Printed vacuum pump muffler quiets the lab

printed-vacuum-pump-muffler

[Joel] made a brilliant improvement to his shop. If you think about it, most folks would hear a loud vacuum pump and either tolerate it or put in some ear plugs. But [Joel] heard a loud vacuum pump and thought: hey, I can fix that! His solution was to design and print his own muffler.

He did a bit of research on the topic and found that design complexity runs the gamut based on the application. For instance, you don’t want to affect the airflow of a vehicle’s exhaust too much or you will take a horsepower (and efficiency) hit for it. In this case the vacuum pump making all the noise has a relatively low airflow so that is not a concern. What he ended up doing is designing a baffle that will help cushion the vibrations in the airy by piping it through a maze of channels. The end result drops from about 92 dB to 82 dB. That might not seem like much, but decibel measurements aren’t linear so it ends up having a great effect. Hear for yourself in the video after the break.

Comments

  1. I have done the same thing with a large aquarium air stone.

  2. Will Morris says:

    I`d like to take this opportunity to correct a slight misconception here:

    Although the decibel scale is logarithmic (i.e. the amplitude is proprortional to 10 raised to the decibel value), the human ear also has logarithmic response so the volume you perceive IS proportional to the decibel scale – that`s the whole point of using it.

    Therefore, although -10dB means a tenfold decrease in amplitude, the perceived volume only decreases to 82/92~89% of its initial value – a decrease of about 11%.

    • nato says:

      I believe a -10 dB difference doesn’t mean a tenfold decrease in amplitude (of pressure), but a tenfold decrease of power. I think the amplitude of pressure corresponding to a -10 dB difference would be a reduction factor of sqrt(10). Somebody please correct me if I’m mistaken.

      On the perception front: As you said, the human ear’s response to volume has a logarithmic response, but a -10 dB difference will correlate to the perceived volume being approximately halved.

      • Will Morris says:

        It is correct that the amplitude is the square root of the power, however, normally when we say `dB` we mean `dB SPL` which is proportional to the amplitude (not the power) and is the scale to which our ears response is proportional.

        It`s still a great hack of course and plenty worthwhile, I just thought it was worth clarifying an oft-misunderstood concept, for the benefit of everyone :o)

        • Will Morris says:

          God damn it, you`re right – I justed busted out the old textbooks and our ears ARE proportional to intensity and not amplitude, just like you said. Crap now I`ve polluted these comments with a mistake, exactly what I was trying to prevent!

          • Will Morris says:

            Right I`m getting well confused now:

            On one page it says that dB SPL is the log of the power ratio and one the next it says that it falls off with 1/r – thats a contradiction. How can power go with 1/r in three dimensions!?

          • nato says:

            So your use of the word “proportional” might be an oversight, or it might be intentional and contributing to the confusion. Our hearing is not proportional to intensity or amplitude. Our senses generally respond logarithmically to stimuli, and with hearing volume specifically they certainly respond logarithmically.

            On the issue of the relationship between dB and power or amplitude: dB can be expressed as a relationship of power ratios or as a relationship of amplitude ratios. With the power ratios you multiply the base 10 log of the ratio by 10. With the amplitude ratios you multiply the base 10 log of the ratio by 20. Checkout http://en.wikipedia.org/wiki/Sound_pressure#Sound_pressure_level Where it has the squared ratio of pressures, just think of that as power–since it’s a ratio, the same terms in the numerator and denominator would divide out and equal one.

            As for the 1/r relationship: wikipedia addresses this in the same article at http://en.wikipedia.org/wiki/Sound_pressure#Distance_law

            “No”, I didn’t just edit wikipedia. : )

          • Will Morris says:

            See that`s the thing I f now don`t understand as Wikipedia says that dB SPL falls off with 1/r but if dB SPL is a measure of intensity then it must go with 1/r^2 or it violates conservation of energy!

            I wish I could figure this out.

          • flink says:

            Long ago, I spent an entire two weeks on log functions for electronics. At the end of it I was more confused about it than when I started!

            Just roll with it and smile. You aren’t alone ;-)

    • Alex Rossie says:

      Importantly though, 85dB is the general maximum sound level you can listen to for protracted periods of time without having hearing damage.

      I think the guidelines were 85dB for 8 Hours as the cut off, so this hack is VERY useful.
      Still 82dB is louder than I’d like to endure for any period of time then again I listen to music at about 65-75dB

    • draeath says:

      However, in this case you can clearly hear that the majority of the energy removed was higher-frequency sound, which is perceptually louder than lower frequency sounds. Even though it’s a ~ 10% cut like you say, it’s perceptually a huge difference.

  3. FrankenPC says:

    Yeah, but how much damping effect does a 3-5 foot rubber hose have?

    • medix says:

      Better yet, run the house through a window and pipe the noise outside. I’ve seen this done with much larger pumps quite often, though more for ventilation of pumped-out gasses, etc..

