0:12

That has to do with the acoustically very large room.

Sabine said the very large room can be

characterized by the reverberation time or period,

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

Where and in the last lecture,

we found that the reverberation period of

this room was estimated by myself very roughly.

And then, we got about 0.9 second.

But our measurement system is very expensive.

Estimate reverberation period, 0.7 second.

Okay. And if you look at

the reverberation period of various famous room,

you can find that interesting number.

For example, the whole famous Vienna Symphony Hall,

that has a volume of

15,000 and reverberation unoccupied

is about three second.

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

two second, unoccupied, three second.

And Birmingham Symphony Hall reverberation period

is about 2.5 and it is not occupy,

but if it is occupy,

it is about two second.

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

When it is not occupied, it is 2.5.

Dallas case, 2.9 and 2.8. Stuff like that.

So, what you can see here,

the reverberation period of most of the Symphony hall is

ranging somewhat in 2.52 second.

So, it is more than we expected, okay?

If you go to the very old Catholic,

Cathedral, things like that,

it is more than sometimes three seconds.

So, you feel very revolved around a soundfield.

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

which is published in 2004.

Okay. Now, in some cases,

most cases I believe,

we would like to increase reverberation period in most cases.

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

Can you move up this?

4:44

Then, what we have to do is for example,

in this hall, we have Alpha 181,

maybe I say, this is the ceiling.

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

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

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

And then, we have to decide which one will effectively

reduce or increase the reverberation period, okay?

So, if I rewrite this again,

reverberation period is actually

6:34

this reverberation period is also function

of frequency or wavelets, right?

As you can easily anticipate,

the reverberation period will increase as wavelengths decrease.

Okay. And also, note that the absorption coefficient is

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

the absorption coefficient is also

functional frequency and it will increase as frequency increase.

So, what you measure over here using the instrument,

is the reverberation average in sense.

Okay. But sometimes as an architecture acoustician,

you may want to increase

the reverberation period at certain frequency or certain frequency band.

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

10:13

First, we have to make diffuse field, right?

So, how to make a diffuse field.

You need to have

the sound field that has to be

uniform irrespectable to the position where you measure the sound.

So, the reflection has to be very random.

Therefore, usually to measure diffuse field,

you need to have some box that has different size.

Okay? Which sometimes will called reverberation chamber.

14:10

Yeah. If you add this one over there,

that is what we measure, right?

Of course there is some little difference in the area.

Yeah. Maybe if you want to take

out what could be absorbed by this area,

but this is approximately true.

So therefore, we can measure alpha,

that is the absorption quotient of the specimen

by A specimen minus A,

open area window, divide by S. Therefore,

from the equation over there,

15:28

we can write Aspecimen,

let say in short Asp that is equal to 0.161,

we divide by T60 specimen,

and A0 is point 0.161,

we divide by T60 empty.

Therefore, the alpha is equal to 1.161V

divide by S,

one over T60 specimen minus one over T60 empty.

And then you can measure alpha for

any frequency you have interest, okay?

Well, this imaginable system

requires big reverberation chamber.

If you don't have that kind of chamber,

you need to find out some alternative measurement way.

An alternative away to measure the absorption quotient.

Okay. What would be the alternative way?