Reducing noise from a grinder

[sach27]

We are a plastic sheet extruder and we grind our scrap in a 40HP plastic grinder which grinds the sheets/thermoformed skeletons to small pieces. The grinder is old and generates a lot of noise. More than 110db while grinding.

We did build a sound enclosure around it to reduce the noise but did not do much difference. We reduced around 10 -15 dB. According to regualtion it should be 85dB. We used a Filled EVA barrier with foam and a filled EVA sheet. This sheet is used in automotive companies as a sound deadning material.

The noise seems to be very high frequency since if I stand next to the enclosure I can feel the sound vibrations in me and the ground. I also tested Melamine 2", 1" foam ,2" vinyl quilted barrier, they did not show much reduction in dB.

I am seeking some advice from acoustical engineers or anybody who has dealt with this problem before.

 

[Skogsgurra]

"noise seems to be very high frequency since if I stand next to the enclosure I can feel the sound vibrations in me and the ground"

That indicates low frequencies rather than high. Heavy, massive material like wood and concrete works a lot better with such frequencies. If it is transmitted through floor. then damper feet can work.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[aspearin1]

Agree with Skogsgurra... Vibration dampening at the feet was my first thought as well.

Aaron A. Spearin
ASQ CSSBB

http://www.Engineering6ss.com

"The only constant in life is change." -Bruce Lee

 

[sach27]

The sound enclosure should be flexible and removable. The current one we have has wheels and made of angle iron and expanded metal. and we have the sound deadning material glued to it. Concrete may not work because of the flexibility issues.

Is there any particular kind of wood which is a good barrier.

 

[sach27]

The material I currently use does weigh a lot. It is 1.5lb/ft.sq. with a specific gravity of 1.8.

I conducted an experiment to find out which material reduces the maximum decibels. I have a 6"x6"x6" box where I put 1"/2" foam/quilt, entirely covered. Made a small hole to put in the sound meter. Held it next to the grinder. I get a reduction of 4 -5 decibels. I tried one building a wooden box, and got a reduction of 8 dB...which is better. I read online that wood resonates and vibrates with low frequency sound. I can hear and feel the loud noise. I can hear the noise more compare to feeling the vibrations while I go away from the source. Is it high or low freq?

Thanks skogsgura and aspearin1

 

[Strong]

I suggest that you mount machine on isolation pads or elastomer mounts and flexibly connect any power wires/conduit, instrument wires/conduit, and feed/outlet trays. Consider using a flexible enclosure that allows as much containment as possible. A ventilation fan with silenced air openings may be needed. Here is an example enclosure that you can purchase ready to assemble or get the materials and make it yourself:

http://www.soundseal.com/barricade/sound-enclosures-portable-acoustic-screens.shtml

Walt

 

[Tmoose]

I'd take the sound meter's output (A weighting scale, for better representation of what humans hear) and feed it into a computer's sound card for recording and saving. Then I'd download one of the free FFT programs and analyze the file's frequency content.

I'd suspect A 6 inch square panel is a different animal than the panels on your enclosure. Geometry can over-ride material properties pretty quick.

There may be noise generation by the grinder's panels, in which case stiffening those panels may be quite effective.

If the noise is airborn because the grinder chews with its smouth open, then a flap of several layers of lead filled rubber sheet might help contain it, fortified with an inlet box with stiff panels lined with 4 inch thick absorbent material.

By "stiff" I mean having first few resonant frequencies numerically well above the predominant sound frequencies. Since achieving panels that stiff can be quite a challenge, the default position is to have panels with lots of mass, which provide proportionally lower response when panel natural frequencies are well below forcing frequencies.

 

[kacstic]

Tmoose provided an alternative approach that could be attacked easily. The noise emitting by your grinder could be causing your grinder panels to act as essentially loudspeakers.

In order to decrease their vibrations, you increase their resonant frequencies about 1.4 times the predominant frequency measured. However, adding mass which sometimes adds stiffness will likely not shift the panel resonances enough. Actually the added mass could be acting as a damper, decreasing vibration amplitude at the resonance but not shifting the resonant frequency.

To stiffen the panels, you could add brackets that are positioned near the antinodes of the resonances (1st & 2nd). This would stiffen the panel without adding too much mass and thereby increasing your resonant frequency.

Panel resonant frequencies, I think, should be well above the forcing frequencies even if the higher resonant panel frequencies are not relatively strong compared to the first 2 or 3.

 

[ssn596]

I worked at a plastic outfit also. Grinders there were larger than yours. They resembled sawmill chippers (why they called em grinders and not chippers I know not). Chippers and planers are quite similar. A friend who is in wood products said that for all considerations what worked the best for the "houses" they put around chippers and planers were of 2x 4 construction sheet rocked both sides and for extreme noise double sheet rocked both sides if needed.

Not portable I admit, but then make em big enough to get a forklift in when you hafta tear it down.

Dan Bentler