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Old 18th July 2010, 10:56 AM   #1
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Default Custom Built PA subwoofer

Hi all, new to the forums, but from what i've read you guys all really know your stuff, so I'd value your input on my design for a PA subwoofer for my band to use.

I am currently using this driver and amp combo in a sealed box as part of my home built jukebox, but as a PA sub for band use, it just doesn't cut it. Since money is quite tight at the moment due to an impending wedding, I have to use the driver and amp I've already got, despite their obvious unsuitability for the task being asked of them.

The TS specs given are specified as being with the 4 ohm voice coils wired in parallel, giving a 2 ohm load, but the amp is not two ohm stable, and I have a 600 watt (bridged into 8 ohm) amp I might want to use instead. If I wire the voice coils in series to get an 8 ohm load, will this alter the TS specs in such a way that my design is no longer tuned the way I want??

WinISD gives me a graph that shows a peak of 3.6dB around the 30Hz mark, and the -3dB point is about 24Hz. If you think I should be aiming for a different graph, let me know. In fact, i'd appreciate ANY suggestions or comments on the design or how I could improve it, bearing in mind I'm stuck with the driver and amp(s) I have.

Also if any more information would help, just ask!

Cabinet - 18mm MDF
Internal Dimensions - 700x700x450
Box Volume (Unadjusted) - 220 Litres
Driver Volume - 4.25 Litres
Plate Amp - 1.7 Litres
Port Volume - 4.2 Litres
Bracing Volume - 7 Litres
Box Volume (Adjusted) - 203.35 Litres
Ports - Two ports, 102mm inside dia.
Port Length - 220mm
Tuning Frequency - 28.57Hz

Driver - Response 300 Watt RMS Dual Voice Coil 12" Subwoofer
Dc Resistance: 1.7 ohms
Vas: 79.454L
Qms: 3.238
Qes: 0.446
Qts: 0.392
Sensitivity: 88.192 dB @ 1 watt & 1 metre
BL: 9.992TM
Fo: 28.77Hz
Sd: 514.72 sq cm

Plate Amp - 350 Watt RMS into 4 ohms

Thanks in advance!
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Old 18th July 2010, 04:42 PM   #2
djk is offline djk
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That is a total non-starter for a PA subwoofer.
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Last edited by djk; 18th July 2010 at 04:51 PM.
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Old 18th July 2010, 05:07 PM   #3
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A horn will be usable.

Throat area = 250cm
Mouth area = 3000cm
Length = 250cm
Rear volume = 15L

290L net volume, probably about 360L gross. This might be around 2' x 2' x 3' (external).

Average is about 100dB/W/1M in 2Pi (on the floor), 42hz~120hz (will need about 3dB of cut around 110hz).
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Old 19th July 2010, 03:51 AM   #4
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Thanks for the response. My experience has only ever been with sealed and traditional ported enclosures for auto and home theatre use. I'm doing as much reading as possible at the moment to try and get my head around tapped horns, since I think it would be the ideal thing for me.

The designs I have seen mostly look complicated, but would I be right in saying that you basically start with a horn throat thats about a quarter of Sd, and fold the horn around so that the rear of the driver is 'tapped' into the horn quite close to the mouth? Also that the horn mouth should be about twice the driver Sd.

I am having trouble working out how you work out the length of the horn, though... Any help would be greatly appreciated.
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Old 20th July 2010, 05:12 PM   #5
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"I am having trouble working out how you work out the length of the horn, though... Any help would be greatly appreciated. "

Use McBean's Hornresp program. A horn must be a minimum of 1/4 wavelength of the lowest frequency you want to reproduce.

The driver you have will not do the job you want it to in a tapped horn (otherwise I would have suggested same). It would be fine for a home theater subwoofer tuned for 20hz or so, but not for PA.
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Old 21st July 2010, 12:05 PM   #6
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Quote:
Originally Posted by shaggerz View Post
..I am having trouble working out how you work out the length of the horn, though... Any help would be greatly appreciated..
Hi shaggerz,

Played with your DVC driver where the coils are connected in series:

b
Attached Images
File Type: gif CB-PA-8-Ohm.GIF (63.1 KB, 274 views)
File Type: jpg CB-PA-8-Ohm_154L-OD_Horn.JPG (361.7 KB, 274 views)
File Type: jpg CB-PA-8-Ohm_96L_T-TQWT.JPG (334.1 KB, 264 views)
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Old 21st July 2010, 12:10 PM   #7
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Thanks but those pictures are too small to read could you post larger ones? or email them to me? will pm address.
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Old 21st July 2010, 02:22 PM   #8
bjorno is offline bjorno  Sweden
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Quote:
Originally Posted by shaggerz View Post
Thanks but those pictures are too small to read could you post larger ones? or email them to me? will pm address.
Read GM's hint on how to enlarge= the thumbnails : post #15 and #16:

25-30hz tapped horn recommendation please... TB w8-740??

b
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Old 21st July 2010, 02:46 PM   #9
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Click on the thumbnail then on the four arrow cluster in the bottom corner to enlarge, again to close.
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Old 21st July 2010, 06:25 PM   #10
djk is offline djk
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It is as I suspected, the traditional horn is better suited for the parameters of this driver, yielding about 5dB more output overall, and better bandwidth too.

I would be in favor of a tapped horn, if I could use a different diver.
(bearing in mind I'm stuck with the driver and amp(s) I have.)
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