fan noise versus speed - Page 1
Forum section: electronics / general

author: Simon
author_rank: Unknown
timestamp: September 08, 2019, 10:16:43 am
content: while it is easy enough to find information on fan laws and the power required to drive same fan at different speeds etc I am struggling to find any similar universal laws for the noise of a fan. Do such laws exist or is it a perculiar characteristic of any particular fan?

author: Rerouter
author_rank: Super Contributor
timestamp: September 08, 2019, 10:29:59 am
content: this is the closes to what your asking that I'm aware of, at the 9:00 markits not quite linear, but probably could be modeled as a 2 slope curve,I would imagine its a function of blade velocity, higher speed = higher drag = more audible noise from the vortexes

author: Simon
author_rank: Unknown
timestamp: September 08, 2019, 10:32:51 am
content: I see. Well i am writing a report for a 1st year uni course so i need to state an accepted physical law or take my own measurements in the middle of a car park.

author: MosherIV
author_rank: Super Contributor
timestamp: September 08, 2019, 10:33:36 am
content: Most fans have a dB specification.One problem with a single universal equation for fan noise is that it is highly dependant on the application. Eg enclosure inlet - is it small round holes, slats, is there a foam filter/muffler.

author: Simon
author_rank: Unknown
timestamp: September 08, 2019, 10:36:57 am
content: Quote from: MosherIV on September 08, 2019, 10:33:36 amThe fan in question is 85.6dB at 4'750rpm, but they affer no other noise information so it sounds like i need to benchmark it myself. Yes the application will make a difference but i need to abstractly show the advantages of runnig a fan at a lower speed rather than on/off. Clearly 85 dB turning on and off is much less pleasant than running at 50% speed. Subjectively i know which i prefer but I need numbers for my report.

author: Rerouter
author_rank: Super Contributor
timestamp: September 08, 2019, 10:40:09 am
content: What I mean is its a starting point for working out what exactly is happening, and then describing it,As the fan speed increases, you have a higher airflow, so turbulent noise increases against the shroud,As the blade speed increases you increase the frequency of any oscillating vortexes,As the rotation rate increases the noise from the loading of the motor increases,And the effective viscosity of air increases as you try and move it faster through a restriction, as its turbulents start to get in the way of the moving air streamFor the exact specifics, your going to need to sit down and try and start fitting this into a general model of how it changes as you alter parameters, e.g. blade pitch increases airflow vs RPM, the number of blades decreases efficiency, so resulting in more drag, longer blades give more drag but in a non linear fashion, the width of the motor dome is like accelerating that shape through the air, it has drag based on velocity,Etc, etc, etc, put down all the effects you can think that may cause a fan to be noisy, then start isolating how that changes vs rpm.

author: Simon
author_rank: Unknown
timestamp: September 08, 2019, 10:43:15 am
content: i am not designing the fan. Just demonstrating that running an off the shelf fan slower makes less noise. if there is no universal law then i will have to measure an actual fan and use those figures in my report.

author: coppice
author_rank: Super Contributor
timestamp: September 08, 2019, 10:44:47 am
content: When looking at fan noise in terms of its annoyance, don't be fooled by simple power figures. A fairly loud broadband noise like whoosh is usually less annoying that a more structured noise of the same power level. As you increase the speed of a fan you not only increase the power level of the noise, you also change is character, as things like blade vibration and turbulence come into play. This is probably why many fans seem to abruptly get more annoying above a certain speed, as some vibration mode kicks in around that speed.

author: Rerouter
author_rank: Super Contributor
timestamp: September 08, 2019, 10:49:57 am
content: well that makes things much easier, noise sums are fractional, so using the charts in that youtube clip,1 fan running at 100% has 44.1db of noiseto match the same airflow would take 2 fans running at a 50% duty cycle2 fans running at 50% sum to 30.8db of noisehttp://www.sengpielaudio.com/calculator-spl.htmThus a fan running at 50% speed is quieter on average than a fan at 100% with a 50% on/off cycle.Either reference that data, or similar data, repeat it yourself, or find it in a datasheet,Seems easy enough?

author: mc172
author_rank: Frequent Contributor
timestamp: September 08, 2019, 10:56:15 am
content: Quote from: Simon on September 08, 2019, 10:43:15 amWhilst it may be true for a fan mounted in free space, that a fan running at a lower speed produces less noise, in a useful system it may actually make more noise. This is because fan noise is largely a function of efficiency.On a fan curve, which is unique to the inlet and exhaust nozzle geometry and area of that particular system, there is only one point, and therefore one speed, at which the fan achieves maximum efficiency. If you are sitting right at this point, reducing the speed will likely increase noise.Running fans in free space has no real world value.This might help you:https://www.mne.psu.edu/lamancusa/me458/11_fan.pdf

author: Simon
author_rank: Unknown
timestamp: September 08, 2019, 11:08:13 am
content: Quote from: mc172 on September 08, 2019, 10:56:15 amthanks, I know but I am not analysing an instalation. I and simply arguing in my report for speed control and mentioning lower noise. I know that I am oversimplifying the noise issue but it's not the central set peice. I would have to attach the fan to something to stop it taking off so my standat pallet will act as the in situ thing that will disrupt straight airflow.

