I just got some new Yorkville LS1208s up and running in the theater today. The only other time we've had subs in this room was a pair of borrowed EAW SB1000s in the back corner of the house for a big earth-shaking effect where it didn't much matter what parts of the room "played along". The LS1208s are for live music and dance playback, and I have them in a more conventional spot corner loading in front of the proscenium wall. Problem is, there's a pair of big return ducts right there at floor level in the corner both left and right, and they rattle a lot. I did not today get as far as trying to notch out the worst frequencies, but I'm guessing that'll be far from a silver bullet anyway. These sections of duct are also the only spots we can't easily access the outsides of, but I can take the grilles off and crawl inside easy. Does anyone know about a treatment that could work in this situation? What's made for use inside an existing duct?
Duct Rattle From Subwoofer
- Thread starterColin
- Start date
Brenden Friedel
Active Member
You’re not going to really be able to notch out the freq. causing the rattle. What may be smart is diagnose what it rattling or move the subwoofers to a different spot or consider flying the subs
You can wrap the outside of a duct with lagging, but I wouldn't put anything inside of it. There are products that are meant for going inside of the duct, but they present a major maintenance issue with dust/debris/bacteria/mold growth inside of your ducts. Most of the mechanical engineers in my firm refuse to put sound absorbing materials inside of the ductwork itself for these reasons.
BTW, most of the stuff that's sold as "duct liner", which is intended to go inside of a duct, will be a lightweight fiber-based material. That won't do much for dampening low frequencies. You need a dense, mass-loaded vinyl type of product to dampen the vibrations.
BTW, most of the stuff that's sold as "duct liner", which is intended to go inside of a duct, will be a lightweight fiber-based material. That won't do much for dampening low frequencies. You need a dense, mass-loaded vinyl type of product to dampen the vibrations.
BillConnerFASTC
Well-Known Member
Did you say you cannot get to outside of duct? Even with spray urethane? It might not take much at the center of the wide sides to stop it's vibration. From inside of duct, maybe just a screw into framing if you can locate it. Try a little and see if it makes a difference.
Part of reason I work so hard for round duct.
Part of reason I work so hard for round duct.
I would consult a good sheet metal contractor before doing anything else. They may fabricate some bracing to go inside at key locations, or come up with a way to run some screws from the inside into surrounding structure. I like Bill's expanding foam idea too, provided it is used on the outside of the duct, not the inside. Filling from a few, well placed holes might do the trick. If the voids are large, you might need help from an insulation contractor.
Don't make a new problem by adding much of anything to the inside of the duct. That could cause turbulence noise and reduce the air flow.
Don't make a new problem by adding much of anything to the inside of the duct. That could cause turbulence noise and reduce the air flow.
Thanks, all good input. I'll take a closer look at access on Monday. Cutting holes in the building just gets complicated here with lots of concerned parties. There's definitely a discrete way to do it from inside a closet on one side, but not sure about the other. Not a big void to fill under the ducts, but top and sides my guess is bigger.
@Colin Pardon me while I butt my nose in: Carefully temporarily removing a section of baseboard molding allows a 1/2 or 5/8 inch hole to be neatly bored with an auger, Forstner, or something neater and a thin flexible camera to be snaked in (complete with LED illumination) for a thorough peek a boo after which a copious amount of expanding foam could be inserted through the same, small diameter, opening prior to replacing the baseboard, totally hiding your intrusion.
I used to work with an even older geezer who taught me to use an extremely thin, yet WIDE, pry bar, along with a few carefully chosen scraps of 1/8 inch Maso' or ply to pry wall moldings off leaving the originally installed finishing nails / pneumatic brads behind in the wall. Removing the molding in this manner leaves its visible surface(s) unblemished complete with any fillers used to hide the original countersunk finishing nails intact.
With the undamaged section of molding removed any manner of tools, with protective shims, can be used to pull the nails or brads from the wall and discard them. Once the work was completed "behind the scenes", the original molding could usually be tastefully reattached using 19 gauge pneumatically driven brads. Vic' Lardy discretely employed this method time and time again, especially when working with moldings from the late eighteen / early nineteen hundreds which were long of of production. I'll butt out now and crawl back in my cave.
Toodleoo!
Ron Hebbard
Is it the sheet metal of the duct shaking in and of itself, as in the traditional theatrical thunder machines, or is it two pieces of the same rattling where they join together? Is it making noise because of sound waves traveling through the air, or because the subwoofer cabinet itself is shaking the floor and ductwork?
