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Old 26th September 2005, 07:08 AM   #11
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Default Re: D'Appolito Fudged

Quote:
Originally posted by john van ommen
I agree with the other posters; redesigning the Thor Enclosure in Mathcad is a great idea.
Augspurger's tables are quite crude... i expect that much more can be had from these drivers.

dave
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Old 26th September 2005, 09:25 AM   #12
Byrd is offline Byrd  South Africa
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Default Re: Re: D'Appolito Fudged

Quote:
Originally posted by planet10


i expect that much more can be had from these drivers.

dave
Any Proposals?
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Old 26th September 2005, 09:41 AM   #13
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Sigh. I was hoping you wouldn't ask us this. Because, frankly, I think these drivers are a downright pig to get the best out of. Unless I'm feeling unusually stupid today, which is always a possibility. I've been fiddling in MathCad, trying to see what gives. Firstly, for your viewing pleasure, I present Dr. Joe d'Apollitio's Thor, as modelled by Martin King's MathCad software.
This is approximate, but the uncorrected, anechoic reality will be close.

It's not as bad as I anticipated, as from the driver specifications I was less than optimistic that they would work at all in a conventional TL. Credit where credit is due for getting a reasonably decent response from it. However, Thor does have a number of problems. That rising response (add 6db overall for the drivers being wired in parallel by the way) is going to need a wad of correction to bring it down into line. I assume it has this built into the crossover, which I have yet to look at. Either way, no wonder many users find it clear sounding! Note also that this is not falling away in lock step with room gain -you're going to get a boost somewhere no doubt, but this is hardly going to be consistant. I also think that 0.78lbs/feet of stuffing is way too much. It should be less -around 0.3lbs per foot is the max that should be used in my view; if you need more, the TL is incorrectly designed.

To be fair to Thor's great (and he is) designer, this is one of the better 'traditional' TLs I've seen, and he only had aprroximate, transitional tables to work with. Also, many people would probably like this sort of sound -it's going to be crisp and highly detailed whatever you do with it. However, I too strongly disagree with his notion that reducing the size of the enclosure doesn't make a difference. It does, and I'm highly dubious about the measurements / calculations that suggest it doesn't. I'm still trying to get something that I would consider satisfactory out of these drivers in MathCad, and it's not easy. I don't personally think they suit the conventional TL taper, though I could have simply missed a magic combination. A straight pipe looks similar to the conventional taper. So far, I've had the most promising results out of the good old TQWT, but still nothing I'd build yet. I'll let you know later when I hit on something that works.

Any other thoughts or cabinet dimensions guys?
Best
Scott
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File Type: jpg thor fr.jpg (62.4 KB, 5622 views)
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Old 26th September 2005, 10:31 AM   #14
Byrd is offline Byrd  South Africa
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Hey Scott - Is that the response of just the driver or does it take into account augmentation by the rear wave.

D'Appolito measured the Thor's f3 @45 Hz which is supported by the graphs where the graphs show combined response with augmentation. Where there is no augmentation the responses look similar to your predicted response. The question is is this an anechoic response or "room response". I would imagine it should be anechoic.

The Seas Thor article is not very clear when it comes to what kind of measurements these are. Very naughty.

edit
I should read things before opening my mouth / moving my fingers

Also just for reference this is the commentary on Depth in the article.

Quote:
The internal depth, d, is then computed as follows:
To get the external depth you must add the thickness of a 1' front baffle, a 0.75' internal baffle, and a 0.75' rear panel for an overall depth of 18.75. This number was considerably deeper than I wanted and would lead to a rather large and heavy enclosure.

At this point I made a number of arbitrary decisions. I chose a line taper of 3:1 and limited the overall depth to 13.5'. This led to the internal layout of the line shown in Fig. 1A. Placing the interior baffle at an angle produces the desired taper. A side, but important, benefit of the interior baffle is that it adds
greatly to enclosure rigidity, effectively clamping the side panels together and largely eliminating side-wall vibration. The resulting layout has a throat area of 61.875 in2, which is roughly 1.6 times the combined diaphragm area of the two drivers, and an exit area of 20.625 in2. As you will see, this departure
from Augspurger's recommendation has little effect on f3.
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Old 26th September 2005, 11:09 AM   #15
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Quote:
Originally posted by Byrd
Hey Scott - Is that the response of just the driver or does it take into account augmentation by the rear wave.

