Thread: Moving the DC outside

If you want to get an idea of how much noise is coming from the motor and impeller then just block the inlet, that eliminates most air flow noise. The main noise in the workshop (and you can hear it outside the closed workshop as well) is from the sound of air flowing into the intake ports - and that's with a 150mm bell-mouth intake which should have the least edge flow noise (as opposed to something with sharp non aerodynamic edges) Most noise from the impeller and motor comes from low frequency rumble which can make a cabinet resonate. Intake noise increases as flow rate increases. I'm running a 3HP DC, levels are around 86db with that port open and 64db with it closed, the Router table air intakes are even louder - those readings are from inside the workshop.

Today I reassembled the full DC inside the shed and took some sound measurements, again standing 1m away.

64db background noise (same as yesterday)
85db with the DC on and the door open a few inches
77db with the DC on and the door closed

Interestingly, that's pretty much the same as I had measured with no soundproofing and only the bag/filter in the shed, weird?!

Just to be sure, I split the DC back into its two halves again, and with only the bag/filter in the shed, measured 74db (door propped open) and 69db (door closed), pretty much confirming my previous measurements.

77db with the DC in the shed is too loud and I am running out of space to add much extra weight or baffling on the walls. So it looks like I'll be sticking with keeping the motor and impeller in the garage, and adding a baffle to vent the shed so I don't have to run with the door wedged open.

Those readings are interesting? I assume that you had the intake ducted into the shed when doing the test above.

Here are my readings for a 3HP 4 bag DE mounted in a timber frame (lots of treated pine to cut down panel resonance) Fibre Cement clad housing - no other lining - no exhaust baffle.

Using a standalone dedicated noise meter.

Normal background noise at the moment is around 41dB.

DC cabinet door open @ 1m = 87dB - this door is one full side of the cabinet.

Door closed @ 1m = 62dB.

On the fence line @ 3m from DC cabinet (opposite side to the exhaust ports) is 58dB.

The side of cabinet where the DC exhaust exits @ 5m = 60dB, the fence line on that side of the cabinet is a further 11m away and the DC almost inaudible there.

Some of this noise with the cabinet closed is from the Intake port in the closed Woodwork shed. At 1m from the 6" intake port it is 87dB, the total pipe length to that first port from the DC is 1.8m.

We can run the DE at night and you can't hear it inside our house which is 3m from the DE cabinet. The cabinet is mounted in a long walkway between the WW shed and the house, this is not an ideal location as it concentrates and bounces any noise down that relatively narrow walk way.

Couple of comments.
Fibre cement is a very good noise suppressor - much better than sheet metal.

There is nothing absorbing the sound inside the enclosure.
Suggest a couple of layers old carpet on the floor and one on the ceiling.

A well made baffle should be much better at sound reduction than a crack in a doorway.

Check that the ducting running through the wall of the enclosure is not touching the enclosure wall.
Maybe when you had the DC inside the enclosure it was touching whereas before it might not have been?.
I cut my ducting holes 50 mm oversize and cut foam rings to bridge the gap.

Originally Posted by MandJ

Those readings are interesting? I assume that you had the intake ducted into the shed when doing the test above.

No, I haven't yet modified the intake to 6" ducting so was t able to connect it up for the test. When I did the test with the full DC in the shed, it was with a naked input, and the ducting into the garage was left in place but disconnected.

Originally Posted by BobL

Couple of comments.
Fibre cement is a very good noise suppressor - much better than sheet metal.

There is nothing absorbing the sound inside the enclosure.
Suggest a couple of layers old carpet on the floor and one on the ceiling...

Perhaps I should have used FC as the internal cladding inside the shed rather than MDF?
I will try and find some cheap scrap/surplus carpet as internal lining.

Hi Ian,

i suspect a couple of things that you could try to suppress noise further.

- Try caulking all the gaps, I built a compressor box a while ago, with acoustic foam and plywood. Because i needed to get the compressor in and out of a tight space i built it in a way that i can remove all the walls if needed to move the box at a later stage. When i tested it the first time noise levels was almost identical to what it was just sitting on the floor. Realised there were gaps everywhere caulked everything and remeasured and reduced everything by about 13 db.
- Also depending on how well your mdf fits together when you close the door, might be worth adding a few strips of rubber where they touch, as any vibrations would be transmitted directly to the outside.

Also these are amazing, they killed any vibration my compressor made, obviously might not be as effective with your concrete slab but could reduce by another couple of dbs

https://www.bunnings.com.au/whites-o...-pack_p3961547

Also in terms of sound absorbing, you don't need much just enough to reduce the amount of energy bouncing around and compounding as the waves crash into each other. I suspect even if you stuff each of the corners with old jackets/clothing/socks etc it'll give you a good reduction in noise levels.