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 08, 2019, 11:56:18 pm
content: Proper noise measurements are made in an anechoic chamber, with the sound level meter at 1m distance from the source.You'll want to readAnatomy of the Silent Fanatsilentpcreview.com. They also have some (old but good) data on the sound level, volume, and RPM relationship on various fans (Nexus D12SL-12,Noiseblocker M12,Scythe Gentle Typhoons(were my favourite for filter fans), on the 2012 batch, for example). Lots of fan and noise stuff there, although the data is basically limited to under 25 dBA (as anything above that is too noisy to consider "silent" in any way).Even noise level alone isn't sufficient, as thespectrumof the sound generated matters a lot to us humans. A wide-spectrum hum, like white noise, is much preferable at the same sound levels as a high-pitched whine (or with any spectral spikes).On any given fan, increasing the fan speed will increase the noise level. However, the amount of air moved, and the amount of pressure differential a given fan can create, depends on both the RPM and the fan shape. So, you definitely cannot make claims like "a 800 RPM fan is always quieter than a 1200 RPM one", because it depends on the fan sizes and blade shapes. Similarly for RPM and airflow, or pressure differential.I have a ceiling fan that at roughly 70 RPM (guesstimate) is absolutely silent except for a very slight wide-spectrum noise generated by the blades, but pushes enough air to make a big difference to me on hot days. Then again, it has five blades and a diameter of 1.3m.I used to build "silent"/quiet computers, and experimented a lot with PC case fans. The one rule of thumb I trust, is to use the largest diameter fan you can, if you wish to reduce the noise generated. Everything else varies from manufacturer to manufacturer, blade design, and even fan to fan.

author: Red Squirrel
author_rank: Super Contributor
timestamp: September 09, 2019, 12:08:52 am
content: There are various noises a fan produces as well so worth considering each one. The actual airflow noise, the mechanical noise, ex: ball bearings or other moving parts, and electrical noise, which may or may not be audible and will probably depend a lot on speed. These noises may also resonate inside the case or whatever it is attached to.Could make an interesting paper to break down each of these noises separately to see how they change based on voltage given to the fan. Or even compare different ways of controlling speed. Unde volting it, vs using the pwm pin on a fan that has that feature. That would probably affect mostly the electrical noise.

author: mariush
author_rank: Super Contributor
timestamp: September 09, 2019, 12:19:39 am
content: The number of fins and the shape of the fins will affect the amount of noise. There's fans optimized for static pressure and fans optimized for pushing air around - these can behave differently as you lower the voltageThickness (15mm vs 25mm vs 38mm etc) will affect noise..If the fan has ball bearings or other types will also affect noise.Some ball bearing fans can be more silent at higher speeds, and could make a sort of rumble at lower rpms.Some PWM fans may whine if the voltage gets too low...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 09, 2019, 02:49:49 am
content: Quote from: Simon on September 08, 2019, 10:32:51 amStrong recommendation:Pick a different subject.There is not anything "accepted" or "law" about a fan, directly.While the fundamentals they work on are well understood, the minutiae of their construction and operation are at best something that can only be simulated.Both fluid dynamics and mechanics are notoriously unyielding subjects. The systems of equations are very nonlinear, often prone to chaos at the slightest touch of a feather (literally: a falling feather creates turbulent air currents). There is nothing in the macro scale, turbulent flow regime of a fan, which needs to be consistent or sensical.You might however turn it around, and illustrate all the many ways in which something so trivial and simple, is actually immensely complex -- this would rather drift away from the simple concept of "state a physical law" however.You could drill down into the many laws underpinning a fan, though finding a level that's both accessible enough (for your audience and yourself!), may still be challenging. Not to mention, interesting enough for a report; not that that's a big hurdle to clear.On the subject of fan noise -- perhaps concentrating on the fan itself wouldn't be so wise, but taking it instead as a given, you could discuss, for example, how the sound intensity (maybe frequency response too?) varies with distance, and with surroundings (reflections, absorptive materials), maybe how this affects the design of some machines--ah, but then you need to find sources discussing it, which you probably can, but they're just going to be superficial articles, not real design analyses with materials and measurements. Well, for a 1st year report that is probably still fine, you're more demonstrating that you can synthesizeareport, following the general rhetorical form and avoiding grammatical and logical gaffes, more than producing quantitatively useful analysis. Basically, your refs might be shite, but as long as your reporting of them is right, it should grade okay.Tim

author: Rerouter
author_rank: Super Contributor
timestamp: September 09, 2019, 06:38:06 am
content: I'm going to point at:QuoteWhen its a pain to simulate, just build and measure

author: Simon
author_rank: Unknown
timestamp: September 09, 2019, 06:42:57 am
content: As I said the report is not about the noise. It's just something I can toss in there to make the project sound even better, so I will not dwell on it much.

