[Brainchild]

Could someone help out interpreting material's gas flow properties

Originally Posted by Hannes_F

Hi Andre,

sorry, maybe I did not get what you mean.

I am using material with low flow resistivity, around 5000 Pa*s/m². Cheap stuff.

Test these values in the Porous Absorption calculator:

Absorber thickness 100 mm
Air gap 100 mm
Start graph at 30 Hz.
Absorber flow resistivity 5000 or 16800 or 45000 Pa*s/m².



First, we see that the absorption is definetely different in the bass range.

Then, if we compare the 5000 Pa*s/m² with the 16800 Pa*s/m² curve we see that below ca. 135 Hz the absorption is better for the the more resitive material. Tests have shown this as well like Ethan Winer's site shows.

However if this leads to a "more is always better" thinking this could be an error. If you watch the curves closely you will see that there is a frequency range between 135 and 700 Hz where the fluffy 5000 Pa*s/m² material absorbs better. Moreover, the 45000 Pa*s/m² material aborbs worse than the 16800 Pa*s/m² material.

This is definetively counter intuitive. But if you think about it then denser material should be good for insulation, and this can include a good portion of backscattered energy.

Now enter an absorption width of 300 mm and a gap of 200 mm. Surprise - the 5000 Pa*s/m² is best!



If this is true then my conclusions are:

- If you are restricted to panels in the range of 10 cm or 4 " then material in the range of 16000 - 25000 Pa*s/m² is probably your best choice. With other words OC 703 or 705. (But do yourself a favor and don't ignore the demand for an air gap behind them).

- If you have more space then consider thicker absorbers with bigger gaps, filled with fluffy glass wool of 5000 Pa*s/m² or even less.

Now how relevant are those simulated values?

#1 Download the software ZORBA by Marshall Day Acoustics for a double check. It says the same if you activate "alpha normal". If you check their website you see that they verified their simulations with experimental data and the two are very close.

# 2 I am not finished yet with my studio but the traps that I installed so far behave exactly as they should according to these simulations.

My conclusions:
- More is not always better.
- It always depends (i. e. on the frequency range, the absorber width and position, the gap width)
- I try to know what I am doing. Read, read, read (I mean books, scientific articles, papers - not necessarily forum opinions).The links to the literature are everywhere. Start at the bottom of this site: http://www.bobgolds.com/AbsorptionCoefficients.htm . Read all the BBC articles about their acoustics tests.
- I try to understand the theory but also run tests in between.

Somebody who does not know what he/she is really doing may end up spending much money with mediocre results. Cruel but true :-)

Hope this helps
Hannes

Originally Posted by avare

I am the one who recommended, not suggested, using 3.5" material with an airspace in post #3 in this thread, with the technical explanation for the recommendation. When several people remarked upon my recommendation, I clarified it in post#9 with a link to thread that discusses porous insulation and sound absorption. It had nothing to with cost or value point.

The absorption of a porous absorber is dependent upon three things:

Location
Thickness
Gas flow resistance

Location is important because porous absorbers work on impeding the velocity of the air particles caused by the sound waves. If the location is where there is no particle velocity, then the of the absorber is nil.

Thickness affects how wide a frequency range the absorber is effective at. The thicker the absorber, the wider the frequency range. With absorbers against a wall, this being effective down to lower frequencies.

Gas flow resistance is important as for best absorption, the resistance of the absorber should equal the impedance of air (406 Rayls). If the resistance is too low, then the sound travels through the absorber. This is shown in the first drawing in post #29 in the thread I linked. If the resistance too high, then sound will be reflected, as shown in the second and forth drawings. The reflection occurs within the absorber itself. This is shown in the attachment, along with the equations detailing the real and imaginary components, of the sound wave and impedance. The resistance is a function of the gas flow resistivity and thickness of the absorber.

Unfortunately, almost no insulation companies provide gas flow resistivity data on their products, so we (as in studio designers) use material density as an indirect indicator of gas flow resistivity and scrutinize test data for clues.

With the Ultratouch test data (repeated in my previous post), it shows almost no increase in absorption in the 125 Hz band when the material thickness is increased by over 50%. If there is no internal reflection due to high gas flow resistance, then absorption would increase. This is being taken a clue that the gas flow resistance is too high.

As I wrote two paragraphs above " The resistance is a function of the gas flow resistivity and thickness of the absorber. " Since the type of material has been defined, this leaves adjusting spacing and thickness to achieve desired results.

The use of spacing is common in commercial facilities. Eric Desart used it in Galaxy Studios' absorbers. Peutz used it in the wall treatments for the Heineken Music Hall. In the latter case the ratio was 2:1 for space to material thickness) Neither of these are "value point" projects.

If the OP uses thicker Ultratouch instead of spacing with 3.5" material, he will get inferior acoustic results while spending more money. The recommendation for a space is based on physics and the restraints (what material) of this particular project.


The attachment is from the Bruel Acoustics web page. Unfortunately the page uses frames. If you want to read the document, which deals with his Standing Wave Tube apparatus, log onto the web page Bruel Acoustics, click on "Technical review and products information" and then "Technical Review 97-01: Standing Wave Tube SWT". If the name Bruel seems familiar, it could be because he is one of the founders of Bruel and Kjaer.

Andre

Originally Posted by avare

Several things are going on in this thread and rather than quote I will attempt to comment separately.

There is no advanatage to combining, or layering, different densities od acoustic material.

Filling a space (including a corner) is one way to get the required gas flow resistance at the required wavelength away from the wall. Cost becomes the factor and look at the cost of the material! 4" 703 last time I checked was $4/ft^2. Roxul Safe n Sound (general home insulation) at 3.5" thick was 37 CENTS/ft^2. The second product has the required GFR for thicker absorbers. A filled corner with it will cost a fraction of layered/spaced 4" 703! Do the math for prices in your part of the world.

Diffuse field absorption is nominally effective down to about 1/15 of the wavelength of the material thickness. Normal incidence absorption down to about 1/7 of the wavelength. Most peopel do not know the differenc between diffuse field and normal incidence absorption and refer to thte diffuse field, as that is what measured in reverb chambers.

Dig around on STudiotips.com Whealey's gas flow resistance and absorption calculator. It works with normal and specific anges of incidence for sound waves.

I hope this helps.

etc.