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post #1 of 31 Old 09-03-2013, 08:45 AM - Thread Starter
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Hi,

Just to confirm if I on the 2nd Sub and there is no increase in db is it because of phrase issue.

Thanks
Marcus
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post #2 of 31 Old 09-03-2013, 11:52 AM
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I'll give a very general answer to your very general question: yes.
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post #3 of 31 Old 09-03-2013, 03:20 PM
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Quote:
Originally Posted by Marcus Gan View Post

is it because of phrase issue.
Phrase, no, phase, maybe. But to have no increase would be almost impossible. Phase issues can result in cancellations at some frequencies, but only some, so there would still be a net increase. Reversed polarity, which is 180 degrees out of phase at all frequencies, would result in a net loss.

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post #4 of 31 Old 09-03-2013, 04:01 PM - Thread Starter
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Thanks, I use the pink noise on my Avr and looking at the RS meter not even 1 db increase. I think is reverse polarity. Sub is 1 JTR S1 & DIY TC Sound 5400 so I just need to switch the polarity on DIY sub right.

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post #5 of 31 Old 09-03-2013, 05:00 PM
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Quote:
Originally Posted by Bill Fitzmaurice View Post

Phrase, no, phase, maybe. But to have no increase would be almost impossible. Phase issues can result in cancellations at some frequencies, but only some, so there would still be a net increase. Reversed polarity, which is 180 degrees out of phase at all frequencies, would result in a net loss.

Right. But if he's using his AVR test tones, and his db drops with both subs as opposed to one, thats almost always gonna be a 180 phase issue.

Edit: I think we're saying the same thing lol
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post #6 of 31 Old 09-03-2013, 05:18 PM
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Another possibility is that after calibration the AVR set the overall level as before, trimming the sub output to match the target output levels.

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post #7 of 31 Old 09-07-2013, 10:44 PM - Thread Starter
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Hi Guys,

OK, I reversed the polarity and now when I switch on the 2nd sub there is an increase of 3 db is that normal now.

Thanks
Marcus Gan
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post #8 of 31 Old 09-08-2013, 12:39 AM
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Yes....how are your subs laid out in room?

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post #9 of 31 Old 09-08-2013, 09:12 AM
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Quote:
Originally Posted by Marcus Gan View Post

Hi Guys,

OK, I reversed the polarity and now when I switch on the 2nd sub there is an increase of 3 db is that normal now.

Thanks
Marcus Gan

Yes 3-6db at the listening position is a normal increase for adding a second sub that is identical to the first. Normally when each sub is placed in opposite ends of the room one sub will need the polarity reversed 180 to be in phase. This is just a generalization...not all rooms are the same which is why its best to try every setup combination possible.
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post #10 of 31 Old 09-08-2013, 09:54 AM
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Quote:
Originally Posted by basshead81 View Post

Yes 3-6db at the listening position is a normal increase for adding a second sub that is identical to the first. Normally when each sub is placed in opposite ends of the room one sub will need the polarity reversed 180 to be in phase. This is just a generalization...not all rooms are the same which is why its best to try every setup combination possible.


What about if you place the 2 subs in the 2 front corners (Left and Right front corner), what should the Phase be set at?
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post #11 of 31 Old 09-08-2013, 10:52 AM
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At 0, coz they are on the same side of the room. Not the best of ideas to place that way. One front and one in the rear yield far better FR as you excite room modes from opposite locations. In this case you may have to reverse the polarity of one of the subs. But it is not a must as your room acoustics dictate the set up.

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post #12 of 31 Old 09-08-2013, 02:08 PM
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Originally Posted by mantaraydesign View Post

What about if you place the 2 subs in the 2 front corners (Left and Right front corner), what should the Phase be set at?
Set them to the same phase/polarity and they should cancel your room's 1st & 3rd width modes (i.e., peaks & dips at 38Hz and 113Hz across your seating area). If you can move them 1/4 room width (45 inches) in from the side walls, then they should also cancel your room's 2nd width mode (75Hz). It's a no-cost way to improve seat-to-seat consistency in the bass range.

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post #13 of 31 Old 09-08-2013, 03:24 PM
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Originally Posted by sdurani View Post

Set them to the same phase/polarity and they should cancel your room's 1st & 3rd width modes (i.e., peaks & dips at 38Hz and 113Hz across your seating area). If you can move them 1/4 room width (45 inches) in from the side walls, then they should also cancel your room's 2nd width mode (75Hz). It's a no-cost way to improve seat-to-seat consistency in the bass range.



