Folded plenum to tame cyclone noise?

[Mike Desch]

Here we go again--another cyclone question.

Since there has been some excellent advice on SMC regarding what many of you have done with taming the noise of a cyclone, I thought I would throw this out for discussion.

Somewhere recently I read a thread about using a folded plenum on the return air to the shop. reminded me of some old work-related issues that my feeble mind had already forgotten.

Should the folded plenum have only one 180 degree turn or should there be at least two 180 degree bends to mostly eliminate the sound path?

Should the plenum insides be lined with something to reduce noise?
Maybe old pile carpet (cheaper than "titty foam" or the expensive real foam stuff).

What has worked for those of you who have gone this route?

I was thinking of using a return air plenum that was about 5 times larger than the size of the inlet air (to reduce the air velocity back into the shop).

Ok guys and gals, come forth and expound on your experiences and preferences ...

[Brad Gobble]

from what I can tell from my clearvue the sound comes from the impeller and radiates out through the housing around it.

My own plenum is 6" flexible ducting wrapped in insulation then enclosed in 3/4" mdf. Still noisy as hell. If you could wrap / attach something like a blanket directly to the cyclone and the impeller housing you might have a chance.

I built an insulated closet around mine.

[Bartee Lamar]

I built my return plenum inside a wall of the enclosure.

Here is link to ALL the pics (https://picasaweb.google.com/bartee/CycloneEnclosureRoom#)

This is how the majority of the air be routed out of the enclosure. I will be putting up 1/2" ply on the outside of the wall. This will extend up to the ceiling on the top. I am going to leave a 2' opening at the bottom of the wall for the air to escape. I am really not sure this is necessary since I will have MANY air leaks that will just have to remain.

https://lh3.googleusercontent.com/_oHfKrHmaVuo/SLm5M0_WAEI/AAAAAAAACho/BBX6-K6tU4Q/s800/IMG_2728.jpg

This is the exhaust for the enclosure. This allowed me to baffle the noise. Just a note, different materials will kill different frequency of sound. so I used MDF on one side of the wall and plywood on the other. ( It is also what I happened to have around)

https://lh5.googleusercontent.com/_oHfKrHmaVuo/SM2eGun1huI/AAAAAAAACts/bB1wgt9egIs/s800/IMG_2792.JPG

[Dan Friedrichs]

Very nice looking installation, Bartee. I saw in the album that you measured the sound level before - did you take a measurement after building the closet?

[Bartee Lamar]

After building closet and doing everything db level is about 70. It is much more low end noise which is not bothersome. I really do not realize it is running after a while.

As you can see I tend to over engineer everything, but this time I think it really paid off.

The amazing thing is this is directly under the master bedroom. My wife takes naps while I work !!!

My cyclone DC has been one of the best investments in my shop.

[Mike Desch]

Thanks to all who have replied.
I already have the cyclone in an enclosed closet, but not sure I did the right "construction."
I put the beast in a 36x42" closet framed with 2x4s. used 1/2 inch drywall both inside and outside including the ceiling, but then added 1/2" sound board to the inside.

From some of the posts I have read here and on another Forum, I'm not sure that I should have added the inside drywall or the soundboard.

Thanks for your comments?

[Doug Shepard]

Not a folded plenum but I just went with a 6" thick filter through the closet wall with my ClearVue and dont have any noise concerns. I really hear more air rushing through the ducts than noise coming from the cyclone.
187475187476

[Ken Deckelman]

Jim Becker posted a great picture of this in post #18 here: http://www.sawmillcreek.org/showthread.php?161929-Another-DC-Closet-Doubting-Myself-Again

I also asked a question about if it was OK to use the 1/2" foam board I already had to make it but I never got an answer:(

[Phil Maddox]

The more turns the better for sound - 2 is usually plenty. Line the plenum with rigid insulation (fiberglass duct board, not foam) to REALLY cut down on the noise. Carpet will help to some degree but not as much as the insulation. My SDG is a low pitched sound which is hard to control but not as irritating. Is your noise high pitched? Is it mostly air related noise?

PM

[Alan Schaffter]

Not to be a downer, but I hope everyone realizes that the more ducting, fittings, filters, plenums, bends, sound absorbing material, etc., etc. no matter where, end-to-end, in a DC system the more static pressure resistance. The higher the static pressure resistance, the lower the CFM. The lower CFM, the less dust you collect at the source. Its just physics.

