what makes a fan loud?

[ku]

I was just wondering, what part of the fan makes it so loud? Is it the motor? The propellers?

 

[yellowvespa]

Noise is a result of the following:

Blade angle, RPM, blade design (leading edge). The motor is 99.9% silent. I've just switched a spare computer over to a squirrel cage design fan as an experiment. One fan for the entire case and CPU. Got it from a wrecking yard and at low speed it puts out more than any ten case fans but the best part is that it's sooo quiet.

 

[GtPrOjEcTX]

pics of that design?

 

[BreakApart]

Squirrel Cage fan

 

[Wolfdog]

Probably the biggest cause of sound problems is obstruction of airflow. Some fans are just poorly built. Those fans usually illicit a loud whine. Those are mostly the incorrect use of lubricant. They already use a very thin layer to begin with. Air obstruction is probably the biggest one though for a good quality fan.

As a good example take the case that I am using right now. I just bought two enermax variable fan speed fans. One for the front and the other in the back. When they are running outside the case at full rpm they are almost inaudible. They do push quite a bit of air. Once they were placed in the case though, it sounded like a vacume cleaner sucking up a big hair ball. I was able to turn down thier rpm's in the short term, but some case modding must be done to get it up to snuff. I have them running at 2200 rpm, and they are still noisy. Instead of using a fan grill in the front and the back, they just cut round holes in the case itself. The holes themselves inhibit the fan from doing its job, and run up the case noise. I feel the need to bring out the cordless drill and make those holes bigger. The larger holes will improve air flow, and lower the ambient noise from the air being pulled into the case.

 

[etalns]

Soemtimes if you install the fan wrong and chip off part of one fan you're gonna get nasty amonuts of noise.

 

[FlowerMan]

Yup, airflow and turbulence make fan noise. The motor is often times negligible in terms of noise.

 

[ku]

ok, so it's the turbulence that causes much of the noise right? so why do same size that exert the same amount of CFM and yet produce different dBAs?

 

[anime]

the blade contour design and angle affect that--because all of that affecting the turbulence hence affect the noise (sometimes even the smoothness of the blade surface also a factor)
Ever wonder why US military so edgy on keeping the super silent submarine propeler a secret?

 

[Jhhnn]

The vast majority of the noise is created by air turbulence in most instances, either by the blades or by obstruction. Some noise is generated by the bearings, particularly ball bearings, and some is generated by the rpm sensing circuitry in some fans. Impeller balance and materials also factor in. All this noise can be amplified by the case or heatsink acting as a sounding board, similar to the way stringed instruments work.

Panaflos, for example, use excellent blade design&balance, quiet high-tech sleeve bearings and no rpm sensing circuitry to achieve their results....

Some manufacturers obviously fudge the figures to tell you what they think you want to hear, or actually what you don't want to hear.... and total noise doesn't tell the whole story, frequency and waveform shape are important in determining the annoyance factor.