I am not understanding what you are saying at all. But I have attached a picture of my room size. The ceiling height is 9 Feet tall.


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post #14 of 31 Old 09-08-2013, 03:34 PM
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Originally Posted by mantaraydesign View Post

I am not understanding what you are saying at all. But I have attached a picture of my room size. The ceiling height is 9 Feet tall.



They should both be set the same in that configuration...however I would put one sub in the opposite rear corner. 43ft is a really long span.
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post #15 of 31 Old 09-08-2013, 03:47 PM
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ok, moving 45 inches from the side walls make the 2 subs seems kind of close together.

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post #16 of 31 Old 09-08-2013, 04:19 PM
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Originally Posted by basshead81 View Post

They should both be set the same in that configuration...however I would put one sub in the opposite rear corner. 43ft is a really long span.


I will give this a try as well to see which method sounds the best.
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post #17 of 31 Old 09-08-2013, 05:10 PM
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Originally Posted by mantaraydesign View Post

I am not understanding what you are saying at all.
If you blow across an empty Coke bottle, you can get that small chamber to resonate (make that boooooooh sound). If you enlarge that chamber to the size of your room, it will still resonate (of course at different frequencies than a Coke bottle).

The 15-foot width of your room will have resonances (room modes, standing waves, whatever you want to call them) at 38Hz, 75Hz and 113Hz. Sounds at those frequencies will exhibit big peaks & dips along the width of your room (meaning the bass will sound very different from seat to seat).

Placing a pair of subs on opposite sides of the room (like you have them now) will cancel the 38Hz and 113Hz modes. Moving them in to the quarter-points of room width will additionally cancel the 75Hz mode. Those seat to seat variations will be minimized. The auto EQ in your receiver will thank you for the improved consistency.
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Originally Posted by mantaraydesign View Post

ok, moving 45 inches from the side walls make the 2 subs seems kind of close together.
Does that matter if that placement gives you bass that is smoother AND more consistent across the seating area. Remember to place the middle of your subwoofer drivers at the 45-inch marks, then re-run the auto-calibration in your receiver.
MemX likes this.

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post #18 of 31 Old 09-08-2013, 06:21 PM - Thread Starter
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Originally Posted by braveheart123 View Post

Yes....how are your subs laid out in room?

Hi Braveheart,

They are both infront between the R/C & L/C. appr 1/4 to the left and right.
However when I received my Catalyst 12 I will place both sub in the center as speaker stand and place the Catalyst 12 horizontally on top. That's my best option because my room to narrow on 10 feet wide.

Marcus
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post #19 of 31 Old 09-08-2013, 06:41 PM
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Quote:
Originally Posted by sdurani View Post

If you blow across an empty Coke bottle, you can get that small chamber to resonate (make that boooooooh sound). If you enlarge that chamber to the size of your room, it will still resonate (of course at different frequencies than a Coke bottle).

The 15-foot width of your room will have resonances (room modes, standing waves, whatever you want to call them) at 38Hz, 75Hz and 113Hz. Sounds at those frequencies will exhibit big peaks & dips along the width of your room (meaning the bass will sound very different from seat to seat).

Placing a pair of subs on opposite sides of the room (like you have them now) will cancel the 38Hz and 113Hz modes. Moving them in to the quarter-points of room width will additionally cancel the 75Hz mode. Those seat to seat variations will be minimized. The auto EQ in your receiver will thank you for the improved consistency.
Does that matter if that placement gives you bass that is smoother AND more consistent across the seating area. Remember to place the middle of your subwoofer drivers at the 45-inch marks, then re-run the auto-calibration in your receiver.


Excellent information you are given me!

The only question I have left is if I move the 2 subs away from the corners, will I loose some dB sound levels or loose volume gain?
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post #20 of 31 Old 09-08-2013, 06:44 PM
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Quote:
Originally Posted by sdurani View Post

If you blow across an empty Coke bottle, you can get that small chamber to resonate (make that boooooooh sound). If you enlarge that chamber to the size of your room, it will still resonate (of course at different frequencies than a Coke bottle).