Originally Posted by Ian Dobson

No, I haven't yet modified the intake to 6" ducting so was t able to connect it up for the test. When I did the test with the full DC in the shed, it was with a naked input, and the ducting into the garage was left in place but disconnected.

if I read that correctly then there is a problem with your noise readings. With no ducting, you have to block the impeller inlet if you are going to test the noise level with the motor and impeller inside the DC cabinet, otherwise you are simply reading the intake sound level of the air being sucked into the impeller, when you duct the impeller into the shed, then you are actually reading the sound of the complete DC inside the cabinet.

Sorry to go on about this but just to clarify post #54:

You are always going to have intake noise, it will change with the amount of air flow the DE is capable of, the type size and length of ducting and where the duct is within a machine - or not - and the shape of the duct intake.

So measuring that intake noise is a fruitless exercise when trying to see how loud the DC is inside an EXTERNAL cabinet. The intake sound will be contained - for the most part - inside the workshop, some will escape into the external DE cabinet and find it's way out the cabinet vent port, but in my case it's minimal. If you have a lot of low frequency noise from the impeller or resonance in the cabinet that can't be reduced then these and other noises have to be addressed with an exhaust vent baffle. I did't need any - that is another reason I heavily cross braced all interior surfaces, it breaks up internal cabinet reflections and panel resonances. Like I said, I don't use a baffle and I have no lining, but as BobL said, thick fibre cement is pretty dense, the reason I used it, but like most big panels they need to be braced to stop low frequency resonances. Everything was not only glued, but screwed down to within an inch of it's life, then sealed again with silicon sealer but silicon mainly for extra weather proofing more than sound reduction.

Thanks. I believe that blocking the impeller inlet with the whole DC in the shed will fail to take into account the noise from normal airflow through the bags. Currently the air duct into my shed ends at a rectangular transition which interfaces with the bag housing, and there is no straightforward way for me to connect this to the impeller inlet without making irreversible changes to the ducting. I might be able to run a 4" Flexi from the impeller inlet then internally back through the 6" ducting, at least that would remove the majority of air intake noise from the shed measurement.

Then your smaller DC must substantially different than mine, the noise is made at the pick up and then reduces as it travels to the DC, my Lathe port produces 87 DB and travels only 1.8M to the DC intake, it is virtually inaudible from the DC housing, there is more intake noise coming through the walls of the shed than from the DC housing exhaust port

Originally Posted by MandJ

Then your smaller DC must substantially different than mine, the noise is made at the pick up and then reduces as it travels to the DC, my Lathe port produces 87 DB and travels only 1.8M to the DC intake, it is virtually inaudible from the DC housing, there is more intake noise coming through the walls of the shed than from the DC housing exhaust port

My DC impeller must be much noisier than yours.

I just went into the garage and measured the following, at 1m away from the DC motor/impeller, and 1m above the floor. (Note at present the impeller ouput is
ducted through the window to the bag/filter unit in the shed.

1. DC input connected to my TS with 3m of flex hose: 79db

2. DC input disconnected and naked: 85db

3. DC input plugged (with duct tape covering the input flange): 85db

With the input plugged and no air flow through it, there is no inrush noise from the port but the impeller is making more noise than with ducting connected. I expect there would be a lot of turbulence internally as the motor strains to drive air around the impeller loop, and the motor would be subjected to a much higher load resistive torque as a result.

This would very likely be at a different frequency to the inrush noise and your enclosure with the FC sheet cladding may be much better at suppressing it as such.

Those readings are similar to mine - sound does not add up the way you think, adding the intake noise to the impeller/ motor noise will only show a very small increase, yet each can be deafening on there own - what I'm saying is that you can't measure the effectiveness of the DC housing with the complete DE "inside the housing" and measuring DB with the impeller intake port open (inside the cabinet).

Yes there is a difference in DE impeller noise (with the complete DE in the housing) and with the intake open and then blocked - but the way you are measuring it won't show the DE housing reduction correctly until you pipe the impeller intake into the WS - until then you are guessing how much remote intake noise will end up at the cabinet exhaust vent.

However I'll leave it at that and let you build it the way you think will work best, I know how annoying it is when someone (me in this case) harps on, and I'm sure you'll get it sorted one way or the other.

Originally Posted by Ian Dobson

With the input plugged and no air flow through it, there is no inrush noise from the port but the impeller is making more noise than with ducting connected. I expect there would be a lot of turbulence internally as the motor strains to drive air around the impeller loop, and the motor would be subjected to a much higher load resistive torque as a result..

There is actually less load on an impeller motor when there is less or no air going through the impeller. This is demonstrated by the much lower currents drawn by an impeller motor when the impeller is blocked.
What happens is the air remaining in the impeller just goes around and around.
The more air that passes through the impeller then more work the motor has to do.

3m of flex will reduce the air flow substantially and the flex itself will absorb some nose so the SPL measurements in steps 1 - 2 above makes sense.

Measurement 3 is curious as it should be the quietest of the the 3.
It sounds like the motor impeller combo is itself vibrating.
It could be a motor bearing?
It would be interesting to see what the SPL measurements would be if you could dampen the vibe on teh impeller by attaching/hanging/placing some weights on the impeller.