At part one, we studied the how the sound is reflected and transmitted when sound meets the flat surface of discontinuity like this. But, at part two, we will introduce the more complicated case, more sophisticated case. What if the sound, the generated sound makes the obstacle look like this, for example like this, or like this depending on the size of obstacle, the wave, the degree of scattering of wave will be different. Therefore we will talk about at chapter four, scattering, and diffraction For example, I generate a sound, what if there's a big wall. Then the sound is properating and then there will be some acoustic pressure fluctuation, therefore even if I cannot see someone over here, this guy can hear my voice. That we call the diffraction, and how this diffraction is related with the scattering. And also, if you see this case, when I shout over here, then it meets the flat surface of this continuity, and there will be some transformation. That is, this surface would vibrate, therefore it is related. Therefore, the radiation scattering and diffraction are all related together, so we will Attempt to explain all these three concept with a unified way, okay? That's will be handled in chapter four, and in chapter five, we will talk about acoustic wave in small or large space. When I generate a sign, for instance, in this room, and sound is generated and propagated in many directions, and depending on the wavelengths, this space could be regarded as a large space. Suppose, I am shouting in a big concert hall, then, comparing with the size of a concert hall, the wavelengths I generate for example, this kind of sound this is one kilohertz approximately. The wavelength is about 0.34 meter, therefore this wave we'll look at ought to be the concert hall very big size. But, suppose I am blowing small bottle, like your beer bottle, then the sound is generated. Then the acoustic wave that is generated by the presence of beer bottle will be the wavelengths, that is generated by the beer bottle would be much much larger than beer bottle, so that case, we can say the acoustic, that is a small space in terms of wavelengths. So we will talk about acoustic in a large space, as well as a small space. So in this chapter, you can understand the Helmholtz resonator. Like, how this kind of bottle will generate a sound, and a musical various musical instruments, and I blow this and the musical instrument generate a sound, this is the case when we have a small acoustic space so, we here a relatively large acoustic space. Or we will talk about, the how to design the concert hall or lecture hall using the concept we just talk about. So in this case has the space is much, much larger than compare with the wavelengths of interest, and we will also study. A famous Sabine's theory, reverberation time or period, how this measure is related with the quality of the concert hall, thank you.
