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Old 22nd June 2009, 10:17 AM   #1
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Default 2nd Order Dome SUB

The idea is to augment the Low frequencies range by adding a good sounding fit looking SUB to be located in a (aprox 20+m x 15m) room. The place doubles as a training daily rehearsal music room and also as an small auditorium. Utilised mostly by a single multinstrumentist musician and sometimes by a small ensemble or different formations between them.

Half of this room is where the piano, the keyboards, violin, microphones and the rest of the instruments are located, the other half is where the audience sits.
The present amplification is achieved by a pair of full range DAS speaker boxes.
The Subwoofer ideally would produce from the lowest freq possible up to 100-120 Hz. The xover point will finally be chosen depending of the Sub interaction with the room and the rest.
It would be actively crossovered with its dedicated amp.

The driver is a JBL-2268HPL which looks impressive as a driver concept, and had the luck to already have it here. Its specs are in this page

It appears that the best position for such use of the Sub is at the room's center, hence the enclosure would need to be visually unobtrusive and for that we'd -so far- are going for a (ported) plywood made Dome, with the driver pointing upwards for better omnidirectional sound dispersion. Similar to an hemisphere.

Dome designs of the Second order type allows the 18" driver to be placed flat on the top pentagram, see the 2nd order shape in this guide to build on paper. In these drawings is easy to spot the flat pentagram surface on top.

Also there is a page that does Domes for housing with good ideas for the structure build.
Additionally as a Dome speaker, it would be easy to add internal bracing, resulting in a very firm structure.

-Dimensions: Since the driver is originally used on JBL's VRX918s,
which is a squared box measuring: 508 mm x 597 mm x 749 mm D (20.0 in. x 23.5 in. x 29.5 in. D), the idea is that the dome internal volume at least equals that. I have yet to find a way how to calculate such volume plus the bracing and each side
(triangles+ pentagrams) wooden affirming structure. Our idea is not having to use complex modeling, specially because these differential drivers use double coil and I dont know if the Xmax is applied the same in those equations... Maybe someone could enligthen here?

-I am sure there exist software that finds all the precise dome side's measures to precisely draw the plywood needs. Just found this page, have yet to dive in...

Any advice, correction, related information or comments would be very much welcomed, particularly those which could either improve the results or provide a more scientific approach
or a clue to what expect in terms of sound (low range) or tips for the actual build are very much wellcome.
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Old 22nd June 2009, 05:53 PM   #2
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Looks good.

Which amp are you going to use?
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Old 22nd June 2009, 11:25 PM   #3
GM is offline GM  United States
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Default Re: 2nd Order Dome SUB

Quote:
Originally posted by djcl.ear

The Subwoofer ideally would produce from the lowest freq possible up to 100-120 Hz.

The driver is a JBL-2268HPL ........Its specs are in this 'page'.

..........with the driver pointing upwards for better omnidirectional sound dispersion. Similar to an hemisphere.

..........a squared box measuring: 508 mm x 597 mm x 749 mm D (20.0 in. x 23.5 in. x 29.5 in. D), the idea is that the dome internal volume at least equals that. I have yet to find a way how to calculate such volume plus the bracing and each side
(triangles+ pentagrams) wooden affirming structure.

Our idea is not having to use complex modeling, specially because these differential drivers use double coil and I dont know if the Xmax is applied the same in those equations... Maybe someone could enligthen here?
By lowest', I assume you mean a 27.5 Hz piano note, so you want a sub tuned at least this low.

Which pdf on this 'page' has its T/S specs?

First we either need accurate specs to determine if it is suitable for up/down orientation or have JBL tell you.

WRT calculating a geodesic dome's volume (Vb) for a sub box, using a simple hemisphere's formula is close enough since you will be tuning it to suit by experimenting with its vent length.

I have no idea what you want to know WRT the driver's Xmax. I mean Xmax is just a distortion point in a driver's stroke.

GM
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Old 23rd June 2009, 05:22 AM   #4
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By "2nd order Dome" I meant Frequency 2 Dome

Thank you Chris 661 for your interest. The amplifier is a rather small one but I guess sufficient for the usage. Its a 275 w RMS at 8 ohms.
In future upgrades, improving middle and treble amplification and their speakers appear as more critical and hence prioritary.

