  This paper presents an acoustic study of a standard NACA 0012 aerofoil with
additional self-oscillating passive flaplets attached to the trailing edge. The
tests with varying geometries of the flaplets were conducted in the anechoic
wind tunnel at Brandenburg University of Technology, at chord based Reynolds
numbers, $Re_c = 100,000 - 900,000$ at three geometric angles of attack
$\alpha_g = 0^\circ, 10^\circ$ and $15^\circ$. It was observed that all flaplet
configurations reduce tonal noise and that the key geometric parameter to
reduce this noise component is the width of the flaplets. The narrowest
configuration tested almost completely removed the tonal noise, leading to an
average overall sound pressure level reduction of up to 9 dB across the entire
$Re_c$ range at $\alpha_g = 10^\circ$. It was also observed that, in the low
frequency regime, a further noise reduction can be achieved by tuning the
natural frequency of the oscillating flaplets. The thereby affected frequency
range in the noise spectrum moves to higher frequencies when the natural
frequency of the flaplets is increased and vice versa. Hence we show a novel
way to target specific frequencies in passive aerofoil self-noise cancellation.
