Is it possible to predict the behaviour of a porous absorber used as a gobo somewhere in the room?
I use this porous absorber calculator:
http://www.acousticmodelling.com/porous.php

If I try to predict the behaviour of the absorber placed on a boundary (with or without a moderate air gap) I obtain a good and smooth curve of the absorption coefficient throughout the entire (effective) range of the absorber.
But if I set the air gap at a value of 3000mm ( 3 mt) I obtain an uneven curve, something like a lot of comb filtering. Is this irregular behaviour what really happens when I use a gobo in the middle of a room, or am I using badly the 'air gap' parameter?

According to the results, moreover, if I increase the thickness of the absorber in the calculator (let's say from 180mm to 240mm) the frequency response become smoother (always uneven, but smoother).

The gobo becomes acoustically transparent at wavengths longer than the dimensions of gobo.

The simulation are based on models following the 1/4 wave length rule.
The ondulation show you the ramdomize behavior when the 1/4 wavelength is not respected.
Don't worry under 100hz the prediction can be far from the reality measured.
So build, measure and adapt if the working is not optimal.

Thank you very much to both Avare and dinococcus. Both your replies seems to suggest that in my attempted simulation the quarter wavelenght rule is not followed. Sorry for the very noob question, may I ask why? Does this rule not apply when the absorber is far from a boundary?
Again, sorry for the basic question (put in the wrong way, maybe), if you have some link to clarify me the question I will gladly read it.

Ok for the under 100hz prediction, that's clear, I will simply experiment...

For some professionals on GS, the 1/4 wavelength rule is a myth.

The simulator show only the ramdomize behavior when you are outside the model.

This simulation is not the reality.

Oh, ok... so I should probably ignore the calculator in this case.

And what about @ avare reply?
"The gobo becomes acoustically transparent at wavengths longer than the dimensions of gobo."
I'm a complete beginner, just trying to (inordinately) gather informations and understand... but this is not what I expected to hear. Confusing for me, it's not what I commonly read about porous absorption (or gobos made of porous absorber)...

well its not a flat response you need in the absorber, its the positioning of it in the room, and how it makes the response in the room flatter.

As you increase the distance from the walls, the air gap resonance tuning decreases in frequency, and thus, start effecting the low frequency standing waves in the room. This comb filter effect can come in handy, depending on the standing waves you are trying to tune out in the room, or more importantly, increasing the bolt area (the area of flat response in the room where it would be optimal to mix in)...

So your panel simulator data when subtracted by the physical placement, subtracts from the propregated standing waves in the room, causing a change in the bolt area.

here is a real room simulator that you could use to find those annpoying standing wave frequencies, so that you can develop panels that subtract from this depending where you place them in the room.

https://amcoustics.com/tools/amroc

thank you audiospecific!
actually, I've already tried to calculate modes in my room (with other tools), even if in my case I have some additional challenge, since the room has an irregular shape. Just to clarify, I do not pretend to do my room treatment with gobos. Just these days I'm try to build some pressure based limp to address modes under Schroeder, and I already have lots of absorption, and some scattering-diffusion. The gobos are meant to add some absorption during recording of certain sources (vocals, guitars), without doing this additional absorption "permanent" in the room (or simply because a gobo in a certain position could not be permanent, i.e. in front of a window or a door)
Anyway, I understand that even in building gobos I should consider the overall modal response of the room.

Anyway, I have to say that, because of my lack of understanding and skills in acoustics, I do not figure out how to conciliate your advice with dinococcus and avare ones. I feel more confused than before

Sorry I missed this: the depths that you are writing about are also way outside the range the Delaney-Balzey equations were intended for.

The model of the calculator I'm using is the Allard & Champoux. But, anyway, the calculator should not warn me if I insert some data that is outside the range of the model capability? Sometimes, I received some warning from the calculator, and the calculator refused to give me a reply, but not in this case...

My apologies. It is frustrating to maybe gsr warnings. At the professional level I assume people are competent.

my apologies

What for? My reference meant the people who do not know and do not ask.

Oh, ok, I'm not that guy. I do not know, but I do ask. So, hopefully, I'm entitled to use a calculator and do some questions if I do not understand it

My question was: what do you mean when you say that "The gobo becomes acoustically transparent at wavengths longer than the dimensions of gobo" ?

For "dimensions" you mean "thickness"?
And: are you talking of a gobo made of just porous absorbent material?

And, if I understand what you are saying (but of course, I am not sure), is a gobo placed far from a wall mostly acoustically transparent, for a great part of the frequency range of interest, and then useless?

It wii start to become ineffectivr at wavelengths longer than the width. Eg. 570 Hz for 60 cm.

this is suprising for me, the width is the main parameter? And does not the depth (thickness) ot the gobo influence its effectiveness?

Width in relation with the distance.

The "transparency" will be not the same at 0,5m, 1 m or 3 m of distance for a object with 60 cm side.

the effect of thickness is somewhat intuitive. The effect of lemgth and width is not. That is with width being the smaller of the two.

Perfect, thank you both.


Ok, so: The gobo is less transparent and more effective in the low frequency range (given the same width, height and depth) if placed at a smaller distance from the microphone/instrument. Right?

The distance of the gobo from the wall affect the sound in a way that cannot be predicted by current models (if the distance is relatively high) but just evaluated in practice. I've correctly understood this point, too?

The gobo is more effective if he is near the source of the sound for the direct sound but less effective for the sound bouncing on the wall and returning towards you.
It's depends of the size of the gobo.

excellent, thanks