or characteristics of room any,

which can be regarded the Acoustic Large,

can be measured by simple time constant,

which is reverberation time or reverberation period.

I think I already mentioned very briefly what it means,

but today I'd like to derive this,

and I will conclude.

This will be point

one six one and sizable and open area window.

AS can be regarded as summing up or

the absorption coefficient and area of the surface.

And let's see how we can get this resolved.

Under what kind of condition we can get this result.

Okay? Suppose we have a room that look like this,

and maybe in the lecture hall you

have stairs and you have some seats, whatever.

And those can be regarded as for example, some surfaces impedance,

say Z_1 and you have

another Sophy's Z_2 and probably some other sophistries Z_3 and so on so on.

And suppose you have orchestra playing over here that radiates sound,

and this sound will meet

the surface distribution of impedance mismatch that I cannot hear Z_1,

Z_2, Z_3, and it of course will be

reflected back and forth like that,

so eventually all the sound will be mixed up in this space.

So, if you look at the energy that is of course in general,

function of space and time,

it is consisted by two components as we know.

One is the kinetic energy,

which is one half [inaudible] and the other one would be what we call potential energy.

This is just the energy we derive by squeezing the unit volume of compressed air.

And then, we can find the kinetic energy

by squeezing the unit volume of air by the pressure,

we'll get this result.

Okay. And if you see the certain volume over here, right away,

within a certain period of time of this energy that

I can see that sort of mean energy inside of this volume.

And during that period of time could be regarded as,

one over delta v, integral dv,

and then I also take the average with

respect to time from t to t plus T,

capital T is the period,

I mean, time duration that I'm going to take on average.

And E(r,t)dt.

If you look at

this energy which delta v

is very large compared with wavelength cube of interest,

and Lambda is wavelength,

and also the T is

large compare with the frequency that I have interest.

This energy would look like