Need Help with Sound Reduction

[JHCC]

So, my wife just moved her yarn shop into a new space, which is just about perfect except for one thing: there’s a large air conditioning unit mounted on the ceiling that sounds like an airplane taking off. 

I’ve measured the noise with a decibel meter on my phone, and the level jumps about 20 db when the thing kicks on. Naturally, this makes conversation difficult and does not contribute to the relaxed atmosphere we’re trying to encourage. 

The majority of the sound appears to be coming out of the end closest to the wall, where the filter is mounted. 

It seems to me that if I add some sound-absorbing panels to the ceiling and wall of that space (like the stuff they use on the walls of recording studios) and some kind of sound-absorbing baffles to its sides and bottom (leaving enough space between them that the airflow into the unit isn’t blocked or constrained), that should help a lot. The panels can attach directly to the wall and ceiling, and I should be able to hang the baffles off the mounting frame that holds up the AC unit.

Here’s the kind of material I’m thinking of for the wall and ceiling:

 And here’s the “wood wool” material that I’m thinking of for the baffles (not as effective as the foam, perhaps, but better suited to the store’s aesthetic):

Anyone here have experience with this kind of thing? I’m open to suggestions.

[Latticino]

What you have described sounds like fan noise from the interior of the unit.  You have photographed the wrong side to evaluate equipment type, but from what I can see it appears to be a fairly standard fan coil unit with heating and cooling coils (though heat may be decoupled to a perimeter system).  There are a couple of installation/design problems that I can see from the photo regarding noise control:

1. No vibration isolation in the unit supports (should have neoprene vibration isolators on the threaded rod).
2. No flexible duct connection on the supply and return duct connections (actually there is no return duct at all, see below)
3. I usually like to see at least one acoustically lined return duct elbow at the return inlet to help attenuate fan noise running opposite to airflow (at least there is no compressor noise, which would be a lot worse)
4. This type of unit typically is not hung exposed in a finished, occupied space.  It should have been selected with all panels internally lined with acoustic material (the "skin" of the unit).  I suspect this is a budget unit and does not have this option.

Your selection of the "wood-wool" material should help.  Other acoustic attenuation materials will also help as well, including fabric wall panels and/or hangings.  Adding vibration isolation to the hanging rods is pretty easy and you should be able to do that yourself.  Put your hand on the unit while it is cooling to see if you feel any vibration.

Another non-intuitive option is to run the fan continuously. The change in background noise level is typically the most disconcerting, if the fan runs continuously it becomes background white noise and can more easily be ignored.  Many of these units have variable or (3) speed fans.  Provided the airflow is sufficient to prevent freezing of the evaporator cooling coil, you may be able to switch to running it at a lower fan speed.  This can greatly affect the fan noise generated.

There are other issues with the unit design/selection (including lack of connection to the exterior for code required mechanical ventilation), but I don't want to get too far into the weeds.

[JHCC]

Thank you so much, Latticino. I was hoping you’d chime in.  

Here’s the entire assembly:

1 hour ago, Latticino said:

You have photographed the wrong side to evaluate equipment type

This might help:

[Frosty]

Oh my, it almost looks like it was designed to be a resonator. 

I'm going to keep an interested eye on the thread and my ignorant ideas out. 

Frosty The Lucky.

[Latticino]

Looks to be an American Standard Dx Cooling only split system fan coil air conditioner.  These units are essentially the same as Trane, but from their "light commercial" line.  This is not really a product designed to be hung, exposed in a finished occupied space.  I'm not an acoustic engineer, but the suggestions I've already made should help.  I'm not sure how much pull you have for updates if your wife is just the tenant.  Only other thing I can suggest is to ensure the fan impeller is running smoothly and is not imbalanced.  Should have no other real noise generators other than the fan, so anything you can do to reduce RPM and vibration from that will help.

Good luck.

[JHCC]

Thanks, Dan. I'll see what we can do with the fan settings, vibration isolators, and sound-absorbing material as described. I'll keep you posted.

25 minutes ago, Latticino said:

I'm not sure how much pull you have for updates if your wife is just the tenant.

Very little, and if we did have any such pull, we'd probably use it to get a bathroom on the ground floor. Currently, customers and staff have to take the stairs up to the second floor or down to the basement, which isn't great for those with limited mobility.

[Steve Sells]

Be aware anything mentioned will only deal with reflections of the sound, it isnt dealing with the source of the sound itself it will still have the noise

[Frosty]

Yeah, the tenant landlord thing is why I didn't suggest putting it outside, even if you had to build a weather proof box.

Is there a way to deal with the source of the sound itself Steve?

Frosty The Lucky.

[Latticino]

Steve,

Umm, not exactly.  Slowing the fan speed (adjustable sheeve, VFD, ECM speed control, multispeed fan jumpers or switch...), if possible, will definitely deal with the source of the sound. Typically landlords of spaces like this don't bother with proper air balancing and just run the fans full out, even if they don't need to be.  Just have to be careful not to run the speed too low since there is the possibility of freezing the Dx evaporator coil (see manufacturer's install guide).

Also adding vibration isolation can deal with the source of the sound if the fan vibration is migrating through the hangers to the building structure or from the unit to the supply ductwork.  That is why I told him to lay his hand on the unit while running to see if he can feel any vibration.  Properly balancing the impeller can also make a difference if it is not running correctly.

