Noise Reduction Dilema

Check via google for 2 or 4 stroke tuning pages.

There are many tips and spreadsheets to calculate and build exhausts to reduce noise at high performance.

Vmixa024:

Not sure what you mean by "vacuum motor", (a motor that is driven by vacuum, or a motor that produces vacuum) but I'll assume you mean a motor driven device to produce vacuum, as in a vacuum cleaner, although some techniques of sound reduction would apply to both.

Any enclosure must be lined with sound absorbent material but in the case that the motor is externally cooled, that must be allowed for, but baffle any openings, and the baffles would be lined with sound absorbent material.

You can typically fashion a muffler out of a multi-baffled "chamber" that is lined with sound absorbent material making it large enough to produce no noticeable back pressure.

Isolate the motor assembly with rubber mounts to avoid sound transmission to the base or supports.

Try looking at acoustic insulation as may typically found lining the engine compartments of boats.

As the others have said, is it an electric motor or do you have an air inlet, exhaust outlet...

I guess you must have an air outlet if its generating a vacuum?

John.

First step - remove the filter and measure the noise with a spectrum analyser - that way you know what you're dealing with and what measures are needed. Tonal sounds require very different solutions to broadband noise.

You've already discovered the first rule of noise reduction - sound down, back pressure up (flow down), so decide what your limits on flow reduction are; different "mufflers" have different back pressure performance - a helmholtz resonator has very little impact on flow, whereas a triple pass with perf has an enormous impact.

The rest of the guys have given good ideas of some of the tools available e.g. baffling your inlet/outlet - this is done on the airflow from vehicle test chambers, so there's plenty of knowledge out there.

And remember - sound is energy. If you reflect it back into the chamber, the noise in the chamber goes up, so an acoustic chamber will be needed - again plenty of those out there of various design.

I know this doesn't answer your question, but I hope it gives you the tools to work it out. Have fun...and let us know what happens.

Is this an issue of noise within the enclosure or noise external to the enclosure during motor operation?

To clarify a few things

This vacuum pump is driven by DC 12 volt, very small, eccentric vacuum pump. This vacuum pump is mounted on a PCB with other components. The overall assembly is then enclosed within an enlosure.

There are many constraints to the problem, the muffler must be within a dimension of length 5cm with diamter of 2 cm.

Because we do not wish to change the dimensions of the system, adding an enclosure to the vacuum pump is not feasible (not enough room).

I've trying adding some sound absorbant material (fiber glass batt) within the overall casing, but there's just not enough room for the overall dimensions.

The mufflers seem to work (just running the pump in a room and compare the pump with the muffler vs one without), but when i put that inside the casing, the sounds actually amplifies.

Its unfortunate but I think we might have to either replace the existing vacuum pump or replacing the overall casing.

Thanks for the ideas guys/girls

Is the muffler inside the enclosure too? If so, you may be getting

a) shell noise - where the skin of the muffler acts as a loudspeaker, or

b) standing waves from the tailpipe noise...if this contains wavelengths that are multiples of the room dimensions you get ampilfication - it's how woodwind instruments work. Changing the dimensions of the muffler even slightly could help with this as it's very frequency/wavelength dependent.

Then there's structure-borne noise, but I think someone else mentioned that.

Ohhhhhh..... its tiny then!!!

Blimey how much noise can you tolerate?

Would a peristaltic pump do the job as they are quieter than a diaphragm or any other type of vibration pump,,,

What do you mean by a very small eccentric vacuum pump??

Couldn't you use a venturii vacuum generator?

John.

It sounds like you may well be dealing with resonant frequencies, since if adding the case makes the sound louder it could be because its addition creates a resonant system (Adding, removing or redistributing "solidly" connected parts will change the resonant frequency of the assembly).

Try mounting the vacuum pump to the PCB on "soft" mounts. Try a flexible connection between the pump and muffler, with the muffler also on "soft" mounts. Affixing a weight (a piece of lead for instance) to the pump will also change the resonant frequency and might help. In the low volume manufacture of A/C units for instance, it is common that a section of plumbing might be in resonance with the compressor and will vibrate, adding to the noise and also resulting in eventual failure from metal fatigue. This is addressed typically by attaching a weight to the offending tubing. (In a high production volume situation, it is more efficient to vary the plumbing and/or its supports if any.)

