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Old 05-29-2006, 08:40 PM   #15
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Just finished an overhaul of the front AC for noise. I started w/my noise meter & did a Before test = 64.5 dBA (A-Weighted sound for closest to the way the human ear responds) w/the fan on high and either AC on or fan only (due to the low vibration frequency of the compressor, it doesn't add to the A-weighted sound level).

I tore the unit apart, removing plastic grille, plastic plate, 6 screws holding lower plate on supply plenum. The duct from the squirrel cage fan visible once the lower plate is off was not taped off at all to the plenum, and the plenum was not taped off to the front or rear supply ducts; thanks WRV! I removed the plenum (requires taking one of the three 3/8" headed flange screws that sandwich the AC unit to the roof out as it punctures both the lower plate & the upper rear edge of the plenum).

Also, there is a 110V elec box screwed to the PS of AC cutout obstructing the return air path. Since air noise is proportional (sort of) to air velocity, obstructions increase air velocity. In addition to the elec. box, a giant wad of wiring is in the way of airflow on the PS, front, both increasing velocity, and putting movable junk in the air path to vibrate.

I moved the elec. box up and to the rear where it is as out of the air path as possible. I moved the wad of wiring (looked like about 30' of paired 28ga) to above the new elec. box location; trimmed the remaining unused wiring out (about another 4' of useless junk), zip-tied the rest of the wiring into tight bundles over the front of the supply plenum and to the passenger side thereof as applicable. Then I did some sheet metal origami to get the return plenum's ~4"x4" top duct hole to mate to the AC unit's squirrel cage ~4"x4" air exit as best as possible. Then I taped off the plenum to the AC unit & plenum to the front & rear supply ducts & reinstalled the lower plate & plastic plate/grille. Note that the tape job will have to be cut to remove the plenum in the future for service if disassembly is required.

Now I've got 60.7 dBA, a reduction of about 4 dBA exclusively from making a sanitary & complete installation with the wiring out of the air path and the ducts sealed. Sweet. Now I have an idea for a new grille/deflector to try out, but that'll take a bit of fabricating.
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Old 05-30-2006, 12:22 AM   #16
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EM - OK, 4 dBA reduction...numbers are numbers. Now, how does it sound? When you go in and turn the AC on can you say, "WOW, this is a LOT quieter"? Or, is it more of a thought like, "Hmmm, sounds a bit quieter"? (Who says being hard of hearing is a bad thing)? I used to have a feel for noise numbers but it has been too many years and I have forgptten what it really sounds like. I seem to recall changes at that level are barely perceptable. I suspect most of the forum can't associate the noise data with physically hearing also.
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Old 05-30-2006, 09:57 AM   #17
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Steve- Great question.
Most people cannot distinguish 1dB difference at all. Some cannot tell 2dB. Most can tell 3dB, tho meaningful hearing loss might wipe out that capability. Almost everyone can tell 4dB even w/hearing loss, when the noise levels are juxtaposed (better that way... or this way?).

Noises more than 3dB apart default to the louder one only, i.e. they don't add but the louder one overpowers the lesser in calculations and mostly in human response, even to you might distinguish the lesser. The compressor noise is one such- due to the very low frequency it doesn't seem to have much power, so it doesn't add in the overall noise level, even tho I can tell when it kicks on and off while the (much louder) fan is going. Translating to this problem, if you couldn't get 3dB+ difference, it wouldn't be worth a great effort as far as noise goes.

When thinking about motorhome noise, I think about environment. While driving, it obviously wouldn't matter at all if you ran gen & roof airs on high while the coach is going 60mph down a concrete interstate; road noise kills all. But while parked, I want to be able to talk w/out shouting (i.e. talking or TV competes w/AC). In ambient noise levels above 50dB, normal hearing persons raise their voice, while below they don't. Since the low fan noise after finishing the AC installation for WRV is about 56dBA, that means raising voice for any AC-operating condition to feel you can be comfortably heard. If the AC was 65dB (WRV slap-dash, cheezoid style, w/fan on high), you would have to raise your voice a fair amount, close to shouting for women (men are ~3dB louder than women as a rule so we can be heard above higher background w/out shouting than women; I'm sure there is a deep philosophical point somewhere in this neighborhood, but I have to admit it is entirely lost on me whatever it is).

So I've added a comfort level by dropping from ~65dB to ~61, but I haven't made the AC noise inconsequential by any means. And listening in the coach before, then a coupla hours later in the after condition, most folks wouldn't recall the before in a sufficiently detailed way to say "aha, nice fix," tho some would hear the difference. I'm looking for another 3dB to drop out of the raised voice issue entirely into "normal" speech levels (~58dB fan-high); hoping to get that from a clean looking deflector that would replace the plastic grille. If I do the live-in-front-run-rear-AC trick referenced above, after finishing WRV's AC install for them, that alone would do the trick. But son Jason just bought a house in Las Vegas; need all the AC you can get there!!!!!!!!
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Old 05-30-2006, 11:36 AM   #18
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Have looked at several different coaches, most have double discharge registers as Alpine uses. All quieter.
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Old 05-30-2006, 04:52 PM   #19
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Ok, I admit that I sometimes miss the obvious. I have an 05-06 36' with dometic roof ac's with heat pumps. It seems that the fans have only one speed and that is high. Where is the switch for low/hi control? I would love to know as our units are way loud.

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Old 05-30-2006, 05:49 PM   #20
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Jerry, You will find your fan speeds in the thermostat. I have a 34' Alpine with a single thermostat w/ zone 1 & 2 which dictates which A/C is on or both if I wish. The fan speeds are controlled in the thermostat. Find your book or the single sheet that came with the coach in reference to the thermostat. It can be confusing until you understand it and then it is quite simple. Of course that pertains to most everything we do except in my case computers. They are always confusing.

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Old 05-31-2006, 05:29 PM   #21
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Thanks for the info Ted. I thought that leaving the fan in "auto" would give me correct speeds. Aparently I need to set the speed myself with this unit.
Have never been to Pa. Looking forward to it when I retire in a year or two. Understand it is a beautiful state.

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Old 06-06-2006, 01:17 PM   #22
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Sound tests are done. The solid grille deflector instead of the OEM plastic grille gave no discernable difference
The primary sources of sound then devolve to the noise created by air rushing around either the 100+ sq. inches of deflector perimeter or 70 sq. in. of grille opening, and the compressor.
My front compressor has a loud, low vibration that causes a drumming effect on the roof. The rear one does not.

While fooling around, I found that the optimum sound level for daytime is achieved by closing 1) bedroom vents, 2) the accordian door to bedroom about 80%+, and 3) the sliding door behind the fridge about 95%+, and running the rear AC on low fan. For night time, I can reverse the theory, running front AC w/the first & third thru 8th vents (counting from the front) closed. Closing second vent from front caused a loud whistling that could be heard throughout the coach, even w/doors closed, due to the high pitch. But I'm still left w/the front compressor drumming whether fan is low or hi, and regardless of doors closed. I'll have to think about that some more. For some reason the front AC is mounted differently than the rear (3 long screws instead of 4, single ply foam under leading edge @ front AC, vs. double @ rear), so maybe the mounting isn't right.

For those interested in better efficiency of AC units, I'd suggest replacing the painted AC cover on the roof (for those whose covers are painted) with a new one that is white, or painting the cover white, for less heat gain on the compressor coils.
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