dipole sub bass and line arrays

[exipnos]

Hi there,

This is my first post so please be gentle and help a newbie

I've purchased a large amount of cheap NSB drivers from partsexpress to build some line arrays. I will use these in a dedicated HT room 15 feet wide and 23 feet long. I plan on doing a 7.1 system and will do identical line arrays for all channels even though its overkill for the surrounds.

The line arrays will be OB and use 16 NSB per tower. The tweeter line is either going to be 8-10 pt2 planars or 36 of the small 1" dayton neos.

Now to the problems and questions.

1. The line arrays are OB and therefore the NSB will only play to about 150Hz. This makes it impossible to crossover to a subwoofer in a good way. I will therefore have to build the arrays as a 3way system.

2. I was thinking of using 4 12" woofers per tower to match the NSB line sensitivity. The woofers will go in the base of each tower and placed in a v configuration 2 woofers high on each side of the V. The woofers are also dipole.

3. Woofer specs say they play 20-5000Hz. Since I have 4 woofers per tower could I expect them to play as low as 20Hz? I was wondering if each tower can both take care of the region where the NSB drop off and also the subwoofer region.

4. Since I will have 28 12" woofers in my room I hope that I won't have to use a subwoofer. I know placement of speakers and subwoofers are not the same but since this is dipole I hope room modes is not an issue.

I was planning on going active on the whole system so hopefully crossover design is simplified.

Will this work. Is it a good setup for HT?

Cheers,

Exipnos

[Paul H]

Hi there,

This is my first post so please be gentle and help a newbie

I've purchased a large amount of cheap NSB drivers from partsexpress to build some line arrays. I will use these in a dedicated HT room 15 feet wide and 23 feet long. I plan on doing a 7.1 system and will do identical line arrays for all channels even though its overkill for the surrounds.

The line arrays will be OB and use 16 NSB per tower. The tweeter line is either going to be 8-10 pt2 planars or 36 of the small 1" dayton neos.

Now to the problems and questions.

1. The line arrays are OB and therefore the NSB will only play to about 150Hz. This makes it impossible to crossover to a subwoofer in a good way. I will therefore have to build the arrays as a 3way system.

16 NSB's will hit around 105 dB as low as 150 Hz on an open baffle. Lower frequencies will definitely require help, so the 3-way is a good idea.

2. I was thinking of using 4 12" woofers per tower to match the NSB line sensitivity. The woofers will go in the base of each tower and placed in a v configuration 2 woofers high on each side of the V. The woofers are also dipole.

4-12" woofers will only play loud down to 40-60 Hz, depending on their xmax, etc. I have 2-15" (AE-IB15's) per speaker, and still have and use a sub.

3. Woofer specs say they play 20-5000Hz. Since I have 4 woofers per tower could I expect them to play as low as 20Hz? I was wondering if each tower can both take care of the region where the NSB drop off and also the subwoofer region.

That range is a bit of a meaningless spec - which drivers are you looking at?

4. Since I will have 28 12" woofers in my room I hope that I won't have to use a subwoofer. I know placement of speakers and subwoofers are not the same but since this is dipole I hope room modes is not an issue.

I'm sure that with 28 drivers room modes will be an issue - but have no idea what specific issues they would be

I was planning on going active on the whole system so hopefully crossover design is simplified.

Will this work. Is it a good setup for HT?

Cheers,

Exipnos

If you're planning on going active as in buying some behringer components it will be relatively simple - buy, plug in, initial set-up, take in-room measurments to adjust. To design and build a custom active crossover is another very complicated matter entirely (if you have no experience in this area).


Your problems with this setup will be:
1) Dipole speakers require massive excursion and/or surface area to play down to 20Hz at good movie levels - you're better off with a separate enclosed sub, or two, to hit the very lowest registers.
2) The centre speaker? A line array doesn't work well as a centre, as it's horizontal dispersion when tipped on it's side is poor.
3)Space - do you have space for 4 surround speakers that are 5-6' high?

I'd suggest building a pair for your mains first and deciding where you want to go from there.

Paul

[exipnos]

The woofers are some cheap stuff that could be found at www.hifikit.se. Its the Pro Audio SW 1230 or Pro Audio SW 12.

There is a guy that buit some dipole subwoofer that uses 4 of these woofers.

http://www.hififorum.nu/forum/topic.asp?TOPIC_ID=30297&whichpage=1

On this page is his measurements.

http://www.hififorum.nu/forum/topic.asp?TOPIC_ID=30297&whichpage=4

Supposedly pretty good even in the low end stuff.

[Paul W]

Dipoles are great, but they are normally placed at least 3 feet from the nearest wall. Space could easily be an issue with a 7 channel system. (Perf'd screen front projector?) Even in a good sized room, the humans could be engulfed by speakers.

I agree with Paul...don't try to go below 40-50Hz with dipole bass...you'll need box subs or IB for movies. Consider "regular" high output forward facing woofer 50Hz-150/200Hz to help prevent cavity resonances with the woofer playing that high in a V or W baffle.

[cjd]

I would also recommend not using the NSB's below 200Hz open baffle. They can "go" lower, but as they get pushed their midrange goes away ever so slightly. But, just my few cents.

Otherwise, these guys are right - for HT, you'll need a box sub still.

C

[exipnos]

All the front speakers will be placed behind a "procinium" thats built with acoustic transparent fabric. The screen will also be acoustical transparent. The front speakers will be about 2 feet in front of wall. But the whole wall will be treated. The rest of the room will also be treated. The rest of the speakers will in in fake columns. Yes these speakers will be closer to the wall 1 - 1.5 foot maximum. But I will try treating these positions as much as possible.

I know that closed sub is usually needed but if you see the measurements on the 4 12" dipole sub it really seem to go low enough with enough SPL. Do I really need closed sub if I have 7 of these bass dipoles?

