Mass and vibration transfer

J

jrstar01

Audiophyte
I have a subwoofer placed in a cubby hole intended for firewood. A few inches above the cubby hole is another cutout that has a few suspended shelves with display pieces (beer steins, vases, etc.). All of this structure is framed and drywalled, including the fireplace mantle and such. I built the shelving areas and the firewood cubby holes myself, and they are quite solid.

The old sub was a snug fit in the cubby. Literally pressed against the top of its hole. This would result in occasional vibration of the stuff on the shelves. There is even a small crack in the compound just above the cubby.

The old sub has given up the ghost and will be replaced.

Let's say that potential new sub "A" weights 60 pounds and will fit in the opening without touching the sides or top. Potential new sub "B" weights 100 pounds and also fits the opening with room to spare.

Will the extra mass and inertia of Sub B result in less vibration being transferred into the surrounding structure? Could this result in a meaningful reduction of NVH from the shelves? Or will the spl from a modern sub that's NOT 15 years old cause all that stuff to shake anyway?

FWIW, the listening area is an open concept deal that's about 9000 ft^3, plus there's a staircase open to a loft, and the back of the listening area has three large wall cutouts open to a sunroom. There is all kinds of volume and several places for sound waves to get lost. Yeah, also contemplating a sub on each side of the fireplace. Thanks!
 
BoredSysAdmin

BoredSysAdmin

Audioholic Slumlord
FWIW, the listening area is an open concept deal that's about 9000 ft^3, plus there's a staircase open to a loft, and the back of the listening area has three large wall cutouts open to a sunroom.
Woohah
 
TheWarrior

TheWarrior

Audioholic Ninja
Hi,

The vibrations are caused by the subwoofer which in turn causes other objects to resonate at specific frequencies. A big, sturdy, well braced sub won't resonate itself, but any loose objects or large wooden structures (regardless of how thick they are) will resonate.

Getting a bigger sub with a shelf full of glass items is a safety concern. Most of us have to compromise with a living room being our HT, but get your breakables far away!
 
tyhjaarpa

tyhjaarpa

Audioholic Field Marshall
Platfoam under your sub would help to avoid resonance directly moving from sub to construction and vibrating things. I myself have build platforms for my sub / towers using these and they work fantastically! How ever this does not avoid things vibrating from air pressure / sound waves your sub will produce.
 
KEW

KEW

Audioholic Overlord
You need to address both vibrations from direct contact (through house frame), and airwave vibrations.
A platform like tyhjaarpa suggests reduces direct contact considerably.
Reduction of airwaves is counter-productive, so you really need to address the items on the shelves.
I don't know what might work for your decor, but you can dampen the shelf surface with something like a sheet of felt or neoprene:
http://www.amazon.com/gp/product/B003HEYL5Q/ref=pd_lpo_sbs_dp_ss_3?pf_rd_p=1944687622&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B001GAD5G4&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0D552DXGGN94RT0W65HJ

Or you can address individual contact points by putting felt or foam pads or something like Blu-tack:
http://www.amazon.com/Blu-Tack-060968-Reusable-Adhesive-75g/dp/B001FGLX72/ref=sr_1_1?ie=UTF8&qid=1455281885&sr=8-1&keywords=blu-tack.
That has worked well for me.

Without knowing exactly what items you have and what they weigh, it is impossible to know what would work best for you (and regardless, you'd end up doing some trial and error).

Make sure your shelf does not slope so that objects travel when they vibate (felt will usually stop audible vibrations, but objects can still "wander".
 
J

jrstar01

Audiophyte
Thanks for the comments.

I'm probably over-analyzing this. The old sub, which was 18x20 on the top, was pressed firmly against the drywall in the top of its hole. I mean full contact across the entire surface area. Any vibration or cabinet resonance on the top was going STRAIGHT into structure.

Either of the new subs I'm considering won't be pressed against the top. Cabinet vibration would have to travel down through the feet to get into structure. Big difference. (Platfoam isn't possible. I'm dealing with .25 to .75 inch gap on the top. Securing the stuff on the shelves is on the to-do list though.)

I guess I just need to get about the business of choosing between the two subs and move on.
 

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