      • Dax says:

        He did it wrong by using rounded corners. The point is to reflect the sound backwards in the channel at every turn so the energy would get absorbed on many more reflections, so the maze needs to have sharp 90 degree turns. Rounded corners act as waveguides.

        Each lengh of channel between turns must also be off-tune to the main frequency of the noise so it won’t develop standing waves.

        • AlanMcR says:

          Not to mention printing in jaggies to cause the sound to bounce around in the channels. They are a pain to mill, but trivial to print.

        • josh says:

          Technically, to do it “right” you need to properly model the wave channel to dissipate as much sound energy as possible, given the structure. There are programs that can handle acoustic modelling in that kind of environment, although they are rather expensive.

          Source: My dad used to do R&D for a muffler company, and showed me the ropes. He also modelled the church that we installed a sound system at.

          • josh says:

            Aside the “do it right according to industry standards” this was a damn cool hack. And saves his ears, at that.

          • Dax says:

            Dunno about industry standards. I’ve tried out different things for muffling fan noise in PCs by ducting them, and what works best is hard turns and flat baffles in front of vent holes to reflect the sound back.

          • Blue Footed Booby says:

            You can still do it the way they designed suppressors for firearms before computers: either lots of math, or trial and error.

    • Sometimes not much. Didn’t you and a friend ever play telephone with a garden hose?

      • FrankenPC says:

        Exactly! The string is tight. Allows waves to traverse the distance. With a rubber hose, the elasticity should absorb the sound and convert it into mechanical motion.

  4. Quazi says:

    why wasn’t it printed as a single piece?

    • snow says:

      because you cant print “overhanging” structures. or in theory you could use a filler but the how would you get the filler out?

    • draeath says:

      Typical 3D printing requires the area to be printed to be supported by material underneath. The lid would only have such support above the baffles, which is how the baffles were printed.

      There are ways around this by using a filler that can be melted or otherwise removed via solvent, but that’s a patent minefield so you don’t see it used often.

    • kj6epl says:

      It could be done with a redesign. With an FDM printer, you can create a “bridge” between to towers or walls, as long as it is a straight line. However, you can’t create a curve without support under it. So in this case, you would need to modify the design a bit, namely 2 changes: 1) The exhaust ports would need to be rectangular and span the width of the channel underneath it. 2) The hose nipple would need to have a rectangular boss underneath it, if only one layer. If the author would upload it to Thingiverse, I could make the said modifications to make it a one piece print.

  5. vonskippy says:

    So how does he “know” that all those channels do anything at all?

    I’d like to see a comparison between his box, and a simple empty/hollow box. My guess is the extra tube and the expansion chamber is all that’s making it less noisy, and the channels do little to nothing to add to that effect.

    • Whatnot says:

      What is odd about it is that the sound would also go through the side and not somehow only travel in a straight line along the airflow, so he’d need to make the sides of the box sound absorbing for it to properly work I would think.
      And yeah I think a silicone tube would adsorb better than (semi-)hard ABS and be more effective, especially if you bend it a few times.

      But I’m no expert since I tried so many things to reduce the sound of various noise-making machines and the results were always so-so rather than a great success.

  6. lloydatkinson says:

    Learn something new everyday: dB isn’t linear.

  7. Bacchus says:

    Oh. It’s a vacuum pump muffler.

    I thought it was… something else…

  8. Whatnot says:

    Hah, the short video already made me want to scream from the annoying sound, and it might be less piercing with the muffler but it’s still very very annoying.

    I think ti’s also the size of these things, the smaller the noisier, but small means you can put a whole enclosure over the thing with some sound insulation.

  9. Todd Carney says:

    In addition to lowering the sound volume, this (and maybe most) muffler seems also to lower the pitch. That’s what gets to me in noise from such sources: the treble-frequencies. This printed muffler seems to filter those a bit. I’d like to try something similar to this for my 2hp, 30-gallon “oil-less” compressor. I think I’d have to use conventional fabrication methods, though. I hate all oil-less compressors. Piston-type, belt driven ones are much easier on the ear–but not the pocketbook!

  10. In the name of... says:

    I wonder if soft foam rubber milled out would have been better for the inside baffles of this thing? Acoustic baffles are usually soft to absorb some of the sound energy a little everytime it strikes a surface, rather than just passing it along.

  11. roboman2444 says:

    Wouldnt creating a rather rough surface on each of those baffles help even further?
    There was a trick that some mini sumo builders did was covering the front/sides of their robot with a fluffy or furry material. This would really hurt the opponents that used sonar.

  12. Fallen says:

    Should be stuffed with some fiberglass (or polyfil). You can then convert (some of) the sound adiabatically to heat. Just like in a real muffler ;)

    (Or in some speaker boxes where it’s desirable to dissipate the rear wave, like a transmission line)

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s