Noise transmitted thro the ground

[maxh]

Hi everyone,

Has anyone got some experience of stopping ground borne noise transmission ?

I have a problem with a pump in a wet well which is transmitting noise to a dwelling some 50 yards away.

The noise is definitley coming through the ground as you can hear it in the walls. It is a problem as the nightime noise level is less than 18 dBA in the bedroom and at the problem frequencies we see a rise of 5 to 10 db depending on ground conditions (wet or dry).

The problem frequency is approx 300 Hz which is 6X the blade passing frequency of the pump (2 blades). There is also a lower harmonic at 150 Hz which is very apparent.

It has been suggested that this is a mechanical vibration problem, but I am not sure as the frequencies are quite high. If it is just a mechanical phenomenom we could fit anti-vib mounts, but would solve the problem at the frequencies we are seeing.

Any comments would be appreciated ?

 

[MikeHalloran]

I'd replace the motor bearings.



Mike Halloran

http://www.deangelomarine.com

 

[Tmoose]

Does the pump supply water for the house with the noise complaint? Blade pass related frequencies sometimes (usually?) are pressure pulsations/variations in the water, so pump mounting treatments don't help much. The pressure pulses run along the pipe like those bulls in Spain.

Can you feel vibration on the pipes or tanks in the dwelling when the pump runs? The pressure pulses must get some mechanical panels moving to create sound.

 

[maxh]

Hi Guys, thanks for the thoughts,

Mike

We suggested changing the bearings, but, the pump manufacturer says they are OK (well they would wouldn't they) and the problem has been apparent since day 1 of the installation and yes I know that bearings can be no good at day 1 and this has been pointed out.

The thing that supports the bearing contention is that the noise can be described as a trundling noise !

Tmoose

As for vibration, you can feel the vibration on the wet well covers, but the measured vibration peaks don't coincide with the airborne noise peaks.

The pipes run along the road in front of the house and do not enter the property at all.


I still don't understand why we should measure airborne noise peaks at 150 and 300 hz which are quite high multiples of the pumps natural frequencies.

Max

 

[MikeHalloran]

Check this thread:

http://www.eng-tips.com/gviewthread.cfm/pid/407/qid/94299

Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[Tmoose]

RE: harmonics - the blade pass pressure variation is probably not a 50 Hz sine wave. More like a train of pulses happening at 50 Hz. The FFT of a pulse train contains tons of even and odd integer harmonics, all happy to excite any resonance they can.
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I'd measure the vibration on the pipes, especially near the dwelling. They probably know all about the pressure pulsations the pump is spitting out. Oh, I bet they are buried, not in a chase, so breaking their connection to the ground may not be possible. So, reducing the pressure pulses may be the best path. Is the impeller full sized, or was the OD trimmed to match requirement. If full sized, what is the cutwater-to-impeller clearance? More clearance between cutwater and impeller OD is sometimes useful in reducing the pulse. "Increase the clearance to 15% of the impeller diameter" keeps popping into my head.
It is probably too big a pump to use a cheap VFD to adjust the speed, but changing speed a little can be pretty effective too.

A side branch resonator might be an economical to plumb into the pump discharge line. I never got the chance to play with regular pulsation dampers to see what they did to higher frequencies.