Can bad sub-placement be corrected with "helper subs"? - AVS Forum
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post #1 of 11 Old 12-20-2012, 11:33 PM - Thread Starter
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I am in the process of my theater construction and the design I am going with, is not very flexible in regards to subwoofer placement. I have two JBL S1S-EX which is to be placed in my screen wall. I can only vary the placement a little to the left and to the right.

If I setup my system and find out that I have very large seat to seat variation with this placement and nulls in one of the rows, will I be able to address this by adding balancing subs in the side/back of the room without moving the "main" subs behind the screen? (My DSP supports individual delay for each sub). I was thinking about going with the Procella subwoofers as balancing-subs as they have minimal depth.

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post #2 of 11 Old 12-21-2012, 07:35 AM
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Yes

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post #3 of 11 Old 12-21-2012, 08:24 AM
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Yes, BUT you may have a hard time getting a decent sonic result; it will be tricky and may not yield good results.

Moving your main subs closer to a corner and varying the exact distance could make a lot more difference.
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post #4 of 11 Old 12-21-2012, 09:10 AM
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If you haven't read it, check out the Harman International white paper by Todd Welti, Subwoofers: Optimum Number and Placement here:

http://www.harman.com/EN-US/OurCompany/Innovation/Pages/WhitePapers.aspx

There's other good stuff there as well.

I've got four 18" JBL subs in my room, and I can place them at the midpoints of the walls, one of the optimal positions for multi subs. The notion of adding two additional subs in your room can have merit, though there are likely a few caveats.

1. It's preferable but not mandatory to have the subs matched. If they're matched, there's less futzing about because their characteristics will be similar and predictable.
2. If the subs can't be matched, then having two similar pairs is good enough. (see below)
3. Utilizing two different pairs creates some added complexity from the standpoint of different output characteristics and response curves. It's more work but not impossible to EQ this effectively.
4. Mixing four different subs in a room is a PITA, and it's good you're not looking at this.
5. Since midpoints of the walls seem out of the question, then corners is the next bet, as commsysman points out. Get those front subs as close to the corners as you can. Since you're in the building stage, it's better to do it now, even if it sets you back a few days to do it.

My four JBL subs consist of two S1S-EX units and two 4645C units. They all have the 2242-HPL 18" driver in them, so they pretty close to an exact match. Placed at the midpoints of the walls, there's not a dead spot or a boomy spot in the whole room. Due to the small size of my HT (15x17x8.5) my Synthesis® calibrator recommended having no door in the left rear corner; rather I left a heavily curtained opening so the excess LF waves could bass dump into the adjacent music/fitness room rather than kill us with concussive pressure. (OK, that's an overstatement. tongue.gif )

Read Welti's paper, referenced above, and good luck!

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post #5 of 11 Old 12-21-2012, 02:05 PM - Thread Starter
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Nice to know that its possible.

Filecat: I have read the papers, but as far as I remember they assume that all walls/ceilings are equally reflective etc, so I dont know if their studies will work in my room. I know there are some other studies which recommends random placement.

I actually have 4 S1S-EX's, but I cannot fit them all behind the screen and there is not enough space in the back of the room. I can change my design so I am able to have three S1S-EX's behind the screen and then use smaller subs in the back/sides of the room if the seat to seat needs improvement. I dont know if there are drawbacks by using an uneven number of subs in the screenwall. But I love the JBL subs so I would rather use as much of them as possible instead of selling them.

By the way, is there a reason why you did not choose the corner placement for your subs? Your current configuration is not nearly as effective as the corner placement.

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post #6 of 11 Old 12-21-2012, 08:12 PM
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Here's a little info to the OP.

I have my personal experience with that. The Rythmik FV15HP is placed at the room corner. The bass is not ideal. I tried to hook up an additional Klipsch SW450 to the setup. The SW450 is placed nearfield, just behind my couch.



As you can see from the REW graph, the dual sub setup gives a smooth response from 20Hz to 60Hz. Although it look nice on graph, the bass sounds a little muddy. My local AV installer suggest NOT to do that as it spoils the sound quality. He said quick, tight and accurate bass is what we need, not just a smoother frequency response.

