Pachyderm - Acoustic Modeling questions

Plug Ins
,
1

First off, thank you Arthur for making the Pachyderm plugin!

Secondly, I am new to acoustic simulations and I have a rather lengthy question regarding some settings / overall best practices.

What is the typical best practice for choosing acoustic properties settings when modeling a suspended ceiling absorber/island? From what I see, most manufacturers will provide the absorption / Sabine coefficients, but not much else. Realistically, if there is enough of a gap between the absorber and the ceiling (lets say > 2ft), some sound waves will be reflected, others absorbed, and some will also be transmitted through the surface (with some attenuation).

It seems possible choices are:
Option 1) Ignore any sound transmitting through it, and just use the absorption coefficients. Depending on the room setup, this may or may not be a reasonable approximation.

Option 2) Use the absorption setting and a semi-transparency setting. At first I was thinking this transmission setting was meaning the Power Transmission coefficient, but I realize it says, ā€œ% non-absorbed transmitted energyā€, so I think it is something else. I am now thinking this combination would have some rays pass through completely unobstructed, and some rays reflect or absorb, which is not really what Iā€™m after, but perhaps can be an approximation?

Option 3) Use the absorption setting and transmission loss setting. This may be redundant though, so alternatively would I set the absorption to 0, and only adjust the transmission loss value? Any guidance on setting reasonable transmission loss values to correspond to something like a mineral wool ceiling absorber if all I have is a ā€œtypicalā€ datasheet from a manufacturer with absorption values and maybe an overall density?

In general, how is the math/probability behaving when all of those settings are used together? e.g. if absorption coefficient is 0.6, and transparency coefficient is 0.7, for a single ray, what is the probability of reflection vs. absorption vs. transmission (and would that transmitted ray have the same energy or be reduced because of the absorption value)? Or if using transmission loss, does the absorption coefficient still get used, or is that ignored?

Any answers or guidance is appreciated. Thank you!

2

Hi,
I almost didnā€™t find this, so please when you have a question concerning Pachyderm - call me with @Arthur. (Arenā€™t I lucky that Iā€™ve been here long enough to have that particular tagā€¦)

For your case indicated above - know that generally speaking, everything we do is necessarily an integrated sum of all the frequencies we includeā€¦ and because of the way filters work, we will never truly isolate a single frequency in practice (though we can calculate that in theory sometimes). Furthermore, Geometric Acoustics is an approximate method, so we tend to calculate in discrete ranges of frequency, such as octave bands. This is only one of many simplifications that all GA software do (and to use the software correctly, you need to understand the simplifications of GA algorithms).

So, I would say use Option 1. (Options 2 and 3 will not represent the phase related behavior correctly - using absorption coefficients, at least the reactive characteristics of the material are handled empirically).

There is something else you can do, though. You are correct that there is comb filtering with a porous absorber with a deep airspace. I would direct you to try the materials designer in Pachyderm (under materials, and click call absorption designer). You can then actually calculate the absorption in fine detail of an Acoustical Ceiling Tile. As long as the ceiling runs wall to wall, it should be valid (being an infinite absorber apparent):

If you know how to script in python, you can get even more detail out if it - like so:

image
image1088Ɨ562 46.2 KB

image
image1052Ɨ544 88.4 KB

In fact, the interaction of the wave with the airspace is incredibly intricate.

One thing you can do which will enable you to do more interesting work with this - With the material designer, you can make a ā€˜smart materialā€™ which will reference the correct absorption coefficient per angle of incidence.

image
image2791Ɨ1608 497 KB

If you click ā€˜create smart materialā€™, you will note that the sliders are replaced with the directional graph in the lower left hand corner.

Hope this helps.

Arthur

3

Thank you so much for the detailed response.

For the scenario I was thinking of, the ceiling absorbers would not completely cover the ceiling (maybe somewhere around 50% only), but from what you said it sounds like the option 1 is still the way to go.

I will certainly have to explore the absorption designer more.

In general (if for example I had two adjacent rooms), does setting a TL value for an interior wall make that surface ignore the absorption coefficient, or is the absorption coefficient now accounting for both absorption and transmission and the TL value determines the amount of energy dissipated? I am trying to understand how the program determines whether to absorb/eliminate vs transmit, or reflect a ray. I know for the power coefficients when looking at sound incident on a surface R + T = 1, but that T effectively contains absorption and transmission.

Thank you!

4

Hi,

I wrote that material a very long time ago, so I donā€™t remember exactly what I did, or how, but Iā€™m pretty sure it follows this kind of template:

R = I * (1-alpha) * (1-tau) * (1-s)

Where R is the specular reflection, and s is the scattering coefficient, and of course, this would mean T (the transmitted sound) is the following:

T = I * (1 - alpha) * (tau)

Arthur

5

Thank you!

1 Like