8th Class Science Sound Reflection of Sound

       Reflection of Sound

When we shout in a well or in an empty room, we hearoursound after sometime. This is known as the reflection of sound. When the sound strikes any solid object, it reflects back. If the surface is a smooth one, the reflection is more, and if the surface is rough, the reflection is less. On the rough surface, some part of the sound is absorbed by the surface. There are two phenomena of reflection:

(i) Echo

(ii) Reverberation

         Echo

If a sound produced is reflected back after some time on striking a solid surface, it is known as echo. The condition for echo to occur is that, the object should be at a minimum distance of 15 m. The distance for an object of 2 sec echo return would be 343 meters, as the speed of the sound is 343 m/sec. In most cases of human hearing, the echo is about one-half of a sec or about half the distance. The sound grows fainter with the distance.

          Reverberation

The repeated reflection of sound is called reverberation. The reflection time in case of reverberation is approximately 0.1 sec, which is much lesser than the echo. In case of reverberation the sound cannot be recognized. It is also defined as the collection of reflected sounds from the surface in any closed space, like auditorium. In an auditorium, it is desirable that the reverberation should be of such an extent that it helps to overcome the inverse square law drop-off of sound intensity. If the reverberation is excessive, it makes the sound runs together with loss of articulation, that is, the sound become unrecognizable and garbled.

Reverberation is resulted from reflection of any sound wave from obstacles and its return to the point of listening. That is why we perceive a direct source of an acoustic sound and its multiple reflections from the closest objects.

The signal with highest intensity comes first and is followed by the sound with lower intensity. In real situation, the sound impulses are lengthier than the time of arrival of first reflections. Hence, reverberation is superimposed on a source sound.

       There are various phenomenon of reflection of sound. The reverberation is one of them. In case of reverberation the sound is not distinct and cannot be understood properly. This is the reason, while constructing the auditorium, the reflection time is taken care off . What is the reflection time in case of reverberation?

(a) Less than 0.1 sec

(b) 0.1 sec

(c) More than 0.1 sec

(d) 0.5 sec

(e) None of these

Answer: (a)

Explanation

The reflection time in case of reverberation is less than 0.1 sec.

        Echo is one of the phenomena of reflection of sound. When we go to the mountain top and shout loudly, we hear our own sound after some time. What should be the minimum distance of the object for the echo to occur?

(a) 10m

(b) 15m

(c) 20m

(d) 50m

(e) None of these

Answer: (b)

Explanation

The minimum distance for echo to occur is 15 m.