D'Appolito measured the Thor's f3 @45 Hz which is supported by the graphs where the graphs show combined response with augmentation. Where there is no augmentation the responses look similar to your predicted response. The question is is this an anechoic response or "room response". I would imagine it should be anechoic.

The Seas Thor article is not very clear when it comes to what kind of measurements these are. Very naughty.

Oh no, this isn't the driver response curve, you can get that from Seas. Nope, this is the real deal: MathCad predicted system response curve (anechoic). In other words, it's the combined driver and cabinet frequency response up to 1KHz (above that, only the drivers contribute).

Note that phrase arbitary. In his defence, the tables are not great, and were quickly superceeded by Martin's work, but I'm a little surprised at Joe with that -never mind he should know better, he DOES know better. There is actually an improvement with the larger cab, though it admittedly is small. When I modelled Thor, I actually also modelled the original dimensions. Things look a little smoother, but the basic shape of the curve, with the dip below 100Hz remains -it's a characteritic of the cabinet/driver combination.

In room response plots I take with a grain of salt, and view them only as of curiosity value -evey room is different, so it might be perfect in one room, and lousy in another, like any other speaker. You just never know. OK, so a speaker isn't going to be placed in an anechoic listening room either, but it is the only way that shows exactly what the speaker itself does, without external influences. The graphs refered to in the Thor article are not up to much in my view, as they are scattered around, rather difficult to read, and it's unclear as to how they were made / measured.

Thor could, and I'm sure does, work well in some circumstances (a 25' - 35' long room is going to help here), and I reckon that lots of people will like the sound it gives. Rather than redesigning the cabinet (though I'd cut out the nonsense of 0.78 lbs to the cubic foot of stuffing for a start and take it back to 0.3 lbs) I've just taken a look at the crossover, and I notice there's a 1mH inductor in the hot lead to the bass-driver's and their zobel. It might be worth playing around some with the value of this, as we regularly do for compensation circuits using single-driver systems. You could also try adding a resistor in parallel with it. As Thor is just a 2 way, it shouldn't be too tricky to ballance it out.
Cheers
Scott
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Old 26th September 2005, 04:37 PM   #16
MJK is offline MJK  United States
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I have never bothered to model the Thor enclosure, I criticized the design in the past without actually doing the homework, so I am very interested in Scott's plots. But I have a couple of questions/comments for Scott.

1. Did you combine the two drivers into one equivalent driver and then use the actual enclosure dimensions to model the speaker's response? If yes, did you place the equivalent driver at the midpoint/tweeter location?

2. The plot you produced with the sagging low end response is for the speaker radiating into 2 x pi space. Put this design out into a room and the bass/midbass will be an additional 3 to 4 dB down due to baffle step. If you mentally sketch this corrected plot, how does the simulation's low end response compare to the published audioXpress measurement data?

3. If I remember correctly, the audioXpress article had some near field measurements for the driver and terminus, how do these plots compare to the individual calculated responses?

Thanks for running the sims, very interesting results.
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Old 26th September 2005, 07:05 PM   #17
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You guys have probably seen this but, just in case, the Thor was measured in the Seas anechoic chamber.

http://www.seas.no/kit/thor.mling.pdf
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Old 26th September 2005, 07:22 PM   #18
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Quote:
Originally posted by catapult
You guys have probably seen this but, just in case, the Thor was measured in the Seas anechoic chamber.

http://www.seas.no/kit/thor.mling.pdf
Well, that pretty much confirms it.

Quote:
Originally posted by MJK
I have never bothered to model the Thor enclosure, I criticized the design in the past without actually doing the homework, so I am very interested in Scott's plots. But I have a couple of questions/comments for Scott.

