AC Beat Frequency

[Cate&Rob]

When our Atwood AC is working hard (compressor running continuously) it develops a beat frequency at about 1Hz . . . audible only inside the trailer. I have removed the interior cover and can’t find anything loose (pressing against various brackets and panels does nothing). I have removed the exterior cover and can’t find anything loose, but even more confusing . . . the beat frequency is not audible outside . . . right at the AC with cover removed!

When the compressor is cycling on and off (normal “light load operation) this noise does not happen.

Cooling ability seems normal at all times.

Any ideas ?

Rob

[NoPeeking]

I'm not an acoustic engineer, but is it possible vibration from the AC becomes harmonic at another point... say refrigerator, theatre seating, TV mounting? The audible frequency may be some distance from the vibration source.

Mike

[Yoda]

When our Atwood AC is working hard (compressor running continuously) it develops a beat frequency at about 1Hz . . . audible only inside the trailer. I have removed the interior cover and can’t find anything loose (pressing against various brackets and panels does nothing). I have removed the exterior cover and can’t find anything loose, but even more confusing . . . the beat frequency is not audible outside . . . right at the AC with cover removed!

When the compressor is cycling on and off (normal “light load operation) this noise does not happen.

Cooling ability seems normal at all times.

Any ideas ?

Rob

Must be a 70's throw back any hippies around?

What little I remember from college - could it be dynamic harmonics setting up in the lower case / fan shroud? Did you press on the fan shroud? You can try adding Automotive self stick sound proofing to a few spots and see if that changes it. The only other place would be motor/fan vibration reflecting off of the fan shroud downward (echo chamber). My guess in under continuous load it heats up differently and something changes. Just brainstorming. Last did you make sure all screws and fasteners were tight?

I did that to my house big kitchen SS sink as when we first installed it - it had a bad reverberation when filling with water. Put the stuff on the bottom and sides and the noise went away. I got mine at the local auto parts, but Amazon has a lot of choices.

Hope this helps

Keith

[Country Campers]

I recently learned that one can adjust the air flow with the directional vents to achieve the same airflow from each vent. Being as the ac duct is a closed system this adjustment is made to achieve maximum efficiency. This is made possible with the use of an air flow meter, which I now have to adjust mine. I wonder if this could be the issue with the "humm" you are hearing. It also could be a loose piece of foil tape in the duct work.

Brian

[Cate&Rob]

Thanks for the ideas . . . looking for a natural frequency being lit up by the compressor. My initial idea as well. The difference is that this only happens after a while with continuous run of the compressor. I am wondering If the compressor is starving for refrigerant . . . but cooling performance seems normal. No connection points to measure system pressures or add refrigerant . . . that I can find.

Rob

[Jlawles2]

Rob, you might want to check the lines running from the condenser to the evaporator. As the unit runs hard for long periods of time, the lines could condensate and freeze causing the flexibility to go away. When this happens harmonics from the compressor will travel through the hard objects at mach chicken (higher than through air). While checking things also verify the torque on the hold down bolts on the AC to ensure it has not compression set the seal or the seal gone out.

[TucsonJim]

Rob - Have you tightened the four main bolts on that Atwood? I'd start there. If you get a chance, check out this installation video for the Atwood Air Command that e-Trailer produced about 4 years ago. It might prompt you on areas to check for the oscillation.

https://www.etrailer.com/tv-install-...t15025-21.aspx

Jim

[TedS]

Maybe it's the fan and compressor running at 1Hz speed difference. Dry bearings could drag the fan speed down a little.

[Scott'n'Wendy]

I like Jim and Ted's suggestions. Check mounting points including isolation rubber, and maybe the bearing could be giving an issue after it heats up after running a while.

[Jkwilson]

A data point for you Rob since I have the same ACs. I've never experienced what you are describing.

[Cate&Rob]

A data point for you Rob since I have the same ACs. I've never experienced what you are describing.

Hi John,

I was hoping that you would chime in with your knowledge on the same AC unit. We have never (in 8 years!) used our AC as much as in the last two weeks. Heat in Florida has been brutal for this time of year. I was wondering if this was “normal” after several continuous hours of compressor running . . . I don’t think it is. Now . . . what to do about this .

Rob

[Jkwilson]

Hi John,

I was hoping that you would chime in with your knowledge on the same AC unit. We have never (in 8 years!) used our AC as much as in the last two weeks. Heat in Florida has been brutal for this time of year. I was wondering if this was “normal” after several continuous hours of compressor running . . . I don’t think it is. Now . . . what to do about this .

Rob

Do you just have the one or do you also have a bedroom AC?

[Cate&Rob]

Do you just have the one or do you also have a bedroom AC?

Just one AC . . . so, nothing to compare to.

Rob

[Yoda]

Just one AC . . . so, nothing to compare to.

Rob

Brainstorming here
Could you be getting an imbalance sound between the compressor motor and the fan motor similar to a twin-screw boat when the props are out of sink? Loose mounts might cause that as folks have mentioned. I know - go back to my corner

Keith

[Jlawles2]

Rob, I know the home AC units (especially window units) will get a harmonic in them as they run for extended periods of time. I think it has to do with the steady state of the compressor and the compressed fluid creating a surge wave in the tubing. Remember steady state power is not steady frequency. The frequency of the power grid is cyclic, it has a period to it where it fluctuates between about 58 Hz and 62 Hz.

You may not have noticed it in home AC when in climates in the northern climates where heat and humidity are not as high.