Sound, Class 9th Notes

THESE NOTES ARE PREPARED ACCORDING TO JKBOSE SYLLABI IN SIMPLIFIED AND CONCISE  PATTERN

Q. Define sound? Explain how sound waves traverse in the medium.
                             Or
How compressions and rarefactions are produced near source of sound?

                              Or

How does the sound produced by a vibrating object in a medium reach your ear?

                             Or

Explain how sound is produced by your school bell?


Ans: The sound is form of energy which gives us sensation of hearing.

The sound is produced due to vibrations in different objects. The sound travels in the form of waves.

The sound waves are longitudinal waves. When a sound wave travel in medium, the particles of medium vibrate back and forth in the same direction as wave.

At one point particles of medium come closer, there is decrease of volume and increase in density which causes compression.

At alternate point, the particles of medium move apart from one another, there is increase in volume and decrease in density which causes rarefaction.

Thus sound waves travel in the form of compressions and rarefactions.

Q. Why is sound waves called longitudinal waves?

Ans: The sound waves travel in the form of compressions and rarefactions. The particles of medium vibrate in direction parallel to direction of sound wave. So it is a longitudinal wave.

Q. Why are sound waves called mechanical waves?

Ans: The sound waves are called mechanical waves because they need material medium like solid, liquid or gas to travel.

Q. Does sound waves follow same laws of reflection as light does. Explain?


Ans: The sound waves follow same laws of reflection as light because:

The sound waves get reflected when they fall on any surface.

The angle of incidence is equal to angle of reflection.

Q.  Discuss the various parameters of sound waves?       

                                        Or
What are the various factors which define sound waves?   

                                       Or
Define following terms, a. Wave length, b. Frequency, c. Time period,  d. Amplitude

Ans: The various factors which define sound waves are:-

a. Wave length: The distance between two compressions ot two rarefactions is called wave length.

It is denoted by lambda or  "λ" and its SI Unit is meter.

b. Frequency : The number of oscillations per unit time is frequency

It is denoted by Nu or "ν" and its SI unit is hertz.

c. Time period: The time taken to complete one oscillation is callef time period.

It is denoted by "T" and its SI Unit is seconds.

d. Amplitude: The maximum displacement of particles of medium from original position is called amplitude.

It is denoted by "A" and its SI Unit is meter.

Q. Which wave property determines,
  A. Loudness           B. Pitch

                          Or

Q. What is loudness of sound? What factors does it depend on? 

Ans: The loudness is determined by amplitude. More amplitude means more loud sound.

The pitch is determined by frequency. More frequency means higher pitch.

Q. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend

Ans: Sound waves need material medium to travel. The moon does not have the atmosphere, hence I will not be able to hear any sound produced by my friend.




Q. Guess which sound has a higher pitch: guitar or car horn?

Ans: The guitar will have higher pitch. Higher pitch means more frequency.





Q. How are the wavelength and frequency of a sound wave related to its speed?

Ans: The Speed of the sound is directly proportional to the frequency and the wavelength of the sound wave.

Speed = Wavelength x Frequency

If one or both of them increases, the speed of the sound wave will also increase.

Q. Give to practical applications of reflection of sound waves?

Ans: Megaphones, Horns  Trumpets are designed to send sound waves particular direction.

The stethoscope works on the principle of reflection of sound.

Concert halls, conference halls, cinemas are designed as such that there is proper reflection.



Q. What is Echo?

Ans: Echo is the reflection of sound waves from mountains, high buildings. It occurs in form of repeatation of sound.

In echo, the reflected sound is produced after 0.1 seconds.

Q. What is reverberation? How can it be reduced?

Ans: The phenomenon of persistance of sound after the source has stopped producing sound is called reverberation.

To reduce reverberation, the walls of hall are covered with sound absorbing material

The floors are also carpeted.



Q. What is the audible range of human ear?

Ans: The audible range of human ear is 20 hz to 20,000 hz. The below 20 hz sound is called infrasonic sound and above 20,000 hz is called ultrasonic sound.



Q. What is sonic boom?

Ans: Sonic boom is explosive noise caused by shock waves when an object producing sound moves faster than sound itself. Eg supersonic jets.


Q. How Bats use ultra sound to catch prey?

Ans: The ultrasound waves emitted by Bat are reflected from prey.

These reflected rays are detected by bat to know location and nature of prey.


Q. What are the different applications of ultrasound waves?

Ans: The ultrasound waves have many applications: 

i. It is used to clean parts like spiral tubes, electronic compounds.

ii. They are used to detect deep cracks in metal blocks used in construction.

iii. They are used in ultra sonography to get images of internal organs.

iv. They are used in SONAR in ships.

Q. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium.

Ans: Given,

Frequency = 220 hz

Speed = 440 m/s

Wavelength = ?



We Know,

Speed of a wave = Wavelength x Frequency

440 = Wavelength x 220

Wavelength = 440/220

Wavelength = 2 m

 

Q. A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source.

Ans: Given,

Frequency = 500 hz

Distance = 450 m

Time Period = ?



