Individual results will vary. I expect among other characteristics, that the shape of the impeller blades might also affect results (e.g. "s"-shaped v. straight blades). I would also expect the impact of the mod on specially designed "quiet" fans to be less obvious.
I had a bit of spare time this weekend, so thought I'd report on a simple experiment I tried.
In an experiment to reduce fan noise, I "took-off" all the sharp edges on each of the support arms of some case fans. These particular fans came from the factory with a non-round/non-oval cross-section.
Admittedly, this is a very limited test, but to my ears, the mod seems to have made an obvious reduction in the noise produced by several fans, especially when they are running at high speed (audibly comparing modified v. unmodified fans running at same RPMs).
Before the mod, my thinking was that sharp edged impeller support arms would likely create more turbulence from the edge-airflow boundary from the fairly close blades...and hence produce a bit more noise. It seems I was right.
I did my testing on some dirt-cheap 120mm Ultra case fans (~$10 on sale) and on some 120mm Cougar case fans ~$12 on sale). I haven't yet tried a mod on a CPU fan which spins at even higher max RPMs (~3500cpu v. ~2000case fan). The Cougars started out much quieter than the Ultra fans to begin with, but I could still tell the difference between a modified Cougar and an unmodified Cougar.
A modified 120mm Ultra was about as quiet as an unmodified 120mm Rosewill at ~1800RPM.
The 120mm Rosewill case fans that I have came with round cross-section support arms, so no mod was possible. The Cougar fans came with a sort-of trapezoidal cross-section support arm, but they still had sharp edges...so I rounded those off. I think one doesn't have to take much when rounding-off the sharp edge...I probably took off more plastic than needed to get the optimum reduction.
You may or may not think it worth it, but I've one system with 7 fans and I'd think every little bit of noise reduction helps. (although, in that system I use mostly unmodified Cooler-Master Excalibur "barometric ball bearing" fans (which I believe are about the quietest 2000RPM ~86cfm "standard" 120mm case fans I've found ~$13 when on sale. They have quite decent output as well.).
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Nice aerodynamic experiment, John. Reduction in turbulence reduce noise, which proved true in your experiment. You should bring it to the next level for the experiment. The new Boeing 787 uses Chevrons for the outlets of its turbofan jet engines. Those are sawtooth patterns which reduce turbulence, and hence reduce noise.
That does sound like a good experiment. I wonder if anyone has done similar in the R/C turbofan/ducted-fan aircraft community. That would be a good indicator of how well it scales down.
Wish I had an audio frequency analyzer. I'd like to see the effects at different RPMs.
I'd think one might also be able to design a set of acoustic baffles, similar to a gun "silencer" that also might be effective but not create too much back-pressure. Something like inside an anechoic chamber, or radar stealth shapes, perhaps for multiple frequencies. Might even be useful for noise reduction at fan intake ports as well (e.g. side case fans, most generic single-fan CPU fans/coolers).
That does sound like a good experiment. I wonder if anyone has done similar in the R/C turbofan/ducted-fan aircraft community. That would be a good indicator of how well it scales down...
Since you mentioned RC planes, I am diving in the topic a bit.
First, I doubt (purely guessing) anyone would use chevrons in an RC plane, because noise is the last thing they worry about, especially when there's no occupant in a model plane. I think flyability or airworthiness is the primary concern of any RC plane enthusiast.
Also, any RC plane that uses true turbojet engine(s), is not a toy for the amateurs, but a miniature airplane for professionals. There are some pretty amazing planes they did, which you can easily find in youtube. Here's an RC A380 that's basically a scale down model of the real thing. http://www.youtube.com/watch?v=-Rt9zX1rZFU Completely blows my mind how well it sounds and flies!
Yeah...those scaled-down models are incredible. It's really horrible when they crash them, too (youtube also has vids of those).
Mostly I just putter around with "foamies". Once you have the engine, prop, and electronics, you can make one in 1/2 an hour from scraps of 1/8" foamboard...in just about any shape you want. There's even video of a flying lawnmower out there. Who needs good aerodynamics when you've got massive raw thrust?
Personally I believe a LOT of "UFO"s that are on YouTube (at least the ones that aren't "photoshopped") are "foamies" with LEDs.
I design and make my own engines and build my own engine control electronics from scratch (except the receiver and transmitters). I've made engines out by modifying old CD motors (rewire and add neodymium magnets and they're incredibly powerful and dangerous with a propellor on them). I've used parts from old motherboards (e.g. the MOSFETs and other 'stuff') to build the engine controllers, battery rechargers, and so on. Motherboard and PSU surface-mount MOSFETs, capacitors, resistors, and more are perfect for this type of thing. It really brings down the cost of the hobby. Some of my online circuit designs are used by others in the hobby.
Damn, didn't know you're an expert on RC too! What didn't you know, John?
Here's an idea for an RC plane engine, at least good for the foamies. Take a foam cup, cut the bottom out, then cut some sawtooth pattern at the narrow end. That's your engine enclosure / duct. Next fit a fan inside the foam duct, with exhaust toward the narrow sawtooth end. I don't think a regular PC case fan is sufficient for this application, probably need something with higher RPM / CFM, and with a round frame if possible. Make a small hole for the wiring if needed, make sure to seal it afterward so that air has only one way in and one way out. Repeat the steps above if you need two or more engines.
There goes your cheap duct fan engine. Just attach it / them to the plane and connect the wires for electricity. Though you probably need a pretty big plane to scale up to a foam cup engine. Make yourself a jumbo foam jet with 4 of those will be pretty cool. May be I will make one just for fun if I have some free time for nothing else.