How Can Passive Serrations Quiet Aircraft Engines?

Imagine flying on a plane and hearing a softer hum, as if the engine were wearing headphones. That’s the promise of passive serrations, a technology that adds tiny "teeth" to the leading edge of a compressor blade. In an Italian lab, researchers 3D‑printed blades with up to nine teeth, about 10% of the blade’s chord, and tested them in turbulent airflow that mimics the rotor‑stator interaction inside a jet engine.

Using a semicircular microphone array covering elevations from 15° to 90° and azimuths from 0° to 180°, they recorded the sound emitted by the blades from many angles. By analysing frequency spectra and autocorrelation, they found that serrated blades cut noise by 2‑6 dB in the 1‑4 kHz range, especially at mid‑high frequencies. The reduction was consistent across all measurement planes, showing that the tooth shape affects not only sound intensity but also its direction.

These findings link turbulence characteristics to detailed acoustic maps, paving the way for quieter engines and more comfortable flights for everyone.

Passive serrations work by altering the pressure distribution along the blade's leading edge. When turbulent air hits a smooth surface, velocity fluctuations generate vortices of sizes comparable to the turbulence scale, which in turn emit sound. Introducing teeth creates micro‑features that break these vortices into smaller structures, shifting acoustic energy toward higher frequencies where the human ear is less sensitive.

In the experiment, turbulence was generated by a passive grid, allowing control of the integral length scale (≈ 0.02 m) and velocity fluctuations (≈ 0.5 m/s). The PSD (Power Spectral Density) showed energy peaks in the 1‑4 kHz band, corresponding to typical intermediate‑scale vortices. Autocorrelation confirmed a reduction in the temporal coherence of the pressure field, an indicator of less organized noisy vortices.

Beamforming reconstructed three‑dimensional SPL maps, revealing that serrations not only lower sound levels but also modify directivity: pressure lobes shift toward higher angles, reducing lateral exposure. These observations suggest that, by optimizing tooth geometry (spacing, amplitude, count), one can tailor an acoustic profile for different engine configurations, integrating noise reduction with aerodynamic performance. Moreover, 3D printing enabled testing of unconventional shapes, opening the door to bio‑inspired designs, such as hawk wings, which exploit natural serrations to control flow.

In summary, passive serrations represent a passive, lightweight, low‑cost strategy to mitigate engine noise, with potential applications not only in aviation but also in wind turbines and industrial compressors.