[Archived phorum post from squ1.com v2 by clarice]
Hi everybody,
I'm thinking about absorption coefficients and I have some doubts:
1) If the rule absorption + transmission + reflection = 1 is also applied to acoustics how can an absorption coefficient be higher than 1?
2) Are the absorption coefficients from the Ecotect help "section material properties - sound absorption" refering to absorption + transmission? Or are they Sabine absorption coefficient?
3) The absorption coefficients from the "material assignments panel - acoustic data" are Sabine absorption coefficients? Or are they absorption + transmission? Can I use absorption coefficient from the help file "section material properties - sound absorption" directly in the acoustic data from the material assignment panel?
Can anyone please give me a hand with this?
Thanks in advance
Regards,
Clarice
Re: absorption coefficients
[Archived phorum post from squ1.com v2 by Olivier]
Clarice,
I have never really used the acoustical properties of Ecotect.
I think that absoption coefficient should never be above 1.0 (100%) since it represent a ratio - open window = total sound absorption = 1.0.
I think yes - you should be able to use the data from the example Material Properties - Sound Absorption table in the Acoustic Data window of the Material Properties. I guess Sabines units could be higher than 1.0, since it is not a ratio, but a rate. I don't know. I am a bit confused too as the difference between Sabine unit (rate of absorbtion) and Abs. coef (ratio of abs. against an open window).
I cannot see where you would have to put any transmission or reflection data. I would think that the reverse of the absorption coeff. would be the reflection value [1 - Abs. Coef = reflect.]. As to the transmission, I do not know. Seems like it would have to do with the density of the material itself, as entered in the other window of the material properties. Andrew would know.
Sorry I can't be of more help. I took my acoustics class 5 years ago...
olivier
absorption coefficients - still puzzled!
[Archived phorum post from squ1.com v2 by clarice]
Hi Olivier,
Well aparently we are both puzzled about the absorption coefficients.
I still don't have an answer for this subject. Couldn't find anything in the help, tutorial or squ1 website.
Andrew or Caroline could you please give us a hand on that?!
Look forward to hearing from you
All the best
Clarice
Re: absorption coefficients
[Archived phorum post from squ1.com v2 by Andrew]
Hi Clarice and Olivier,
Once again, sorry for the delay.
1) As Olivier said, the absorption coefficient is basically the ratio of the actual sound absorption compared to that of an open window of the same area.
However, many reverberation room experiments have shown that some materials actually have an effect _greater_ than that of an open window at certain frequencies. There are lots of theories as to why this may be the case, however it is a room-wide effect and may be the result of destructive interference, edge effects or peculiar combinations of many different phenomenon.
Nonetheless, you will find materials in the published literature that have absorption values at certain frequencies of greater than 1.0.
2) & 3) All absorption coefficients in the help file are given in Sabine and, yes, they are intended to be used directly in the absorption coefficient fields in the materials dialog box.
BTW: When we talk about room acoustics, an absorption coefficient is all that energy that is not reflected back into the space. Thus all sabine values are absorption + transmission.
Hope that helps and is not too late...
Regards,
Andrew
Dr. Andrew Marsh
Square One research Pty Ltd
just to check
[Archived phorum post from squ1.com v2 by clarice]
Now I got it,
Thank you again Andrew! I didn't realize Sabine coefficients were refering to absorption + transmission. But anyway I used the coefficients directly in the material dialog box. So it's not too late
I got very puzzled with some frequencies with values greater than one so I thought everything was wrong but apparently everything is correct. Good!
I suppose absorption coefficients from comercial brochures cathalogs, etc, are also Sabine - when there's nothing saying the contrary, am I correct? Can I use coefficients from Sonex cathalogs directly in the material box, for example?
Regards,
Clarice
Re: just to check
[Archived phorum post from squ1.com v2 by Andrew]
Hi Clarice,
As a guess I would say yes that the figures in catalogues are likely to be sabine if they don't specify otherwise. I always find a good way to decipher such things is to double check with the units for figures -- usually suppliers will at least provide the appropriate units of measure if nothing else useful / descriptive...
Regards,
Andrew
Dr. Andrew Marsh
Square One research Pty Ltd
Re: just to check
[Archived phorum post from squ1.com v2 by clarice]
Hi Andrew,
Thanks again for the answer I'll check the units! Good idea.
Regards,
Clarice
Re: absorption coefficients
[Archived phorum post from squ1.com v2 by milan]
what about the total absorption coefficients used for some of the objects such as chairs? which unit are they in? how should they be converted for use in ecotect ?
Re: absorption coefficients
[Archived phorum post from squ1.com v2 by Olivier]
Milan,
I think the Total Absorbtion coefficient are also in Sabine.
You can enter your Sabine values straight into the Acoustic Data window of a material, for each frequencies.
Have a look at the help file under "sound absorption" or "material properties".
olivier