dual subs (off center woofers) and placement - AVS Forum
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post #1 of 13 Old 11-27-2012, 08:50 PM - Thread Starter
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I have read and heard that if you place dual subs equidistant to the main listening position it will smooth out FR. So is it the box that should be placed equidistant or the woofer driver or the box port?

for instance, I am considering purchasing two pb12-nsds as pictured below.
http://www.svsound.com/components/com_virtuemart/shop_image/product/PB12NSD-nogrill-fron4ed56045490fb.jpg

The woofer is off centered in the box. So if the box is placed on the front wall equidistance from the main listening position those woofers will no longer be equidistance since they are off center in the box. If the woofers are placed equidistance from the main listening position the box will not be centered.

Does this matter and if so what should be equidistance from main listening spot woofer, port or box?
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post #2 of 13 Old 11-27-2012, 09:26 PM
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Read and heard...and the only rule is equidistant? That minor a difference in symmetry in subs is fairly meaningless but try moving them around and see if you can tell the difference...

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post #3 of 13 Old 11-27-2012, 10:20 PM
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Depends on your room and placement of the subs more than their symmetrical placement in regards to each other. Describe your room and where you are thinking about placing them would provide more detail. Search for articles/threads on sub placement.

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post #4 of 13 Old 11-27-2012, 10:43 PM - Thread Starter
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My understanding is each subwoofer will interact with the room and create peaks and nulls. The idea of having two symmetrically placed subwoofers is so that they there peaks and nulls align thus balancing out the frequency response

Room is dedicated theater room in basement 13ft wide x 18ft long by 8ft tall
It is sealed off with 3 mdf columns spaced 5ft apart. It has 2 mdf columns on back wall.

I was planning on putting the subs on the front soundstage wall at 1/4 room width and 3/4 room width points based on reading suggested sub spots that offer good fr balance.

So should woofer centers be at the 1/4 and 3/4 room width points or center of sub box or should the sub ports be at this point. I'd like to try and balance the frequency response throughout the room as much as possible using dual subs.
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post #5 of 13 Old 11-28-2012, 05:59 AM
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Check out some of the Whitepapers over at Harman - Subwoofers: Optimal number and location.

http://www.harman.com/EN-US/OurCompany/Innovation/Pages/WhitePapers.aspx?CategoryID=White%20papers

This may help you with starting points for your room, then go from there.

I am also a big fan of the 8033 for sub optimization
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post #6 of 13 Old 11-28-2012, 06:22 AM
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Originally Posted by proudx View Post

My understanding is each subwoofer will interact with the room and create peaks and nulls. The idea of having two symmetrically placed subwoofers is so that they there peaks and nulls align thus balancing out the frequency response
Room is dedicated theater room in basement 13ft wide x 18ft long by 8ft tall
It is sealed off with 3 mdf columns spaced 5ft apart. It has 2 mdf columns on back wall.
I was planning on putting the subs on the front soundstage wall at 1/4 room width and 3/4 room width points based on reading suggested sub spots that offer good fr balance.
So should woofer centers be at the 1/4 and 3/4 room width points or center of sub box or should the sub ports be at this point. I'd like to try and balance the frequency response throughout the room as much as possible using dual subs.
Placing the subs equidistant to the listening position will ensure they are *timed* properly, (i.e., that the waves propagated by the drivers will arrive at the LP at the same time.) This may, or may not be important in any given room. It depends on the interaction of the subs with the room. It is more important to place the subs where they provide the flattest frequency response. This can only be determined by trial and error, and it requires measurement capability. Short of that, the subwoofer "crawl test" is the next best tool to use for subwoofer placement. However, that becomes a much more complex process with multiple subwoofers.

In addition, many modern receivers and pre/pro's offer the ability to adjust the levels and DISTANCES for more than one sub. Therefore, if the dual subwoofer placements are not equidistant, the difference can be equalized with the Distance settings.

The offsets of the driver in the box will have miniscule and inconsequential effect on the results.