The idea is you don't want your total assembly (including the pump) or any separate part of it to have a resonant frequency (or harmonic) with the pump as this will amplify the sound output.

THIS IS HOW YOU CAN BUILD AN ABSOLUTELY SOUND PROOF CONTAINER FOR YOUR MOTOR, WITH ACCESS DOORS AND EXHAUST PORT. (A LARGER VERSION OF THIS CONSTRUCTION WILL GIVE YOU A SOUNDED STUDIO FOR AUDIO OR VIDEO PRODUCTION.) The instructions assume the motor is to be used in an air environment and are from a sounded studio that I actually designed and built for a producer.

The process is fairly complex, so the problem had better be really worth solving. Sound is propagated through a gas or fluid (such as air or water) and anything solid directly exposed through a gas or fluid to the sound on one side and ambient gas or fluid on the other.

a)The outer room.

1. Out of any material at hand (brick or block walls would be good, and a strong roof), build a sold outer box or room to house an inner box or room that houses your motor. Leave a hole in the roof for exhaust piping and another for air conditioning/heating/air supply.

2. Leave an access-door sized opening in the side of the outer box big enough to move your motor in and out of the room. See the section on doors that follows.

3. Use the existing floor of your motor's current building location for the floor of this box.

4. Coat all six surfaces on the inside of this outer box with rubber coating material or bitumen to seal the air outside away from the air inside. No holes (with the exception of the holes for the exhaust and air supply as designed below).

5. Line the outer box with aluminum foil fastened to the rubber with glue and fiberglass wool fastened to the aluminum foil with glue. These remove the higher and lower stray frequencies being produced by the motor.

b)Removing through-floor frequency perpetuation

1. Within the box, place rubber sound attenuators on the rubber-coated floor. Your attenuator sales person will tell you how many and how far apart the rubber floor attenuators need to be based upon the weight of the motor, peripherals, and inner box design combined plus the compression requirements of the rubber attenuators, which usually require a certain amount of compression to reach maximum attenuation.

C) The inner room

1. Following the instructions below, build another box to fit within the first, big enough to house the motor. This will be the motor housing. Match a door opening to the opening in the outer box. The inner room should not be connected to the outer room at any point, or touch it directly.

2. Suspend the ceiling framing, with holes in it for exhaust and air supply not directly below the holes in the outer box, but a distance from them. Use ceiling suspension attenuators (a variety of spring attenuator with hooks on either end, the number, distance, and settings on the hooks determined by the weight of the finished ceiling) to suspend the frame from the outer box's roof joists.

3. At this point, if you wish, you can build a permanent RF cage around the room by covering the outside of the ceiling joists and wall studs of the inner box with small-diameter chicken wire connected to ground rods through the floor.

4. Build the floor on the rubber sound attenuators beginning with a layer of plywood to evenly distribute the weight of the floor, then the floor joists, and finally the finished floor. If the floor is very large and composed of concrete, it should be cracked and the saw openings sealed with epoxy. I like a crack-pattern not more than every ten feet (2.3m) and not less than three feet (1m).

5. Use double 1/2" gyproc drywall on ordinary studs with bottom plates that have been rubber-sealed where they meet your new floor (not necessary to use staggered studs, but you can if you want to place both an outer wall and inner wall on the inner box) to make the walls and ceiling. Place uninterrupted (a single sheet, no breaks or sealed overlaps) plastic vapour barrier between the two drywall layers in the vertical corners. The drywall should be taped and plastered with the first layer mounted vertically and the inside layer mounted over it horizontally on either steel or wood wall studs mounted to the floor plate. Since the two opposed layers cover the taped portions of the drywall joints, the exposed portions are reduced to the pinhole sized overlap holes, which have been plastered anyway. Do not connect the walls to the ceiling frame. Now your sound-proof room has a 1" thick wall with no holes through it.

6. Connect the ceiling frame of the inner box to the walls using a wide rubber or vinyl strip, which has been carefully glued to the walls and fastened to the ceiling frame with ordinary screws. Make sure there are no holes through the strip at the corners (rubber bitumen will suffice to take care of this).

7. Drywall the ceiling in the same fashion as the walls...two layers in opposed directions.

8. After the room has been completed, including the doors, drapery can be hung inside here and there in order to reduce or eliminate the motor's produced-sound die-away time.

d) Exhaust and air ducting/piping.

