DIY 18" PA subwoofer cabinet - diyAudio
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Old 2nd February 2009, 08:31 PM   #1
CruiSE is offline CruiSE  England
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Question DIY 18" PA subwoofer cabinet

Hi All,

I'm new to this forum but not new to sound, music and PA systems. I'm thinking of building a pair of DIY single 18" subwoofer cabinets to go with my pair of Samson DB500a speakers.

I will be using Precision Devices 186 (700w @ 8 Ohm) speaker components, 3/4" birch plywood for construction, and a Behringer EP2500 poweramp in bridged mono (2400w @ 4 Ohms) to power both speakers in parallel. I going for the vented design, would like each sub to go down to around the 30Hz mark and provide loud clear bass for both recorded CD music and a live rock band in a small venue.

I've attached a screenshot of the T/S parameters of the PD.186.

The problem I have is that i'm not really sure how big the cabinets need to be to get the most out of the speaker drivers. I've been using software like WinISD, Boxnotes and AJDesign to try and get a better understanding but i'm still quite baffled on the whole dimensions and the vent side I need. My woodworking skills are up for the task but i'm currently stuck in the design stage.

So any help you guys care to contribute to get me on my way would be most appreciated. Thanks
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File Type: jpg pd186 t:s.jpg (30.9 KB, 710 views)
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Old 2nd February 2009, 10:46 PM   #2
tinitus is offline tinitus  Europe
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PD186 seems like the choice fore you goal

Fore 30hz you need 250-280 liter(netto), four ports of each 100mm, 150-200mm long

Total port area of four 100mm ports are 320cm2, if you want to build slot ports, but its length cant be changed, which is possible with the round port tubes

Could be that with longer ports a smaller box may be possible, but cant help you with that
Anyway, shorter ports mostly seem to work/sound better
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Old 3rd February 2009, 01:43 AM   #3
CruiSE is offline CruiSE  England
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Thanks for the reply. You know what, i've been trying to calculate port sizes all day and was finding it difficult to design a cabinet around two ports with really long lengths each. I never thought of adding more ports to my design and thus reducing the required port length. I think with your help i may be on to something now. Thanks
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Old 3rd February 2009, 01:58 AM   #4
KOA is offline KOA  United States
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when adding more ports for a given box size and port diameter, the length of the ports will increase if I am not mistaken.
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Old 3rd February 2009, 02:44 AM   #5
tinitus is offline tinitus  Europe
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Quote:
Originally posted by KOA
when adding more ports for a given box size and port diameter, the length of the ports will increase if I am not mistaken.
So it is

This driver seem to like a very big port...and I mostly try to keep port length within 100-250mm

I have split it up in 4...but it could also be 3, if thats more convenient

I made it 4, so that each would be 100mm

The area of one 100mm is 80cm2, which means the total area of 4 ports calculated is 320cm2...which gives an ok port length

But the calculated port is not always the optimal...fore that you have to experiment a bit, either with measurements or listening
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Old 3rd February 2009, 04:02 AM   #6
CruiSE is offline CruiSE  England
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You're right about more ports relating to more port length needed. Found that out after reading the reply and punching numbers into WinISD.

It suggests that I use a box with a volume of 152.61 litres and tuned to 27Hz. If I have two 10.20cm circular ports with a cross area of 163.4cm^2, the vent length would have to be 37.66cm.

If I wanted to use rectangular ports like you see on subwoofer models like the Peavey UL118S, how would I go about calculating the length needed?

Thanks
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Old 3rd February 2009, 04:39 AM   #7
tinitus is offline tinitus  Europe
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Quote:
Originally posted by CruiSE

If I wanted to use rectangular ports like you see on subwoofer models like the Peavey UL118S, how would I go about calculating the length needed?

Thanks

Its the same...width x height = area

Only, you use the width of box
But I would split it up in 3 or 4 sections
Makes the calculation slightly more tricky

Personally I would stay away from very long ports
I think the port length should relate to individual ports area...so ofcourse big ports may be longer, but not too much

Another possible option could be triangular ports in the corners
4 riangular ports calculates as 2 squared ports

165 liter is only half of VAS, and sound a bit smallish to me...you have to look out fore overshoot/peak(boxQ)
...I dont think you want that with such big woofer

You will also have some room gain, and normally a smooth gradual rolloff is preferred
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Old 4th February 2009, 01:09 PM   #8
tinitus is offline tinitus  Europe
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Is it fore PA ? I may have missed that, and if so my suggestion might not be valid
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Old 4th February 2009, 02:00 PM   #9
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About 160 - 200 liter would be common for PA-purposes. I suggest you build the G-sub from speakerplans as it was designed for that particular speaker (the 10.5 mm Xmax is a bit overstated) http://speakerplans.com/index.php?id=gsub

Best regards Johan
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Old 4th February 2009, 03:49 PM   #10
CruiSE is offline CruiSE  England
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Great find Johan, thanks
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