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relan 07-30-2018 02:57 AM

Riser Build - Novice Questions
 
3 Attachment(s)
I am building an approximately 8'x6'x13" riser that will sit on my existing wood floor and had a few questions I was hoping to get some help with.

I was trying to roughly follow the multifunction-theater-seat-riser article from audioholics.com though its from 2007 so may be dated.

Layers:
Carpet Pad
1/2 Plywood
U-Boats
2x10
6" Rockwool
1/2 Plywood
Green Glue
1/8 Vinyl Mass
Green Glue
3/4 MDF
Wood Flooring

1) Does the plywood and MDF make sense or should I just use OSB for all the flooring layers?

2) Does it make sense to use Green Glue between the flooring and the Vinyl?

3) What barrier should I use to keep the insulation from coming through the gap created by the U-boats? Should I use a plastic sheet beneath the U-boats and seal it along the edges of the riser or does that defeat part of the point of having the air gap? I didn't quite follow this part of the original article.

4) 2/3s of the back of the room are open to another room. Does it make sense to vent the back of the riser in places that aren't next to a wall? I assume no?

5) If the seats partially cover the vents does that defeat the benefit of the vents?

6) Is creating 1.5" ports in the front of the riser in each bay useful? Most of the other build threads I have looked at other then the article don't seem to do this.

Any additional feedback welcome.

Thanks!

drunkpenguin 07-30-2018 09:35 AM

I'm not sure what the point of the ubolts are, but that riser will weigh a lot and if you are putting in on top of a wood floor expect to find dents in the future if you ever remove that riser.

DaveClement 07-30-2018 10:02 AM

1) Plywood and OSB can all be used somewhat interchangeably. Plywood has some structural advantages, particularly if it gets wet, but it usually costs more. OSB is certainly good enough for this application, as it is the primary material that is used in subfloors. I would NOT recommend MDF. MDF has much less strength, particularly if it ever gets wet. It works for building non-structural speaker cabinets and book cases, but it should never be used for something that you have to stand on.

2) Green glue should be used between two hard and somewhat porous panel type layers (drywall, OSB, plywood, etc.) It would probably not be effective with a soft material like vinyl. You generally want at least two 3/4" layers of OSB or plywood or three layers of 1/2”. To save money, roofing felt can be used instead of Green Glue to provide some damping between the layers. You don’t necessarily need all of the performance of Green Glue, since the goal isn’t to keep sound out of the riser, it is to keep it from resonating like a drum.

3) I wouldn’t worry too much about the gap on the bottom where the U-boats hold the framing above the floor. Unless you are using blown in insulation, it isn’t going to go anywhere. You’ll probably be covering the gap with carpet or something anyway.

4) It makes more sense to have the openings next to a wall, since that is where sound waves concentrate. Compare this to putting a speaker in a corner, which makes the bass louder. If you don’t have a wall to put the riser against, it will still work, just not as well.

5) As long as air (in other words, sound) can reasonably get in, you should be OK. If you put a couch with very short or no legs on top of the vents to the point that it was blocking the air flow, they would be far less effective.

6) 1.5” ports are probably too small to do much good.

At first I skimmed through the material and focused on the questions. After reading through the materials, I have a few more thoughts. I assume that the top of the list starts at the floor and the bottom of the list is at the top of the riser

I’m not sure why you would need carpet pad or 1/2” plywood on the bottom layer. You can set the U-boats directly on the floor. (Or the framing, if you are not using U-boats.)

Regular old fluffy fiberglass should work just fine for this application. You don’t necessarily need rockwool or any other “fancy” insulation.

Mass loaded vinyl (MLF) is used to add mass. There are much cheaper ways to add mass, like another layer of OSB or plywood. You can also use drywall if it sandwiched between layers of OSB or plywood. MLV can be as over $2.00 per pound. Drywall can be as cheap as 18 cents per pound. OSB or plywood would be a little more, but far cheaper than MLV. I don’t see the point in more mass in this application. You need it to help block sound in walls an ceilings, but you are not really trying to block sound here. After all, you are putting in vents to let the sound in. You are really only looking at making a sturdy surface that won't resonate or vibrate. I would go with my earlier thought of two layers of 3/4" OSB or plywood, or three layers of 1/2", or some other similar combination that is around and inch and a half thick.

relan 07-30-2018 10:29 PM

Quote:

Originally Posted by DaveClement (Post 56564284)


4) It makes more sense to have the openings next to a wall, since that is where sound waves concentrate. Compare this to putting a speaker in a corner, which makes the bass louder. If you don’t have a wall to put the riser against, it will still work, just not as well.