Thanks also GM for your questions, that i'll try to answer the best I can.
Quote:
By lowest', I assume you mean a 27.5 Hz piano note, so you want a sub tuned at least this low.
That would be ideal, but I understand the difficulty of achieving such goal.
One important part of the limitation is the desired Sub shape (Dome) and to frame that i'll try to post pictures of the actual room soon. Have in mind the place is located in a remote valley with a impressive natural landscape. He hungs several world-music acoustic instruments on the back wall as part of the decoration.
Joakin mainly amplifies his violin and keyboards. The former is his main instrument and he passes it thru a powerful effect processor that he runs in varied configurations during concerts. I'd say it gets to low freqs, though haven't heard them too low but that may very well be because of amplification limitations...
I understand the driver by (anechoic measure?) produces usable 35 Hz (-3dB). And to my limited knowledge in this, this figure could be bettered with an increase in the enclosure internal volume? and accordingly increasing the length of the port?.
My strategy is to build the box the better quality possible (firm) using at least the VRX vol measures (see the attached file) or bigger if someone in the know confirms that doing this will improve the low range AND if at the same time the SUB keeps being unobstrusive on the suggested size. Then with the enclosure already built, find the length of the port using a PVC tube to varying lengths and measuring a low tone looking for best results.
I understand the port (vent) location is not relevant, is this correct?

Quote:
Which pdf on this 'page' has its T/S specs? First we either need accurate specs to determine if it is suitable for up/down orientation or have JBL tell you.
The tech data is in the Altec Lansing site. The link on the first post was from the VRX (my fault). I understand the 2268HPL is the same (H means 8 Ohms) version as 2268H, minus a "tire" in the ferrite magnet.. The actual T/S parameters are here


Quote:
WRT calculating a geodesic dome's volume (Vb) for a sub box, using a simple hemisphere's formula is close enough since you will be tuning it to suit by experimenting with its vent length.
Well, probably yes. What volume would you recommend?
Is it true that the typical calculations just indicate the minimum size and hence erring on a bigger size would not diminish the enclosure low freqs capacity?

Quote:
I have no idea what you want to know WRT the driver's Xmax. I mean Xmax is just a distortion point in a driver's stroke. GM
I've read in this forum about the problem that 23 Xmax produces at attempting calculations with enclosure design software. The differential drivers have two coils acting push pull, so they will not be comparable with Xmax from traditional single coil drivers. I understand the problem, but I have not seen a solution or a proposed equation (or software) for differential drivers...
Since I am interested in this driver technology and plan to use in the future I'd wellcome related findings.

Glad to finally get to discuss all this.
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File Type: pdf vrx918s.pdf (49.0 KB, 25 views)
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Old 24th June 2009, 10:18 AM   #5
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"I've read in this forum about the problem that 23 Xmax "

That is the mechanical damage limit, not the x-max

x-max is usually specified as the 70% Bl point, which is less than 10mm on the reference you listed.

http://audioheritage.org/vbulletin/s...ad.php?t=11346

The 2268 was discussed here:

JBL 2268HPL as opossed to JBL2242H
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Old 24th June 2009, 07:28 PM   #6
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Hi,

FYI:
Taking a lower x-max (=8mm where BL is 1/sqrt(2) x BL(nominal)) into consideration and if designing a T-TQWT that is 1 Hz safe (21.5 mm = thus less than 23 mm).

Maximum achievable SPL at 20 Hz in '2PI' is about 110,5 dB/1m/118W compared to the maximum SPL in the passband that is around 112 dB.

For this IMO this Hi SQ design you have to pay the penalty of operating the driver in a low efficiency:
1W in results in a passband SPL of about 93 dB ref 8 Ohm instead of a closed box efficiency of 96 dB.

About 3 dB is traded for BW together with a box size requirement of 338L = 11.94 cu ft. See the pictures 1(3)-2(3)

After listening and comparing with identical (= tapered+volume-length) TL-subs as in picture 3(3) an OD-TL = offset driver TL (zero offset-TL shown), I probably will no more design TL subs.

I always try to trade SQ and BW over what ever SPL that could be achieved.

b

1(3)
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Old 24th June 2009, 07:30 PM   #7
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2(3)
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Old 24th June 2009, 07:49 PM   #8
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3(3)
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File Type: jpg 3-od-tl -comp.t-tqwt.jpg (93.4 KB, 125 views)
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