What other things you have to consider also? If you look at this point, you usually see there is a cup look like that, and you're suppose to fill your specimen in the cup. But sometimes, you cut the specimen, and put inside of a cup, then you have certain room between your specimen and the cup. That will reduce the accuracy of your measurement of RPA. Maybe you want to reduce the gap, then what you do is you make a little bit bigger specimen, and try to push it in, that will change the density of your so specimen. Then, you measured RPA R, will not be same as the true RPA. So, what I normally do is I have like a 10 different specimen, and then measure it, and average it. So, you have to average a lot of things, like a 10 different specimen, at different pmax, and pmin, minimum. So, you need to do a lot of experiment. If you just measure the absorption quotient by doing one single measurement, nobody believe it. The next question is how to measure maximum and minimum. I draw over there, maybe I use the microphone over there. But maybe what you can do is to use many, many microphone over there, and you can certainly do it, all right. And these days microphone is not so expensive compared with 20 years ago. So, you may have this kind of system you may design. But even if you install many microphones, you have to be very careful about the mounting of a microphone. Do you want to mount a microphone like that? Or you want to mount a microphone like that? Or do you want to mount microphone having some small gap over there? Depending on the way of mounting your microphone to receive the signal is different. So, actually because you know the acoustics, if you mount a microphone like that, you will have some archefact due to scattering. So ideal one is you have to flash mounted microphone. But perfectly flash mounting microphone is not possible. So, what I recommend you to do is to put the small O-ring over here to reduce the any undesirable scattering effect, or any undesirable vibration due to the microphone. And then finally, what you have to do is you have to calibrate it. Calibration simply means that you excite the system with the certain frequency, F1 and F2. And what you have to be able to observe over the here is the sound pressure that you expect to have. So, you have to calibrate it. But in old days, if you ask your supervisor, ''Hey, I did 20 microphone.'' Then I'm sure your supervisor will not like your idea. So, in old days, what we did is, so tube, and we have a speaker, and inside those speaker, we use this kind of small tube. And at the end of tube, we put the microphone, and we can move this tube so that we can measure continuously at any point or inside of tube, the sound pressure. That is what normally used in conventional standing wave apparatus. And other measure is possible. Using same standing wave apparatus, and you use two microphone measure. Because you know the position say P1(X=X1), I'm sorry this, okay, this is good. Can be regarded as the one is due to instant wave, and the other one is due to reflected wave. And measuring some pressure in other point, this is one and two and then, X = X2. And I can write, this is P0, jkx2 plus reflected R, that is RP0 exponential. Sorry, this is plus. Yes, this is plus. Plus jkx2. And because you have two microphones, you can obtain in a straightforward transfer function H12, that is simply P2 over P1. Right, transfer function? And this transfer function has a very good characteristics with respect to noise. Because we are comparing the signal P1 and P2. If noise of P1 and noise of P2 is not correlated, then this function does not really affected by the noise. And this is what? This is this. So, I can write P1, that is P0 plus this and also, P2 is P0 like that. So, I can write that this is one plus R exponential jkx1. And over there, that is one plus R exponential minus jkx2. And we know this value and know that value, then therefore, we can measure the absorption coefficient. That is quite a straightforward way to measure the absorption coefficient. So, in this lecture, we introduce three different ways to measure the absorption coefficient. One is using reverberation period, and then second, using standing wave apparatus. Third, in this case, we are using a scanning microphone. We use a microphone, using this kind of small tube and then moving. And the way to know the distance is very simple, you put a scan over there. And then, you can read the scan from here. So, that's not difficult. And third way is using transfer function. I'd like to ask you, which one would be best? It's not easy to say why it won't work, but look at how much money we use for these three different case. This is the most expensive case. Therefore, intuitively, we could say the first measure would provide us best measurement because we don't have to worry about the mounting problem. How to mount the specimen, things like that, as long as we can provide a diffuse fill effectively. That this method would be the best. But you have to be very careful to measure using this method because the wall has to be rigid. In our laboratory, you can see the small reverberation chamber. And the ideally, wall has to be very thick and rigid. But you see the chamber, you will see it at the basement, it's not very rigid. But if you measure the vibration of wall at a frequency that you are going to use, then you can certainly argue that, "Oh, this wall is relatively easy by seeing the vibration of wall." Things like that. And the second method is very classic, an old method. That is very cheap because you need only one microphone, and rigid wall too, and a speaker. You can measure it. Third measure based on transfer function H12, which is simplest case. But you have to be very careful for mounting your microphone. All right. And also, I'd like to mention some of the. Okay, no problem. No problem. In your book, you will see the many different type of formula for reverberation period. For example, the Sabine's formula, and the Olin's formula, and the Millington's formula, and the Fitzroy formula, and that is different with the Sabine's formula. And those formulas is good for different sound field. So, depending on the sound field you want to consider, you select the appropriate reverberation period of formula. I think this is what I intended to talk to you in this lecture.