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Bookshelf Speaker Acoustics in a Cabinet

901 Views 12 Replies 6 Participants Last post by  Dennis Erskine
Greetings to all. I've very much enjoyed reading these forums and while some of the more advanced "in's and out's" of room acoustics makes my head spin, I've been trying to fight through it and garner a basic understanding.

I understand that LCR speaker placement in a cabinet is frowned upon. Unfortunately, I find myself in a bit of a pickle, if you will. My wife and I are buying a new home which should be complete around September or October. It's your typical "tract home" type of setup, so customization is pretty limited. It's a two-story home with no basement, but it does have an upstairs gameroom. There is a bedroom off of the gameroom that measures roughly 15'-6" X 12'-0" X 9'-0". We plan to make this our new media room.

Due to layout constraints and mostly spousal pressure, I'm forced to orient my planned seating arrangement towards the short end of the room. The same spousal pressure is also forcing me to build custom cabinet across the entire "front" of the room to house all of my equipment. This room will have two cased openings on the right-hand side into the game room, as I had to agree to leave the area with an "open feel". I hope to fill these openings at a later date, but that's another discussion.

My question is this: I plan to use three NHT SB-3 (bookshelf-type) speakers as my LCR front speakers. My current cabinet design includes "false columns" to enclose the L & R speakers with the C in a section of cabinet either above or below the 50" plasma display. Since I'm designing the cabinet myself, I'm pretty flexible on the materials and method of construction to use. My initial idea for the enclosures (top, bottom, and rear) around the speakers was to sandwich two layers of 1/2" MDF with Green Glue, then attach a layer of 1" Insulshield, then fill the remaining space around the speakers with batt insulation.

Please bear in mind that I'm not looking for audiophile quality here, but I do want good performance if I can get it. The cabinet doors will be covered with a MOB-type fabric, but what else should be done around the fronts of the speakers?

Thanks in advance for any help that you guys can provide me.
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Stand in your room and speak to your wife. Cut the bottom and top out of a cardboard box. Stick your head the in box and speak again. Thanks exactly what will happen to your speakers in a cabinet.
So there's really no way to achieve even a decent sound with this approach? Rats! My plan for the L & R speakers was to essentially build them inside of false columns. My idea stemmed from what folks have done with their side surround speakers that I've seen in these forums, except that in this case the face of the speakers would only be protruding 6 inches in front of the face of the surrounding cabinets, leaving 4 inches of speaker "inside the cabinet".

Dennis, given my room size limitations (only having 12ft front-to-back to work with), are there any suggestions that you could offer me?

I'm sure that my room layout would make much more sense if I could at least post a rough outline for you guys to look at. This design is far from set in stone, so if this is a dumb way to go (which at this point I'm 99% sure that it is), then it sounds like I need to go back to the drawing board.

At this point, I've been running this room through my mind so much that I'm starting to get designer's block. Any help you guys could give me would be very much appreciated.
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Ok, after obsessing about this for the last few hours and the numerous google searches that I've done as a result, I stumbled upon a photo that will be very helpful in trying to convey what I'm wanting to do for my front speakers. My "false columns" as they are currently sketched up are very similar to the application shown at newton.org.

The site lists Dennis as the consultant for the project, so now I'm confused. Is this type of speaker mounting NOT considered a "cabinet". I must have done a bad job of explaining my idea in my first post. I'll scan one of my sketches tomorrow and link it so that you guys can actually see what I'm talking about.

I need sleep.
Dennis that cardboard box example is great. I propose a part 2. While your head is in the box have your wife stuff all the remaining space in the box with bath towels leaving only your mouth visible.

Now that's how the cabinet might sound if you surround the speakers with dense insulation.
FWIW, I was going to put my JBL E30 bookshelves in a "cubbie". I decided against it, but I think it could have been OK. I called JBL and they told me that as long as the speaker is front ported (which the E30 is) it should be OK. I was instructed to make sure the front of the speaker was at least flush with the cubbie (Yours protrudes, so thats better). Then fill the area around the speaker cabinet with insulation. I was then going to cover the entire opening with speaker grill cloth from Partsexpress.