The 15-foot width of your room will have resonances (room modes, standing waves, whatever you want to call them) at 38Hz, 75Hz and 113Hz. Sounds at those frequencies will exhibit big peaks & dips along the width of your room (meaning the bass will sound very different from seat to seat).

Placing a pair of subs on opposite sides of the room (like you have them now) will cancel the 38Hz and 113Hz modes. Moving them in to the quarter-points of room width will additionally cancel the 75Hz mode. Those seat to seat variations will be minimized. The auto EQ in your receiver will thank you for the improved consistency.
Sanjay,

Your calculations are nice, simple predictions, but rarely do they translate into real world rooms.

A: You are assuming a perfectly rectangular room with perfectly reflective walls, floor and ceiling. I've never encountered one of those.
B: Tangential and Oblique modes also come into play. They could reinforce, or cancel, axial modes.
C: The listening position is the most important factor for determining whether a mode is a cancellation or a peak. Move the LP a foot or two left or right, front or back, or raise it a foot, and you'll get completely different FR's.
D: In-room measurements are the only way to verify what the room mode calculators predict.

If the room is 43 ft. long, I don't think I would place a sub on the rear wall and a sub on the front wall. If the LP is 10 or 12 ft from the front of the room, that would put the front sub 10 ft away and the rear sub 33 ft. away. Using level-matching would make the rear sub 6 to 10 dB higher than front sub. System headroom will suffer badly.

If the the room is 43 ft. long, I suggest placing the two subs at the mid-sidewall positions. This will ensure that they can be gain matched, and should provide the best headroom capability. It won't provide the best room reinforcement, but it should provide the smoothest in-room response.

mantaraydesign, what subs do you have? Where do you plan to place the seating?

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post #21 of 31 Old 09-08-2013, 06:48 PM
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Oh my mistake I didnt realize the distance of the seating position.
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post #22 of 31 Old 09-08-2013, 06:58 PM
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Originally Posted by craig john View Post

Sanjay,

Your calculations are nice, simple predictions, but rarely do they translate into real world rooms.

A: You are assuming a perfectly rectangular room with perfectly reflective walls, floor and ceiling. I've never encountered one of those.
B: Tangential and Oblique modes also come into play. They could reinforce, or cancel, axial modes.
C: The listening position is the most important factor for determining whether a mode is a cancellation or a peak. Move the LP a foot or two left or right, front or back, or raise it a foot, and you'll get completely different FR's.
D: In-room measurements are the only way to verify what the room mode calculators predict.

If the room is 43 ft. long, I don't think I would place a sub on the rear wall and a sub on the front wall. If the LP is 10 or 12 ft from the front of the room, that would put the front sub 10 ft away and the rear sub 33 ft. away. Using level-matching would make the rear sub 6 to 10 dB higher than front sub. System headroom will suffer badly.

If the the room is 43 ft. long, I suggest placing the two subs at the mid-sidewall positions. This will ensure that they can be gain matched, and should provide the best headroom capability. It won't provide the best room reinforcement, but it should provide the smoothest in-room response.

mantaraydesign, what subs do you have? Where do you plan to place the seating?

Craig



I am using the (2) SVS PB12-Pluses with the 800 watts amp for each sub.

The seating position from the front wall is 20 feet.


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post #23 of 31 Old 09-08-2013, 07:32 PM
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20 feet? What size screen do you have?

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post #24 of 31 Old 09-08-2013, 07:39 PM
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20 feet? What size screen do you have?

It is the 120 inch projection screen.
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post #25 of 31 Old 09-08-2013, 07:44 PM
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Assuming you have a 1080p projector, I would be sitting closer:

http://myhometheater.homestead.com/viewingdistancecalculator.html

THX recommends 13.4 ft.

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post #26 of 31 Old 09-08-2013, 07:46 PM
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Assuming you have a 1080p projector, I would be sitting closer:

http://myhometheater.homestead.com/viewingdistancecalculator.html

THX recommends 13.4 ft.

Craig


Yes, it is the 1080p projector.

Thanks for the great advice about THX recommendation!
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post #27 of 31 Old 09-08-2013, 07:57 PM
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The only question I have left is if I move the 2 subs away from the corners, will I loose some dB sound levels or loose volume gain?
Yes. But when you run your receiver's auto calibration, it will re-set the subwoofer levels so that they're back to where they were.

