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On the one hand you can see that there are certain fluctuations and the values &#;&#;should only be used as a rough guide. Nevertheless, a certain linearity can be seen for each material type. Any flow resistivity can be achieved with almost all materials. Depending on the type, this requires a different material density.
The most important finding when comparing glass wool vs. rock wool: rock wool must be about 50% heavier than glass wool to achieve the same flow resistivity. For example, we achieve the value of Pa * s / m² with 35-40kg / m³ rock wool, or with 20kg / m³ glass wool.
Caruso Iso Bond, on the other hand, is very similar to rock wool. The flow resistivity of Pa * s / m² can be achieved with both materials with a material density of 40kg / m².
But now finally to the actual questions of today's article. Let's start with:
With this question you can already see how I would approach the material selection: first we determine the correct range for the flow resistivity. And then we use the material table to see which material with which weight can be used to achieve this flow resistivity.
And don't worry if you are still undecided about the absorber depth. After the following examples, we will once again address the question of how thick the material may be at the various points.
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Since it is very time-consuming to impossible to acoustically measure all materials and all combinations at home, I have come to appreciate a (free!) Online tool. Of course, every simulation is only an approximation. But to get a feeling for the effects of different absorber depths and flow resistances, I know of no better and easier way than this calculator: http://www.acousticmodelling.com/porous.php
There are some limitations to keep in mind when using this tool. For one thing, I assumed an angle of 0 degrees for the simulation, i.e. we assume a vertical angle when the sound hits the wall. This is the case, for example, when we think of the wall behind the speakers. In practice, this angle changes somewhat for absorbers on the side walls, depending on how large the width of the room is and how large the listening distance is. In my experience, the 0 degree bends are the &#;worst&#;, i.e. a low degree of absorption is displayed. If we can increase the angle in practice, the values &#;&#;should always be better than the simulation for 0 degrees, since the sound travels more through the absorber at an oblique angle of incidence and is therefore better damped.
The tool only calculates with the absorber depth and the flow resistance. There is no way that the material density is taken into account. In this respect, the values &#;&#;are to be treated with caution and can differ slightly in reality.
Another assumption of the tool is the use of an infinitely large absorber wall. The output values &#;&#;are only achieved if a sufficient number of absorbers are placed side by side without gaps. I think it makes sense that we cannot conquer a 100 Hz wave (with a wavelength of 3.40 m) with a single absorber in the size of 1.20 m x 0.60 m. At low frequencies, we should be aware that we have to apply large areas. At high frequencies, i.e. if the absorber is larger than the wavelength (for example, 1 kHz has a wavelength of 34 cm), we can already achieve good absorption with a single absorber.
For the sake of completeness, the technical parameters that I used for my curves: air temperature: 20 degrees Celsius, air pressure: Pa, Angle of Incidence: 0 degrees, Porous Model: Allard and Champoux ().
To classify how well the curves of the free tool compare with professional software, I performed the same simulation with the Soundflow software from AFMG (second graphic). The specific weight is also taken into account in the calculation. I used the density of the respective rock wool here. At low frequencies, the degree of absorption is somewhat higher compared to the calculation without weight.
With Soundflow I also used an angle of incidence of 0 degrees and an infinitely large area as parameters. Bies was used as a model.
Using the following examples, I would like to give you a small guideline to find a reasonable flow resistance.
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