Since the wave energy E equals ρwgH2/8 or
ρwga2/2 (H = 2a), wave spectra in the earlier literature
were expressed in terms of E and called wave-energy
spectra. However, it has become common practice to
drop the term ρwg and to plot a2/2 or, simply, a2 along
the vertical axis. The wave-energy spectrum is
thus usually regarded synonymously with the “variance
spectrum”.