The formula is valid for either straight or staggered perforations. An approximation for the value of a, when you do not know the value of b, is:

a = d[(const./P1/2) -1]

The value of the constant is 9.5 for staggered and 8.9 for straight perforations.

We can predict from the value of TI the amount by which sound waves at the very high frequency of 10 kHz are attenuated in passing through the sheet, according to the curve in Figure IS, and from this we can develop a curve for the attenuation at lower frequencies. (See Part Two, Section II).