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An assessment of fan setup, turbulance and vibration in duct noise reduction.


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An assessment of my fan setup, with turbulence, vibration and general noise reduction.

Written by wufo.

Id like to start off by saying these are my personal views.....don't take anything by fact, but merely suggestions and info.

If anyone would like to add to this then be my guest the more info in one place on things like these the better, but saying that most of this info is out there already but for people like me this is a starting point to pinpoint your problem if you have any.

I'll tell you a little bit about my inline fan and the problems I've come across.
I'm currently running an S and P TD silent range 500 so that's 500cfm for those that cant work it out by the name.

The duct size is 150/160.
It cost me about £150 including VAT, in all it's a quiet fan; the write up on it says its 17dB on its lowest speed, it comes in two specs,

1. Like mine a two speed depending on what way you wire it up, low speed 17dB at 430 cfm and 22dB at 570 cfm.

2. One speed, but you can add a speed controller to it at 22dB at 580 cfm.

But it doesn't really matter what fan you have if you're hitting walls like me because the fan runs at 17dB that doesn't mean that's all your gonna hear.

I started off with 5 inch aluminium ducting for my six inch fan....I know... dick head, but I bought my ducting before my fan, then decided to go for the next fan up, so i just thought id put a slit in the duct then put it over my fan and tape it all together. I thought it was ok but my fan was abit loud, so I made a silencer....then another for the other side, then my mate gave me about 1 meter of six inch duct so I used a piece on either side of my silencers to bring them to the same size as my fans.....they work.......well maybe at quieting the ducts, but the parts that attach to either side
of the fans where noisy as shit. And no matter how many blankets if wrapped around i couldn't deaden the sound.
I was making everything myself and making every mistake possible, I didn't do any research so I thought I was doing it right......wrong...dickhead again.

So some of the info i give is from my own mistakes for running before I can walk.

*speed controls work on fans to lower the speed of the fan in turn this lowers noise but it doesn't cure the turbulence it just creates slower air, and slower air follows the contours better and moves more fluidly over bumps and imperfections making it sound quieter, but turn the speed up and it starts to vibrate....just think of speed bumps in the road, if you go over them slow your ok but try drive over one at 50 mph and you know about it.

*A muffler box works well, but get rid of the air turbulence and vibrations and it works even better, I've seen so many pics of home made ones and they all look different but the same principles apply, just put your fan in a box packed with sound
Deadening material, whether its polystyrene balls foam wotsits or expandable foam, they all work the same. Then seal all the gaps so its practically air tight.

*Even if it is just the intake and outtake that is noisy, the problem started way before the fan, go back to the filter.
So your filter is 6 inch, don't do what I did and get smaller duct it increases velocity, we are trying to lower it.

*From your filter run your ducting stretched out tight....remember air likes to bounce off the ridges inside and create turbulence, don't give it an excuse. If you're using steel duct that's even better because there's less friction on the smooth surface inside.

*There is insulated ducting and acoustic ducting, make sure you buy the right one, insulated ducting will do naff all.

*try not to use square or rectangular ducting unless you are buying all the correct reducers but even then round ones are way better they have a smaller pressure drop per unit area and are the most cost effective and provides maximum air-carrying capacity with minimum pressure loss. The low frequency sound is well contained in a round duct, the flat sections of a square duct behave like a drum easily transmitting low frequency duct rumble.

*A straight duct for a distance of 3 to 6 duct diameters from the fan discharge should be used in order to develop a full dynamic head, turning sooner causes system effect losses. And putting the same before the fan maintains a smooth air flow into the fan.

*lower duct pressure drop reduces fan speed, which also reduces fan noise.

*A fan discharge diffuser should be used before air is released into the atmosphere.

*Always use reducers, they reduce turbulence a lot, it's all to do with the shape. After reading around I've come across info that says if your using 6 inch(eg) then you want a reducer that is twice as long as the diameter tapering down to the next diameter 5 inch(eg) at 15 to 60 degrees per side, so it will be 12 inch long. Don't buy one that goes straight down from 6 inch to 5 inch with a sharp square...that creates a lot of friction/turbulence, even the ones with a gradual angle aren't that good because the angle taper is only about 1 to 2 inch, for a real nice air flow use the info above as changes in duct diameter should be gradual to reduce friction and turbulence.

(I've made one out of empty 2L pop bottles to try, its a brilliant free materiel to use as a test piece before you go out spending money you might end up wasting and with the right strength why not use them. By the way empty pop bottles are my second best thing ever after baby wipes, I can find a million uses for them.)

*Don't have 90 degree elbow bends in the ducting try to curve it nice and big as possible, at least twice the radius as the duct diameter.

* Elbow entrance should be at least two times the inlet diameter distance from the flex connector at the fan...the fan entrance is a high velocity duct.

*The first elbow in the duct leaving the unit should be no closer than two feet from the unit to maximise resistance and noise.

*at the end of the duct at its final stage of the air leaving try a larger expansion than the duct, this lowers velocity pressure.

*Attenuators: devices for reducing the amplitude of a source of energy.

A silencer/muffler, home made or bought are all the same a 6 inch (eg) mesh pipe with acoustic material wrapped around about an inch and a half thick to deaden the sound then sealed in with another clip close pipe on the outside. Most of the shop bought ones now have no mesh inside the acoustic reduction material like they use in recording studios is glued inside the external duct, its more effective.

*A velocity reduction box.....or what ever they are called.
Say they go from 6 inch duct(eg) always double the diameter they open up to so in this case it would be 12 inch, using our 15 degree gradual size increase, we go from 6 to 12 inch then triple the duct diameter so its 3 feet long then cone back down to 6 inch, this slows the air velocity but keeps the air speed the same reducing turbulence later on.

*Later down your ducting you can then increase the ducting size (velocity reduction box) before it enters the fan.

(I'm going to try this directly on the fan and with another piece of duct about 3 feet long in-between velocity reducer and fan, I'm also going to try one on the outlet side too, before the silencer and then again after the silencer to see which is better.)

* hang the fan if possible this stops the vibration and the sound travelling in all directions.

I've also seen 40 degree diffuser cones with loads of small holes in that are put inside ducting to break the air up before it hits the fan I suppose one could be put on both sides to break the air as its going out too......this is the next thing I'm going to make.

I've also seen a sort of bowl inside a duct with small holes in and a bigger hole (1 inch eg) to direct air flow down the vent.

I'm on a mission to get my fan as quiet as possible, so I'm going to be trying a lot of these suggestions, if anybody actually reads this and finds it useful or would like to try any, please let me know how you got on.

Please...please do give negative responses, as I said before don't take all this as fact but suggestions.

Thanks a lot.....Wufo.
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