, d = 1.5 km = 1500 m
Time, t = d/v = 1500/700 ≈ 2.14 s

Q12. Establish the relationship between the speed of sound, its wavelength, and frequency. If the velocity of sound in air is 340 m s–1, calculate
  (i) wavelength when frequency is 256 Hz.  
(ii) frequency when wavelength is 0.85 m (5 Marks)

Ans: Relationship between Sound speed, wavelength and frequency 
Speed = distance/time
 V = wavelength/time 
V = wavelength × 1/time  
Frequency = 1/time 
V = wavelength × frequency
1) Wavelength = speed/frequency = 340/256 = 1.32 m
2) Frequency = speed / wavelength = 340/0.85 = 400 Hz 

Q13. Describe the applications of ultrasound in medical and industrial fields with examples. (5 Marks)

Ans: Ultrasound, with frequencies above 20 kHz, has various applications:

1. Medical Applications:
   • Echocardiography: Uses ultrasound to reflect off heart tissues, creating images to detect heart abnormalities.
   • Ultrasonography: Images internal organs (e.g., liver, kidney) or foetus to detect abnormalities like kidney stones or congenital defects.
   • Kidney Stone Treatment: Breaks kidney stones into fine grains using ultrasound, which are then flushed out with urine.
2. Industrial Applications:
   • Cleaning: Ultrasound detaches dirt from hard-to-reach parts (e.g., spiral tubes, electronic components) in a cleaning solution due to high-frequency vibrations.
   • Flaw Detection: Detects cracks in metal blocks by reflecting ultrasound off defects, ensuring structural integrity in buildings or machines.
     These applications leverage ultrasound’s ability to travel along defined paths and reflect off surfaces.