anti-vibration mount for extractor fan

[sm_jamieson]

Hi, I'm mounting a bathroom extractor in-line fan in the loft, on the ceiling joists. Any idea for a DIY anti-vibration mount to minimise the noise conducted to the ceiling ? Cheers, Simon.

[Grunff]

How about this type of thing:

[sm_jamieson]

thing:

> --

Maplin also have them, so I should be able to pick them up without yet another online order and delivery ! Simon.

[Andy Hall]

kind of things are good. But.......

It is very important to select the right type for a) the mass of the object and b) the rotational speed.

There is a range for any given mount, but the mount needs to be substantial enough to support the weight of the item but not so substantial that it is too firm. In those cases, it won't dampen the vibration.

[sm_jamieson]

It's one of these:

weight given, I'd have to weigh it, but it is fairly light plastic. Simon.

[Andy Hall]

Then you need really light weight mounts or they won't do very much.

I have a larger Vent Axia unit that I use to ventilate my servers and networking equipment.

It's very well balanced, but I addressed the remaining vibration by taking a small paving slab and putting a rubber mat on that and just laying the fan on it. Works well.

[sm_jamieson]

OK, I'll do something like that - a slab, some type of lightweight foam, and strap the fan down using some kind of rubber straps, so no screws have to pass through. I may have to mount the fan itself on another board, since it's footprint is very small. Hey, I could put this sandwich in a shallow box with foam under and around the top plate. Simon.

[Andy Hall]

Really all that you need to do is to prevent vibrations being transmitted to the woodwork.

Doing this puts in place a small mass (the fan), a poor means of coupling (the foam) and a large mass to try and move. The large mass is not going to move a lot.

You could try gluing the mounting board to the foam and foam to block using construction adhesive etc.

[fred]

In article , sm_jamieson writes

Dangling from a couple of pieces of bungee cord would be a simple option.

[Andrew Gabriel]

If it wasn't so well balanced, a part-inflated bicycle inner tube replacing the rubber mat works extremely well (with a suitable mounting surface resting on top).

[Harry Bloomfield]

sm_jamieson submitted this idea :

Could you not suspend its weight from the roof timbers, then use some very flexible duct down to the ceiling to prevent noise transmission?

[Lobster]

Isn't there also an issue with having too much of an anti-vibration mount (eg, something soft and inflated with air) in that if a device which is intended to be mounted firmly ends up essentially hanging freely unsupported in space, it will ultimately shake itself to bits?

David

[Andy Hall]

kind

That depends on what it is an how it's balanced. If it's well designed, there shouldn't really be much if any vibration.

For example, I have my air compressor on an arrangement of four antivibration mounts. This is because it sits in the loft of my workshop and I don't want it transmitting noise and vibration to the timber frame and wooden floor.

The compressor is belt driven by a motor and has a cylinder with piston (obviously). Overall, the weight is about 80kg. I knew the speed of the motor and of the compressor, so the natural resonant frequency can be determined. Most of the vibration comes at the speed of the compressor rather than the motor since the rotating parts have greater mass and although well balanced, the forces associated with air compression inevitably cause vibration.

What then has to be done is to determine a set of mounts that will take the weight between them (i.e. 25% each here) and be useful at the frequency involved. The design of the mount and material used influence this.

practice, there is very little transmission of noise and vibration at all. The compressor does heave slightly on the mountings as it starts, but this is to be expected.

I used the same technique for mounting a dust extractor. This is a large cyclone machine weighing about 80kg and having an impellor fan of about 400mm diameter and 100mm height, driven by a 2kW motor. However, it is very carefully balanced indeed and there is little vibration when running. There's a small amount of initial kick when the motor is started, but that's about it.

The whole point is that the mounts must be chosen to support the item adequately while at the same time not being too firm and hence an ineffective dampener.

It's even better to select a fan etc. that is well balanced and which doesn't need much dampening in the first place. If it's that bad, it will shake itself to bits anyway.

[Andy Hall]

I've tried that in the past. Roof timbers can make a great musical instrument if vibration is coupled at the right place.

[Harry Bloomfield]

Andy Hall presented the following explanation :

Not tried it, but my thinking was that the roof timbers would have much more weight on them and be less likely to vibrate, as well as having the physical separation.

[Andy Hall]

I see what you're saying.

I think if it were suspended it would be reasonable, mounted and there does seem to be noise transmission, despite heavy tiles above.

[The Natural Philosopher]

Either use rubber grommets, or mount to a piece of wood and use a rubber block (or glue with silicone?) to isolate that from the structure. Adding lead round the fan shroud will help too.

[sm_jamieson]

Thanks for all the ideas folks, I'll try a few and see what works. Simon.

[Andrew Gabriel]

Come back with marks out of 10...

[Ian_m]

I bolted together about 10 pieces of leftover carpet (30cmx20cm ?) and bolted the in-line fan to the top layer of carpet. Bolted the whole lot to battens mounted on the ceiling battens. Worked fine and quite.