I’m not sure why you would need carpet pad or 1/2” plywood on the bottom layer. You can set the U-boats directly on the floor. (Or the framing, if you are not using U-boats.)


Mass loaded vinyl (MLF) is used to add mass. There are much cheaper ways to add mass, like another layer of OSB or plywood. You can also use drywall if it sandwiched between layers of OSB or plywood. MLV can be as over $2.00 per pound. Drywall can be as cheap as 18 cents per pound. OSB or plywood would be a little more, but far cheaper than MLV. I don’t see the point in more mass in this application. You need it to help block sound in walls an ceilings, but you are not really trying to block sound here. After all, you are putting in vents to let the sound in. You are really only looking at making a sturdy surface that won't resonate or vibrate. I would go with my earlier thought of two layers of 3/4" OSB or plywood, or three layers of 1/2", or some other similar combination that is around and inch and a half thick.

Thanks for all the feedback much appreciated!

The riser will be against a partial wall so I could vent the portion bordering the wall. Is there some issue with not uniformly venting and having some bays vented and others not? I assume its bonus how much venting as you can do? Since its open to another room in the back I feel like it would look better to not have a bunch of vents on the back side not against the wall.

I was somewhat worried that the U-Boats would long term damage the floor. Maybe they would just worst case scuff it? They seem to be rubber so I assume they wouldn't scratch it or pull the oils out of the floor. I was going down the better safe then sorry route.

Is there some advantage of 3 layers versus 2 for the flooring? Is it just the resistance to water with the plywood as you mentioned?

I assume the wood floor is just tongue and groove and floats on top. I wonder if there needs to be roofers felt or something under the wood floor without impacting the install. I don't feel like my current floor is creaky.

Thanks again for the response I appreciate it.

relan 07-30-2018 10:35 PM

Quote:

Originally Posted by drunkpenguin (Post 56564136)
I'm not sure what the point of the ubolts are, but that riser will weigh a lot and if you are putting in on top of a wood floor expect to find dents in the future if you ever remove that riser.


The U-Boats are 2 x 2 x 1.5" U shaped rubber pieces made by Auralex that you attach to the studs that I believe isolate the riser from the floor. (I assume others make them as well?)

DaveClement 07-31-2018 09:26 AM

For clarity, in order to be a "stud", the framing member has to be running vertically. When it is running horizontally, to support the floor, it would be called a "joist".

U-boats are indeed designed to isolate the floor from the structure below. There are many valid applications, usually when you are "floating" a floor to isolate vibrations and sounds so that they don't travel out of the room and into the building structure, and visa versa. This is normally done for the entire floor, for the same reason you would build independent walls or use chips and channel. I don’t see a lot of benefit from isolating the riser from the floor below. Vibrations will transmit through the floor in all the areas that are not covered by the riser, and you are deliberately letting sound into the riser to the floor under it.

As for venting, vents into bass traps that are near corners of the room are more effective. Vents that are not at corners should still provide benefit, just not as much. Areas that are not vented won’t add benefit. This isn’t a black and white issue, and using a riser as a bass trap is just a way to get something “for free” from a structure that you are already building.

You might be interested in this thread that I just commented on. https://www.avsforum.com/forum/19-de...does-work.html

I can’t speak directly to U-boats damaging a floor or not. It all depends on what kind of floor will be under the riser. Carpet would likely get crushed and may not spring back years later, no matter what you put on it. Areas that are not covered may fade more than areas that are covered by the riser. I can say with certainty that rubber can discolor some types of flooring. I once had a cart with rubber wheels on a vinyl floor and the coloring in the wheels leeched into the vinyl flooring, permanently staining it.
More layers of flooring, particularly if you use Green Glue between the layers. Is less likely to resonate and cause discolorations in the sound. If you bang on the top, you don’t want the riser to sound like a drum. A dull muffled “thud” is much better.

If your existing floor is stable, you really don’t need to do anything to it, other than perhaps putting down something to provide a slight cushion and to protect the finished floor underneath if you ever plan to expose it again.


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