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Old 20th January 2012, 02:11 AM   #1401
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Quote:
Originally Posted by weltersys View Post
As far as tapped horns, the minimum impedance is usually quite close to the DC resistance, so is important when choosing an amp to drive the load.
The first low-impedance dip of traditional basshorns is usually very close to the DC resistance value too. The second dip is usually much higher. A 25% increase isn't uncommon. But, yeah, every basshorn I ever measured had impedance minimum in the passband that was very close to DC resistance. So I suppose I could have just slapped an ohmmeter across the leads but I always thought it was probably best to use that first dip as the Zmin value.
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Old 20th January 2012, 02:37 AM   #1402
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Originally Posted by Djim View Post
I also tried to explain how HornResp seems to be using some sort of steps in finding its lowest impedance point for its calculation and David explained to me in post#1379 it is limited by the graphic resolution of screen pixels.
This is what David says about the algorithm used to calculate:

Quote:
Originally Posted by David McBean View Post
Voltage Eg is automatically adjusted by Hornresp to maintain a constant 1 watt input, even though the load impedance changes with frequency.
The Hornresp model maintains power constant, not voltage. This makes impedance a non-issue with respect to drive voltage. The simulated drive voltage could be said to fluxuate, tracking the impedance curve.

We need to know Zmin when comparing measurements. To set the drive voltage for a reference power level we need to figure impedance.
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Old 20th January 2012, 06:29 AM   #1403
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Originally Posted by Wayne Parham View Post
Essentially, I take the speaker outdoors and use a swept sine to measure impedance. Then I examine the chart for Zmin, minimum impedance above cutoff. Next I calculate the drive voltage required to attain a specific reference power level. Then I set the (RMS AC) voltage drive to that reference value, e.g. 28.3v for 100 watts if Zmin is 8Ω. Then I position the microphone and run a sweep to get a response chart that shows SPL as a function of frequency.
Hi Wayne,

When you calculate the drive voltage required to attain a specific reference power level, do you simply use the magnitude of Zmin and consider it to be a pure resistance, or do you treat Zmin as a complex impedance having both resistive and reactive components? The voltage will be slightly different for the two cases.

The attached screenprint generated using the default Hornresp record compares the true 1 watt / 1 metre response (black trace) against the response obtained by setting Eg to the voltage required to deliver 1 watt into the chosen Zmin value (gray trace). As you can see, there is a reasonable difference between the two. The only point at which the two traces exactly coincide is at the frequency of the minimum impedance within the nominated passband. For this particular example, the minimum impedance happens to be at the very top of the selected bandwidth (at around 800 hertz).

Kind regards,

David
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Last edited by David McBean; 20th January 2012 at 06:33 AM.
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Old 20th January 2012, 02:55 PM   #1404
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The reason for the difference is mostly due to one curve being generated by a constant voltage source and the other being driven with constant power. Remember that when we measure, we're dealing with amplifiers that are essentially constant voltage sources.
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Old 20th January 2012, 04:30 PM   #1405
60ndown is offline 60ndown  United States
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so, is there a winner? i wont read 140 pages but i might build the best single sheet box.

is there a link to the plans/ driver??
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Old 20th January 2012, 05:50 PM   #1406
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Originally Posted by 60ndown View Post
so, is there a winner? i wont read 140 pages but i might build the best single sheet box.

is there a link to the plans/ driver??
If you go back to post # 381, jbel writes:

“Yea, the dual lab smokes the ss15 below 40hz, ss15 smokes the dual lab 50hz on up. With that whole 'iron law' thing, you can have any 2, loud, low, small.”

The dual Lab 12 design uses less than a single sheet of plywood, and has far less distortion than the SS15 using a 3015LF.

Plans are here:
FREE SUB PLAN: Dual Lab12 (Front Loaded) by Welter Systems

I have kept a pair of the 2x12" for smaller gigs, and have made a pair of tapped horns double the size of the 2x12" that I use for most shows.

The larger Keystone TH have more LF output of a pair of the 2x12" (and are lighter than a pair), and more upper output than the SS15, the best of both worlds, but take more than a sheet of plywood.

Art Welter
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Old 20th January 2012, 06:06 PM   #1407
60ndown is offline 60ndown  United States
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sure would be nice to see some photos of the construction of a dual lab 12, ive built dozens of subwoofers over the years but cant quite make out how the build is supposed to go from the drawings i can find and photos of finished boxes?
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Old 20th January 2012, 07:26 PM   #1408
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Originally Posted by 60ndown View Post
sure would be nice to see some photos of the construction of a dual lab 12, ive built dozens of subwoofers over the years but cant quite make out how the build is supposed to go from the drawings i can find and photos of finished boxes?
It is a basic BR cabinet, six sides, two triangular corner ports, all the details are in the link.

The only thing unusual at all is the ports are fairly large and have a 45 degree angle on the outside and inside so the grill frame becomes part of the last 3/4".
That detail keeps the cabinet sounding basically the same at any power level below Xmax.

I used shells from old FLH cabinets to make my 2x12", the destruction details would have been interesting, but I did not photograph them.

The placement of the in/out connectors made putting a cabinet divider difficult, if I were building from scratch I'd add one.

The photo below shows the front of four cabinets with the grills removed.
You seem to be looking for 31 Hz response, the ports could be lengthened slightly, response would flatten out and drop a bit, at the expense of a little 50 Hz area output.

Art
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Last edited by weltersys; 20th January 2012 at 07:29 PM.
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Old 20th January 2012, 08:15 PM   #1409
60ndown is offline 60ndown  United States
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ok thanks i think i get it?

box is 21" deep, ports are 16.5 deep (longer is lower tuning) with 30 sq" area each?

a 3D sketch-up sure would be nice

RAM Designs: T-line Box Design for True Bass 8" Subwoofer - YouTube
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Old 20th January 2012, 08:55 PM   #1410
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Originally Posted by 60ndown View Post
box is 21" deep, ports are 16.5 deep (longer is lower tuning) with 30 sq" area each?
Not quite, Seth H. Bass Box Pro sketch did not include the front baffle recess, you can see that in the thumbnail in the very first post, included below.
The interior depth is 20", interior height is 21 (22.5-1.5").

Reading through the two pages of posts should answer any other questions.

This is a very basic box, as long as you use big flared corner ports and keep the interior volume close to the same, it will perform fine.

There are now many drivers that cost less than the Lab 12 with similar TS parameters (and more Xmax) that would work well in this size box.

Art
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Last edited by weltersys; 20th January 2012 at 09:25 PM.
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