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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:
- 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/m3 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 = | I2f 2H3 | * 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
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