We have a learned about radiation characteristics using two essential radiators. One was breathing sphere and the other one was trembling sphere. And then we learned about the radiation characteristics using baffled piston, hoping that the baffled piston's the radiation characteristics certainly resembles radiation from breathing or trembling sphere. And what we found from the previous lecture, the radiation characteristics of baffled piston, we have a piston that is oscillating and assuming that there is a baffle that sound radiate. And what we found was, it resembles what we learned from the radiation characteristics of a multiple sphere when we observe a sound at central line when the zie equal zero. Radiation characteristics baffled piston at the centre line, depending on where we observe the sound, compare with radiator sound size which can be represented by kz, the radiation characteristics of baffled piston. This really follows in the far field the monopole radiation. Okay, because the piston is radiating like this and if you think about the L particle over here, L particle over here will not move in this direction because to satisfy the boundary condition over here baffled piston meaning that there is no flow fluid oscillation in this direction. Therefore the radiation in this axis, along this axis would be very very different with the characteristics of radiation in this axis. Therefore there is a directivity pattern depending on the size as well as the observation points. So in summary, the radiation characteristics of a baffled piston are the same as the radiation characteristics of a monopole system when ka is very large or small compared with the wavelengths and is large compared with the wavelength, meaning that k is much much greater than one. There is no directivity with pattern when kr is very, very large what we observed in the far field resembles radiation characteristics of monopole. In other words if we see the sound radiation in the far field it doesn't matter whether the radiation comes from monopole or dipole or trembling sphere. That's the essential concept, that we strongly recommend to you to understand in this lecture series.