author: BravoV
author_rank: Super Contributor
timestamp: September 09, 2019, 06:46:02 am
content: Quote from: Simon on September 08, 2019, 10:16:43 amHow about this so called "Fan Laws" published by Comair Rotron ?->Establishing Cooling Requirements: Air Flow vs Pressure, scroll down to the formulas near bottom.

author: Simon
author_rank: Unknown
timestamp: September 09, 2019, 07:09:54 am
content: I have come across the flow, pressure and power laws and that is the basis of my project being an improvement for the purposes of the report. But noise, that is the first time I see it stated. But as others say is this a universal?

author: BravoV
author_rank: Super Contributor
timestamp: September 09, 2019, 07:16:00 am
content: Quote from: Simon on September 09, 2019, 07:09:54 amNot sure, as its above my paygrade.But looking at the source who published it, looks pretty convincing imo, also read carefully as it says .. it is useful for "predicting" the result, I guess that is good enough as this fluid dynamics and mechanical science as Tim said above, is really dizzying to digest, especially for avg. joe like me.

author: Simon
author_rank: Unknown
timestamp: September 09, 2019, 07:20:51 am
content: Yes it is just theoretical. Usually you use them to estimate small changes on a particular fan. I doubt they take into account the hub. I guess the ideal fan has a 0 size hub. I have noticed that the larger hub fans tend to make more pressure because all of the blade is at maximum lineal velocity.

author: BravoV
author_rank: Super Contributor
timestamp: September 09, 2019, 07:26:59 am
content: Quote from: Simon on September 09, 2019, 07:20:51 amDefinitely, and more like academic stuffs.Anyway, I guess this will be a good starting point in your quest then, hope it helps.

author: IanB
author_rank: Super Contributor
timestamp: September 09, 2019, 07:28:40 am
content: Hi Simon, there will be no universal "law" regarding noise vs speed, but as a first approximation you might assume that for any given fan the amount of noise produced is proportional to the operating power (noise is a function of power dissipation in turbulent air). Since the affinity laws say that power is proportional to the cube of the speed, it follows that the emitted noise might also be assumed to vary with the cube of the speed. Therefore small increases in speed produce big increases in noise.What you can deduce from this is that using speed control on fans is a really good way to reduce the noise level. For instance, on my typical laptop if the fan is running slowly it is nearly silent, but if it runs up to full speed it sounds like a jet engine. As a result it is best to use temperature control and run the fan just fast enough to provide enough cooling and no faster.If you need hard data or evidence for your report I am not sure where you can find it. Your best bet would be to seek out manufacturer data sheets for specific fans which might give some indication of noise vs speed in a typical installation.

author: IanB
author_rank: Super Contributor
timestamp: September 09, 2019, 07:32:09 am
content: By the way, noise is commonly expressed in decibels which is a logarithmic scale. That is why the log appears in the table above. So when I say the noise might be proportional to the cube of the speed, that means the absolute sound pressure level, not the decibel measure. I didn't think of that when I was writing, so I thought I should clarify it here.

author: Simon
author_rank: Unknown
timestamp: September 09, 2019, 10:16:07 am
content: Well as it is I am having to be a bit theoretical. I can't estimate the savings easily as no one can tell what the usage profile of the vehicle will be. So I am just taking the example of the fan only being used at half it's rated speed instead of bang bang control 50% on 50% off. On my 850W fan I can demonstrate a saving of nearly £8000 over the 40'000 hour life of the fan

author: Rerouter
author_rank: Super Contributor
timestamp: September 09, 2019, 10:22:15 am
content: You can define a limit, at what duty cycle does the cost savings reverse, that would be a solid counter argument followup I would think. e.g. at 0.1% bang-bang, how does it compare when you look at similar energy consumption vs the equivalent PWM, If you really wanted to stick the point you could even graph the net gain in airflow vs power consumption with duty cycle control vs bang-bang, as that spin up energy is lost every time in bang-bang

author: Simon
author_rank: Unknown
timestamp: September 09, 2019, 11:26:54 am
content: Yea, a brushless speed controllable fan is already cheaper in volume than a brushed one when you consider the per hour cost (40'000 hours versus 10'000) so the controller costs under £100, so any speed regulation will save money but yes your idea is good I may do that.