Craig

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post #7 of 13 Old 11-28-2012, 08:14 AM - Thread Starter
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Originally Posted by craig john View Post

Placing the subs equidistant to the listening position will ensure they are *timed* properly, (i.e., that the waves propagated by the drivers will arrive at the LP at the same time.) This may, or may not be important in any given room. It depends on the interaction of the subs with the room. It is more important to place the subs where they provide the flattest frequency response. This can only be determined by trial and error, and it requires measurement capability. Short of that, the subwoofer "crawl test" is the next best tool to use for subwoofer placement. However, that becomes a much more complex process with multiple subwoofers.
In addition, many modern receivers and pre/pro's offer the ability to adjust the levels and DISTANCES for more than one sub. Therefore, if the dual subwoofer placements are not equidistant, the difference can be equalized with the Distance settings.
The offsets of the driver in the box will have miniscule and inconsequential effect on the results.
Craig

From a timing perspective then, if my receiver Denon AVR-4810CI only has 1 distance timing for one sub, where would the distance be measured from, the center of the woofers of the subs or the center of the sub boxes? In other words, is the bass energy/sound wave coming from the woofer center or center of sub box?
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post #8 of 13 Old 11-28-2012, 01:01 PM
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Really the timing is from when the receiver emits the test tone until the sound hits the mic. It's very very common for the electronics in the sub itself to add milliseconds (and feet) to the measurement.

But AFAIK, to produce deeper tones, the cone's action is pretty much pistonic, so the sound emanates from the whole cone at once. But the delays from the sub's electronics will far outstrip any physical measurement difference between different areas of the sub driver.

If you're using the autosetup, AFAIK, the receiver can only emit the test noise then identify when the first sound gets to the mic. It's incapable of identifying different arrivals (although arrivals delayed by room interface will be "seen" as frequency response changes). If your subs are equidistant from the main listening position, they'll both arrive at the same time (assuming identical subs) and if they're not equidistant, I'd expect the "measured" result to be accurate for the closest sub.
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post #9 of 13 Old 11-28-2012, 02:19 PM - Thread Starter
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Originally Posted by JHAz View Post

Really the timing is from when the receiver emits the test tone until the sound hits the mic. It's very very common for the electronics in the sub itself to add milliseconds (and feet) to the measurement.
But AFAIK, to produce deeper tones, the cone's action is pretty much pistonic, so the sound emanates from the whole cone at once. But the delays from the sub's electronics will far outstrip any physical measurement difference between different areas of the sub driver.
If you're using the autosetup, AFAIK, the receiver can only emit the test noise then identify when the first sound gets to the mic. It's incapable of identifying different arrivals (although arrivals delayed by room interface will be "seen" as frequency response changes). If your subs are equidistant from the main listening position, they'll both arrive at the same time (assuming identical subs) and if they're not equidistant, I'd expect the "measured" result to be accurate for the closest sub.

i'd be using SVS pb12-nsds. They are identical subs but the driver is offset to the left a few inches and my main listening position is in the middle of the room width 2/3rds back. Since the driver is offset to the left in the box the center of the subbox placed at 1/4 room width and 3/4 room width will result in the offset woofer drivers not being equidistance to center of room main listneing area. So you are saying the driver in the cabinet should be equidistance for timing or the subs box center? guess the better question is where is the energy for timing measurement coming from, the center of the sub box, or the center of the woofer driver.

If the woofers are centered to make them equidistance to the center of room width listening area, the sub box will not be equidistance. The sub cabinet on the right will be closer to the right wall and the sub cabinet on the left will furhter from the left wall.
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post #10 of 13 Old 11-28-2012, 03:17 PM
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You are focusing on something that is simply not important. It's the combined frequency response of the two subwoofers that is important. If you need to set them up so one is 6 ft. away and the other is 12 ft. away, because those positions provide the best frequency response, that would be far better than 2 subs with equidistant drivers.

In your scenario, with the offset drivers, you also have offset ports. There is no way to get symmetrical positioning of both the drivers and the ports. They both contribute to the output, but in different frequency ranges. How would you plan to optimize that aspect? Answer: You can't, so stop worrying about it.

Get yourself some measurement gear if you really want to be OCD about setting them up.