[Phil Maddox]

If the return is kept at 5x intake as Mike stated - that would mean a 16x16 opening or an 18" round duct. Should be short enough to not cause a lot of loss. Lots of other areas causing more problems that this will.

[Mike Desch]

Hi Al:

Maybe I misread your post and maybe you misread mine. I am talking about the "return" air after the cycle is through sucking it all in. The only fittings are the return air plenum from the DC closet thru a heavy door--no piping at all.

- - - - -

Phil:

Thank for the double check on the "turns" in the plenum, and especially the advice to use rigid insulation!

[Alan Schaffter]

Hi Al:

Maybe I misread your post and maybe you misread mine. I am talking about the "return" air after the cycle is through sucking it all in. The only fittings are the return air plenum from the DC closet thru a heavy door--no piping at all.


It doesn't matter- EVERYTHING after the blower just like everything before the blower MUST be considered part of an end-to-end system! The "return" air is still in the system and the cycle is not through sucking, until the air exhausts into the room at ambient pressure. In a simple (most extreme) example - if you cover the exhaust, suction stops as well. Every thing you add (pipe, plenums, filters, etc.) to the outlet side of the blower also affects CFM at the intake- CFM IS/MUST be (physics) the same at the inlet as it is at the outlet.

[Dan Hintz]

Alan,

In principle I agree, but we're talking diminishing returns here (no pun intended). If my return is 4" in diameter, then I have a huge amount of restriction. If it's 6", I'm much better. 8", a bit better, but the increase in efficiency from 6" is getting somewhat small. The move from 8" to 10" is going to be kinda titchy. Jumping to 18", I doubt I could measure much difference in restriction between it and a 10" return with even decent quality measurement tools, and even so, would I care? If I had to hazard a complete guess, I imagine that 18" pipe is reducing efficiency in the sub-1% range compared to an open room leading to the collector's 5" inlet.

[Aaron Rowland]

Mass is the best sound killer. If I had to have a cyclone in my shop I would build a concrete block enclosure around it. Mine is out side of our Brick house in a wood enclosure. It's right next to a garage type roll up door but very little noise comes back in. None from the duct work except moving air noise. Power tools drown it out so I have a light thingey to tell me it's status. If you have a wood enclosure your best bet is heavy carpet glued or stapled to particle board on all 6 sides. For air inlet use a labyrinth lined with carpet.

I made a small diy shop vac with a small cyclone. To kill the noise of the vac motor I used Helmholtz resonators and carpet. Very effective compared to my Ridgid vac.

The noise on all seem to come off the impellers and radiate from the metal blower housing. If the manufacturers would cut serrated slots in the impellers tips it would reduce noise . Note, please no patents rip off on this idea. It's been done.

[Alan Schaffter]

Alan,

In principle I agree, but we're talking diminishing returns here (no pun intended). If my return is 4" in diameter, then I have a huge amount of restriction. If it's 6", I'm much better. 8", a bit better, but the increase in efficiency from 6" is getting somewhat small. The move from 8" to 10" is going to be kinda titchy. Jumping to 18", I doubt I could measure much difference in restriction between it and a 10" return with even decent quality measurement tools, and even so, would I care? If I had to hazard a complete guess, I imagine that 18" pipe is reducing efficiency in the sub-1% range compared to an open room leading to the collector's 5" inlet.

But not all posters here are not talking about round duct- they are talking about a rectangular plenum. An 8" diameter duct has about the same cross-sectional area (50 sq. in.) as a 3.5" X 14" (16" O.C.) stud bay. But look at the surface area for a given lenth. The circumference of an 8" diameter duct is 25" but the perimeter of the rectangular stud bay is 35"- the difference is almost 50% more! and that can really add up if your plenum is long. Plus you have 4 corners that add to turbulence. Add to that two or more sharp 90 degree bends ("a folded plenum") instead of curved duct, and finally, line some or all of it with sound absorbing material that in most cases will add a considerable amount of of friction! Oh, I almost forgot, people then add another filter or two that restricts flow even more. Add up all this overhead and go to your fan curve and see what you have done!!

I just wanted folks to know. As evidenced by at least one poster (sorry Mike) some folks are not aware that every source of SP resistance "after" the blower has just as much impact on CFM as every source on the inlet side.

