Center Channel In Enclosed Space - AVS Forum
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post #1 of 11 Old 01-23-2012, 09:28 PM - Thread Starter
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I'm in the process of lowering the mantle shelf above my fireplace so that my TV can be mounted above it, and not be too high.

I have the JBL LC1 center channel which has a rear facing port.

What I want/intend to do is build a shelf/"cubby" on the underneath side of the mantle shelf wherein the LC1 would slide into. This would allow the TV to be mounted even lower to the mantle shelf, as the speaker will not take up space on top of the mantle.

My concern is the rear facing port. the Speaker is ~11" deep, and the mantle shelf under which it would be mounted is ~14' deep. How detrimental is it for a rear ported center channel not to have much space behind it?

I suppose I could not enclose the entire sides of the speaker shelf/compartment thereby allowing some air/release. My AVT has audyssey. Will that correct for the port being close to the wall/back of the built shelf??
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post #2 of 11 Old 01-23-2012, 10:16 PM
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Um, hmmm, well......It is going to change the sound charactoristics of the speaker. Audyssey may correct for it but, that placement is not ideal.

Sounds good!
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post #3 of 11 Old 01-24-2012, 06:20 AM
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Quote:
Originally Posted by Docj04 View Post

I'm in the process of lowering the mantle shelf above my fireplace so that my TV can be mounted above it, and not be too high.

I have the JBL LC1 center channel which has a rear facing port.

What I want/intend to do is build a shelf/"cubby" on the underneath side of the mantle shelf wherein the LC1 would slide into. This would allow the TV to be mounted even lower to the mantle shelf, as the speaker will not take up space on top of the mantle.

My concern is the rear facing port. the Speaker is ~11" deep, and the mantle shelf under which it would be mounted is ~14' deep. How detrimental is it for a rear ported center channel not to have much space behind it?

I suppose I could not enclose the entire sides of the speaker shelf/compartment thereby allowing some air/release. My AVT has audyssey. Will that correct for the port being close to the wall/back of the built shelf??

Aside from any other acoustical issues the enclosure would cause, plugging the port with a high enough crossover (say 80hz- speakers are rated to 55hz) nothing would be coming out of the port anyway.
Roll off of 80hz (or higher) xover would eliminate most of the output from port, plugging would finish the job.
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post #4 of 11 Old 01-24-2012, 06:57 AM - Thread Starter
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Quote:
Originally Posted by jackbuzz View Post

Aside from any other acoustical issues the enclosure would cause, plugging the port with a high enough crossover (say 80hz- speakers are rated to 55hz) nothing would be coming out of the port anyway.
Roll off of 80hz (or higher) xover would eliminate most of the output from port, plugging would finish the job.

I thought of this, also. i do actually crossover at 80hz, and can't imagine that a center channel duties typically dip much lower than that anyway. Also, I intend to build the shelf so that there is an air gap between the back of the speaker and the wall (albeit only about 1"), as well as make the sides open/not solid to allow for air flow/escape.

Is it an accurate statement that the center (especially XO'd at 80 or 90) would be pushing minimal air out the port anyway?
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post #5 of 11 Old 01-24-2012, 11:23 PM
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That is fairly correct. And when you build the shelf, use fairly thick wood. 3/4 inch or 1 inch to keep any harmonics to a minimum.

Sounds good!
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post #6 of 11 Old 01-25-2012, 07:49 AM - Thread Starter
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Quote:
Originally Posted by karlsaudio View Post

That is fairly correct. And when you build the shelf, use fairly thick wood. 3/4 inch or 1 inch to keep any harmonics to a minimum.


Thanks for that reply. For aesthetic purposes, I would be using 1x12" board to build the shelf.

Also, would there be any benefit to lining the back of the shelf with a thin sheet/panel of mineral wool to improve harmonics?
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post #7 of 11 Old 01-25-2012, 08:35 AM
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Maybe? Not sure on that one. Give it a whirl and see if there is a difference! It could dampen some higher frequencies escaping the rear of the speaker through the port. Keeping them from reflecting back out the front of the enclosure. Creating other problems.

Sounds good!
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post #8 of 11 Old 01-25-2012, 09:51 AM
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Quote:
Originally Posted by jackbuzz View Post

Aside from any other acoustical issues the enclosure would cause, plugging the port with a high enough crossover (say 80hz- speakers are rated to 55hz) nothing would be coming out of the port anyway.
Roll off of 80hz (or higher) xover would eliminate most of the output from port, plugging would finish the job.

Umm..... I have crappy Onkyo speakers with 2 - 5" midwoofers and a rear 1.5" port. I have the speakers x-over at 80hz and there is a TON of air that moves through that port during loud music or bass heavy movie scenes. So I'm not sure that's an accurate statement.

Shawn
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post #9 of 11 Old 01-25-2012, 01:11 PM
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Quote:
Originally Posted by Docj04 View Post

Is it an accurate statement that the center (especially XO'd at 80 or 90) would be pushing minimal air out the port anyway?

You can expect the F3 of the speaker to dramatically rise with sealing the port.

It comes down to the specific woofers and the cabinet to know exactly how it changes but be assured it changes it quite a bit.

You can stuff the port and hear for yourself how the speaker will change. It won't sound the same. Better or worse is your call.

Have Fun!
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post #10 of 11 Old 01-25-2012, 01:54 PM
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Quote:
Originally Posted by flickhtguru View Post

Umm..... I have crappy Onkyo speakers with 2 - 5" midwoofers and a rear 1.5" port. I have the speakers x-over at 80hz and there is a TON of air that moves through that port during loud music or bass heavy movie scenes. So I'm not sure that's an accurate statement.

My answer was based on the assumption that the spec (manufacturers) of -3db at 55hz the port would be tuned somewhere around 58-60hz and with the 3rd or 4th order roll off at xover there wouldn't be much coming out.
If OP has the ability to measure with sine waves or sweeps he could find out the lower limit or as kmannth has said try it and see how it sounds.
Try it before building enclosure, no harm will come and can easily be undone.
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post #11 of 11 Old 02-02-2012, 06:55 AM - Thread Starter
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Well the enclosure was built out of 2x10" boards. I allowed for about a 2" air gap b/t the back of the center channel(and port) and the back wall behind the speaker.

Also, I did not make the sides of the enclosure go all the way back against the back wall, so that there is air flow/escape all around.

Finally, I buil the enclosure leaving about 2" on either side of, and top of the speaker with the intent of stuffing some 2" mineral wool panels between the speaker and the inside of the enclosure. I am thinking (hoping) that this may mitigate any negative acoustic impact of the enclosure to begin with.

Any cont'd opinions on my design, and my rationale explained in this post are apprecited.
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