1. Did you combine the two drivers into one equivalent driver and then use the actual enclosure dimensions to model the speaker's response? If yes, did you place the equivalent driver at the midpoint/tweeter location?

2. The plot you produced with the sagging low end response is for the speaker radiating into 2 x pi space. Put this design out into a room and the bass/midbass will be an additional 3 to 4 dB down due to baffle step. If you mentally sketch this corrected plot, how does the simulation's low end response compare to the published audioXpress measurement data?

3. If I remember correctly, the audioXpress article had some near field measurements for the driver and terminus, how do these plots compare to the individual calculated responses?

Thanks for running the sims, very interesting results.

Hi again Martin we seem to be doing this sort of thing quite a bit of late! What was the last the Hedlund horn (shudder) wasnt it?

Dealing with your queries in order:

1) Yes to all points (it seemed the best way forward hope I was right in thinking that?). I combined the drivers as per your site article on modelling two drivers. The enclosure dimensions were taken from the audioXpress pdf file the frequency plot I posted was for Thor as it stands, not Joes originally calculated dimensions (which I also plugged in, with fairly similar results), and I took the tweeter location as the assumed drive centre. Only changes made in the driver section was to double vad, halve the impedance and remember to take the +6db for the drivers being paralleled.

2) It looks very close I think, especially compared to the graph our friend has provided a link to. With the audioXpress article, its only when you look further that you realise its been disguised by the way the graphs are presented. The measurements look far too flat to be taken at face value, suggesting octave smoothing 9applies to the other graph too), and I note that a very wide dynamic range has been squeezed into a very small space the oldest trick in the book when you want to hide something. The crossover appears to have some compensation built into it, though nowhere enough in my view to sort out the overall rising response. So if we say that cancels out the circa 3 - 4 db lost through baffle-step, and look carefully at the original graphs, what MathCad predicts looks pretty close. Figure 18 pretty much confirms it if you look at the sagging woofer pair curve, as does the anechoic chart in the above link.

3) It does, and theyre a joke. The problem with the plots Im looking at now is that they were all apparently done with different levels of stuffing all over the place, and also different values in the graphs. Theres no consistency at all. Overall, once you sift through the irrelevance, they seem to agree pretty well with the calculated responses in your worksheets, but again, the presentation appears to disguise problems unless you look very carefully.

That said, Ive been fiddling around with different values, and I still cant hit on anything TL wise that Id consider building. A TQWT taper appears to be best, but I simply cant tame that sagging bass below 100Hz with either a conventional taper or straight pipe, leading me to believe that these drivers simply dont appear to be suited to a TL. Any thoughts Martin? Perhaps we should do the community a favour and come up with something for these drivers that actually works.

Cheers
Scott
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Old 26th September 2005, 07:59 PM   #19
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Default Re: Re: Re: D'Appolito Fudged

Quote:
Originally posted by Scottmoose
In his defence, the tables are not great, and were quickly superceeded by Martin's work
Just a historical note: Augspurger's work and Martin's were being done at the same time. When Augspurger 1st presented his paper at AES NY in 1999, i was already working on the 1st published version of Martin's work. There was lots of time for Joe to see & use the MJK worksheets for the Thor.

I've also used both pieces of software... from a development point of view, the MJL worksheets are way more versatile than Augspurger's (if i was a SW developer i'd be embarrased to ask money for something as crude as TLwrx -- windows users might not be as spoiled as i am thou)

dave
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Old 26th September 2005, 08:13 PM   #20
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It may seem from the above that i'm bashung George abit -- not at all. His worl is very much complementary to Martin's, and the 2 validate each other. To alarge extent George has also gotten the attention of a different audience.

Martin on the other hand has made his work much more accessible and has continued his research & interaction with others, whereas George has gone onto other things (leaving rick Schultz to explore the space opened up by the GA SW). Martin is to be applauded for continuing to push the frontiers & to dramatically improve our understanding of box design.

dave
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