Sanjay
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post #28 of 31 Old 09-08-2013, 09:18 PM
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Originally Posted by craig john View Post

Your calculations are nice, simple predictions, but rarely do they translate into real world rooms.
In the REW thread, folks have been using quarter-point and mid-point subwoofer placements with measured results that validate the predictions. One example:
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Wow, check out my sub only response (no smoothing) just by relocating it to the middle of the room as Sanjay suggested several posts ago!! No Audyssey correction either (since my Audyssey calibration was done with the sub in the right corner):




Here's the mid front wall location vs. right corner with Audyssey:



Another big THANKS is in order to Sanjay for his recommendation!
The poster above had a 14-foot wide room, for which calculations predicted his first width mode at 40Hz. So I suggest he move his source of sound pressure (subwoofer) to the location where SPL was lowest (modal null at midpoint of room width). They cancelled, as confirmed by subsequent measurements (above).
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A: You are assuming a perfectly rectangular room with perfectly reflective walls, floor and ceiling. I've never encountered one of those.
I've never encountered one of those rooms either, so the only thing I assumed was parallel side walls between mantaraydesign's subs and his seating location. The poster in the example above (jkasanic) had two open doorways only on one side of the room, but it was still worth trying to cancel the 40Hz modal dip (didn't cost him anything to move the sub to where I suggested).
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B: Tangential and Oblique modes also come into play. They could reinforce, or cancel, axial modes.
Those modes will be there anyway, so I don't understand what you are trying to say. Because there are other modes in a room, should he not even attempt to cancel a couple of width modes using placement?
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C: The listening position is the most important factor for determining whether a mode is a cancellation or a peak. Move the LP a foot or two left or right, front or back, or raise it a foot, and you'll get completely different FR's.
Why would that happen? If a width mode is cancelled, then those peaks & dips are cancelled (or at least minimized) along the width of the room. If those peaks & dips are gone, then why would you get a completely different FR as you move left to right? The whole point of cancelling those modes is to improve consistency across the width of the room.

Sanjay
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post #29 of 31 Old 09-08-2013, 10:07 PM
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The seating position from the front wall is 20 feet.
Good grief, that's far away. I agree with Craig about moving closer (muuuuuuuch closer).

BTW, do you have a single row of seating or two rows? If single, how wide is the seating (couch or theatre chairs)?

Sanjay
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post #30 of 31 Old 09-09-2013, 01:17 PM
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Quote:
Originally Posted by sdurani View Post

In the REW thread, folks have been using quarter-point and mid-point subwoofer placements with measured results that validate the predictions. One example:
The poster above had a 14-foot wide room, for which calculations predicted his first width mode at 40Hz. So I suggest he move his source of sound pressure (subwoofer) to the location where SPL was lowest (modal null at midpoint of room width). They cancelled, as confirmed by subsequent measurements (above).
I've never encountered one of those rooms either, so the only thing I assumed was parallel side walls between mantaraydesign's subs and his seating location. The poster in the example above (jkasanic) had two open doorways only on one side of the room, but it was still worth trying to cancel the 40Hz modal dip (didn't cost him anything to move the sub to where I suggested).
Those modes will be there anyway, so I don't understand what you are trying to say. Because there are other modes in a room, should he not even attempt to cancel a couple of width modes using placement?

My point is that the important response is the *combined* response of all modes together.
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Why would that happen? If a width mode is cancelled, then those peaks & dips are cancelled (or at least minimized) along the width of the room. If those peaks & dips are gone, then why would you get a completely different FR as you move left to right? The whole point of cancelling those modes is to improve consistency across the width of the room.
The "width mode" isn't cancelled across the entire width of the room. A "standing wave" is a series of peaks and nulls across the dimension. As you move from left to right, you move from areas of peaks to areas of nulls. Try playing a 40 Hz tone and walking across the width dimension. You can literally stick your head into and out of the peaks and nulls. Take an SPL meter with you on your journey across the width of the room and you'll be able to measure these peaks and nulls. So, whether the listening position is in a peak or a null, or neither will have as much impact on the measured response as the subwoofer positioning.

In your example a null was removed at the listening/measuring position with the subwoofer re-positioning. That is a good thing. But the nulls were likely caused by the corner positioning, and may well have been improved just by getting the sub out of the corner. But at other, (arbitrary), listening/measuring position(s), those nulls still exist, as do the concomitant peaks.

Craig

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