[Phil Maddox]

Alan - OP is talking about a return that is 5x the inlet size - 18" round duct or 16x16 opening, not 3 1/2 x 14.

[Phil Maddox]

Mass is the best sound killer. If I had to have a cyclone in my shop I would build a concrete block enclosure around it. Mine is out side of our Brick house in a wood enclosure. It's right next to a garage type roll up door but very little noise comes back in. None from the duct work except moving air noise. Power tools drown it out so I have a light thingey to tell me it's status. If you have a wood enclosure your best bet is heavy carpet glued or stapled to particle board on all 6 sides. For air inlet use a labyrinth lined with carpet.

I made a small diy shop vac with a small cyclone. To kill the noise of the vac motor I used Helmholtz resonators and carpet. Very effective compared to my Ridgid vac.

The noise on all seem to come off the impellers and radiate from the metal blower housing. If the manufacturers would cut serrated slots in the impellers tips it would reduce noise . Note, please no patents rip off on this idea. It's been done.

Aaron - the question was not about the material for the enclosure, it was about the return air path (plenum). You can pour concrete a foot thick but if you have a return to the space, that will almost always be the area where most of the noise comes from. CMU is a waste of time, you don't need that much sound attenuation for this purpose.

Overkilling an enclosure is going to leave you unhappy. If you are returning air to the shop, that is the biggest noise offender - 30" thick walls won't help with this. If you must empty the drum or clean the filter by accessing the cyclone from the shop, the door will be the second biggest culprit - again, thick walls won't help with this.

My suggestion - 5/8" drywall, both sides of steel or wood studs, fiberglass insulation in wall. Seal as many penetrations as possible. If you need access from the shop - cheap prehung steel door with weather stripping. Insulate the return air path with rigid fiberglass. Carpet or something similar (free is best) on interior walls will help - aforementioned rigid 'glass is best - homosote (soundboard) is good also. couple of turns in the return.

Good luck.

[Alan Schaffter]

Alan - OP is talking about a return that is 5x the inlet size - 18" round duct or 16x16 opening, not 3 1/2 x 14.

But others weren't . . . as evidenced by the photos. 5X should have about zero impact . . . . unless it is covered with one or more layers of dense filter or sound deadening material.

As far as containing the sound- completely enclose the DC in a sealed sound absorbing chamber - that won't work! or make it hard for sound to escape a large enough opening. Sound can be "bent' by wind over long distance, but at 1126 fps it won't be bent much by DC air moving at only 67 fps or less, so it will generally travel in a straight line and won't bend around corners well. It will bounce around a corner, however. If the the surfaces of a air passage are lined with material that absorbs rather than reflects sound and the passage is bent enough so there is no straight path from end to end- sound will not make it from one end to the other. Some corridors in "that Langley agency" have short curved sound tunnels- walls, floors, and ceilings covered in carpet- if you are at one end, you can't hear someone talking at the other! Bottom, line, a DC enclosure that is properly insulated and has a large offset opening, will stop as much sound as a completely enclosed one.

[mike finch]

I built a room around my dust collector, and used a folded sheet metal duct as an air return.

I had my local heating contractor make the duct for me. The cost of the materials and my time to make it out of wood was more than the HVAC guy wanted.

The outlet to the room is 24"x24" with a grill and air filter in it. The sheet metal duct measures 24"x24", makes a right angle turn and becomes 24"x19" and goes for 5'. I placed a couple of extra pieces of Roxul sound insulation into the duct to make a little maze.

The room has sheet rock outside, Roxul sound insulation inside, and pegboard inside.

I don't think I have increased the static pressure of the system too much, as the air coming out of the grill is not moving all that fast.

It did deaden the sound in the shop considerably. Once I built the enclosure and added the air return, I was able to hear a very loud whistle coming from some of the ductwork and blast gates. I sealed many of these leaks, but not all of them. Most of the sound I hear is on the input side of the dust system. Some of my blast gates leak in a place that I cannot caulk. It is not worth my time to make the closet quieter at this point until I can get rid of the leaks/noise outside the closet.

The cyclone is a 5hp and I think that I can afford some air loss and still have the CFM necessary to move the dust. I have noticed that my system sometimes performs better when I open an extra blast gate.

