of the frequency spectrum will be needed to 
forecast background noise levels as soon as 
these levels have been adequately correlated 
with detailed sea conditions. 
The needs of the naval architect are also 
beginning to be felt. The seakeeping char- 
acteristics of the old as well as the newer 
more advanced ship designs are dependent 
upon wave period as well as height so that 
in order to forecast ship speed and motion 
both should be known, 
The present code evolved to fill a need, 
but the need has increased such that another 
evolutionary step is required. The Wave 
Forecasting Program responding, in part, 
to this reports significant wave heights. 
Even this, however, is not sufficient for all 
needs. For the sake of brevity as well as com- 
pleteness of information a new code is needed 
which clearly distinguishes between sea and 
swell, indicates the period about which the 
maximum spectral energy is concentrated, 
and covers a more practical increment of 
wave heights. The latter is especially needed 
over the range from 2 to 20 feet. 
Wave Height Observations 
Wave heights are presently being deter- 
mined with varying degrees of accuracy by 
bottomed pressure elements, upward looking 
fathometers, buoy housed accellerometers, 
various mechanical and electrical devices, 
and visual observation, By far the greatest 
number of day to day observations are made 
by the latter method, They are made from 
merchant vessels, naval vessels, weather 
ships, and oceanographic research vessels 
from which the observers height above the sea 
varies from 6 to 60 feet. This factor alone 
introduces a tendancy for observers close to 
the sea to exaggerate wave heights, and those 
higher above the sea to underestimate them. 
235 
In addition to this source of error 
is the fact that the resulting average of 
the wave heights observed is computed 
from individual observations ranging in 
number from 0 to, hopefully, 50. Where 
the greatest number of reports fall within 
this range is left to the readers imagina- 
tion. It has been stated that ''at least 
fifty wave heights (estimates) are needed 
before any confidence can be place in the 
computed average wave height." 14 For 
instance, on the basis of 9 observations 
from which the significant wave height is 
computed to be 14.2 feet all that can be 
said theoretically is that the true value 
lies somewhere between 11.0 feet and 
19.8 feet, or a possible error of 28 percent. 
Nearly three times greater than the desired 
error for acoustical purposes. On the basis 
of 50 observations, however, the true value 
would theoretically lie between 12.6 feet and 
15.9 feet, or a possible error of 10 percent 
which places it within the desired acoustical 
error range. 
It is obvious that wave height obser- 
vations must be instrumented if the desired 
accuracy is to be achieved. The need for 
wave recorders on mobile platforms such 
as ships and planes is continually increas- 
ing. This need will be partially filled by 
the shipboard wave recorder now being 
developed for the Hydrographic Office, 
Whether it will provide the accuracy needed 
for acoustical work is yet to be seen, 
Wave Forecasting 
Twenty-four hour wave forecasts are 
issued daily by Fleet Weather Central in 
Suitland, Maryland and Honolulu, Hawaii, 
as well as the Fleet Weather Facilities in 
charge of ship routing at Norfolk, Virginia 
and Alameda, California. The service is 
also provided by several private concerns on 
