Changing Pitch Noise at inlet

[mdrummond]

I have been given the task of quieting a vacuum drum on a machine. It's a rotating drum with a pattern of holes covering the entire drum. It rotates around a shaft with a slot in it. Attached is a vacuum pump. This drum sucks and as it rotates advances paper downstream. The sound is very high pitch. Where would one start trying to reduce the noise level or change the pitch. The drum is 2.5" around and runs at 850ft/s. HELP!

 

[BigInch]

You may have some natural frequency resonance going on. Pitch is dependent on natural frequency resonance of a tube with air flow. Lower amplitude by slowing velocity, either by increasing drum and attached pipe diameters and/or hole size, or all of the above. Drop pitch by increasing length of drum and perhaps also any attached piping. Increase pitch by shortening these lengths. You might have some success by increasing wall thickesses too. Changing the effective resonance length by adding or removing intermediate supports to attached piping may also have some effects. Check natural frequencies of lengths between pipe supports with travel time of air flow. If there is an indication of resonance, add or remove pipe supports. Make distances between supports unequal and away from lengths that correspond to travel time resonance.

"I'm all in favor of keeping dangerous weapons out of the hands of fools. Let's start with typewriters."
- Frank Lloyd Wright (1868-1959)

 

[rcooper]

Your description sounds like a typical design for a siren - something design to emit a loud noise. Could you change the holes to helical slots so that there is not the on/off flow caused by the holes? You could also try shrouding the top of the drum to shield the noise. Let us know how you get on.

 

[rcooper]

How about using a mesh rather than a sheet metal drum?

 

[BigInch]

Isn't the idea for the paper to stick to the drum from the suction created at the holes? I don't know how that would work with a mesh.

"I'm all in favor of keeping dangerous weapons out of the hands of fools. Let's start with typewriters."
- Frank Lloyd Wright (1868-1959)

 

[rcooper]

The mesh would be formed into a drum with suitable reinforcement hoops.

As the mesh drum rotates, the air will be drawn through the mesh in a similar manner to it being drawn through the holes.

This will get rid of, to a large extent, the siren effect caused by the holes chopping the stream of air. It is essentially making the holes very small.

 

[BigInch]

OK, I was visualizing chicken wire mesh.

"I'm all in favor of keeping dangerous weapons out of the hands of fools. Let's start with typewriters."
- Frank Lloyd Wright (1868-1959)

 

[mdrummond]

Thanks for your help guys. I'll be mocking up a prototype later this month. I'll be putting your ideas to work.

Thanks again.

 

[BigInch]

Not the chicken-wire one I hope.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[rcooper]

Only if you can get the right grade of chickens.