Bumps spaced at half a wavelength re-radiate in phase and vanish acoustically.

Evidence

In the case of d = λ 2 , the wavefronts from the bumps and surface are delayed by λ and so arrive back in phase. Thus the bumps disappear, and the surface behaves as if it were flat. That is, it behaves like a specular reflector. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2078