We Know,

Time Period = 1 / Frequency

Time Period = 1 / 500

Time Period = 0.002 Seconds.

Q. In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature

Ans: The sound does travel faster in Iron than water and air at a particular temperature.

Q. An echo is heard in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 m/s?

Ans: Given,

Time taken to reach the reflecting surface = 3/2 = 1.5 s

Speed of Sound wave = 342 m/s

Distance of reflecting surface =?

We Know;

Speed = Distance/Time

342 = Distance/1.5

Distance = 342 x 1.5

Distance of reflecting surface = 513 m

 

Q. Why are the ceilings of concert halls curved?

Ans: The ceilings of concert halls are curved so that the sound waves spread equally in all directions after reflection.

It also ensures that the sound waves are reflected down uniformly towards the people.

 

Q. What is the range of frequencies associated with (a) Infrasound? (b) Ultrasound?

Ans:

a. Infrasound= The range of frequency of Infrasound is 0 Hz to 20 Hz

b. Ultrasound= The range of frequency of Ultrasound is 20000 Hz to Infinity

 

Q. Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room?

Ans: Each person has its own pitch in the voice which is dependent on the frequency of the sound waves. It will help us to identify a friend in a dark room.

Q. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?

Ans: Flash is a light wave whereas Thunder is a sound wave. Since, light travels faster than sound, therefore thunder is heard a few seconds after the flash.

Q. A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 m/s.

Ans: Given;

Lowest Frequency = 20 hz

Highest Frequency = 20000 Hz

Speed of Sound wave = 344 m/s

Lowest Wavelength = ?

Highest Wavelength = ?

We Know;

Speed = Wavelength x Frequency

344 = Wavelength x 20

Wavelength = 344/20

Wavelength = 17.2 m



Again,

Speed = Wavelength x Frequency

344 = Wavelength x 20000

Wavelength = 344/20000

Wavelength = 0.0172 m



Thus, the range of hearing is 0.0172 to 17.2 m wavelength.

 

Q. The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?

Ans: Given,

Frequency = 100 Hz

i.e. The source vibrates 100 times per second

Now, Number of times it vibrates in a minute = 100 x 60 (1 Min = 60 S)

i.e. 6000 times.

 

Q. When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day?


Ans: We know that the speed of sound is directly proportional to the temperature. On a hotter day, the sound will travel faster and will reach back to us before 0.1s. Hence, we will not hear any Echo if the distance between source and reflecting surface remains same.

 

Q. A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Given, g = 10 m/s2 and speed of sound = 340 m/s

Ans: Given,

Height of Tower = 500 m

Acceleration due to gravity= 10 m/s2

Speed of Sound = 340 m/s

First, we need to first calculate time to reach the bottom of tower,

We Know,

H = UT + ½ GT^2

500 = 0xT + ½ x 10 x T^2

500 = 5T^2

T^2 = 500/5

T^2 = 100

T = √100

T = 10

Therefore, time required by the stone to reach the bottom of the tower is 10 seconds

Now, we need to calculate the time required for the sound waves to reach the top of the tower,

We Know,

Speed = Distance/Time

340 = 500/Time (Here, distance is the height of Tower)

Time = 500/340

Time = 1.4 S



Thus, Time after which splash is heard = Time taken by stone to reach the bottom of tower + Time taken by sound wave to reach top

i.e. 10 + 1.4 =

= 11.4 Seconds

 

Q. A sound wave travels at a speed of 339 m s–1. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?

Ans: Given,

Speed of Sound Wave = 339 m/s

Wavelength = 1.5 cm i.e. 0.015 m

Frequency = ?



We Know that,

Speed of Sound wave = Wavelength x Frequency

339 = 0.015 x Frequency

Frequency = 339/0.015

Frequency = 22600 Hz

The Frequency is the sound wave is higher than the audible range of human ear. Hence, it will be not audible.

Q. How is ultrasound used for cleaning?

Ans: Ultrasound waves have many uses. It is used for cleaning those objects which cannot be cleaned by normal washing.

Objects to be cleaned are put in the cleaning solution inside a machine and the ultrasound waves are passed through the solution. The ultrasound waves remove dirt from the objects.

 Q. Explain how defects in a metal block can be detected using ultrasound.

Ans: Ultrasound waves have many uses. It is used to detect defects in the metal blocks.

Ultrasound waves are passed through the metal block from one side and a detector is placed on the opposite side.

In a metal block with defects, all the ultrasound waves do not reach the detector and are bounced back.

 
Q. Explain the structure and working of human ear?

Ans: Human ear is a sense organ that helps us in hearing.

Structure of human ear: The human ear consists of following parts;

I. Outer Ear: It has Pinna, ear canal and ear drum.

II. The middle Ear: It has hammer, anvil and stirrup.

III. The Inner Ear: It has Cochlea, Auditory nerve.

Working: 

The Pinna collects sound waves, which passes through canal.
These sound waves fall on ear drum which vibrates.

The middle ear has hammer, anvil and stirrup which amplify the oscillations about 60 times.

The inner ear has cochlea where the waves are converted into electric signals.

The electric signals reach brain for interpretation through auditory nerve.  

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