When more than one wave is present in a system there exists the possib
ilty of a resonant interaction. Resonant modes become nonlinear at sma
ller amplitudes than nonresonant modes. If the nonlinearity causes inc
reased growth rates then it may be that, for a time at least, the beha
viour of the resonant modes will be the dominant feature. In shear lay
ers resonant triads can be found where two oblique modes resonate with
a plane wave and this case has received much attention in the literat
ure. For a given plane wave, the resonance condition selects oblique m
odes of a certain wave angle and agreement has been found between pred
icted wave angles and those measured in experiments. In this paper it
is shown that resonance conditions can also be met between two planar
waves in a Blasius boundary layer, where one of the waves is the usual
unstable mode, and the other is a higher-order damped mode. The effec
ts of wave modulation are modelled by performing a spatial analysis bu
t allowing the frequency to become complex. It is found that for certa
in complex frequencies the strength of the nonlinear resonant interact
ion coefficients is greatly increased. Experiments have been performed
in a low-turbulence wind tunnel in which disturbances with modulated
and unmodulated sections were introduced into the boundary layer over
a flat plate. It was found that disturbances with the frequency and mo
dulation predicted by the theory do indeed show a much greater suscept
ibility to nonlinear breakdown than nonresonant disturbances.