Intake Sound Deadening?

[Mechaniker]

Could anyone please explain to me how the perforations in the acoustic panels in the intake of the CFM-56 are made. Looking at them I notice that many of the small holes are not full thickness through the material, in some cases appear to be in a random pattern and and other areas are missing entirely; I am trying to understand what manufacturing process and specification would allow this. Surely in an aircraft that requires tolerances and accuracy to an infinitesimal degree if the plans specify holes in a particular pattern they should be formed fully in the correct pattern or not formed at all??
Is anyone able to shed some light on this?
Hals und Beinbruch!

[A Squared]

Strictly uninformed speculation, but might it be that they are intentionally random because a random arrangement is more effective in doing whatever they do than a regular pattern of the same?

[Mechaniker]

Thanks for that, what I said was not entirely true though, the holes do appear to be in a specific pattern but in some areas whether the holes are formed fully or not seems to be a bit random. The photo below sort of shows this, you can see at the edges areas where the holes are missing and a band in the centre where they are missing too. I would imagine on the engine below, as they are on the engines I see at work, holes that appear to be full thickness are only partially formed if you inspect it closely.

[lomapaseo]

I'm not sure what holes you are talking about in the rather large photo

There are structural mounting holes for rivets and sound deadening holes for acoustic treatment. The number of holes for attachment are a structural analysis thing (vibration lmiting is key) and the other holes is more of enough that fit and various depths to trap the noise frquencies and amplitudes.

I may have completely missed your question so feel free to ignore my ramblings.

[barit1]

While I don't pretend to know the whole story, I know just enough acoustics to be dangerous.

Each cavity seems to be sized to "trap" a specific audio wavelength. The size of the hole and depth tunes the chamber like an organ pipe. Some tuned for takeoff (high fan rpm, high audio frequency), and some for approach noise (low rpm, low audio frequency).

Semi-educated guess on my part.

[HazelNuts39]

I seem to recall that the intake silencing panels consist of a perforated sheet bonded to a honeycomb layer. The honeycomb cells together with the holes act as Helmholtz resonators, tuned to attenuate a particular range of frequencies, by varying the ratio between the volume of the honeycomb cells and the size of the holes. I would expect those frequencies to be in the range emitted during approach, because for takeoff the exhaust noise is usually dominant. I'm just guessing that some of the holes appear to be closed but are really filled by resin because they are facing the walls between the honeycomb cells. They would then not be random but would fit the hexagonal pattern of the honeycomb backing.

[Mark 1]

It's been a while, but I was once involved in the design of them.

Behind a porous facing sheet are usually honeycomb cells that allow a plain acoustic wave to propagate to the hard surface and reflect back to the front surface again.

The facing sheet is designed to a specified acoustic impedance such that roughly half of the incident acoustic energy is reflected on the surface and the other half propagates up and down the honeycomb cell. The cell is tuned by it's depth such that the reflected wave is about 180 degrees phase shifted from the incident wave and thus cancels out that target frequency (usually blade passing frequency and harmonics at the approach power setting).

The facing sheet controls the acoustic impedance by the percentage porosity and discharge coefficient of the holes which should average out over each honeycomb cell. Dynarohr is a woven facing sheet that essentially does the same task with a woven sheet.

Repair schemes for facing sheet damage usually allow them to be filled with epoxy and microballoons over the damage area and to hell with the acoustics.

I believe various methods are used to perforate the facing sheets including laser, EDM and punching; each with a different discharge coefficient of the hole.

Some aircraft have had very deep intake liners to target the 'buzzsaw' noise of supersonic fan tips, which has lower frequency character than regular fan noise. This type of liner is quite rare as most aircraft will approach with subsonic fan tips (below 65-70% N1).

[Cardinal]

Thought about this thread doing the preflight this morning, figured I could offer a little more detail. You can barely makeout the honeycomb lurking behind the perforations, and clearly see how the holes are simply filled near the walls. CFM56-5B A319.

[lomapaseo]

Thanks for that pic. If that is what was meant in the first post it pretty much matches what has been said earlier. The acoustic stuff is handled by having enough holes available per square foot for acoustic purposes (open to the face). At the edges of this area enough stiffness must be provided to dampen any vibratory forces from those spinning fan blades. So the structural requirements simply fill in portions near the rivets to meet the structural needs.

[Mechaniker]

Excellent guys this answers what i'm getting at, thanks to cardinal too for taking the photos.
Cardinal's photo clearly shows that the perforated sheet above the honeycomb is completely perforated during manufacture and then, as Mark 1 says, some areas are then filled with epoxy causing localised areas to be covered over, the epoxy is done so well that it looks like the holes have been formed this way rather than filled with epoxy!
Thanks everyone

[Beeline]

Repairing the area does bring into account a maximum surface area of acoustic loss on most parts of the engine including the exhaust.

The CFM is a great intake, rip the top layer off and seal the edges, obviously within reason and MM limits.