However, I do know a few people who have additional seal sub placed nearfield for greater bass experience. They are using high quality sealed subs (such as Rythmik F15 or F12).
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post #7 of 11 Old 12-23-2012, 06:28 PM
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Quote:
Originally Posted by JonasHansen View Post

Nice to know that its possible.
Filecat: I have read the papers, but as far as I remember they assume that all walls/ceilings are equally reflective etc, so I dont know if their studies will work in my room. I know there are some other studies which recommends random placement.
I actually have 4 S1S-EX's, but I cannot fit them all behind the screen and there is not enough space in the back of the room. I can change my design so I am able to have three S1S-EX's behind the screen and then use smaller subs in the back/sides of the room if the seat to seat needs improvement. I dont know if there are drawbacks by using an uneven number of subs in the screenwall. But I love the JBL subs so I would rather use as much of them as possible instead of selling them.
By the way, is there a reason why you did not choose the corner placement for your subs? Your current configuration is not nearly as effective as the corner placement.

I have a music only 5.1 JBL Performance Series set up that's in a room with only two walls. It's a step down family room that's open on two sides into a total area of 15,000 cubic feet. I'm using four HTPS400 subs in this area, none of which are in the corners, for both practical and theoretical reasons. The bass is very solid and uniform, and the overall soundstage, depth, and width are stellar.

In our HT, I'm not that keen to have the subs in the corners, so the reason is personal choice and technical application. I'd counter that "not nearly as effective" is a valid subjective judgment, but I don't think the science is there to support it. To me, corner placement creates a few problems of its own, including excessive gain at resonant frequencies. Welti's assessment of the difference between corner and midpoint placement is they're the top two (actually top 3--see below) choices, with four subs being the optimal number either way. The JBL Synthesis® calibrator who came out to do the final dial in was happy to see midpoint placement: subs at the midpoints, bass traps in the corners. I'm a bit of a bass whore, but I don't really like boomy bass, which seems to be easier to avoid with midwall placement.

Interestingly, the JBL synthesis® demo room at Harman's Northridge Campus has subs and Everest II systems sitting atop each other deep in the corners. I guess this is largely due to the way the room was built, but it sure seemed weird. It was plenty loud, but a bit "phasey, boomy, and muddy." smile.gif Of course, I told them it sounded great. tongue.gif


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post #8 of 11 Old 12-24-2012, 09:53 AM
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Quote:
Originally Posted by JonasHansen View Post

I can only vary the placement a little to the left and to the right.
Can you place the subs at the quarter-width points on the front wall (i.e., one fourth of the room width in from the left and right walls)?
Quote:
Originally Posted by JonasHansen View Post

If I setup my system and find out that I have very large seat to seat variation with this placement and nulls in one of the rows, will I be able to address this by adding balancing subs in the side/back of the room without moving the "main" subs behind the screen?
Do you have measuring gear so that you can find out whether the locations for the helper subs are helping or hurting seat to seat variation?

Sanjay
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post #9 of 11 Old 12-25-2012, 07:16 AM
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The DIY speaker build thread had lots of people who went multiple subs, I am starting that process just now also, http://www.avsforum.com/t/1446659/multi-sub-help-diy-triangle-sub-w-dayton-audio-rss460ho-4-18-24-sides-x-34-face-x-32-tall-5-5-cu-ft

This webpage will definitely help you out in real world analysis, http://mehlau.net/audio/multisub_geddes/
Quote:
Subwoofer setup after Earl Geddes (GedLee LLC)

The multisub setup described on this page has two big advantages over other methods: only three subs are needed and you don't need to put the subwoofers at specified locations. Nonetheless there are some basic rules that have proven to yield best results:

Put one sub in a corner close to the mains. The second sub is a lot more flexible as to its location, but it should not be in a corner. Side wall or back wall, near the midpoint is a good idea. Put the third sub wherever you can that is not too close to the other two. It's a good idea to get one of them off of the floor.
Subwoofer requirements

Because we are using 3 subwoofers, they do not need to be as powerful as a single subwoofer. Any decent active subwoofer (ported or closed design) with a 10" or bigger driver will do. It should have controls for

level (continuously variable)
low pass frequency (continuously variable)
phase (switchable or continuously variable)
parametric equalizer (optional)

Measurement equipment

Measurement microphone
Microphone preamp
Computer
Realtime FFT analyzer software
White noise signal (download - WAV, 5.1MB)

Calibration procedure

Make sure that the main loudspeakers are not high pass filtered as they act as additional low frequency sources, which is desirable.

