Hi

Air flow resistivity and optimal absorption depends on the situation.

For a bulk absorber (like mineral wool), you want specific flow resitivity to be density*wavespeed (rho*c = kg/m3*m/s = kgm/s2*s/m3 = Ns/m3).

I always muddle flow resistivity with specific flow resistivity, so I hope I get it right this time. The difference is that the specific flow resistivity = flow resistivity multiplied with the absorber thickness.

http://acta.fih.upt.ro/pdf/2011-4/ACTA-2011-4-04.pdf

For a sheet absorber, the optimum value depends on where it is placed

• For a freely suspended sheet, it should be 3.2*rho*c
• For a thin sheet in front of a wall, it should be about 2.2*rho*c

Simply put, the absorber must stand still for it to provide any flow resistivity - it will otherwise move with the acoustic particle velocity and in doing so, provide less absorption. There are several ways to accomplish this.

• Surface weight, where its 2*pi*frequency**surface_weight >> rho*c.
• It can be rigidly anchored to a structure
• A sheet absorber, e.g. a curtain, can be pretensioned using mechanical springs and/or have weights sown into the bottom edge.

Absorption is about correctly tuning flow resistivity to the situation. Therefore, you can get absorption out of any material or construction that meet the above outlined criteria, e.g. microperforated glass/metal, concrete, wire mesh screens and so on. 

A good book for the above is Noise Reduction Analysis (Physics) by Uno Ingaard (ISBN-10: 1934015318).

Here is also a previous discussion on the matter

https://www.researchgate.net/post/Is_it_possible_get_Air_Flow_Resistivity_for_acoustics_material_using_any_modelling_software

Sincerely

Claes

Hi

it is not flow resistivity but specific flow resistivity (i.e. Flow resistivity times thickness) that matters. 

Optimium values are

• rho*c for bulk absorbers
• 3.2*rho*c for sheet hanging free
• 2.2*rho*c for sheet absorbers backed by a wall,

where rho is density and c is the wavespeed. 

Furthermore, an absorber must stand still to provide pressuredrop. This is accomplished by

• inertia where 2*pi*f*m > 3*rho*c
• stiffness where the absorber is connected to something rigid to keep it still
• preloading where the sheet is loaded using springs and/or weight sown into the sheet lower edge. 

Any material that meets the above provides absorption, e.g. wire mest screens, microperforated glass, plastic or metal, concrete, etc. 

A good book on the subject of absorber design is Uno Ingaard Noise Reduction Analysis (Physics)

http://www.amazon.com/Noise-Reduction-Analysis-Physics-Ingard/dp/1934015318/ref=sr_1_2?s=books&ie=UTF8&qid=1449699676&sr=1-2

sincerely

claes

Hi,

The easiest connection between flow resistivity and sound absorption coefficient is Delany-Bazley model. You can find it here:

http://apmr.matelys.com/PropagationModels/MotionlessSkeleton/DelanyBazleyModel.html

If You want to use more detalised model (taking into account porosity, characteristic length, turtousity and so on) please refer to Johnson-Allard model or other. You could find models, input parameters and some simple theory in excellent book of T. Cox and P. D'Antonio "Acoustic absorbers and diffusers".

Adam

thank you Mr.Adam