0:12

That has to do with the acoustically very large room.

Â Sabine said the very large room can be

Â characterized by the reverberation time or period,

Â which is related with size of room and open area window.

Â Where and in the last lecture,

Â we found that the reverberation period of

Â this room was estimated by myself very roughly.

Â And then, we got about 0.9 second.

Â But our measurement system is very expensive.

Â Estimate reverberation period, 0.7 second.

Â Okay. And if you look at

Â the reverberation period of various famous room,

Â you can find that interesting number.

Â For example, the whole famous Vienna Symphony Hall,

Â that has a volume of

Â 15,000 and reverberation unoccupied

Â is about three second.

Â Okay, so, the Vienna Hall reverberation period when it is occupied,

Â two second, unoccupied, three second.

Â And Birmingham Symphony Hall reverberation period

Â is about 2.5 and it is not occupy,

Â but if it is occupy,

Â it is about two second.

Â And Boston Symphony Hall is about 1.9 second when it is occupied.

Â When it is not occupied, it is 2.5.

Â Dallas case, 2.9 and 2.8. Stuff like that.

Â So, what you can see here,

Â the reverberation period of most of the Symphony hall is

Â ranging somewhat in 2.52 second.

Â So, it is more than we expected, okay?

Â If you go to the very old Catholic,

Â Cathedral, things like that,

Â it is more than sometimes three seconds.

Â So, you feel very revolved around a soundfield.

Â And this table was adapted from Beranek Concert Halls and Opera Houses,

Â which is published in 2004.

Â Okay. Now, in some cases,

Â most cases I believe,

Â we would like to increase reverberation period in most cases.

Â And then, we have to select the area that we wanted change, okay?

Â Can you move up this?

Â 4:44

Then, what we have to do is for example,

Â in this hall, we have Alpha 181,

Â maybe I say, this is the ceiling.

Â And Alpha 282, this is the area on the back side, and Alpha three,

Â A3, which would be the area covered by you guys and then,

Â Alpha four, A4, which would be at the bottom, things like that.

Â And then, we have to decide which one will effectively

Â reduce or increase the reverberation period, okay?

Â So, if I rewrite this again,

Â reverberation period is actually

Â 6:34

this reverberation period is also function

Â of frequency or wavelets, right?

Â As you can easily anticipate,

Â the reverberation period will increase as wavelengths decrease.

Â Okay. And also, note that the absorption coefficient is

Â also functional frequency normally even if we have a same of or whatever,

Â the absorption coefficient is also

Â functional frequency and it will increase as frequency increase.

Â So, what you measure over here using the instrument,

Â is the reverberation average in sense.

Â Okay. But sometimes as an architecture acoustician,

Â you may want to increase

Â the reverberation period at certain frequency or certain frequency band.

Â All right? Also, this equation says there are some way to,

Â 10:13

First, we have to make diffuse field, right?

Â So, how to make a diffuse field.

Â You need to have

Â the sound field that has to be

Â uniform irrespectable to the position where you measure the sound.

Â So, the reflection has to be very random.

Â Therefore, usually to measure diffuse field,

Â you need to have some box that has different size.

Â Okay? Which sometimes will called reverberation chamber.

Â 14:10

Yeah. If you add this one over there,

Â that is what we measure, right?

Â Of course there is some little difference in the area.

Â Yeah. Maybe if you want to take

Â out what could be absorbed by this area,

Â but this is approximately true.

Â So therefore, we can measure alpha,

Â that is the absorption quotient of the specimen

Â by A specimen minus A,

Â open area window, divide by S. Therefore,

Â from the equation over there,

Â 15:28

we can write Aspecimen,

Â let say in short Asp that is equal to 0.161,

Â we divide by T60 specimen,

Â and A0 is point 0.161,

Â we divide by T60 empty.

Â Therefore, the alpha is equal to 1.161V

Â divide by S,

Â one over T60 specimen minus one over T60 empty.

Â And then you can measure alpha for

Â any frequency you have interest, okay?

Â Well, this imaginable system

Â requires big reverberation chamber.

Â If you don't have that kind of chamber,

Â you need to find out some alternative measurement way.

Â An alternative away to measure the absorption quotient.

Â Okay. What would be the alternative way?

Â