Chapter 12 apply. Each spectral band must be multiplied by a dif- 
ferent correction factor as given by equation (12.21). The power 
spectrum on the bottom of figure 39 is then the power spectrum of 
the free surface. The true power in each band will lie between the 
dashed lines 90% of the time and the solid curve is the best esti- 
mate of the power spectrum. 
Figure 40 is a comparison of the pressure power spectrum with 
the free surface power spectrum. It shows that the low period end 
of the power spectrum has to be amplified very much more than the 
high period end. The minor wiggle in the pressure spectrum at a 
period of 5 seconds may even be an important secondary peak in the 
free surface record. The free surface record will be more irregular 
and choppy then the pressure record. The spectra also show that 
the "significant" (or "characteristic") period of the free surface 
wave record will be lower than the "significant" (or "characteristic" ) 
period of the pressure record. 
It is now possible to see where the 10% to 25% error described 
by Snodgrass [1951] comes from when the "significant" (or "character- 
istic") period is used along with the "significant" height to zo from 
a pressure record to the waves at the free surface. The "significant" 
height is crudely proportional to the square root of ee and the 
"significant" height of the pressure record is crudely proportional 
to the square root of Epmax’ rhe "significant" period method of 
pressure record analysis multiplies (Bomex) by a constant 
amplification factor [cosh (1 °H/e) I(+,,H)], for a fixed py 
which depends on the choice of the "significant period. This choice 
varies from analyst to analyst on the same record. 
93 