1. Air conditioning ducts should be mounted onto the air supply/heater/conditioner and any inside fan through a piece of corrugated duct and the rest of the ductwork should be fiberglass lined with at least two bends in it between the inside and outside openings. Tightly rubber seal ducts where they pass through the ceiling and roof.

2. Exhaust ducts should be connected to the muffler or exhaust ports via corrugated piping and then into fiberglass lined ducts having at least two bends out through the roof and tightly rubber sealed where they pass through the ceiling and roof. The corrugations prevent most of the motor vibration from being transmitted to the outside, and the length of duct, two bends, and fiberglass lining takes care of the remaining noise.

e) Doors.

Inside doors are hinged to open inward, and outside doors are hinged to open outward. Because they are intended for the passage of large items, you may prefer to use double doors. Doors must have sounded doorstops on all sides that fit corresponding doorstops built into the door frames and either similarly-stepped separating astrigals or where the doors meet each other. Latch-controlled door bars hold the doors shut. A rough cloth coating around the space between the inner and outer sets of doors is usually a good idea. Sounded doorstops are built with wood stepped to create a right-angled zig-zag step key-fit between the door and its frame or the connecting double. Thus, sound going through the cracks around the doors run into the first part of the stop, and are forced upward to the second, and outward to the third, and upward and outward again before being allowed to escape, which by then leaves nothing to escape anyway, since sound is stopped by being forced to travel twice past the right-angled stops.

f) Windows

If windows are desired, they should be built of heavier glass (at least 4 mil), all four sides resting on rubber strips in their frames. Sounded windows have to at least be double (i.e. two or more separate panes), and mounted at a slope with reference to each other, so sound frequencies cannot be evenly transferred between them. A rough cloth wrapped around foam rubber should occupy the fames between the glass.

Good luck.

Mark

OOPS!

In response #9, under e) the door section, I forgot to mention that the doors should also be constructed of similar double overlap drywall on frames. I added a pair of diagonal tension cables in the door frame and a caster wheel in the bottom of each door to help support the resulting heavy door swing. If you wish, a vinyl or rubber strip can also be added to the perimeter of the doors inside of the inside box's door to deflect some of the sound away from the cracks.

Mark

Hi Mark,

This is a very small vacuum pump (dimension of width: 6cm by 4 cm with height of 4.5cm). There's not much room to work with on the PCB board where the vacuum pump is attached to. I think I will try other exhaust filter sizes (waiting for the samples to come in) Hopefully the amplication of the noise within the enclosure of the overall system will be less. Still trying not to change the existing enclosure, but as of now it seems we require bigger enclosure in order to add the sound absorbant materials as suggested by you guys.

Funny that a little pump like that can make so much noise.

Since the pump is only big enough to enclose in a box, 5 variables can help:

1. No air contact (seal all air holes) between inside & outside air around the box unless it's through sound dampened ducts and/or door(s) with stepped stops, etc.

2. Thick walls on the main (inner) enclosure, or at the very least, two sets of walls (inner & outer) that are not parallel at any point, and connected directly to each other in as few places as possible or through 'staggered stud'-style upright supports.

3. Removing high & low frequencies by insulating the box (inside or outside) with foil & FGW (Fiberglass).

4. Sound dampening material on the inside of the box (rough cloth, acoustic ceiling tile, etc.)

5. Placing the box on rubber sound attenuators to stop through-ground sound propagation.

Good luck.

Mark

Mark:

You invested a lot of time here and everything you said were good and effective techniques of soundproofing.

But you seem to be picturing some huge pump and assembly.

The dimensions are only about 2 3/8 in. x 1 1/2 in. x 1 3/4 in. high: that is roughly about the volume of a pack of cigarettes.

Well said Greg....

I think a lot of us didn't have the nerve to tell Mark that its not a walk-in compressor room it is only a tiny thing!!

John.

It is, however amazing to discover just how noisy those little pumps can be. Anyone who has experienced a fishtank aerator compressor with the adjustment at the open position knows that the amount of air delivered hardly seems worth the accompanying noise.

A sounded box can be of any size... all the way from a full-blown studio down to something very small (say 6" X 6" for a cigarette-package-size pump). Of course, if it's that small, it doesn't need specially-purchased sound attenuators. Just putting some of that cushion rubber on the bottom of the box would accomplish preventing the machine vibrations from transferring through to the floor in the form of noise. Inch-thick (1/2" ply with 1/2" gyproc/drywall liner), rubber-sealed walls with sound attenuating insulation (aluminum foil, acoustic ceiling tile material) will do the rest. The box could be made with a fitting, removable lid and a periscope-shaped air delivery port: _|^--

to prevent air-bourne noise from moving directly out of the box.

