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A fill up exercise for class 9 Physics from the chapter “Sound”

Dear students,

Find below some of the fill in the blanks questions  from the chapter sound. (Class 9 Science) Practisinf fill in the blanks typwe questions helps better comprehension and thorough reading of the text book. As a follow up you can try making more fill in the blanks type questions yourself from the textbook.



  1. ______ is a form of energy which produces a sensation of hearing to our ears.
  2. λ is a symbol of representing _____________.
  3. __________ is the reciprocal of frequency.
  4. Mechanical waves are of two types; _____________ and ______________.
  5. Sound waves travel in the form of ____________ and __________________.
  6. Transverse waves propagate in the form of _________ and _____________.
  7. The maximum displacement of the wave from mean position is called ______________.
  8. Number of vibrations completed in one second is called ________________.
  9. The distance between two consecutive compressions or two consecutive rarefactions is called ______________.
  10. Sound of frequency below 20 Hz is called _____________
  11. velocity of sound = ___________ x wavelenth
  12. Sound cannot travel in _____________
  13. Sound of frequency greater than 20 kHz is called ___________
  14. _______________ is used to check abnormalities of fetus.
  15. SI unit of frequency is _________.
  16. ____________ is a device used to measure the depth of sea.
  17. SONAR’s full form is __________________________________
  18. Sound travels fastest in ____ medium
  19. Loudness of sound depend on _____ of vibration.
  20. Audible range for human beings is from _ Hz to ___ Hz.
  21. Bats detect their prey using ___________– waves.
  22. Sound heard after reflection from an obstacle or boundary is called ________-.
  23. The persistence of sound after the source has stopped vibrations is called ________.
  24. ___________ is a device used by doctors and its working is based on multiple reflection of sound.
  25. Loudness of sound is determined by the ___________ of vibrations.
  26. Checking the functioning of heart with the help of ultrasound is called ________________
  27. The outer ear is also called ________
  28. Another function of human ear is to _____________ our body.
  29. _______ of ear collects sound from the surroundings.
  30. Ultrasound are produced by some animals like __________, _____________ and _____________.
  31. Electrical signals from ear are send to the brain via _____________
  32. The three bones of the ear which amplifies the vibrations are _______ , ______________ and ____________.
  33. ______________ converts sound into electrical signals.
  34. The smallest bone in ear is __________ which is also the smallest bone in human body.
  35. ____________ are regions of higher density and pressure when sound propagates through a medium.

Super Hero crossing Mach1 speed and then?!!

Charles posted this interesting Question on a Super Hero crossing the Mach 1 Speed. Visitors are requested to respond with your ideas.

“I’m working on a novel with a “superhero” protagonist.  The guy puts on his spandex, and takes off across state, say. 🙂
OK, as he speeds up, can he tell when he approaches and passes Mach one?  I mean, do you think such a character would feel  vibrations as he nears the speed of sound, and can slow so he doesn’t boom people below?  I read where a condensation cloud appears around jets as they pass Mach 1.”

Answer: When an object crosses the speed of sound (Mach 1) boom is heard if he is traveling through air or another medium. No boom will be heard in vacuum no matter how fast he is traveling. (Sound requires a medium for propagation as it is a mechanical wave). When you write a novel, you can just assume special abilities and capabilities to the super hero. (That is why he is super)

Factors affecting frequency of sound produced by a stretched string

Smile asks:

Study how the frequency of sound produced will change in each case with the following strings of length 15cms when the strings are tied between  2 ends-

  • aluminium string
  • copper string
  • cotton string
  • metallic string
  • jute string

Also study how the pitch changes when the strings are made taught and loose. Study how the frequency of sound changes with thickness of the following strings

  • cotton strings
  • copper strings


This seems to be a homework question or a project question. Therefore I am not giving a detailed answer so as not to tamper the basic aim of assigning a project.

The frequency of sound produced by a stretched string depends on the following factors:

  1. the length of the string
  2. the linear mass density (i.e; the mass per unit length) of the string
  3. the tension in the string

When you are using strings of different materials, the factor which changes is the mass per unit length and the same is true when you are changing the thickness.

When you make the string more taut, the tension increases and vice versa.

The question is given for a constant length. Therefore the case of effect of changing length does not come into picture.

The formula showing the relationship is 

it is evident from the formula that the frequency of sound is

  • inversely proportional to the  length
  • directly proportional to the square root of tension in the string and
  • inversely proportional to the square root of linear density of the string.

on proper substitution, the formula can be recast as

and this will be more convenient for you to answer the questions.

I recommend that you try to explore by actually performing the experiments.

Refer to http://www.tutorvista.com/content/physics/physics-iii/waves/stretched-strings-vibration.php for more

why sound travel faster in moist air than in dry air?

Raza Asks:

“Why sound travel faster in moist air than in dry


The density of dry air is more than that of moist air (Wonder why? Just answer me – which is denser – skimmed milk or fresh milk. The cream is lighter and when removed from milk, we get skimmed milk and therefore skimmed milk will be denser than the fresh one with cream content. Just like that the water vapor is lighter than dry air. When moisture is removed from air, its density increases). The speed of sound in a medium is inversely proportional to the square root of its density. Therefore, the speed of sound in moist air is more than that in dry air.

Doppler Effect

Hasan asks:

“What is Doppler Effect?”



Doppler Effect is the apparent change in frequency of a wave due to the relative motion between the source and the listener.

(Please come back soon for more details with figures and videos)

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