Noise Measurement of Vacuum Pump

[JagSTypeElectric]

I would like to be able to measure the frequency of a vacuum pump and then build a Helmholtz resonator to reduce the noise.
Can anyone please help me with an inexpensive set-up to measure the noise and analyze it for frequency?

Thanks!

 

[SylvestreW]

Well, the cheapest method would be to record a *.Wav file and then analyze it on the computer (assuming you have software capable of that - audacity comes to mind).


-
Syl.

http://www.aercoustics.com

 

[GregLocock]

or goldwave.

To be honest if the helmholtz is going to be successful then you should be able to see a sinewave on a scope.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Hatch]

Interesting Question here are my thoughts:

1) In my experience the noise comes from the vacuum pump itself and the motor. The pipe noise is not usually a problem.

2) OK assuming the pipe noise is an issue, the inlet pipe is operating in a vacuum. What speeed of sound do you use to design the Helmholtz resonator?

3) On the discharge side, the flow is actually very low after the initial startup, again, what corrections are required for the speed of sound?

 

[SomptingGuy]

There will never be a complete vacuum, besides the speed of sound is not related to mean pressure. So sqrt(gamma.R.T) should be used.

- Steve

 

[Hatch]

Somptinguy,

I looked up the solution for a Helmholtz resonator and the equation is:

helmholtz resonance freq = v/2PI * sqrt (A/Vo L)

The v in the equation is the speed of sound in a gas so do you have another equation where the speed of sound is not required?

 

[JagSTypeElectric]

Thanks everyone for the input!

I agree that there are two noise components, one is structural through the body of my vehicle (this is an electric Mustang and we need the vacuum pump for the brake assist). and one is radiated through the sound wave which then sets off components around it.

To handle the structural issues, we plan to use a damping material like "B-Quiet". This material is a Butal and aluminum composite that will reduce noise transmission through structural components.

To handle the sound wave, we're using the Helmholtz Resonator.
As I understand it, while the speed of sound is in the equation, that doesn't preclude us from making a resonator that will reduce the noise. Once we find the dominant frequency of the noise, we can "T" a tube of x diameter and y length in the system before the vacuum intake. This should set up a standing wave that will cancel the noise of the wave made by the pump.

My issue is taking and analyzing the data to determine the noise frequency.

Someone mentioned the wave file. I don't know much about these files and how to set up the computer to take them. If I get a USB microphone, do I need an amplifier, is there a shareware program to record the noise? Once I have the file, I think I need to do a FFT (Fast Forier transform) on the file to get the natural frequencies. Does anyone know of any shareware that will do FFT's? Or at least something cheap?

Thank you all for your support!

 

[JagSTypeElectric]

I guess I should have included the equation I'm using:

freq. = (speed of sound/2*Pie)*(area of a tube/(volume in the tube*length of the tube))^.5

This can also be found in wikipedia under helmholtz resonance.

 

[GregLocock]

You have mistranscribed the wiki equation.

Tuning Helmholtz's can be rather frustrating, for a start, if you are talking about low pressure then you won't get much damping.

Chances are the main noise you are trying to cancel is at the number of intake valve vents per second, or some integer multiple of that.











Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SomptingGuy]

Hatch:

My point was that the expression for the speed of sound is not dependent on pressure.

- Steve

 

[hacksaw]

the old vacuum pumps developed discharge noise when the cylinder released the pressure and most used a muffler arrangement, of course one suited to draining condensible i.e. oil vapor

you need to be more specific about your pump style, pressures, capacities etc. to get a better answer from the crew

 

[JagSTypeElectric]

Thanks again for the interest!

I'm not sure how pressure relates to results. Could you further explain this?

I've seen the Helmholtz resonator work on vehicle intakes before and in my experience they can be effective at a specific frequency. I think it can be frustrating if you try to tune out a broad frequency noise. Having said that, it's obviously true that I have not actually done this myself before, I've only watched others who were much more experienced in the area. So I'm humbly listening to everyones input.

Here's more info for those who have asked.

I'm buying the pump off the shelf, and unfortunately I'm not able to get a lot of data on it. However, there is some info.

I'm buying the pump from

http://www.MetricMind.com

If you click on "EV Vacuum pump" on the left side of the page, it will take you to the detail page. I'm using the 70/6E2. Some of the specs are on this page, also a graph of pressure and amperage vs. time to pump up a 2 liter "booster" is attached through the link under the "-.65 bar" label.

http://www.metricmind.com/line_art/70-6e2p.gif

Thanks again for all the help!

 

[VibeAC]

Why don't you just rent a spectrum analyzer and be done!

Trying to develop your own system/methodology for analyzing dynamic signals can be time-consuming, and at the end of the day you have to make sure that your measurements and analysis are accurate (e.g. calibrated).

Contact the Modal Shop

http://www.modalshop.com/

Andrew Gorton, MSc
Noise & Vibration Consultant

http://www.PapadimosGroup.com

 

[Co]

I agree with the above post. Bruel and Kjaer works great and you can always do a FFT in Matlab