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post #11 of 13 Old 11-28-2012, 04:54 PM
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Quote:
Originally Posted by craig john View Post

You are focusing on something that is simply not important. It's the combined frequency response of the two subwoofers that is important. If you need to set them up so one is 6 ft. away and the other is 12 ft. away, because those positions provide the best frequency response, that would be far better than 2 subs with equidistant drivers.
In your scenario, with the offset drivers, you also have offset ports. There is no way to get symmetrical positioning of both the drivers and the ports. They both contribute to the output, but in different frequency ranges. How would you plan to optimize that aspect? Answer: You can't, so stop worrying about it.
Get yourself some measurement gear if you really want to be OCD about setting them up.
Craig

+1. I started to go here but decided just to answer the question posed.

OP, many folks here deliberately change their subwoofer distance setting to improve the way the subs and mains combine in the crossover region. Phase anomalies occur in both main speakers and subs, and getting the phases to match up so that everything adds together is far more beneficial sonically than worrying a lot about distance/ timing of the subs, since AFAIK we identify arrival time by the higher frequencies anyway.
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post #12 of 13 Old 11-28-2012, 07:01 PM - Thread Starter
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Quote:
Originally Posted by craig john View Post

You are focusing on something that is simply not important. It's the combined frequency response of the two subwoofers that is important. If you need to set them up so one is 6 ft. away and the other is 12 ft. away, because those positions provide the best frequency response, that would be far better than 2 subs with equidistant drivers.
In your scenario, with the offset drivers, you also have offset ports. There is no way to get symmetrical positioning of both the drivers and the ports. They both contribute to the output, but in different frequency ranges. How would you plan to optimize that aspect? Answer: You can't, so stop worrying about it.
Get yourself some measurement gear if you really want to be OCD about setting them up.
Craig

def going to take some measurements when i decide on my subs and get them in my house.

I was just going to start at 1/4 room width and 3/4 room width distances looking at audioholics multiple subwoofer setup and calibration guide as they suggest.

They are very clear that multiple subwoofers should be as symetrically placed as possible in a rectagular room. In regards to where to place the subs in the 1/4 room width and 3/4 room width it states
"
Note: The distance from the walls to the subs should be measured at the acoustical center for each sub which is usually near the dustcap of the drivers.
"
So since the pb12-nsd woofer is offcenter, is the accoustical center the center of the sub, or the center of the woofer?


The article goes on to say

"
The goal is to time align the subs to the room which is NOT necessarily the distance to the listening position. In order for more than one sub to get "traction" against the same room mode they must be symmetrically placed to the mode.
"


"Editorial Note about Subwoofer Placement & Combined SPL Output
Placing your subwoofers as symmetrically as possible with respect to the listening area can be useful especially if one favors a particular listening seat but we should attempt to make the bass good for all seats"

"
Figure 1c. 1/4 W Placement
Although this isn't spelled out in the CEA recommendation, Dr. Toole references it in his book as a good solution for two subwoofers but suggests additional subwoofers may be needed. In my experience I've had excellent results placing two subs against the front wall at locations of 1/4 the room width. I've had even better results placing two additional subs in a similar manner against the back wall. This configuration can achieve nearly as good frequency response performance as the 4 Corner placement with nearly as much bass gain as well.


see diagram 1a

http://www.audioholics.com/tweaks/get-good-bass/multiple-subwoofer-setup-calibration-1


Obviously taking measurements are important but getting these things symetrically placed to cancel each others nulls and peaks from room modes is obviously important. its the point of running dual subs, I thought. And if the driver is off center i'm not sure we can satisfy this statement from the article: "In order for more than one sub to get "traction" against the same room mode they must be symmetrically placed to the mode."

i might need to look at another subwoofer that has the drivers and port centered in the box. i know the pb12-plus does, but would 2 pb12-plus subs be overkill in a 13x18x8 room?

