MCQ : Sound - 2 Class 9 Science MCQs & solutions - Free

• Initial Strike: When the key of a mechanical piano is struck gently, the sound produced will be at a certain volume and pitch.


• Second Strike: When the key is struck harder the second time




• Loudness: The sound produced will be louder compared to the initial strike as more force is applied.


• Pitch: The pitch of the sound will not be different between the two strikes.




• Conclusion: Therefore, in the second case, the sound will be louder but the pitch will remain the same.

Here, distance = 3 km = 3000 m 
Time  = 2 sec 
Therefore, speed= distance / time
=  3000 /2
=  1500 m /s 

Sound is only audible to the average humar ear if the frequencies lie between 20 Hz and 20 kHz. The actual range varies from person to person. Sound waves with frequencies less than 20 Hz are called infrasonic or subsonic and those with frequencies above 20 kHz are called ultrasonic. 

• Definition of Ultrasonic Waves: Ultrasonic waves are sound waves with frequencies above 20,000 Hz, which is beyond the range of human hearing.
• Frequencies of Ultrasonic Waves: These waves are specifically defined as having frequencies that exceed 20,000 Hz.
• Applications of Ultrasonic Waves: Ultrasonic waves are utilised in various fields, including:
  • Medical imaging
  • Cleaning
  • Distance measurement
  • Non-destructive testing
• Characteristics of Ultrasonic Waves: They possess short wavelengths and high frequencies, making them ideal for applications requiring high precision.

• Propagation depends on the elasticity of medium: Mechanical waves require a medium to travel through, and the speed at which they propagate depends on the elasticity and density of the medium.


• Can be both transverse or longitudinal: Mechanical waves can be classified as transverse waves, where the particles vibrate perpendicular to the direction of wave propagation, or longitudinal waves, where the particles vibrate parallel to the direction of wave propagation.


• Requires a material medium: Unlike electromagnetic waves, mechanical waves require a material medium for propagation. They cannot travel through a vacuum.


• Speed is comparatively higher: The speed of mechanical waves is determined by the properties of the medium through which they travel and is usually lower than the speed of electromagnetic waves in a vacuum.

The correct answer is D: Speed is comparatively higher

Speed of mechanical waves is actually lower compared to electromagnetic waves in a vacuum. Mechanical waves travel at a speed that is determined by the properties of the medium through which they are propagating, such as elasticity and density. This speed is usually slower than the speed of light in a vacuum, which is the speed at which electromagnetic waves travel.

Ultrasound is the application of ultrasonic waves, which are sound waves with a frequency higher than the human hearing range (above 20,000 Hz). These waves are widely used in medical imaging (such as sonography), industrial inspections, and cleaning delicate objects.

An earthquake produces infrasound before the mainshock wave begins. Infrasound is low-frequency sound less than 20hz.

• Explanation:




• Sound travels fastest in solids compared to liquids and gases because the particles in solids are closely packed together, allowing sound waves to travel more efficiently.


• Among solids, steel is one of the best conductors of sound due to its high density and stiffness.


• When sound waves travel through steel, they encounter less resistance and can propagate at a faster speed.

The human ear retains the sensation of a sound for about 0.1 s.

For a reflected sound to be heard as a distinct echo, the time interval between the original sound and the reflected sound must be at least 0.1 s.

With the speed of sound taken as 300 m/s, sound travels a distance of 300 × 0.1 = 30 m in 0.1 s. This is the round-trip distance (to the reflector and back).

Therefore the reflecting surface must be at least 15 m away from the source/listener for a distinct echo to be heard.

In a room of maximum linear size 10 m, the largest possible round-trip distance to a wall is 20 m, which takes time 20/300 ≈ 0.0667 s, less than 0.1 s.

Since the reflected sound returns before the persistence of hearing ends, it merges with the original sound and is not heard as a distinct echo.

Therefore, echo is not heard due to persistence of hearing.

Reflection of sound is used in many devices. For example ; megaphone, loudspeaker, bulb horn, stethoscope, hearing aid, sound board etc.

Given:
Time of one Oscillation (Time Period) = 0.02 sec
Speed of Sound = 3000 cm/sec
Convert the speed to metres per second: 3000 cm/s = 30 m/s

Use the relation,
v =  λ/T
 λ = v × T
where λ is the wavelength, v is the speed and T is the time period.

Wavelength (λ) = 30 × 0.02 = 0.6 m

Therefore the wavelength is 0.6 m
So, Correct Option: B

Answer: (d) All the above

Explanation: Sound has the ability to travel through solid, liquid and gas.

A note is a sound produced by a mixture of several frequencies. A sound produced by a single frequency is called a tone. 

Sound  travels faster in soilds than in liquids, and faster in liquids than in gases. This is because the density of solids is higher than that of liquids which means that the particles are closer together.

For a distinct echo, the time interval between the original sound and the reflected sound must be at least 0.1 seconds. With the speed of sound at 344 m/s, the total distance traveled by sound must be
344m/s×0.1s=34.4m.
Since this is the round-trip distance, the minimum distance to the reflecting surface is half of this, which is 17.2 meters.