## Calculating early reflection level...Help!

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### Calculating early reflection level...Help!

Hello everyone,

I've made a 3D model of my control room and I wanted to calculate the early reflections level and time delay after placing porous material\diffusion on the walls.
Is it possible? How?

I know that in order to calculate the level\delay of the reflection before treatment
I use these two equations:

20log (Direct Path/Reflected Path) - for level
(Reflected Path - Direct Path) / 343.5 - for delay.

In this topic - viewtopic.php?f=1&t=2923&start=20
On page 2, I've seen this equation:
Reduction of reflected sound level: R = -20 * log r [dB]
But I don't quite get it.

Can someone please shed some light on this topic for me?

Thank you very much!
Srulik E.
Srulik

Posts: 4
Joined: Sat Jul 17, 2010 10:23 am

### Re: Calculating early reflection level...Help!

If you are assuming geometrical behavior and you are not interested in phase information, you can weigh in the reduction of energy absorbed by the walls.

In the case of the equation you quoted, "r"is just the reflection factor, and the result will indicate the extra energy to trim due to the absorption.

For more complex geometrical scenarios you need to solve an image-source model which can also consider a diffusion factor (and good luck in determining that for your walls...).
And for wave behavior, don't even get me started...

All in all, what you describe is an extremely simplified/idealised approach to very roughly approximating how reflections will be recorded at the receiving positions.

But beware; from a perceptual point of view there is much more to early reflection than merely their strength and time of arrival.

Cheers,

Jon.
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jonessy

Posts: 580
Joined: Wed Jan 04, 2006 3:39 pm
Location: Israel

### Re: Calculating early reflection level...Help!

Hi Jon, Thanks for your replay!

So, if alpha represents the absorption coefficient of a given surface, can I calculate the reflection factor just by 1-Alpha?

BTW, if a full scale absorption, alpha=1 means that no energy comes back from the walls, how is it that I saw more than one product that has a higher than 1 absorption coefficient?

Thanks a bunch,
Srulik.
Srulik

Posts: 4
Joined: Sat Jul 17, 2010 10:23 am

### Re: Calculating early reflection level...Help!

Yes. well technically it would be more correct to say that |r(w)|=sqrt(1-alpha(w)),
but since you're looking essentially at the magnitude of a real valued reflection factor, then you can use your simplification.

Also don't forget that it is r(w) and alpha(w) as these are frequency dependent values.

For absorption coefficients greater than one, look around; a lot has already been written/discussed here.
__________________________
Bert is still my hero.
What d'ya know.
jonessy

Posts: 580
Joined: Wed Jan 04, 2006 3:39 pm
Location: Israel

### Re: Calculating early reflection level...Help!

Srulik wrote:Hi Jon, Thanks for your replay!

So, if alpha represents the absorption coefficient of a given surface, can I calculate the reflection factor just by 1-Alpha?

BTW, if a full scale absorption, alpha=1 means that no energy comes back from the walls, how is it that I saw more than one product that has a higher than 1 absorption coefficient?

Thanks a bunch,
Srulik.

Look a little closer, you are defining it wrong.
directedvoice

Posts: 6
Joined: Tue Jul 26, 2011 5:55 pm

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