[JHCC]

There's a really noticeable difference in the sound level from one end to the other, and all of the sound seems to be coming out of the air-intake side (where the filter is). It's a bit too high up to reach easily, so I haven't yet tried putting a hand on it to feel if there's a lot of vibration coming through the outer skin. I'll take a look later, and maybe while I'm up there, I can see if there's a visible model number for me to look up the installation guide.

[Frosty]

If you can reach it with a broom stick that'll let you feel the vibration. Do NOT put your ear to it!

Frosty The Lucky.

[JHCC]

No significant vibration; just a bit of a hum. 

[Frosty]

Just a hum, all way around?

Was it being loud? 

It looks like the unit draws air straight in the end, making the shortest distance between the room and the fan blades and the fan is usually the noisy part of squirrel cage blowers.

On the other hand the intake end of air movers seem to the the loud ends. If you want to quieten my T burners you need a baffle around the T, that's why NARB's Ts are behind the forge body, it's still loud but not nearly so much. 

I'm wondering if muffling the intake wouldn't go a long way towards quieting it. No, not a "muffler" but a large box around the intake with 4 large filtered intakes leaving the face opposite the unit's intake blank. This would leave the bulk of the noise a reflector aimed at the source and having 4x the filtered surface area the fan would have 1/4 the work to move the same volume.

Think of it as a reverse diffusion nozzle. 

Just some thoughts FWIY, my heads full of thoughts and sometimes I gotta let them out or the voices get crabby.

Frosty The Lucky.

 

[JHCC]

12 hours ago, Frosty said:

Just a hum, all way around?

Yes, just like it's quietly pleased with itself.

12 hours ago, Frosty said:

a large box around the intake with 4 large filtered intakes leaving the face opposite the unit's intake blank.

I'm thinking about putting those ridged foam sound-absorbing panels on the wall opposite the intake, on the ceiling between the intake and the wall, and on some kind of thin plywood "floor" between the bottom edge of the intake and the wall. That would leave a ~24" square opening on either side, which could then have baffles of some type placed in front to absorb the sound without blocking the airflow. I'll try to post a sketch later.

[JHCC]

Here's what it sounds like. When it's going full blast, it's about 70-72 db six feet away.

 

[Mike BR]

If it were me, I’d buy a case of cheap drop ceiling tiles and some hot melt glue, and see what worked. 

[Frosty]

That sounds like air movement from here, I don't hear any mechanical noise to "fix." Before I built the diffuser intake I'd try draping a wool blanket over or around it. Wool, especially felt is an excellent noise damper, I damped a couple machines in the soils lab quiet enough to live with. It was that or build a separate building for them. Wool felt is dense enough to physically damp vibration while blocking the sound waves in air. It worked darned well as a curtain even.

I glued 1/2" wool felt to the LA Abrasion drum in the state soils lab and it damped the injuriously loud ring of 12, 1lb steel balls falling IIRC 24" against the steel drum to bearable with foam ear plugs.

It still sounds like air movement rather than mechanical so just test damping mechanical noise in the body. Hence my suggestion of draping or wrapping with a blanket.

Frosty The Lucky.

[JHCC]

Hmm...now where could I get some wool...?

[Frosty]

He wonders with knitted brow. What a fabulous marketing tool! The stranger on the interwebs Felt, Oberlin's largest Bobbin!

Frosty The Lucky.

[BillyBones]

There is a wool gathering at Young's Dairy in Yellow Springs the weekend before Quadstate... I would bet some one there could get you some. 

That is if you can not locate some in say a local yarn shop or something. 

[Latticino]

John,

As previously mentioned, the easiest first method of reducing fan and air noise is to reduce the fan speed to the practical minimum available.  There are a number of design choices in unit selection, location and distribution ducting that I wouldn't have made, but that is water under the bridge at this point.  Strongly recommend you get the unit IOM and see what you can do about fan speed.  There should be some method of adjustment.  Just be careful not to reduce it so far that the coil freezes, which can have a number of bad side effects.

I can't really evaluate the noise source from your videos unfortunately.  I'm giving suggestions based on your evaluation onsite and specifically your indication that the sound is louder at the unit inlet.  Sound attenuating hangings can certainly help (a suspended architectural "cloud" under the unit could also be effective).  The fact that the duct is not insulated and there is no ducting at the return (unit inlet) doesn't help at all.

[JHCC]

33 minutes ago, Latticino said:

Strongly recommend you get the unit IOM and see what you can do about fan speed.  There should be some method of adjustment. 

Will do; thanks. I'll let you know if I have any questions once I get the documentation.

[Irondragon Forge ClayWorks]

When was the unit installed, if recently it looks fairly new. I would ask the landlord to have the installer come and remedy some of the problems.

I can't control the wind, all I can do is adjust my sail’s.~ Semper Paratus

[JHCC]

2020, I think. 

I’ve tracked down the installer’s guide and I’m working on making sense out of it. Probably going to go ahead with the sound-absorbing foam and baffles as a first step, as I’d rather not mess with machinery I don’t understand if I can possibly avoid it. 

[BillyBones]

JHCC, have you any info on who did the installation? There maybe some kind of service agreement with the unit that they will come out and look at it for free. Part of the one with mine came with periodic maintenance checks. They do not do much just make sure it is working ok, any repairs i would of course have to pay for.