MCQ : Sound - 2 - Class 9 MCQ

• 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 the range of human hearing, typically above 20,000 Hz.


• Frequencies of Ultrasonic Waves: Ultrasonic waves have frequencies above 20,000 Hz, which is beyond the upper limit of human hearing.


• Applications of Ultrasonic Waves: Ultrasonic waves are used in various applications such as medical imaging, cleaning, distance measurement, and non-destructive testing.


• Characteristics of Ultrasonic Waves: Ultrasonic waves have short wavelengths and high frequencies, making them suitable for applications where high precision is required.

• 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: Ultrasound is a form of imaging technique that uses high-frequency sound waves to create images of structures inside the body.

• Ultrasound Waves: Ultrasound machines emit high-frequency sound waves that travel through the body and bounce off tissues to create images.


• Ultrasonic Waves: These high-frequency sound waves are above the range of human hearing, typically above 20,000 Hz.


• Application: Ultrasound imaging is commonly used in medical settings to visualize organs, tissues, and blood flow in real-time.

• Accuracy: Ultrasound is non-invasive and provides real-time images with high accuracy.


• Safety: It is considered safe as it does not use radiation like X-rays or CT scans.


• Medical Uses: Ultrasound is used for various purposes such as monitoring pregnancy, diagnosing conditions like tumors, gallstones, and evaluating heart health.

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.

If you have persistence, you continue to do something even though it is difficult or others are against it. 

( As the sensation of sound persists in our brain for about 0.1 s, to hear a distinct echo the time interval between  the original sound and the reflected one must be at least 0.1 s. If we take the speed of the sound to be 300 m/s at a given temperature, in air, sound must go to the obstacle and reach back the ear of the listener on reflection after 0.1s. )

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

Distance between two crests or troughs is called the wavelength. Distance between crest and trough is half of the distance. 

Wavelength λ = AE

By symmetry,

Hence, BD is half the wavelength.

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.