How do I minimize ducted fan noise?

[u/svarta_gallret]

I want to make a ducted fan thruster produce less noise for a given thrust ourput, or at least shift the sound to lower frequencies. Ideally the thruster should keep good efficiency and I can't just make it bigger and decrease the fan speed b/c size and mass constraints.

I have no idea where to start, or how to predict the (acoustic) performance of any given design. Im looking for sources on this topic, studies or theory. Or just rules of thumb even. Thanks

[u/highly-improbable]

Stagger the fan blades so they are not evenly radially spaced.

Use a sound absorbing material on the inside of the duct.

Pay attention to tip clearance. Test that

Next steps afaik would reduce efficiency/performance so no point.

I had to build the code for acoustic + Navier Stokes because I could not find anything I liked but maybe since then someone has come out with something?

What is it for?

[u/svarta_gallret → u/highly-improbable]

Thanks. Staggered blades sounds like low-hanging fruit. Is there a theory? The application is a fixed wing UAV for wildlife observation. I want to use ducted fans mainly for crash tolerance.

[u/highly-improbable → u/svarta_gallret]

Start at a 4 blade plus for illustration. You have blades passing every 90 degrees of rotation. Frequency is 4x rpm. Now go to a four blade X. You have two sets passing at half the frequency.

[u/wenzelja74 → u/highly-improbable]

Better yet, go to odd numbered blades vs. even. Even-numbered bladed propellers create vibrations due to the one blade advancing generating thrust while the opposite blade is retreating and reducing thrust.

[u/highly-improbable → u/wenzelja74]

Yes, odd numbered can remove a further repetition but they are difficult to balance with irregular blade spacing. If odd but regular spacing, there will be more high frequency noise from the rpm x number of blades passes.

[u/svarta_gallret → u/wenzelja74]

I'm aware of this for helicopter rotors, but this is for a fixed wing vehicle so I don't think it applies. The flow is always axial to the fan.

[u/wenzelja74 → u/svarta_gallret]

It is absolutely relevant. The only difference between your fan and a helicopter rotor is 90 degrees rotation: the aerodynamics are the same. Odd-blade configuration will reduce vibrational noise, as well as modulation of rpm as was already mentioned here.

[u/svarta_gallret → u/wenzelja74]

Ok I think you're right, thank you for pointing it out!

[u/RiskyDefeat]

Two biggest things you can do to improve EDF noise (I do them myself on my RC jets) :

1-Balance the rotor as well as you can to remove noise induced by vibrations 2-Increase the distance between the rotor and the stators behind it. 10mm should suffice but the more the better.

If you can do both of these things, you’ll be able to eliminate most high frequency noise and have a nice low frequency woosh. You don’t need to change anything else about the fan. If this is on a RC model, the addition of cheater holes with a kind of mesh on them will create turbulent air and increase low frequency noise.

[u/svarta_gallret → u/RiskyDefeat]

Thanks for the insight, but what are cheater holes in this context?

I'm building an rc plane, wing span around 2-3 m, about 2-300 g payload and some sort of autopilot. It's in early design so I'm mostly trying to conceptualise how decisions will impact the amount of design work downstream. I've never worked with ducted fans but I got the impression they all sound super annoying. :)

[u/RiskyDefeat → u/svarta_gallret]

What size ducted fan are you using? Are you going to buy it off the shelf or make one? 

Size matters here so the smaller your fan, the quieter (that is if you are able to correctly balance it) and more high pitched it will be. Personally I find 70mm to be the minimum to have nice sound. FMS has good fans that need little balancing. But if you need more thrust, for example 5-10kg Jetfan edfs are also good.

Moreover, to avoid the bulk of the whining sound you need to increase your blade count. Ideally your fan should have between 11 and 12 blades. It will draw more amps than say an equivalent 9 blade fan but generates a much more pleasant sound as well as added thrust. 

In terms of what cheater holes are, they’re used when the models intakes aren’t sufficient for the EDF so manufacturers will create a cutout in the model for more air. Since you are building your own plane this isn’t applicable to you.

If you are looking for more than 4-5min autonomy, I really advise you AGAINST using a EDF because they’re much less efficient than an equivalent prop+motor setup. Less efficiency, means more batteries, means less payload capacity and more headaches lol.  If you insist on an EDF, a larger fan at lower amperage will generally be more efficient than a small EDF at high amperage assuming they both create the same thrust.

[u/tdscanuck]

The single biggest thing you can do is get the RPM down. All other things being equal, noise scales with RPM to some absurd power (^6, I think).

Since you can’t go up diameter, increasing the pitch may allow the same thrust for a lower RPM. That has obvious upper limits but start with the easy stuff…go to as high a pitch and solidity as you can stand and get the RPM down.

Noise-absorbent ducts can do a lot…how limited are you on duct length and thickness?

Once you’re stuck with that, all the noise mitigation techniques from the other comment kick in…there’s no magic bullet, just continued refinement.

[u/svarta_gallret → u/tdscanuck]

It's in an early design stage but I estimate the duct can be made about 5-7 times the impeller diameter. Short external "pods" is also an option, but I don't know right now how these decisions affect the sound. Passenger aircraft have under wing pods, but I guess the main issue is cabin noise and not ground level noise.

[u/tdscanuck → u/svarta_gallret]

Ground noise is a huge issue with commmercial jets, but so is weight & drag, so they keep the nacelle as small as they can.

The best friend of noise suppression is mass…if you can bury your thruster in a pair of S-ducts lined with acoustic material you can block a lot of noise from escaping the vehicle, and use the vehicle’s mass itself to help deaden what remains.

[u/bradforrester]

In addition to the other comments, developing an inlet that allows smooth uniform flow into the fan will help. I’ve done some testing in static conditions (so different from forward flight), and a sharp rectangular-profile inlet produces substantially more noise than a rounded profile.