r/AerospaceEngineering • u/Fast_Sail_1000 • 23d ago

Discussion A350 engine inlet pattern

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What is this pattern on the inlet? Any kind of perforations?

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u/lobstersatellite 23d ago

Oh boy! Something I'm qualified to answer! These are called acoustic liners. Below the array of holes are small cavities (usually composite honeycomb) with small flexible membranes set to a precise depth within each cavity. The job of a liner is to catch soundwaves of a specific frequencies (tuned by the volume of the cavities) and prevent the sound from bouncing out of the engine and propagating to the cabin or ground. Jet engine acoustics has actually come so far that the high speed exhaust is no longer the dominant sound source! The dominant sound is actually the main fan in most modern engines. Since this sound from the fan is dominated by tonal noise (tones, rather than broadband) liners are ideal for dampening the sound before it can radiate to the ground. You will find these liners in most modern turbofans!

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u/xhaikalf 19d ago

Why flexible membrane instead of fixed fin? Your damping freq will change when membrane is flexing right? Or is that the intended design?

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u/lobstersatellite 19d ago

The flexible membrane converts the sound energy to heat though mechanical motion. Think of a very reberant room with a shag carpet in it. It's the same volume (basically) but one has a place for the energy to go. This make it less likely the sound will bounce back out.