Sound

Option “B” is correct.
Astronauts float around in space because there is no gravity in space. Everyone knows that the farther you get from Earth, the less the gravitational force is. Astronauts are so far from the Earth that gravity is so small. There is no atmosphere in space, therefore propagation of sound is not possible there. Sound needs a material medium for their propagation. In the space, there is no material medium so astronauts cannot listen to each other.
Hence, an astronaut can’t hear his companion at the surface of the moon because there is no medium for sound propagation.

Option”C” is correct

Option “A” is correct.

Option “C” is correct.
The intensity or loudness of a sound depends upon the extent to which the sounding body vibrates, i.e., the amplitude of vibration. A sound is louder as the amplitude of vibration is greater, and the intensity decreases as the distance from the source increases. Loudness or intensity of sound is measured in units called decibels.

Option “D” is correct.
Waves produced in a resonance tube are sound waves. Two sound waves in opposite direction interfere with each other to create resonance. As sound waves are longitudinal waves, waves produced are longitudinal stationary.

Option “B” is correct.
Like all waves, sound waves can be reflected. Sound waves suffer reflection from the large obstacles. As a result of reflection of sound wave from a large obstacle, the sound is heard which is named as an echo. Ordinarily echo is not heard as the reflected sound gets merged with the original sound. Certain conditions have to be satisfied to hear an echo distinctly (as a separate sound).
The sensation of any sound persists in our ear for about 0.1 seconds. This is known as the persistence of hearing. If the echo is heard within this time interval, the original sound and its echo cannot be distinguished. So the most important condition for hearing an echo is that the reflected sound should reach the ear only after a lapse of at least 0.1 second after the original sound dies off. As the speed of sound is 340 m/s, the distance travelled by sound in 0.1 second is 34 m. This is twice the minimum distance between a source of sound and the reflector. So, if the obstacle is at a distance of 17 m at least, the reflected sound or the echo is heard after 0.1 second, distinctly.
Hence, to hear a clear echo, the minimum distance between the reflecting surface and the observer is 17 m.

Option “B” is correct.

Option “A” is correct.
The sensation of sound persists in our ear for about 0.1 seconds. To hear a distinct echo, the time interval between the original and reflected sound must be greater than or equal to 0.1 seconds.

Option “D” is correct.
The walls and ceiling of an auditorium are covered with fibrous materials to absorb sound and prevent echoes.

Option “B” is correct.
The sensation of hearing of any sound persists in our brain for 1/10th second. This is called the persistence of hearing.