The pictures are lit fairly poorly.

The first picture shows the outside of the room with the air return in the upper left and the duct work entering from the right rear.

The second and third pictures show the return duct slapped to the wall inside the room. Given that this is a shop and a closet I didn't spend a great deal of time making the install look pretty.

The last picture shows the input duct passing through the wall and connecting to the cyclone.

-Mike

[Mike Desch]

WoW!

Thanks to all of you who have replied to this thread. I didn't intend to stir up a hornets nest, but there certainly has been a lot of good information passed out.

But don't stop now -- keep those thoughts coming ...

[Darrin Vanden Bosch]

The more turns there are the more resticted the airflow is

[Larry Edgerton]

As far as the carpet thing, I would be just as concerned with what would be living in the carpet after a time as the dust itself. A bunch of carpet, that will be dirty, in a place where it can not be cleaned. At least I know what is in the wood dust.

Just a thought......

Mine is outside now, but all of this will help me keep it quiet so my neighbor will not hear it. Its too noisey right now, thinking about a cement block enclosure myself.

[Erik Jarvi]

As an ex sound guy (but still does it on the side), the Helmholtz resonator seems the way to go to reduce noise for the best bang for the buck. Ok I'd build an enclosure, too. Take this with a grain of salt I don't have a DC, yet.

[Dan Hintz]

As an ex sound guy (but still does it on the side), the Helmholtz resonator seems the way to go to reduce noise for the least bang for the buck. Ok I'd build an enclosure, too. Take this with a grain of salt I don't have a DC, yet.
Problem with Helmholtz resonators is they're tuned for a very small range of frequencies... I think for cyclones it's a relatively wide range we want to kill off.

[Erik Jarvi]

Problem with Helmholtz resonators is they're tuned for a very small range of frequencies... I think for cyclones it's a relatively wide range we want to kill off.

I agree, now that I've had some caffiene. The enclosure would dampen the high/mid freq's, then the Helmholtz for the low rumble. You'd need alot of mass to dampen the low freq's read concrete blocks.

[Aaron Rowland]

I've used Helmholtz resonators in the past. See my post #15. You can set them up with a high Q or low Q for wider band width Wider doesn't give as much reduction for a given frequency but better overall.. With sound you have to live with a little bit here and a little bit there for sound reduction to get where you want to be. A low freq. Helmholtz may help with rumble.. I've seen mentions of Roxul and resilient channels with off set studs here. That has a big effect. STC is sort of an industry standard on noise. Google for a whole lot of Info. Spray cellulose was 90MM = 52 STC. 210 mm Rock wool = 54 STC. Concrete blocks with a drywall and studs very good. Sound is very hard to contain. I could not get the sound down where I wanted it for my cyclone and gave up after a lot of $ and work. Moved the cyclone outside and now I can hardly hear it run. Out side is good in a mild climate. Heat loss is not as bad as one would think for a hobby operation. Specific heat of dry air is very low. You can set the cyclone up in cold climates with a valve to return the air back inside or when the weather is nice outside.

Summary -I strongly advise putting anything 3 HP and up outside. The CFM and DB numbers published may be a bit off.

[Aaron Rowland]

Just for fun I took some DB measurements in the shop. I have an elcheapo Radio Shack meter. I checked it against a pro -meter once and it's very close in readings. I highly recommend it.

The shop with the radio running but no tools running measured 60-62 Dba.. Various blast gates opened one at a time measured about 74 dba. That's lower then riding in most cars. Trucks- maybe 80-83dba. I have blast gates used for clean up with a 2.5" ports. With out the vac hose the Dba was 100. The small diameter resulted resulted in a high pitch squeal without the hose. With the 30' vac hose they dropped to about 85 Dba. I'm running extremely high SP on my DC set up . About 2-3 times higher then most cyclones so these numbers are on the high side for air movement. The 2.5" cyclone hose out performs both of my shop vacs by a large margin.

Change of subject and back to airflow. After you have your cyclone enclosed and up and running if you want to measure the difference in pressure between inside the encloser and the shop it's easy. Make up a slack tube manometer. Get about 10-15' of 1/4" clear plastic tubing. Stick one end inside the enclosure. The other end bend a U about 2-3' High and fasten to a piece of plywood.. Then fill with water to about 12" high. Mark the center where both water levels meet. Turn on the cyclone and observe the water levels. 1" apart is to much pressure , 1/4" - 1/2" ok. To high, you need to make a bigger hole in the enclosure for exhaust. You can also use this manometer to measure SP in your ducts. That's a whole new thread on how to measure CFM so I won't go there.

