Calculating the Windsheet Thickness
I found a formula on the following website
to calculate the windsheet thickness (WST). The WST and mouth height are defined in the image
to the right. I'll summarize here by saying that a scientist named
Hartmuth Ising developed a number to characterize the mouth of a pipe. The Ising number should be between
2 and 3. I=2 gives you maximum efficiency. As you increase the number, you get more harmonics and anything
higher than 3 will overblow. I've found that I=2 gives good results. Here's the equation for calculating
the WST:
WST =  I^{2}f ^{2}ρH^{3} 
2P 
 I is Ising's number
 f is the frequency
 ρ is the density of air
 H is the mouth height
 P is the air pressure
It's important to be careful with the units in this equation. Using 1.2 kg/m^{3} for the density
of air and putting in a factor to correct for the units, the following equation will give you the WST
in inches.
WST =  I^{2}f ^{2}H^{3}  * 1.554 x 10^{6} 
P 
 WST is the windsheet thickness in inches
 I is Ising's number
 f is the frequency in Hz
 H is the mouth height in inches
 P is the air pressure in inches of water