My goal is to achieve lfe bass peaks from blurays in that room of 109db in the 20hz to 80hz range.
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post #13 of 13 Old 11-29-2012, 07:44 AM
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Quote:
Originally Posted by proudx View Post

def going to take some measurements when i decide on my subs and get them in my house.
I was just going to start at 1/4 room width and 3/4 room width distances looking at audioholics multiple subwoofer setup and calibration guide as they suggest.
They are very clear that multiple subwoofers should be as symetrically placed as possible in a rectagular room. In regards to where to place the subs in the 1/4 room width and 3/4 room width it states
"
Note: The distance from the walls to the subs should be measured at the acoustical center for each sub which is usually near the dustcap of the drivers.
"
So since the pb12-nsd woofer is offcenter, is the accoustical center the center of the sub, or the center of the woofer?
I'm going to answer your question, but first I'm going to reiterate that you are focusing on the wrong thing. You should be focusing on the combined frequency response, not how symmetrically they are place WRT the LP. The answer to your question is: The acoustic center for the frequencies produced by the driver is the center of the driver. The acoustic center for the frequencies produced by the port will be the center of the port. Now, good luck getting both of those to be symmetrically placed to the LP
Quote:
The article goes on to say
"
The goal is to time align the subs to the room which is NOT necessarily the distance to the listening position. In order for more than one sub to get "traction" against the same room mode they must be symmetrically placed to the mode.
"
"Editorial Note about Subwoofer Placement & Combined SPL Output
Placing your subwoofers as symmetrically as possible with respect to the listening area can be useful especially if one favors a particular listening seat but we should attempt to make the bass good for all seats"
"
Figure 1c. 1/4 W Placement
Although this isn't spelled out in the CEA recommendation, Dr. Toole references it in his book as a good solution for two subwoofers but suggests additional subwoofers may be needed. In my experience I've had excellent results placing two subs against the front wall at locations of 1/4 the room width. I've had even better results placing two additional subs in a similar manner against the back wall. This configuration can achieve nearly as good frequency response performance as the 4 Corner placement with nearly as much bass gain as well.
see diagram 1a
http://www.audioholics.com/tweaks/get-good-bass/multiple-subwoofer-setup-calibration-1
Obviously taking measurements are important but getting these things symetrically placed to cancel each others nulls and peaks from room modes is obviously important. its the point of running dual subs, I thought. And if the driver is off center i'm not sure we can satisfy this statement from the article: "In order for more than one sub to get "traction" against the same room mode they must be symmetrically placed to the mode."
i might need to look at another subwoofer that has the drivers and port centered in the box. i know the pb12-plus does, but would 2 pb12-plus subs be overkill in a 13x18x8 room?
My goal is to achieve lfe bass peaks from blurays in that room of 109db in the 20hz to 80hz range.
Your goal should be to achieve FLAT frequency response. If you want to optimize output, corner placement would be your best bet.

The article recommends the 1/4 - 3/4 positioning as a third alternative. The best arrangement, (assuming your room is rectangular and sealed), is the mid-side or mid-front/rear wall placements. Nonetheless, he makes my point as well:
Quote:
If your placement options are limited, you will have to use trial and error to compensate by manipulating electrical delay of one or more of the subs (1 ms per foot) via the speaker distance settings (assuming your processor has multiple subwoofer outputs with independent delay settings and channel trims or can artificially add group delay by engaging one of the subwoofers internal crossover slightly (20Hz-30Hz) above or below your processor's crossover frequency.

If you can't achieve proper integration, you will have to consider moving the sub(s) to a more optimal location. If your subs are properly summing together, you will potentially gain up to 6dB for every doubling of subwoofers (corner loaded) used minus the loss for mode cancellations. Realize however that anytime you vary time delay, phase or EQ for just a single sub you potentially reduce the effectiveness of standing wave reduction or modal reduction. Be careful not to cancel the modal reducing benefits of multi subs while also endlessly chasing your tail in search of the best settings! The goal is to time align the subs to the room which is NOT necessarily the distance to the listening position. In order for more than one sub to get "traction" against the same room mode they must be symmetrically placed to the mode.

Placing subs symmetrically to the LP is not the only way to reduce modal peaks and nulls. In fact, Earl Geddes recommends random placement of subs to impact modal peaks and nulls:
http://mehlau.net/audio/multisub_geddes/

The bottom line in all this is that the *only* way to optimize multiple subwoofers is with measurements and trial and error. Anything that attempts to predict the best placements is, at best, a guess. It can be based on good theoretical science, but until you get the subs in your room and set your listening position(s) in that room, you can never accurately predict how they will measure or sound. The couple of inches of offset of those SVS subs' drivers and ports may, or may not, make a whole lot of difference. It's unpredictable.

Craig

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