If it's the joints in the duct, it seems to me that one ought to be able to more solidly join the segments together somehow working from the inside of the duct without too much difficulty. If it's the whole of the duct that needs stiffening, that's somewhat of a harder problem. Likewise, if it's vibration from the speaker cabinet, a different supporting strategy might solve things nicely (which could be as simple as sticking it on a chunk of foam or some other soft material).
If it's the joints in the duct, it seems to me that one ought to be able to more solidly join the segments together somehow working from the inside of the duct without too much difficulty. If it's the whole of the duct that needs stiffening, that's somewhat of a harder problem. Likewise, if it's vibration from the speaker cabinet, a different supporting strategy might solve things nicely (which could be as simple as sticking it on a chunk of foam or some other soft material).
@DrewE @Colin @Amiers @FMEng and @BillConnerFASTC
I'd suspect the latter: The sub's output is rattling the building and its entirety. Herewith a 'Cole's Notes version of a previous (lengthier and more detailed) post: What feels like a decade and a half ago, I owed a favor to someone and the time came when payment was requested: I found myself generating a series of effects tracks to provide the sound and the FEELING of DIY fireworks in the basement unexpectedly exploding simultaneously seemingly directly below the stage. (There was no basement below the stage, we were already in the 1870 odd basement.) This was true "church basement" dinner theatre. The church was erected in the 1800's on the edge of a farmer's property within approximately 10' of a steep ravine leading to a small bay. The church basement was dug into the previously undisturbed soil where every approximately 1,000 pound boulder in the area was rolled, stacked and mortared to form the foundation for the original single story church; as the village and its parishioners grew, a second story was added; decades later, an attic meeting room and a bell tower.
Cutting to the chase: The group didn't know me from Adam but I was able to borrow a key and access to the basement of the totally empty church for ONE midnight to 5:00 a.m. session as long as I left the place undisturbed, and returned any tables I moved to their original locations prior to the 6:00 a.m. cleaning person's arrival.
Picking up the pace: I borrowed one of the original 'Sen-surround' subs originally developed and built to travel with the film "Earthquake!" as part of an optional equipment package to be received along with the 70 mm prints. For my one and only overnight session I lugged in the bare minimum for my purposes: A manually sweep-able audio oscillator capable of sweeping from appreciably below 20 Hz to slightly higher than 20KHz in one continuous band, the Sen-surround cabinet (complete with its LARGE 'Sen-surround!!! logo stenciled in white paint on the cabinet's sides) and the largest stereo BGW solid state amp I owned. In a church basement theatre there were a limited range of locations to 'lose' the sub without reducing the seat count nor occupying too much space in the cramped, near non existent, wings. I overran my 5:00 a.m. curfew but got to meet the caretaker as we passed in the parking lot.
In my five hours I'd tried several locations and chose two or three frequencies where the sub was truly vibrating anything / EVERYTHING that would sympathetically resonate. Even though the patrons would be exclusively in the basement, I got into my task and checked the church directly above along with the entrance lobby. I found frequencies you could hardly hear but could definitely feel. I stashed the sub then put the oscillator and my BGW back out in my van. At home, I added all the audible tracks that provided the appropriate sounds then laid down my sub tracks for the appropriate 'feelings'. When I auditioned various music and more standard effects for the director's selection and approval, I delayed demoing my final "product" until much closer to opening night when I could position the sub and gaff my cables ONCE. As it happened, our final rehearsal was on the Thursday evening prior to the Friday opening. Immediately above us in the church were the organist and choir director along with their choir of dutiful members primarily between 70 and death. Below them our rehearsal was progressing per normal: The cast rehearsed with near performance projections saving a little in reserve for the crowd anticipated the next evening; door bells / chimes were the real deal and sounded at level, as did real rotary dial phones of the era rang from a 25 Hz 90 volt source to sound 'right' when rung. Pre, interval and post music were played 'top 'n tail' at reduced levels for purposes of cuing then quickly faded out as we had no audience in attendance. The church and choir had become used to this approach anytime our rehearsals were going to collide with other rentals or church functions including wedding rehearsals.
Bottom Line: To say the organist and choir directly above us were unprepared for when we added the sub tracks and ran the explosion cue at our anticipated performance level would be an understatement at best. Unprepared; they dang near collectedly defecated their undies. So many people from various and sundry places in the building: Headed for the exits / stared down stairs for signs of smoke or flames / bellowed 'Is everyone O.K. down there? I don't recall anyone above basement level venturing down to actually see if everything / everyone was actually still intact.
Ya; back to where this novel started: The sub rattling the building's got my vote / sitting it atop some manner of mechanical decoupling's likely urinating in the wind.
@MNicolai @TimMc @FMEng @Ancient Engineer : A devalued Canadian penny for your thoughts?
Toodleoo!
Ron Hebbard
