In this week we talk about how wave is propagating in space. What do I mean by space? The space is rather complicated concept. So, let's start with the one dimensional case. Suppose I have this kind of a string and if I excite over here, the wave is propagating as you can see over here. But if I increase the length of this one dimensional space and if I excite this, the pattern of the wave propagation in one dimensional case would be very different. What if I propagate or generate a wave in three dimensional space like this room? Suppose I excite this three dimensional space by this whistle. For example. This is a one kilohertz of sound and wavelengths of one kilohertz sound is about 34 centimeter. Then 34 centimeter wavelength will regard this space as very large space. So in this case, wave propagate with the speed of sound 343 meter per second and it hit the wall and the back and so on. So, immediately this sound will be filled in this space. Therefore, how fast the sound or energy will be filled in this space or how fast the sound will decay in this space would be very interesting measure that might characterize the sound property of this room. Okay. Say then, very brilliantly find out the primary measure that can characterize the sound propagation in space would be reverberation period. Reverberation period what he found out, is proportional to the volume of this room and inversely proportional to how fast the sound will decay in the room. So if the reverberation period is large, then the decay of the energy of the room will be very small. If the reverberation period of this room is small, then energy is decaying rather fast manner. So, he introduce reverberation period as a major or the physical quantity that characterize of the room. So, in fact reverberation period has been used very widely to characterize the room acoustics.