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23rd September 2011, 02:21 AM  #11  
diyAudio Member
Join Date: Jun 2008
Location: N.E. Ohio

Quote:
Gee where do we start.... 30l at 50hz lets see  there are double 18" boxes that do 140dB and I'm sure the displacement is well under 2l. What about the Danley stuff that puts out ~150dB???
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Regards, Dan 

23rd September 2011, 02:39 AM  #12 
diyAudio Member
Join Date: Mar 2004
Location: Nashville, TN

We just built 18 definite purpose boxes using 2x FaitalPro 18xl1500 at work. In a ~300L box with a single center vent and an f3 of 37Hz, we measured 146 @1M quarter loaded using pink with a lab fp3400 in bridged mode. That was at max output of the labgruppen and with headhoom to spare in the drivers. So, it's totally doable.

23rd September 2011, 02:43 AM  #13 
diyAudio Member
Join Date: Feb 2008
Location: Jackson,michigan

Well, If an 18" has a surface area of 254 with a excursion of 1" it would take 7.28 of them to do 50hz.
So 16 of them is about what I figured would get you to the 30hz range. But that is at 1" excursion. But if you concentrate the energy with a horn of some sort you may get some more gain out of them. jer 
23rd September 2011, 02:51 AM  #14 
diyAudio Member
Join Date: Feb 2008
Location: Jackson,michigan

Also don't forget the room gain.
the Dipole requirements I think were double that, But 8 or 12 of them stacked up in dipole configuration per side just might do the trick !!!! Insane,Ahhhhh,ha,ha,ha,ha,ha,ha !!!!!!! jer 
23rd September 2011, 05:13 AM  #15 
Wizard of Kelts
diyAudio Moderator

Justin:
Here is the chart taken from Richard Small's article for volume displacement for SPL at various low frequencies:
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23rd September 2011, 05:15 AM  #16 
Wizard of Kelts
diyAudio Moderator

Now here is the same chart in inches, which I adapted from the original:
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"A friend will help you move. A really good friend will help you move a body." Anonymous 
23rd September 2011, 05:28 AM  #17 
Wizard of Kelts
diyAudio Moderator

The way it works is that the SPL at a given frequency goes up 12 dB when you quadruple the displacement.
Let's try for 30 Hzwe can always back off down to 40 Hz if we need to. At 30 Hz, 128 cu in gives us 117 dB. So, 512 cu in gives us 129 dB. So, 2048 cu in gives 141 dB. That means that, using closed boxes, we would need the cone area times the displacement to equal 2048 cu inches. Assuming a one inch throw on a PA speaker, that means a cone area in a closed box of 2048 square inches. Considering a 15 inch speaker has a cone area of 130 sq inches or so, that means you would need about sixteen 15 inchers to achieve 140 dB @30 Hz.
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"A friend will help you move. A really good friend will help you move a body." Anonymous 
23rd September 2011, 05:45 AM  #18 
Wizard of Kelts
diyAudio Moderator

Assuming you don't want to do that, there are things you can do.
A. You can move your target frequency up to 40 Hz; B. You can use ported enclosures, which change things in your favor. Let's see what the displacement is for 140 dB at 40 Hz instead of 30 Hz. As we can see, 128 cu inches gives us 122 dB. 512 cu inches gives us 134 dB. 1024 cu inches gives us 140 dB. (Double the displacement = +6 dB). Assuming a one inch travel on a PA speaker, that means 1024 sq inches, or about eight 15 inchers. That is in a closed box. However, if you use a vented box with a Qts of 0.38 or so, you can cut that displacement requirement by 4. Instead of a cone area of 1024 sq inches, (assuming a one inch throw on your PA speaker), you now need only one quarter as much, 256 sq inches. Since most 15's have a cone area around 130 sq inches, you should be able to get this done with two vented box 15 inch PA speakers, with a one inch throw, with a Qts around 0.38, a box volume equal to the speaker's Vas and tuned to 40 Hz or so.
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"A friend will help you move. A really good friend will help you move a body." Anonymous Last edited by kelticwizard; 23rd September 2011 at 05:50 AM. 
23rd September 2011, 06:24 AM  #19 
Wizard of Kelts
diyAudio Moderator

That takes care of the speaker volume requirments. The amp requirements are a different matter.
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"A friend will help you move. A really good friend will help you move a body." Anonymous 
23rd September 2011, 10:33 AM  #20 
diyAudio Member
Join Date: Mar 2010
Location: Salzburg

Excellent, and now
we need a suitable design for Justin's 3015LF
If you can give him 140 dB @ 30Hz from a pair of them, I will be very impressed! Regards, Ben 
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