"First setup the mains and the nearest sub [(the sub nearest to the mains)]. Set your spectrum analyzer to a very low bandwidth but not less than 200 Hz and fairly long averaging time. This will yield a very long average of the sound signal. Take your mic and move it through large spatial positions near but arround the prefered listening position. Be carefull as small bumps of the cable can generate large erroneous signals into the mic. The sweeping has to be smooth. When the analyzer has completed its run you will have a plot of the frequency and spatial averaged low frequency sound field. Try adjusting the sub - never adjust the mains - to see if you can get a better response by changing the gain, the low pass point, the phase and or delay if you have it. The bass should be sagging slightly at this point since you will be adding in two more subs.

Now add in the next closest sub and repeat the entire procedure again. You should be able to improve upon what you had before. If not try turning off the first sub and setting the second one optimal and then add in the first sub. If you still can't get a better response with two subs than with one then you will need to move one of them. You need to get an improvement from the second sub or something is wrong.

Now repeat this process with the third sub. The third sub, when you are close to it should barely be audible. If it is loud, then once again, something is wrong. It should only be filling in holes at this point and not adding any actual level. The level after two subs should be about flat or possibly a slight rise - based on preference. I find a few dB rise at the low end is desirable for best effect.

With the three subs things should be quite smooth when spatially averaged. You can now use any EQ that you have to make a final flattening of the response, but these bands should never be more than a few dB. When you are done, if things go as they usually do for me, your should have a spatial average of about ± 2-3 dB. This can take several hours so don't be impatient. [...] Make sure and write down all the settings!!

Two people helps - one to sweep the mic and another to run the analyzer. Sweep the mike vertically as well as horizontally, but in a wide ellipse. Its not necessary to repeat the same pattern, its only necessary to NOT leave the mic in a stationary position. You can try wider sweeps or narrower ones, but the bigger sweep will likely be better." (Earl Geddes)

Measurements are a must, while the multiple sub theory shows 2 and 4 sub placement, those assume the 4 walls have equal/very similar effects on the low freq, which may or may be true in each room.
-openings; doors/av racks change that
-each wall construction will change that; example it's typical for location in basement with 2 wall foundation and 2 walls stud will change that
-etc

Have fun, and it will take time and patience to put all the puzzle pieces together, as I'm learning.

Now, if I had a "do-over", I'd go to a front wall baffle like JapanDave did with (4) IB drivers.
http://www.avsforum.com/t/1432588/7-x-re-audio-xxx-18-subs-infinite-baffle-sub-ultimate-ib-build


I wonder if you could do same with 4 box subs on your front wall and call it a day?
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post #10 of 11 Old 01-08-2013, 05:28 AM - Thread Starter
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Quote:
Originally Posted by sdurani View Post

Can you place the subs at the quarter-width points on the front wall (i.e., one fourth of the room width in from the left and right walls)? Do you have measuring gear so that you can find out whether the locations for the helper subs are helping or hurting seat to seat variation?

Sorry for the long response time.

The room is not very large (19.6 feet by 13 feet), so mid points are not very good as this will result in the subs being placed directly agains the chairs/sofa. The second row is placed a little less than 2 feet from the back wall and this will make it impossible to place a subwoofer in the back of the room. (Only if I went with smaller "helper" subs).

The only possible positions for the S1S-EX subs are three behind the screen and one sub placed at the left sidewall at the midpoint of the room length.
I have measurement gear (actually I use 6 microphones simultaneously at all seats so I can very easy look at the seat to seat). I have individual processing for each sub too.

How much do you think I will lose in SPL/extension by going from 4 subs to 2? 2 is definately enough for a room of that size, but with 4 subs they should not work as hard and the distortion figures would probably be better.

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post #11 of 11 Old 01-08-2013, 05:34 AM - Thread Starter
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Quote:
Originally Posted by mtbdudex View Post

I wonder if you could do same with 4 box subs on your front wall and call it a day?
Actually - this would be possible in my room I think! Something to consider smile.gif

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