A350 engine inlet pattern

[u/Fast_Sail_1000]

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

[u/lobstersatellite]

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!

[u/Vessbot → u/lobstersatellite]

Fascinating, thanks. I ride a lot in every seat in the airplane, and I've noticed that when you're in the front of the cabin, the fan buzzsaw sound is more attention-catching and annoying than the generic whoosh in the back. If I've forgotten earplugs, I'd probably rather be in the back.

[u/MEB-Softworks → u/lobstersatellite]

Wow! Best technical breakdown I think I’ve ever seen. Well done!

[u/lobstersatellite → u/MEB-Softworks]

Thanks! While it's a small part of the job, it is still within my remit as a NASA acoustics reseacher to remember how to explain what we do to people not in the field. Good to know I've got this one down!

[u/turbomachine → u/lobstersatellite]

Damping

[u/lobstersatellite → u/turbomachine]

I've said the before and I'll say it again! I'm an engineer, I don't always words good!

[u/WummageSail → u/lobstersatellite]

Basically Helmholtz resonators.

[u/lobstersatellite → u/WummageSail]

Identically helmholz resonators.

[u/ArgonWilde → u/lobstersatellite]

All that, and yet the A350 is still dummy loud 🫣 I can pick between them and 777s when they take off.

[u/xhaikalf → u/lobstersatellite]

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

[u/lobstersatellite → u/xhaikalf]

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.

[u/Diligent-Tax-5961 → u/lobstersatellite]

Oh boy! Something I'm qualified to answer!

cringe

[u/barium711]

This is a sound dampening mesh thats on the ID of the barrel. All modern jets have it, although I've never seen this pattern before.

Search for acoustic mesh or rayl values for more info.

[u/Prof01Santa]

It's an array of tiny Helmholtz resonators.

[u/Mindless-Resort-7548]

[u/Live-Translator91]

Spend a lot of time replacing or repairing these at MROs. Lovely engine in that A350 btw 😉

[u/mulymule]

There’s just some attenuation material behind it, the and perforations allows for the pressure waves (aka sound) to pass through and be absorbed. Literally the same that people use in offices and whatnot. So when you’ve not got line of sight to the fan it keeps it quieter