Sound

Option “C” is correct

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 “C” is correct.
A resonance is the sound which is produced by an object when it vibrates at the same rate as the sound waves from another object.

Option “D” is correct.
SONAR is mostly used in naval sheep or submarines.
Hence, it is used by the navigator to see deep see tranches and safer root needed to be taken for avoiding a collision.

Option “C” is correct
If T is the reverberation time of an auditorium of volume V, then: T∝V

Option “C” is correct.
The time for which sound continues to be heard (due to persistence) after the source has stopped producing the sound is called reverberation time . Sabine gave a formula for reverberation time of a good building , which is T=0.16V/A ,
where V= volume of building in cubic metre ,
A= total absorption of building ,
hence option C is correct .

Option “B” is correct.
While crossing a suspension bridge, the soldiers are asked to break their steps. This is so because while marching each soldier exerts a force on the bridge which together creates a frequency and if this frequency matches with the frequency of the bridge, then resonance occurs and the bridge starts moving rapidly with large amplitude and thus, this may lead to breaking off of the suspension bridge.

Option”A” is correct.
i) The phenomenon involved in tuning a radio set to a particular station is resonance.
(ii) Resonance is a condition of forced vibrations. When the frequency of an externally applied periodic force on a body is equal to its natural frequency, the body readily begins to vibrate with an increased amplitude. This phenomenon is known as resonance.
(iii) Loudness is the property by virtue of which a loud sound can be distinguished from a faint one, both having same pitch and frequency. Loudness of sound depends on amplitude of the wave.
(iv) The loudness of sound is measured in the unit decibels

Option “C” is correct.