[Doug Palmer]

I used Dynamat on the inside of my discharge duct and it made an unbelievable difference. It would work on the outside of the duct as well. Dynamat is the liner they put in car trunks and undercarraige to quiet road noise. A little goes a long way. I got mine at a local car stereo installer store, but you may be able to get it at some auto parts stores. Basically it prevents the metal duct work from resonating. Relatively inexpensive solution that dropped my dB from unbearable to 82. Not as nice as the 70 with the closet, but the cost was not bad. There are cheaper alternatives by other manufactures, but you will have to order on line. You can do a test for for a twenty dollar bill.

[Jeff Mackay]

I've been doing a lot of research on soundproofing for another purpose: a home theater. Probably the best source of information I found out there is at the Soundproofing Company (http://www.soundproofingcompany.com/) site. I figured the same principles would apply to a cyclone closet. They have a really good series of articles (http://www.soundproofingcompany.com/library/articles/) if you're interested. No connection, just found it to be a great source of information.

In terms of exhaust, I figure a cyclone closet would have two holes: one for the input duct, and one for the exhaust. You could probably use the principles behind a dead vent (http://www.soundproofingcompany.com/library/articles/the_dead_vent/) for the output.

I just did a quick analysis of the sound level in my own shop. With just a 1.5hp Delta collector running, sound levels reach 102db at about 5 feet from the dust collector. But then my shop is in a crypt with concrete floor and ceiling. It's probably good that I always wear the earmuffs when the dust collector is on...

I'm pretty sure that shortly after I pull the trigger for a cyclone, I'll build a closet for it. And I'll probably use isolation clips with hat channel, multiple layers of drywall, green glue, and fiberglass insulation to quiet things down to a low roar.

[Mike Desch]

Hi again:

FWIW: Here is a reply I received from Roxul about insulating my DC closet.

Thank you for your inquiry, my recommendation would be to have the air gap on the outside of the wall. I would also recommend not covering the inside wall with drywall. By not covering up the insulation, it will allow the insulation to absorb as much sound as possible. If you were to install drywall the sound will reflex off the harder surface and may not be as effective at absorbing sound. It is also important to note that the door may be the weakest area for sound control, having a heavier door with door sweeps can make a big difference in controlling sound attenuation.

Rather different than I had thought, but thought the info might be of interest to those of you planning on taming your cyclone noise.

[Mike Desch]

Hi;

Sorry for the quick post last night, but was in a hurry.

My confusion is the differing solutions recommended by the Soundproofing guy and the Roxul guy.

I had read all the articles on the soundproofing site and was ready to apply double layers of 5/8" drywall and lots of green glue--until I started reading about Roxul. Now, of course, my feeble and failing brain is rather confused about the different approaches to trying to mute some of the cyclone noise.

What do you folks think is the better way to go, or is there a combination of the two that will work even better than either one alone?

Put on your thinking caps and expound with your opinions.

Thanks.

[Ken Deckelman]

Hi;

Sorry for the quick post last night, but was in a hurry.

My confusion is the differing solutions recommended by the Soundproofing guy and the Roxul guy.

I had read all the articles on the soundproofing site and was ready to apply double layers of 5/8" drywall and lots of green glue--until I started reading about Roxul. Now, of course, my feeble and failing brain is rather confused about the different approaches to trying to mute some of the cyclone noise.

What do you folks think is the better way to go, or is there a combination of the two that will work even better than either one alone?

Put on your thinking caps and expound with your opinions.

Thanks.

I used Roxul (special order from Home Cheapot) and used vinyl lattice on the inside to hold it in place. I am not completely finished yet but testing so far has yielded superb results. I will post a link to my Picasa album very soon.

[Chris Tsutsui]

I don't have this problem as I wear ear muffs during any sort of noise...