It's a lot, I know. But I had the impression that this "vacuum motor" is a permanent installation with an important noise problem. And the box is a permanent solution.

Mark

At this sort of size the next thing to try would be to wrap it up in some foam material. Try scrapping an old and cheap armchair cushion, removing the useful polyurethane foam from within and wrapping the pump up in it.

I am now waiting for some samples of the small vacuum pump mufflers.

I'll have to give this solution another shot, at the same time, try to insulate the box.

The vacuum pump itself is not too loud, but since its being used with a medical device, "some people" complain that its loud.

(actually the sound level is about 70 dB, measuring at a very close distance (20cm away from the vacuum pump)) Hopefully using your ideas will reduce the sound to 60 dB that would be the "ideal".

When the mufflers come, I'll keep you posted, keeping my fingers crossed!

Thanks

dB sound level isn't very informative, remember a decibel is a relative measurement...

If its 'A' weighted then it should be written as dBA with the distance from the source specified...

Then we would know what 70 dB is i.e. whether its loud or quiet...

John.

Now, now John, if you're going to get picky, then we should ask him for a spectrum...after all, a tonal 70dB is perceived as much louder than broadband 70dB. Then we could get the NC curves out.

dB is always louder that dB(A), unless all the energy is around 1kHz, in which case dB(A) is meaningless, as you have a tone.

And for the pedantically inclined, it's (with an apostrophe when used as a contraction for it is) dB(A) rather than dBA.

I humbly beg your pardon...

please forgive me for not being lucid in my interpretation of the aforementioned post...

John

Chuckle!

I'd've added a bouncing smilie after the NC curves comments, but I couldn't find one . I do like this one though: LOL

Haha,

you guys are the best,

yes sorry about the vagueness, it is dB in A-weighting, Fast Response, (just want the amplitude), but yes, these samples are taking forever to come, I'm really curious as to how much the improvement would be.

Thanks,

I am also trying to reduce the noise of a pump - but mine is a 2W air pump for a fish tank. It is of about the same dimensions, with a stated flow rate of 900cc/min @ 1bar.

First solution was to use the softest possible material for the supply tubing to reduce vibration transfer to surrounding components.

I am currently working on something more extreme, but which should only add around 5mm to the casing all round. Will keep you informed of progress, but may be a couple of weeks.

Update on pump silencing project:

mounting pump on an old CD drive shock absorber plate reduced transferred vibrations so sound levels @ 20cm from vibrations were reduced by half (amplitude detected: have not got equipment to measure dB accurately)

Have abandoned trials of a vacuum enclosure, as I don't have the neccessary tools.

(Best sound insulator I know is nothing)

Hi guys,

Just to give you and update on my tests.

Results:

After inserting the exhaust mufflers on the vacuum pump, there was no significant reduction in noise, 2 dB(A)

Overall sound is still loud, I even tried mounting this vacuum pump on a damper, still no good. I've contacted the manufacturer about this but they said they will "work on it"

Anyhow I'll think of some more ideas and let you guys know.

Thanks,

vmixa024

Does the end of the exhaust come outside the box?

If you are trying to use the exhaust air to aid cooling, then the air vibrations will still have effect on the casing.

Car exhausts always push the gases clear of the vehicle, as even a tailpipe missing - behind the back silencer box - will increase the noise from the escaping gases.

2 dB(A) is not insignificant! Actually, it's meaningless as you can't weight differences (if before and after are in the same weighting, which they should be, then the difference is in dB. Pedants' corner is now closed)

A drop of 3dB represents a halving of the sound power (or sound intensity) level, since the dB scale is a natural logarthmic one, so you've done bl00dy well.

What pipe size are you using? Reducing its diameter may help, but will increase back pressure and reduce performance, so you'll need to find an acceptable compromise.

Good luck and keep us posted.

Hi

I search silent high speed Brushless DC motors ( or other technology ) for turbo molecular vacuum pumps ; could you tell me names of US motor manufcturers ?

do you also know european & japanese motor manufacturers supplying to turbo molecular vacuum pumps ?

rgds