John Sayers' Recording Studio Design Forum Archive

Hi,
I have found this calculator for porous absorbers ( http://www.whealy.com/acoustics/Porous.html ) and want to check some ideas. My problem is that I only have data for flow resistivity in kPa·s/m² and not in rayls/m (which I need for the calculator).

I found out that rayls are 10· Pa·s/m .
That means, (Isover SPP2 similar to 703) 11 kPa·s/m² = 11 000 Pa·s/m² = 1100 rayls/m - that seems a bit too low for me, since the default setting in the calculator is 16000 rayls/m.

Can you confirm my calculation?

regards
JCR

Fibertex 350 Rockwool 2.2 x 10^4 Rays/m
Fibertex 450 Rockwool 3.3 x 10^4 Rays/m
Fibertex 650 Rockwool, has an acoustic resistivity of about 50000 mks Rayls/m
Fibertex Hd Rockwool 7.0 * 10^4 mks Rayls/m
Fibertex Rockwool Building Blanket Plain 0.5 x 10^4 mks Rayls/m

6000 mks Rays/m is roughly the resistivity of a fluffy fiberglass batt.
Glasswool Ceiling Batts 105mm 0.5 x 10^4 mks Rayls/m
R11 3.5" 89mm fluffy fiberglass pink batt is 4800 mks rayls/m

OC 703 has a resistance of 600 mks Rayls/inch (about 23600 mks rayls / m)
OC 705 has a flow resistance of 770 mks Rayls/inch (about 30000 mks rayls / m)
Bradford Fibertyex 650 (same density as OC 705) is 1270 mks Rayls/inch (about 50000 mks rayls / m).

Air, on the other hand, has a characteristic resistance of 407 mks Rayls


(above from various posts, and http://www.bobgolds.com/AbsorptionCoefficients.htm
below from ir761)

glass fibre (G1) 89 mm batt 4800 mks rayls/m
glass fibre (G1) 65 mm batt 3600 mks rayls/m
glass fibre (G1) 150 mm batt 4300 mks rayls/m
glass fibre (G2) 89 mm batt 7900 mks rayls/m
mineral fibre (M1) 89 mm batt 12700 mks rayls/m
mineral fibre (M1) 65 mm batt 11400 mks rayls/m
mineral fibre (M2) 75 mm batt 16600 mks rayls/m
mineral fibre (M2) 40 mm batt 15000 mks rayls/m
mineral fibre (M3) 83 mm batt 58800 mks rayls/m
cellulose (C2) 90 mm blown 33000 mks rayls/m

Thanks for your answer, the definition 1 rayls = 10· Pa·s/m seems to be wrong then. I guess the factor 10 in the fault (can someone confirm this ? )

The fluffy glasswool have ~5 kPas/m² here. Without the mentioned factor 10 it's 5000 rayls/m, which is similar to your fluffy fiberglass.

By the way, what does the "mks" mean?

regards
JCR

http://www.unc.edu/~rowlett/units/dictR.html

rayl
one of two units of sound impedance. When sound waves pass through any physical substance the pressure of the waves causes the particles of the substance to move. The sound impedance is the ratio between the pressure and the particle velocity it produces. The impedance is 1 rayl if unit pressure produces unit velocity. In MKS units, this means 1 rayl equals 1 pascal-second per meter (Pa·s/m), or (equivalently) 1 newton-second per cubic meter (N·s/m3). Confusingly, the same name, rayl, is used for the corresponding CGS unit, 1 dyne-second per cubic centimeter (dyn·s/cm3). The CGS rayl equals 10 MKS rayls. The units are named for Robert John Strutt, the fourth Lord Rayleigh (1875-1947).

CGS and MKS Units
Scientists have adopted the metric system to simplify their calculations and promote communication across national boundaries. However, there have been two ideas as to which metric units should be preferred in science. Scientists working in laboratories, dealing with small quantities and distances, preferred to measure distance in centimeters and mass in grams. Scientists and engineers working in larger contexts preferred larger units: meters for distance and kilograms for mass. Everyone agreed that units of other quantities such as force, pressure, work, power, and so on should be related in a simple way to the basic units, but which basic units should be used?

The result was two clusterings of metric units in science and engineering. One cluster, based on the centimeter, the gram, and the second, is called the CGS system. The other, based on the meter, kilogram, and second, is called the MKS system.

Thanks for this "ready for press" answer.