However since I'm an audio enthusiast (though not professional), here would be my approach:

1. Sound deaden the sheet metal of your cyclone. You are mostly just adding mass to the thin cyclone sheet metal to reduce the frequency at which it will vibrate. Use 50% surface area coverage and place the deadening material in the center of the panels away from edges. (100% coverage would be ideal, but that's likely overkill) I would not use dynamat for you pay for the brand name, but would go with something online direct that can hold up to high heat and provide excellent adhesion. Like second skin damplifier, or some competing aluminum backed butylene based vibration damping material from parts express, fatmat, etc.

2. If you're looking for the best bang per buck for sound absorption you should get fiberglass soft batting. The thicker or more layers, the lower the frequencies you will absorb. I've done measurements showing regular fiberglass insulation (R13 or something compressed into 3.5" thick acoustic panel) totally beating 4" thick acoustic foam. To solve the problem of fiberglass dust getting into the air, put a layer of polyester batting over the fiberglass, then another layer of burlap which is one of the most acoustically transparent fabrics. Thin muslin will also work. If you're concerned about fiberglass particles in the air like I am, you can use polyester batting only, or just egg crate foam from Uline.com which is pretty inexpensive and double up on the thickness. Also consider fire ratings of the materials you choose, SAFETY, and local codes...

3. Isolation. Since you can't isolate the space to be air tight you must add a "muffler" type of system. The ideal system permits maximum air flow with maximum sound absorbtion. A straight duct that is the smallest diameter allowable with insulation on the inside should make for a good muffler. If you use something soft then this could create friction, so I imagine an aluminum screen, chicken wire or thin plastic could allow air flow, while letting sound become absorbed / reflected. A straight through exhaust system comes to mind. A sharp 90 degree bend in the exhaust might be more restrictive to the air flow than it will beneficial to sound attenuation.

If I had a cyclone and oodles of time on my hand, I would take an RTA measurement to show a spectral waterfall type of plot that shows the frequencies causing the noise. Then with a little engineering one could design a sound treatment system optimized at attenuating the peaks.

Remember, it's not just the cyclone you are have to make quiet, but the room has a big effect on acoustics as well. Something as simple as foam tiles in the corner where your cyclone is can make a difference.. Afterall, a corner acts as sort of a "horn" shape to corner load the sound of your cyclone.

Then an idea for the adventurous is to convert the constant droning sound of the DC to escape a controlled monopole source, then install a microphone, amp, and speaker to produce equal sound that is out of phase. What you will have is active sound cancellation like noise cancelling earphones. Partsexpress sells full range PA amps for cheap, I imagine one could experiment with this and have a turnkey system for $200.

Again, these are just thoughts on the subject so correct me if I'm off...

Good luck peoples.

[Phil Maddox]

Mike - the Roxul guy made some good points about the door and having some sound absorption in the inside walls. However - he probably doesn't understand the sound coming from the cyclone - he is giving you advice that will absorb a lot of high frequencies, but not low. Mass is the key to the lower frequencies. Maybe you have a problem with the higher stuff, I don't know - my 3hp DG is a lower sound.

I have a dedicated home theater - built with double wall construction, RISC clips with hat channel, double 5/8th drywall with green glue - the whole works. Cost a lot of money, works great. In this environment - I think it was worth it...but and this is a BIG but - Waaaaay overkill for your application. You are ducting the air - and therefore the sound - back into the room on purpose - you have to, I understand. This opening will negate all of the other work - Green Glue, double drywall, etc.

I feel like you will get 90% there with drywall both sides, something like homosote on the surface of the drywall (someone suggested carpet) and to line the return area with a couple of turns. That should be reletively inexpensive - that other stuff is going to cost you 5 times the money and get you another 2%.

You can always add another layer of dw and the green glue if you are unhappy.

If you want to go hog wild - I have 4 or 5 tubes of green glue left over you can have if you want just the cost of shipping - which may not be worth it from coast to coast. PM me and let me know.

Good luck.

[Ken Deckelman]

I used Roxul (special order from Home Cheapot) and used vinyl lattice on the inside to hold it in place. I am not completely finished yet but testing so far has yielded superb results. I will post a link to my Picasa album very soon.

As promised, No final numbers yet since i have the door off for painting.

https://picasaweb.google.com/kdeckster/DC#

[Mike Desch]

Nice job Ken!
Wow.
Ken those pics are a wonderful guide to show how you did your DC closet and sound attenuation.
I appreciate you taking the time to post the pics as well as the attached comments.
Very nice job!