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11-05-2018, 10:56 AM #1
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Well now that I'm on a roll...... AC noise reduction
.....and you guys just can't seem to get enough (cough, cough) of my postings, I'll mentioned something else I did.
For whatever reason, my ears are sensitive to sounds in a certain frequency band. And as my luck would have it, that frequency band is happily supplied by that dang Dometic Brisk Air II 15K BTU roof air. Not sure why, but in the last 2 years, I've had a very difficult time sleeping all through the night. Add to that this thing called restless leg syndrome, and it makes for some maddening nights. And that dang A/C unit doesn't help matters. Ever since we went into a Solitude and noticed how quiet that A/C system is, I've wanted to throw rocks at mine. And it irritates my last nerve that I have to crank up the TV volume to 99 in order to over come the A/C's drone. Not to mention that if I'm up trying to calm my legs down and watching TV, the volume is loud enough that it wakes the wife. So........for the last 8 months or so, I've been on a quest to try and figure out how to calm the A/C unit's irritating drone.
First I tried to noodle through to a solution with all manners of sound deadening Rube Goldberg solutions, but that woulda meant probably affixing some thing to the ceiling. That was a non-starter due to not meeting the WAF (wife acceptance factor). So I scoured the internet and that pretty much turned up nothing other than, turn up the TV and live with it.
Then one day I happened on a forum post (don't even know how I got to that forum/post), but there was a conversation between the poster and a Dometic Tech. He suggested that the poster swap out the intake/return air plastic ceiling thingy with a unit that doesn't have that air dump grid. I really didn't give this much thought, but two weeks later we're camping, it's 3:15am, I'm up with the RLS, I'm irritable, and that dam A/C is just laughing at me with its constant droning. THAT'S IT - I CAN'T TAKE IT ANYMORE. So as soon as I could, I ordered the kit.
Being concerned that I'd experience a placebo effect (you know, spend a bunch of money and time and surely what you did made a difference when it hadn't), I dusted off my Radio Shack sound pressure meter. I was mad at myself for having sold a frequency sweep unit that I used to help "tune" audio systems and tame room acoustics. But no matter - I've got golden ears. Shyeah....right.
The kit came with everything one needs to convert the "air dump" intake/air return to just an air intake grate. Before I started tearing stuff out, I measured sound pressure levels in three locations - sitting in the recliner, sitting on the couch across the back of the coach, and up near the stairs going to the bedroom. I took both "A" and "C" weighted readings. Then - I ripped out the old and went in with the new paying special attention to making sure the ducts were sealed properly to the plenum. I also tidied up the wiring which, I know wasn't really necessary since GD does such an awesome (cough, cough) job of bundling their wiring.
Switched the A/C on and????? I kinda was disappointed. I just knew it was gonna be whisper quiet. Alas, it wasn't. So I got out the meter and took readings at the same three locations and to my surprise, there was a 3-4db decrease in sound pressure at each location. Not to get all techie, but 3db is the smallest increment of sound pressure the human ear can discern (Useless trivia: if you want to increase that KC And The Sunshine Band sound by 3db, it will require doubling the amplifier output. So if you're at 4 on the volume knob and the amp is putting out 4 watts, to increase the loudness by 3db, the amp has to double its output now 8 watts. Crank it some more and we go to 16, 32, 64, and finally 128 watts just to get the party really jumpin'. Now you understand why so many audio systems sound like crap when turned up. It's because the amp runs outta steam and goes into clipping and the resulting harshness of distortion ZZZZZZZZzzzzzzzzzzzzzzzzzzzzzzzzzzzz..........
Sorry - I just bored myself to sleep with all that. So where was I ?
Oh - so this new air intake did, in fact, result in lowering the sound pressure. Not a whole bunch mind you, but it did make a difference. But that's not the real story. And this is where it pissed me off in not have a frequency spectrum analyzer. I was perceiving - or thought I was - a shift in the frequency of the "drone". So, I asked the wife to come out and tell me what she thought. Once inside, she immediately said, "Wow! What a difference". When asked to define what she was hearing, she said, "Well....it seems to be a little bit quieter, but the sound is way different. It doesn't have an edginess to it, but sounds smoother, almost a hum like sound."
So was the $39.95 I spent worth it? I kinda think so. Because now when I'm not able to sleep, I still have to have the TV up louder than I'd like, but the A/C's drone isn't in a frequency band that's irritating and allows me to discern the dialog from the TV.
Tom
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11-05-2018, 10:57 AM #2
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- May 2016
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11-05-2018, 11:28 AM #3
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What was the kit part # ? And sorry, I'm a little slow today - what exactly was the part that was replaced ? I can't recognize anything from the pictures ? Thanks !
2022 Imagine XLS 22MLE
Sold: 2015 27RL
2016 Ram 3500 Crewcab Cummins SRW
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11-06-2018, 07:57 AM #4
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11-06-2018, 09:16 AM #5
Hi Tom,
Great to have you back on the forum! If you don't mind, I am going to move this thread to the AC section and add a little more info to the title. This is good information that I am sure others will be looking for in the future.
RobCate & Rob
2015 Reflection 303RLS
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11-07-2018, 06:23 AM #6
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- May 2016
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- Walthourville, GA which is just outside of Hinesville, GA home of the 3rd Infantry Division
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11-07-2018, 06:31 AM #7
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Sorry about that lack of info. The part is here: http://getrvparts.com/dometic-310500...ssembly-cover/
And here's what it looked like before the mod.
The highlighted portion is the dump grille. The above part number eliminates this. This is the intake in our bedroom which I intend to change out at some later date. Theoretically, this "dump" allows a greater volume of cooled air from the A/C to fill the room. Honestly, I don't see any decrease in performance without this dump. In fact, because that cold air plenum is now completely sealed off, I seem to notice an increased air flow from the ceiling vents. YMMV as I don't have a flow meter to substantiate my opinion. And one other opinion.....because of the design of the dump grille on the backside, it results in probably quite a bit of air turbulence. That could be the low frequency "rumble" I was hearing.
OH - and one last thing. Be dang sure that if you install this replacement grille, you follow the instructions on tightening down the loooooooong bolts that secure the A/C unit to the roof and "squish" the A/C unit's gasket. If not done right and the torque is not even, it could result in some vibration noise being transferred to the coach - NOT GOOD! I've got an inch-pound torque wrench, so I was good-to-go.
Tom
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11-07-2018, 07:54 PM #8
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11-19-2018, 07:42 AM #9
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Sorry for the delayed response. I'm not quite sure I understand what you mean by having a different unit. I see that you own a 2018 337RLS and I haven't been in one (ours is a 2016). It's my understanding (assumption??) that the Reflection 5er's come standard with Dometic's Brisk Air II. I guess it's possible that GD has eliminated the "dump" grille and are installing that which I installed. A pic or two would be helpful.
Tom
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11-19-2018, 08:35 AM #10
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Reflection swivel table question
Today, 04:22 PM in New To RV'ing, Purchase Questions & General Concerns