You have to remember that a lot of the very helpful people on this forum design and build high-end dedicated rooms. Dennis designs the best looking theatre's I've ever seen (pictures only, don't know anyone locally with a DE design). You'd never find a speaker in a box in one of his theatres because that's not the best way to go. But...

We all have to make compromises. Check out my gallery and the thread linked in my signature. I'm building a "Media Room" because it just doesn't compare to the HT being built by members here. I'll have 2 windows (although treated with blackout shades), I'll have built-in shelves in the front corners (my wife didn't want an entire fabric wall up front), and I have no columns. That's 3 HT rules broken right there. It's about compromises, and even though my room isn't finished yet (drywall to go up next weekend), I think I'll be quite happy with my room.
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Thanks for the reply, Toeside! Yeah, it's pretty apparent that my little media room is going to be fairly low-end compared many of these guys' dedicated rooms - especially since I'll be constructing it in phases and slowly replacing equipment over the same timespan. However, I feel pretty strongly that with the right design and prior planning that I can still have a good-performing setup to be proud of.

FWIW, I've uploaded version 1 of my floorplan to my gallery. Since I'm a newbie to the forum, I need a few more posts before the system will let me include a link in my message.

I'm still confused as to the difference between installing speakers in false columns that flank a screen in a proscenium (which most people seem to be doing) and installing speakers in false columns that flank a cabinet. Is everyone using a specific type of speaker, or did I just do a poor job of describing my plan in my first post?

My wife will have me committed if I stick my head in a box stuffed with towels and start having a conversation with her.:D
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Would she potentially consider turning the closet (I assume that is the large opening on the right side) into shelving and equipment rack? That would give her the space and 'neat appearance' she wants. Then, you could build a false wall up front and place the speakers behind them to open up the sound a bit. The plasma can still be mounted in the back if you built the wall to recess there like a shadow box but still be acoustically transparent.

Just a thought.
Putting speakers in a 5 sided box is usually a bad idea. The box itself colours the sound.

Putting surround speakers in a 3 sided box (bottom, top, back) is fairly common with columns. Some people surround the speakers with 703.

Another option, if you have EQ or wall mount surrounds, is to put the speakers into a baffle (sometimes called a soffet).

(Oh drat. I just realized I'll have to rethink my column construction again. My surrounds have 30 degree side firing speakers (dipoles, sort of), which would point at the 2x2 front vertical support poles. Hmm...)
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Would she potentially consider turning the closet (I assume that is the large opening on the right side) into shelving and equipment rack?
Unfortunately, that's a chase to route the supply and return ducts down to the first floor ceiling. Believe me, I've wrestled with numerous ideas to get that out of the media room, but there just isn't anywhere else to put it.

Then, you could build a false wall up front and place the speakers behind them to open up the sound a bit.
What about if I were to create a false wall from where the racks are shown out to either side? Would that open the sound up sufficiently?

The plasma can still be mounted in the back if you built the wall to recess there like a shadow box but still be acoustically transparent.
I'm not following you. Are you saying that the walls to the left and right of the plasma would need to be acoustically transparent?
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After reading bpape's and basementbob's replies, I looked again at my floorplan. I should be able to slide the duct chase over enough to create an equipment closet. this should allow me a build prosceniums on either side of the plasma display. I'll re-draw and upload for comments.

Many thanks for you replies!
Yes. The idea would be for the whole wall along the front to be nothing more than cloth and open framing.
Surround speakers (ala dipole/bipole) are designed to be installed on or in a wall (boundary). Monopoles used as surround speakers are also NOT designed to be mounted near a wall or boundary. Bookshelf (regardless of the name) are NOT designed to be installed inside a cabinet or box. Some may have a boundary compensation circuit that will change the timbre of the speaker when mounted in a cabinet. However, that circuit presumes the front of the speaker is flush to the front of the boundary (face of the cabinet) and there are no air cavities to the sides or back of the speaker that can resonate.
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