Flow Resistance, Flow Resistivity , and Resistance Ratio
The flow resistance of a piece of material tells us how easy it is for air to move through the material. The flow resistance depends upon the density of the fibrous material (Ib/sq ft) and the fiber diameter: generally, the heavier the blanket and the finer the fibers, the higher the flow resistance.
And, naturally, thicker layers have more flow resistance than thin ones. With experience, one can even learn to make a pretty good guess at the flow resistance of a material by seeing how hard it is to blow one's breath through the material. But for our purposes, we will rely on the measured values of flow resistance for some commonly available fibrous materials.
There's good news and bad news here, however. The bad news is that the manufacturers of fibrous materials don't worry much about the flow resistance of their products, so it's not always easy to find accurate information on this parameter .
The good news is that the acoustical behavior of our tuned resonant sound absorbers isn't critically dependent on the exact value of the flow resistance of the filling in the air cavity. We can miss the design goal quite a bit and it won't make much difference.
But first we have to discuss how to characterize the flow resistance of a layer of material. It is usually done by means of a resistance ratio that tells how much harder (or easier) it is for the sound pressure to push air through the layer in question than to push it through the air itself.
That probably sounds peculiar, because it may not have occurred to you that sound actually encounters some resistance in moving through the air. In fact, there is a "characteristic impedance" that relates the pressure in a sound wave to the corresponding particle velocity in the air: it is given by the product of the density of the air, p (gm/cc), and the propagation velocity of sound, c (cm/sec):
Characteristic Impedance = pc = 41 cgs rayls. We always relate the flow resistance, R, of a layer of material to the characteristic impedance of the air, pc, by forming the resistance ratio R/pc. If a layer of material has a flow resistance such that R/pc = 1, then a sound wave will not recognize the existence of that material when it is encountered, because it can't tell the difference between this material and air . If the value of R/pc is either substantially greater or less than unity, then the sound wave will "notice" the layer, and tend to be reflected from it rather than entering and passing through it.
