We learned that wave propagation in space has to be seen, in terms of wavelengths. Okay, what if we have this kind of musical instrument? I excite sound over here. Then, inside of this pipe where wave will be propagated. Then the, the wave we'll see this dimension, one dimension, is large compared with the wave that is propagating. But in cross-section of dimension, the wave will regard, this dimension is very short. So in this case, the characteristics of wave propagation would be quite different with what we learned in the last week. For example, wave propagation in, in general room, or Okay, another example would be this. For example, I have this kind of water bottle, and then if I excite this [SOUND]. The sound is generated, then, the, that means, fluid particle is oscillating with the frequency we just, we adjust here. Then that frequency is very low, I'm sure that frequency is about three, is between 300 and 400 Hertz. And the wavelength of that frequency would be very large, compared with the size of this bottle. That means, the characteristics, or property, of a wave that is generated by this kind of scale or this kind of space. Would be rather different with three dimensional act, I mean room acoustics or the way we just see for the case of musical instrument. So this case we call Helmholtz resonator. So in this week we will, we will study about duct acoustics. The wave propagation in musical instrument and wave propagation in Helmholtz resonator.