There is some other guys at that Swedish forum that built similar dipoles but with 8 12" woofers instead of 4. They indicated no problem getting down to 15-20. I will have many more woofers but spread out. I guess if I had all the woofers in one tower it wouldn't have been a problem going really low. Does it matter that my woofers are spread across 7 locations?

Help me understand...

[Jack Gilvey]

There is a guy that buit some dipole subwoofer that uses 4 of these woofers.

http://www.hififorum.nu/forum/topic...297&whichpage=1

Wow, that's really cool. Looks like it's inspired by the Linkwitz Phoenix sub. Beats the hell out of my U-frame , for sure.

[Paul H]

There is some other guys at that Swedish forum that built similar dipoles but with 8 12" woofers instead of 4. They indicated no problem getting down to 15-20. I will have many more woofers but spread out. I guess if I had all the woofers in one tower it wouldn't have been a problem going really low. Does it matter that my woofers are spread across 7 locations?

Help me understand...

Go to Sigfried Linkwitz's site here:

http://www.linkwitzlab.com/page_index.htm

and look up his spreadsheet on "spl max". Put in the parameters for the drivers you're looking at, to start getting some ideas about how much low frequency sound can be had from dipole woofers.

Paul

[exipnos]

okay I downloaded the spreadsheet to play around with but I didn't get far. It asks to input three values that are bolded. I only know what the Xmax is.. What are the other variables?

Effective piston area Sd =
Peak excursion Xmax =
Effective path difference D =

Another similar cheap driver could be found at partsexpress

http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=299-140

This is not exactly the same as the one I was thinking of but maybe I can model that one to get some approximation on my driver. But I still can't find the sd and effective path info.

Anybody?

[ThomasW]

Sd is the cone area
For Peak excursion Xmax, Linkwitz is using the Xmax in his examples.

From the Linkwitz website......

Q14 - What is the low frequency sound pressure level from an open baffle speaker with a given effective piston area and excursion?
A14 - The output depends on the path length difference D for sound reaching the listener from the front and the rear of the piston. For a flat circular baffle of diameter d, with the piston at its center, that path length difference is D = d/2, when measured on-axis and at sufficient distance. For the PHOENIX dipole woofer D = 19" (483 mm) between front and rear cabinet openings. For the PHOENIX main panel, which is folded back and rectangular, the effective path difference is more difficult to determine and approximately D = 12.5"/2 + 3.5" = 9.75" (248 mm).

[Dennis H]

Do I really need closed sub if I have 7 of these bass dipoles?

You'll have to use a custom mixer/splitter if you want to feed the LFE channel to all 7 woofer boxes. Most receivers and prepros only send the LFE channel to the front L&R if you tell them you don't have a sub.

[exipnos]

I'm okay with the custom mixer/splitter since I'm already up to my ears in custom solutions My project sounds more and more like the big wall of china. It will probably take just as long to complete

The whole system will be active and all the crossovers will be done via soundcards on a PC. I will also try DRC and I believe there is software available to set up different bass management.

[exipnos]

I downloaded linkwitz max_spl spreadsheet and I used some of my values. I estimated cone area for one 12" woofer to be about 700cm^2. I then multiplied this by 4 to and used it in the spreadsheet. This is the SPL up to 141hz. First column is frequency, second is monopole, third column is for dipole.

25 97 83
35 103 92
50 109 102
71 115 111
100 121 120
141 127 129

If I use cone area for all the woofers then I get the following data

25 114 100
35 120 109
50 126 118
71 132 127
100 138 136
141 144 146

Is it valid to just calculate the area for all the woofers and not consider that they are spread out in 7 locations in the room. Probably not since there will be cancellations. But do they represent a valid estimate? Can I get something close to these numbers? If so then its probably good enough for my situation.

If I can get decent bass down to my rooms resonance of 24.5hz, then I might be happy with that and not spend more funds on monopole woofer. Yes the dipole will drop off rapidly after that and not give me the really low end stuff but I can probably live with the compromise.

If the above values are close to reality then won't I have another problem in the 100-150hz region? I belive the SPL at those frequencies are much higher then the rest of the array. How do you deal with that?

Cheers,

Exipnos

[Gargoyle]

exipnos wrote: I estimated cone area for one 12" woofer to be about 700cm^2.
Cone area for 12" should be circa 490-530 sqcm.

[exipnos]

My estimate was only based on the area of a 12" circle. I did not deduct for the "central cone area". Yikes thats a big difference between my estimate.

But why such a difference? The 200sqcm difference represents a circle with about 16cm diameter. How big is a normal "central cone area"? Am I thinking wrong in my calculations?

By the way whats the correct term I'm trying to find for the middle of the speaker?

[Gargoyle]

Take a look at spec. sheet of anyone 12" frame woofer. You"ll find the "Effective cone area". More than 16cm diameter circle it is 30cm diameter without twice 2.3 cm for frame and 1/2 of suspension.

[exipnos]

here is the new values if I use 500cm^2 per woofer.

25 94 81
35 100 90
50 106 99
71 112 108
100 118 117
141 124 126

here is the values if I add all 7 locations

25 111 97
35 117 106
50 123 115
71 129 125
100 135 134
141 141 143

The really low tones are less then reference level for sure. I guess I can use this for a while, but I would definately need dedicated subwoofer in the end.

So I guess the next problem will be to find some cheap 12" woofers that would mate well to the NSB line and play OB well between say 40-200hz. Any suggestions?

Final issue. Is it crazy to consider OB dipoles for all the 7 speakers? I thought dipoles where better for minimizing room modes. Is that true? Will I have lots of problems with 3way dipoles in my room or will it be the similar with regular boxes?

Thanks for the info.

Cheers,

Exipnos